Topics

577D1 itching issue - Haven't touched since, currently in storage

DW
 

Some time ago I made a previous post about a 577 D1 with a collector supply issue being partially stuck on despite the variable voltage control parked all the way at the stop. Ever since then I haven't touched that instrument and it is temporarily in storage for a few weeks due to house remodeling.

In AC mode at 6.5V a horizontal line appears 100mV long which cannot be further reduced by the variable collector supply parked at the stop. Raising the collector voltage aggravates the issue in a magnitude that relates to each greater voltage increment I can set. If I short the emitter and base on the fixture the collector supply then goes to 0V.

I find this problem to be only particular in AC, any other setting such as ramped DC or rectified DC the collector goes to 0V or a spot on the display

I checked for any presence of voltage and current on the test fixture at 6.5V AC, I saw about 40mV AC RMS with a voltmeter and about 0.6 mA of current with a 100 ohm resistor shorted across E and C

I tried to park the variac wiper control as far back as I could, improving the 100mV to 10mV big improvement, unfortunately the wiper was off the winding, it would stick, and voltage control was not smooth, the entire wiper has been reset where it is parked entirely over the the beginning of the winding 100mV present again

I tried modifying the brush of the variac wiper to form more of a point instead of the entire area being flat but with no improvement, I was cautioned for doing this so I reformed the brush back to its flat surface. After this point I left the instrument alone and now it has been in storage for about a week, with about 2 more weeks to remain before I can pull it out again

I have been notified of other 577 users who report their collector supply can reach 0V easily (spot on display) at 6.5V which suggests to me I have unique yet minor problem with this instrument. I found the residual voltage seems to originate from the variac after probing it with a voltmeter, wide open it reads 120V and fully off reads about 11V, I will need to recheck this measurement again after I realized I measured to ground and not to neutral from the wiper. No buzzing, burning smells, or visual signs of burning up from the variac. This is weird and I been thinking about it ever since.

Could jumping a clip lead from Neutral to the wiper with the brush insulated from the windings be useful to diagnose why the ollector will not go to 0V or near it? My theory is if this causes the collector to go to 0V or near it then perhaps I have enough resistance occurring from the neutral terminal to the first windings of the variac causing the collector issue I am experiencing, would you agree?

Thanks and happy new year!

KB6NAX
 

Happy New Year, DW,

Could jumping a clip lead from Neutral to the wiper with the brush insulated from the windings be useful to diagnose why the ollector will not go to 0V or near it? My theory is if this causes the collector to go to 0V or near it then perhaps I have enough resistance occurring from the neutral terminal to the first windings of the variac causing the collector issue I am experiencing, would you agree?
That's a bit risky if you forgetfully turn the variac CW. Better to measure resistance between the first turn of the winding and the neutral terminal. Deteriorated connections are accompanied by increase in constriction resistance. Like to know what you find.

Arden

DW
 

I agree with your response Arden, a resistance measurement is less risky and preferable, good call!

When I can pull the 577 out of storage I will perform these checks

I will check the resistance from the neutral to the first winding, then I will check the resistance from the hot to the last winding and see what the differences are. Then repeat the tests with the probe on the wiper instead of the winding.

Thanks for the reply

 

Hi DW,
I get very nervous whenever I start wondering if I can connect a ground to something that I know is directly connected to the AC line. And I am in one of those 115V countries where I was able to get used to being shocked. I'm so scared every time I have to work on 220V wiring that I will do anything to avoid it. I don't know what your mains voltage is but you are risking a very valuable instrument in search of the answer to something that is only a problem to you.

For instance I have used at least five 577s and the flicker and non-zero offset has never bothered me. I did notice it from time to time on various 577s but since it never affected the operation of the 577 it was simply not important. None of this behavior is unusual for a 577. Because you have mentioned this before, and because I have used so many 577s I thought I could convince you from my personal experience that this is not a problem.

With all the 577s I have owned or used I have seen the flicker many times. Because you mentioned it I finally took a close look at it on the 577 I currently have on my bench. I have been using this particular 577D1 (Storage) for two or three years by now so I know it well. The flicker is there on my current 577 as it has been on all the 577s I think I have used. This is no surprise to me. But this time I measured it so you can compare your results to mine.

I thought setting the horizontal deflection to 0.05V/Div. would give me the best chance to see any flicker or non-zero voltages. I then turned the collector voltage to the 1600V range. With the Variac turned down to zero the trace goes 9 divisions across the screen and it flickers faintly in many spots along the trace. That would be the equivalent of a zero voltage of 0.45V.
With each collector voltage setting this is what I get:
1600V: A line 9 divisions across the screen that flickers faintly in many places along the trace.
400V: A line 1 division long at the 4th graticule. Where that ends there is a flickering faint line 3/4 division long.
100V: A line 1 division long at the 4th graticule. Where that ends there is a flickering faint line 1/2 division long.
25V: A spot at the 4th division, a flickering faint line beginning at the spot and extending half a division to the right.
6V: A spot at the 4th division, a flickering faint line beginning at the spot and extending half a division to the right.

The fact that on four collector voltage settings the line or spot is at the 4th graticule tells me I need to tweak an offset or balance trim pot somewhere inside the 577 to move that spot over to the origin.

I recommend that you explore the many different things you can measure with this fascinating tool instead of worrying about the flickering line at 0V. It took me years to do that. Each time I learned how to measure another transistor or semiconductor parameter with my 577 I became a better circuit designer. The versatility of this instrument is so great that I am probably only familiar a third of half of the things it can do. I'm looking forward to all the things I can still be taught by the 577.

You are very lucky indeed to own such a valuable instrument. I can't wait to find out what you design with it.

Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of KB6NAX
Sent: Wednesday, January 01, 2020 11:52 AM
Subject: Re: [TekScopes] 577D1 itching issue - Haven't touched since, currently in storage

Happy New Year, DW,

Could jumping a clip lead from Neutral to the wiper with the brush
insulated from the windings be useful to diagnose why the ollector
will not go to 0V or near it? My theory is if this causes the
collector to go to 0V or near it then perhaps I have enough resistance
occurring from the neutral terminal to the first windings of the
variac causing the collector issue I am experiencing, would you agree?
That's a bit risky if you forgetfully turn the variac CW. Better to measure resistance between the first turn of the winding and the neutral terminal.
Deteriorated connections are accompanied by increase in constriction resistance. Like to know what you find.

Arden




--
Dennis Tillman W7PF
TekScopes Moderator

DW
 

Dennis, good response

Where I live, the mains are 120 and 240 volts. I been shocked by 120 before, it hurts. I also been shocked once by 240 which I found REALLY hurts! I have gained respect for mains voltages though it might not seem like that appears to be the case here. The idea I proposed about checking the variac with jumper leads while energized is obviously asking for trouble, I am actually glad I asked this and got the answer I need.

Thanks for your feedback with 577 collector performance data for comparison. I can see you used the magnification on the horizontal scale but regardless from my observation excellent results from your 577. My results according to my notes indicate at 25V 50mV/division a line appears 7 divisions long, with your response we see a nice comparison happening here.

I should probably explain why I am really posting about this again. My idea was to take everything from the previous post and consolidate the information so it can be easily viewed and someone might have a interesting idea or solution as they can easily and quickly look it over. I got good useful responses as a result but I fear it is at the expense of repeating myself and not letting it be.

Dennis, you make a good point of using the instrument instead of nitpicking of the details. Changing my state of mind I realize the instrument is mostly functional and I should be happy about that. I seem to have put myself in a corner worrying about details instead of enjoying the possibilities and the usefulness I could be enjoying. I will take up on that offer. If I come across something I will be sure to report back here.

 

Hi DW,
I just remembered another detail that may influence the results of what appears when the Variac is turned to "zero". Variac's are a very interesting design for a transformer for two reasons: The enamel insulation is deliberately removed from the same spot on each turn of its primary. This is done intentionally so the rotating wiper brush can make contact with that turn. Depending on where on the winding the enamel is removed the voltage may not be 0 exactly.

For my example assume there are 120 turns on the primary and the input voltage is 120VAC. Where the low side of the mains (0VAC) is connected to the very beginning of the first turn, the voltage will be 0.0VAC. At the end of the last turn where the high side of the mains is connected the voltage will be 120V. But I checked one Variac I have that I can see into and the enamel insulation is removed from each winding at about the half way point in the turn. The brush would already read 0.5VAC at this point. I don't think there is a point where the voltage is exactly 0.0VAC.

This raises another observation about the brush. It must be narrow so it doesn't span two turns at a time else it will be shorting out one turn to the next turn and that will short them together. They would heat up as would the brush which is causing the short. In my example this would cause a short across two turns that differ by exactly 1VAC. But each winding is capable of providing from 1 to 10 Amps under normal load situations for the size Variacs we are likely to encounter. With a brush causing a shorted winding it isn't hard to see that more than10 Amps could flow. To minimize the likelihood of a short the brush comes to a chisel point. The point is slightly narrower than one turn of wire and it is as wide as the removed enamel area of each turn.

Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of DW
Sent: Wednesday, January 01, 2020 6:36 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 577D1 itching issue - Haven't touched since, currently in storage

Dennis, good response

Where I live, the mains are 120 and 240 volts. I been shocked by 120 before, it hurts. I also been shocked once by 240 which I found REALLY hurts! I have gained respect for mains voltages though it might not seem like that appears to be the case here. The idea I proposed about checking the variac with jumper leads while energized is obviously asking for trouble, I am actually glad I asked this and got the answer I need.

Thanks for your feedback with 577 collector performance data for comparison. I can see you used the magnification on the horizontal scale but regardless from my observation excellent results from your 577. My results according to my notes indicate at 25V 50mV/division a line appears 7 divisions long, with your response we see a nice comparison happening here.

I should probably explain why I am really posting about this again. My idea was to take everything from the previous post and consolidate the information so it can be easily viewed and someone might have a interesting idea or solution as they can easily and quickly look it over. I got good useful responses as a result but I fear it is at the expense of repeating myself and not letting it be.

Dennis, you make a good point of using the instrument instead of nitpicking of the details. Changing my state of mind I realize the instrument is mostly functional and I should be happy about that. I seem to have put myself in a corner worrying about details instead of enjoying the possibilities and the usefulness I could be enjoying. I will take up on that offer. If I come across something I will be sure to report back here.





--
Dennis Tillman W7PF
TekScopes Moderator

KB6NAX
 

Hi Dennis,

I made my donation so you have to read this (just kidding :-).

This raises another observation about the brush. It must be narrow so it doesn't span two turns at a time else it will be shorting out one turn to the next turn and that will short them together. They would heat up as would the brush which is causing the short. In my example this would cause a short across two turns that differ by exactly 1VAC. But each winding is capable of providing from 1 to 10 Amps under normal load situations for the size Variacs we are likely to encounter. With a brush causing a shorted winding it isn't hard to see that more than10 Amps could flow. To minimize the likelihood of a short the brush comes to a chisel point. The point is slightly narrower than one turn of wire and it is as wide as the removed enamel area of each turn. <
I studied this on variacs and came to a different conclusion. The maximum power transfer theorem says that the source and load impedances have to be equal for maximum power to be transferred. A shorted turn is far from a matching impedance for the "primary" side, the rest of the variac winding. In order for a large amount of power to be coupled into a shorted turn the resistance of the wire would have to be much lower than what copper provides in order for the reflected impedance to be equal to the source impedance. Think Weller soldering gun. In addition, the brush, overlapping the pair of contact points is a resistor. Intentionally, so it throws the impedance mismatch further off. The result is a very lossy mismatched transformer action, little power is transferred. Also, the brush is large enough to dissipate its heat, most of which is caused by the output load current. The losses are so small you don't realize they are there.

Arden

Bruce Griffiths
 

Matched loads aren't usually used at mains frequencies.

Bruce

On 02 January 2020 at 18:34 KB6NAX <gumbear@...> wrote:


Hi Dennis,

I made my donation so you have to read this (just kidding :-).

This raises another observation about the brush. It must be narrow so it
doesn't span two turns at a time else it will be shorting out one turn to
the next turn and that will short them together. They would heat up as
would the brush which is causing the short. In my example this would cause
a short across two turns that differ by exactly 1VAC. But each winding is
capable of providing from 1 to 10 Amps under normal load situations for
the size Variacs we are likely to encounter. With a brush causing a
shorted winding it isn't hard to see that more than10 Amps could flow. To
minimize the likelihood of a short the brush comes to a chisel point. The
point is slightly narrower than one turn of wire and it is as wide as the
removed enamel area of each turn. <
I studied this on variacs and came to a different conclusion. The maximum
power transfer theorem says that the source and load impedances have to be
equal for maximum power to be transferred. A shorted turn is far from a
matching impedance for the "primary" side, the rest of the variac winding.
In order for a large amount of power to be coupled into a shorted turn the
resistance of the wire would have to be much lower than what copper provides
in order for the reflected impedance to be equal to the source impedance.
Think Weller soldering gun. In addition, the brush, overlapping the pair of
contact points is a resistor. Intentionally, so it throws the impedance
mismatch further off. The result is a very lossy mismatched transformer
action, little power is transferred. Also, the brush is large enough to
dissipate its heat, most of which is caused by the output load current. The
losses are so small you don't realize they are there.

Arden



Chuck Harris
 

On the contrary, the brush is intended to connect to multiple
windings at the same time. To prevent the problem of shorted
turns, the brush is highly resistive

You can think of the brush as a network of multiple resistors
each connecting to a different turn, and combined to form the
output.

Because each adjacent turn is less than a volt, or so, different
than the next, no great amount of heating occurs in the brush.

Yet another reason why a variac may not reach exactly zero
volts.

-Chuck Harris

Dennis Tillman W7PF wrote:
...

This raises another observation about the brush. It must be narrow so it doesn't span two turns at a time else it will be shorting out one turn to the next turn and that will short them together. They would heat up as would the brush which is causing the short. In my example this would cause a short across two turns that differ by exactly 1VAC. But each winding is capable of providing from 1 to 10 Amps under normal load situations for the size Variacs we are likely to encounter. With a brush causing a shorted winding it isn't hard to see that more than10 Amps could flow. To minimize the likelihood of a short the brush comes to a chisel point. The point is slightly narrower than one turn of wire and it is as wide as the removed enamel area of each turn.

Dennis Tillman W7PF

george edmonds
 

Hi all

I have been following this topic for some time, why not do the simple thing and measure with a DMM the voltage between cold end of the Variac and the slider when fully counter clockwise, or am I missing a point. My UK made Variac has a zero volt pad for the brush and termination of the winding.

73 George G6HIG On Thursday, 2 January 2020, 06:14:15 GMT, Chuck Harris <cfharris@...> wrote:

On the contrary, the brush is intended to connect to multiple
windings at the same time.  To prevent the problem of shorted
turns, the brush is highly resistive

You can think of the brush as a network of multiple resistors
each connecting to a different turn, and combined to form the
output.

Because each adjacent turn is less than a volt, or so, different
than the next, no great amount of heating occurs in the brush.

Yet another reason why a variac may not reach exactly zero
volts.

-Chuck Harris

Dennis Tillman W7PF wrote:
...

This raises another observation about the brush. It must be narrow so it doesn't span two turns at a time else it will be shorting out one turn to the next turn and that will short them together. They would heat up as would the brush which is causing the short. In my example this would cause a short across two turns that differ by exactly 1VAC. But each winding is capable of providing from 1 to 10 Amps under normal load situations for the size Variacs we are likely to encounter. With a brush causing a shorted winding it isn't hard to see that more than10 Amps could flow. To minimize the likelihood of a short the brush comes to a chisel point. The point is slightly narrower than one turn of wire and it is as wide as the removed enamel area of each turn.

Dennis Tillman W7PF

 

Chuck, Bruce, and Arden,
All my life I assumed there was little more to learn from how a Variac works. They appear to be pretty simple and a very clever. This has accounted for their incredibly useful life spanning more than 85 years.

The practical side of me says it is a very bad idea to short out a turn on a transformer with 120 (more or less) turns and 120V across it. So tomorrow I will test this by intentionally hooking a short piece of wire from one turn to its neighbor while I am holding the wire in my hand. If you are right the wire won't get hot and I will learn something new. If I am right I will get burned and never trust the three of you again. Either way you have nothing to lose. :)

I will report the results either way.

Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of KB6NAX
Sent: Wednesday, January 01, 2020 9:35 PM
Subject: Re: [TekScopes] 577D1 itching issue - Haven't touched since, currently in storage

Hi Dennis,

I made my donation so you have to read this (just kidding :-).

This raises another observation about the brush. It must be narrow so
it doesn't span two turns at a time else it will be shorting out one
turn to the next turn and that will short them together. They would
heat up as would the brush which is causing the short. In my example
this would cause a short across two turns that differ by exactly 1VAC.
But each winding is capable of providing from 1 to 10 Amps under
normal load situations for the size Variacs we are likely to
encounter. With a brush causing a shorted winding it isn't hard to see
that more than10 Amps could flow. To minimize the likelihood of a
short the brush comes to a chisel point. The point is slightly
narrower than one turn of wire and it is as wide as the removed enamel
area of each turn. <
I studied this on variacs and came to a different conclusion. The maximum power transfer theorem says that the source and load impedances have to be equal for maximum power to be transferred. A shorted turn is far from a matching impedance for the "primary" side, the rest of the variac winding.
In order for a large amount of power to be coupled into a shorted turn the resistance of the wire would have to be much lower than what copper provides in order for the reflected impedance to be equal to the source impedance.
Think Weller soldering gun. In addition, the brush, overlapping the pair of contact points is a resistor. Intentionally, so it throws the impedance mismatch further off. The result is a very lossy mismatched transformer action, little power is transferred. Also, the brush is large enough to dissipate its heat, most of which is caused by the output load current. The losses are so small you don't realize they are there.

Arden






--
Dennis Tillman W7PF
TekScopes Moderator

Ed Breya
 

Chuck called it right - the semi-conductive brush provides interpolation of the output between the discrete steps of the windings. It has to bridge at least two turns at any position, to give a relatively smooth transition between steps. If it didn't, the output would necessarily be discrete steps of voltage, and include "off" (disconnected) states in between, which wouldn't be good. It's a compromise to provide more or less continuous output control, and it does waste some power to heat the brush, and it contributes to wear, besides the mechanical friction. Larger variacs have extra features for better cooling of the brushgear.

The conductivity, abrasiveness, contact force, and shape of the brush have to be just right for best trade-offs between smoothness, power rating, and life. Variacs can be very tricky, but this was all pretty much perfected before any of us were born. Be happy that they work so well at their purpose.

The same sorts of issues arise in other devices where discreteness needs to be handled with smoothness and continuity, like a wirewound pot or rheostat, or the commutator of a motor.

A related, interesting insight can be gained by studying the carbon-pile, a very old-school device.

Ed

Ed Breya
 

On Wed, Jan 1, 2020 at 11:43 PM, Dennis Tillman W7PF wrote:

The practical side of me says it is a very bad idea to short out a turn on a
transformer
Dennis, you should skip your planned experiment just by considering that we're not talking about shorting out adjacent turns on a variac - the wiper is a (carbon and copper, typically) brush, not a metal contact, which are way different from each other. So picture not shorting with a wire, but with a resistance network that bridges the turns, and part of the network goes to the output terminal.

Ed

Albert Otten
 

On Thu, Jan 2, 2020 at 10:06 AM, Ed Breya wrote:


On Wed, Jan 1, 2020 at 11:43 PM, Dennis Tillman W7PF wrote:

The practical side of me says it is a very bad idea to short out a turn on a
transformer
Dennis, you should skip your planned experiment just by considering that we're
not talking about shorting out adjacent turns on a variac - the wiper is a
(carbon and copper, typically) brush, not a metal contact, which are way
different from each other. So picture not shorting with a wire, but with a
resistance network that bridges the turns, and part of the network goes to the
output terminal.

Ed

Jean-Paul
 

Hello all, fine notes on 577 and variacs! The 576 is essential for power electronics design and debugging.
This monster is robust, in 25- 30 years, I have never serviced it except to replace blown out scale illum or fiber alpha display lamps! Inside clean as can be.

1/ See the app notes from GenRad/IEC and Superior Electric for brush care and function:
e.g. https://www.ietlabs.com/pdf/Manuals/GR/GR%20V20%20Variac.pdf
The manufacturers of the Variacs and Powerstats have replacement brushes available.

2/ I swapped my 577 decades ago, results on my 576 (including 100A 176 pulser!):

Step gen ON, < 0.2 div noise and trace with variac at 0. regardless of voltage range setting 15 V - 1500 V
Step gen OFF, perfect round focused dot at 0,0 regardless of voltage range setting.

I doubt if the 576 and 577 differ much in this result. Perhaps the same small variac?

Just the ramblings of an old retired EE

Jon

k1ggi
 

Dennis,
You can confidently place your trust in three other wise men, Karplus,
Tuttle, and Ohm.
https://patentimages.storage.googleapis.com/a3/1e/8c/f0878e160074e6/US200901
3.pdf
Ed, k1ggi

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Dennis
Tillman W7PF
Sent: Thursday, January 02, 2020 2:43 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 577D1 itching issue - Haven't touched since,
currently in storage

Chuck, Bruce, and Arden,
All my life I assumed there was little more to learn from how a Variac
works. They appear to be pretty simple and a very clever. This has accounted
for their incredibly useful life spanning more than 85 years.

The practical side of me says it is a very bad idea to short out a turn on a
transformer with 120 (more or less) turns and 120V across it. So tomorrow I
will test this by intentionally hooking a short piece of wire from one turn
to its neighbor while I am holding the wire in my hand. If you are right the
wire won't get hot and I will learn something new. If I am right I will get
burned and never trust the three of you again. Either way you have nothing
to lose. :)

I will report the results either way.

Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of KB6NAX
Sent: Wednesday, January 01, 2020 9:35 PM
Subject: Re: [TekScopes] 577D1 itching issue - Haven't touched since,
currently in storage

Hi Dennis,

I made my donation so you have to read this (just kidding :-).

This raises another observation about the brush. It must be narrow so
it doesn't span two turns at a time else it will be shorting out one
turn to the next turn and that will short them together. They would
heat up as would the brush which is causing the short. In my example
this would cause a short across two turns that differ by exactly 1VAC.
But each winding is capable of providing from 1 to 10 Amps under
normal load situations for the size Variacs we are likely to
encounter. With a brush causing a shorted winding it isn't hard to see
that more than10 Amps could flow. To minimize the likelihood of a
short the brush comes to a chisel point. The point is slightly
narrower than one turn of wire and it is as wide as the removed enamel
area of each turn. <
I studied this on variacs and came to a different conclusion. The maximum
power transfer theorem says that the source and load impedances have to be
equal for maximum power to be transferred. A shorted turn is far from a
matching impedance for the "primary" side, the rest of the variac winding.
In order for a large amount of power to be coupled into a shorted turn the
resistance of the wire would have to be much lower than what copper provides
in order for the reflected impedance to be equal to the source impedance.
Think Weller soldering gun. In addition, the brush, overlapping the pair of
contact points is a resistor. Intentionally, so it throws the impedance
mismatch further off. The result is a very lossy mismatched transformer
action, little power is transferred. Also, the brush is large enough to
dissipate its heat, most of which is caused by the output load current. The
losses are so small you don't realize they are there.

Arden






--
Dennis Tillman W7PF
TekScopes Moderator

KB6NAX
 

That would not be a logical test, Dennis. The jumper would have to be a resistor to simulate the resistance of the brush. But if you must jumper the turns with a wire do it through a current transformer so you can tell us what amount of current flows in the jumper. Bon chance.

Arden

-----Original Message-----
From: Dennis Tillman W7PF
Sent: Wednesday, January 01, 2020 11:43 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 577D1 itching issue - Haven't touched since, currently in storage

Chuck, Bruce, and Arden,
All my life I assumed there was little more to learn from how a Variac works. They appear to be pretty simple and a very clever. This has accounted for their incredibly useful life spanning more than 85 years.

The practical side of me says it is a very bad idea to short out a turn on a transformer with 120 (more or less) turns and 120V across it. So tomorrow I will test this by intentionally hooking a short piece of wire from one turn to its neighbor while I am holding the wire in my hand. If you are right the wire won't get hot and I will learn something new. If I am right I will get burned and never trust the three of you again. Either way you have nothing to lose. :)

I will report the results either way.

Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of KB6NAX
Sent: Wednesday, January 01, 2020 9:35 PM
Subject: Re: [TekScopes] 577D1 itching issue - Haven't touched since, currently in storage

Hi Dennis,

I made my donation so you have to read this (just kidding :-).

This raises another observation about the brush. It must be narrow so
it doesn't span two turns at a time else it will be shorting out one
turn to the next turn and that will short them together. They would
heat up as would the brush which is causing the short. In my example
this would cause a short across two turns that differ by exactly 1VAC.
But each winding is capable of providing from 1 to 10 Amps under
normal load situations for the size Variacs we are likely to
encounter. With a brush causing a shorted winding it isn't hard to see
that more than10 Amps could flow. To minimize the likelihood of a
short the brush comes to a chisel point. The point is slightly
narrower than one turn of wire and it is as wide as the removed enamel
area of each turn. <
I studied this on variacs and came to a different conclusion. The maximum power transfer theorem says that the source and load impedances have to be equal for maximum power to be transferred. A shorted turn is far from a matching impedance for the "primary" side, the rest of the variac winding.
In order for a large amount of power to be coupled into a shorted turn the resistance of the wire would have to be much lower than what copper provides in order for the reflected impedance to be equal to the source impedance
Think Weller soldering gun. In addition, the brush, overlapping the pair of contact points is a resistor. Intentionally, so it throws the impedance mismatch further off. The result is a very lossy mismatched transformer action, little power is transferred. Also, the brush is large enough to dissipate its heat, most of which is caused by the output load current. The losses are so small you don't realize they are there.

Arden






--
Dennis Tillman W7PF
TekScopes Moderator

KB6NAX
 

Thanks, Albert. That's about as definitive as it can get:

https://patentimages.storage.googleapis.com/a3/1e/8c/f0878e160074e6/US2009013.pdf

Arden

-----Original Message-----
From: Albert Otten
Sent: Thursday, January 02, 2020 2:03 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 577D1 itching issue - Haven't touched since, currently in storage

To refresh memories:
https://groups.io/g/TekScopes/topic/7651993

Albert
........................

Chuck Harris
 

Dennis,

It is a very bad idea to short out a turn on a transformer.

But it is a very good idea to load a turn or two on a transformer...
after all, that is the transformer's job.

Here is what your variac's brush really is:

Turn1 <--Resistor---+
Turn2 <--Resistor---+-----Brush terminal
Turn3 <--Resistor---+

The voltage between turns is around a volt on most variacs.
The resistance of the brush material is chosen so that its
the power dissipation will be 1 or 2 watts, for a typical
variac.

If it is essential that a variac go all the way to zero, there
will be a metallic landing pad for the brush to land on after
it falls off of the last turn.

Absent the landing pad, the lowest voltage will be on the
order of 1/2 turn's voltage.

The variac was originally designed to be a lamp dimmer for
theater use... it replaced the very wasteful carbon pile
rheostats originally used.

This curve tracer application is forcing an oval peg into
a round hole. It sort of fits... if you force it.

-Chuck Harris

Dennis Tillman W7PF wrote:

Chuck, Bruce, and Arden,
All my life I assumed there was little more to learn from how a Variac works. They appear to be pretty simple and a very clever. This has accounted for their incredibly useful life spanning more than 85 years.

The practical side of me says it is a very bad idea to short out a turn on a transformer with 120 (more or less) turns and 120V across it. So tomorrow I will test this by intentionally hooking a short piece of wire from one turn to its neighbor while I am holding the wire in my hand. If you are right the wire won't get hot and I will learn something new. If I am right I will get burned and never trust the three of you again. Either way you have nothing to lose. :)

I will report the results either way.

Dennis Tillman W7PF

Dave Hills
 

What you are seeing may just be stray signal pickup from the open input of the horizontal
amplifier when no DUT is present. Try connecting a 1K resistor from the collector to emitter terminals
of the 177 and move the left-right switch to the appropriate side. On mine this drives the line to a dot
on any voltage range of the peak voltage range switch with the variable collector set to zero%.

Dave

On Wed, Jan 1, 2020 at 12:37 PM, DW wrote:


I agree with your response Arden, a resistance measurement is less risky and
preferable, good call!

When I can pull the 577 out of storage I will perform these checks

I will check the resistance from the neutral to the first winding, then I will
check the resistance from the hot to the last winding and see what the
differences are. Then repeat the tests with the probe on the wiper instead of
the winding.

Thanks for the reply