Reverse Polarity Protection.


Denys VK3zyz
 

I recommend you do install reverse polarity protection.
My v5 board now has a 1.A polyswitch and a reverse diode. I wish it did earlier!! For some reason, I thought I'd put that mod on later, and of course, just after getting it fully calibrated, I plugged the 12V gell cell battery in the wrong way around :(
I've replaced the burnt track feeding the power amp, Q15 and the 5V reg on the control board. It sort off works, but has lost sensitivity and clarity so I think a mixer is suspect. So far, I've not figured out what is wrong.
So, put the reverse protection on FIRST!!!!!


Evan Hand
 

I would agree, as I too had a reverse polarity issue.  For me it was a left over HP power brick; HP has for some reason reverse polarity on the barrel connectors.  I forgot this and plugged it in.  Never got to turn on the rig, as the power supply crow bar shut down the power supply.  Would have fried the rig if I had not wired up the diode as listed in the wiring directions.  The only thing I would add is make sure you have a fuse in line upstream of the diode.  This is really critical if your supply can provide more than 3 amps, and does not have a crow bar shutdown circuit.

73
Evan
AC9TU


Denys VK3zyz
 

That is why I used a polyswitch. It is a resettable fuse :)

https://www.mouser.com/ProductDetail/Littelfuse/RXEF110?qs=hv6pn79dJPSCD61RjaQM8g%3D%3D


Evan Hand
 

Since it is 1 amp, I assume that it is only protecting the board, and not the PA.  When on 80 meters, my v4 and v5 boards will draw 2+ amps total.  That would be close to the trip point of the poly switch that you specified.

On my v5 rig, I have split the fuses, and have 2 1n4001 diodes in series from the 13.5 vdc supplying the rig and the separate feed to the board (red wire on the supplied connector).  The reverse protection is then on the fuse for the PA, since the two series diodes would protect the rest of the board from reverse power.  If i connect the power supply incorrectly, the 3 amp fuse in the PA lead will pop.  This arrangement also allows me to remove the PA fuse and power the rest of the transceiver for testing.  When doing this, the reverse protection is the diodes in series with the power supply to drop the voltage to just under 12 volts.

73
Evan
AC9TU


Denys VK3zyz
 

Ah, thanks for that info Evan.
I'll upgrade the polyswitch. I have them from 50mA to 8Amps here in stock.
Still, I've not got my board going correctly. It is distorted and deaf so I think a mixer is damaged. It is looking like removing the transistors one by one to test them.  Hot air tool and tweezers to work!


David Wilcox <Djwilcox01@...>
 

Just a warning to any hams selling batteries at swaps. Please don’t put red price stickers next to the negative battery pole......

Don’t ask me why.

I now check all batteries I might buy when I get home and mask off a big square around the positive pole and paint it bright red.  Late at night and in haste my eyes and brain don’t coordinate well.  Enough smoke in the shack from my soldering.

Dave K8WPE

David J. Wilcox K8WPE’s iPad

On Mar 27, 2019, at 5:02 AM, Denys VK3zyz <dendad51@...> wrote:

Ah, thanks for that info Evan.
I'll upgrade the polyswitch. I have them from 50mA to 8Amps here in stock.
Still, I've not got my board going correctly. It is distorted and deaf so I think a mixer is damaged. It is looking like removing the transistors one by one to test them.  Hot air tool and tweezers to work!


Evan Hand
 

Sorry to hear about your uBitx board.  I have used these testers:
https://www.eetimes.com/author.asp?section_id=30&doc_id=1329331#

with some success in circuit testing.  Since it is just looking for gain, it is a fairly good indication that the device is not dead.  Been a handy device as it can also check caps and coils, though not too small of values.

The devices are available from a number of manufacturers on the Web.

73
Evan
AC9TU


Tom, wb6b
 

Here is an interesting link to a mosfet reverse polarity protection circuit. I found this mentioned in a post on this group from about a year ago.

https://kc9on.com/ham-radio/station-accessories/reverse-polarity-protection/

Tom, wb6b

----

"Please don’t put red price stickers next to the negative battery pole......"
Ouch!


Denys VK3zyz
 

Yes, they are very useful! I have one and recommend  the addition to any workshop.


 

I have not come across a dud transistor but many issues with soldering.

Check all the mixer diodes. Using the tip of your multimeter probe
gently press the legs towards the board and discover any solder given up in the
board's long travel! Do this on RX with a volume turned up. Any increase in
background noise and you have found it.

Raj


At 27-03-19, you wrote:
Ah, thanks for that info Evan.
I'll upgrade the polyswitch. I have them from 50mA to 8Amps here in stock.
Still, I've not got my board going correctly. It is distorted and deaf so I think a mixer is damaged. It is looking like removing the transistors one by one to test them.  Hot air tool and tweezers to work!


Evan Hand
 

Raj,

FYI, The loss in receiver function was due to a reverse power application WITHOUT the protection diode installed.  That was why he was checking all of the active devices.

73
Evan
AC9TU


Jerry Gaffke
 

The PFET reverse polarity protection is not a bad choice if you need the minimum possible voltage drop.
But it's easy to goof up.  Some schematics for this on the web show the PFET
placed backwards, and thus offering zero protection from reverse polarity.
At power up, the intrinsic reverse diode conducts from drain to source, till the source is Vgs above the gate.
At that point, what is hopefully an enhancement mode PFET starts to conduct, reducing the voltage drop
between drain to source from a diode drop to that incurred by the current through Rds-on.

For a uBitx, I'd recommend instead a diode (most anything capable of dealing with 500ma)
in series with the main 12v into the board at P1 pin 1 (which also protects the Raduino).
We can afford a slight voltage drop there, and this is much easier to understand.

The 12v into PA-PWR on P1 pin 2 for the IRF510's can be a direct connection, since a reverse voltage
on PA-PWR will cause the IRF510's to conduct through their intrinsic reverse diode.
If there is no fuse then some easily repaired board traces (and/or L8,, L9) will blow,
but not your IRF510's.  Diode protection here is ok if you insist, but you will need
a fairly hefty diode, and the diode drop will reduce transmit power by a bit.

A 3 Amp fuse between the power supply and uBitx is highly recommended.

Without reverse protection into the main board, the Raduino can fry because
the ancient LM7805 on the Raduino does not block a negative voltage.
There are newer regulators such as the LM2940 that do.
Replacing the protection diode recommended above with an LM2940T-12,
gives protection for reverse battery voltage, it will also protect the rig
from excessive battery voltage.  (PA-PWR should not go through an LM2940T,
as that will draw too much current.) 

Jerry, KE7ER


But many people who use don't understand how it works, and often circuits found on the web show the FET backwards.

for this scheme show the FET in backwards.


On Wed, Mar 27, 2019 at 04:30 AM, Tom, wb6b wrote:
Here is an interesting link to a mosfet reverse polarity protection circuit. I found this mentioned in a post on this group from about a year ago.

https://kc9on.com/ham-radio/station-accessories/reverse-polarity-protection/

Tom, wb6b


MadRadioModder
 

I far prefer the approach below as you are not crow-baring voltages, blowing fuses, shorting diodes, and letting smoke escape:

 

 

More elegant solution.

 

From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of Jerry Gaffke via Groups.Io
Sent: Wednesday, March 27, 2019 12:08 PM
To: BITX20@groups.io
Subject: Re: [BITX20] Reverse Polarity Protection.

 

The PFET reverse polarity protection is not a bad choice if you need the minimum possible voltage drop.
But it's easy to goof up.  Some schematics for this on the web show the PFET
placed backwards, and thus offering zero protection from reverse polarity.
At power up, the intrinsic reverse diode conducts from drain to source, till the source is Vgs above the gate.
At that point, what is hopefully an enhancement mode PFET starts to conduct, reducing the voltage drop
between drain to source from a diode drop to that incurred by the current through Rds-on.

For a uBitx, I'd recommend instead a diode (most anything capable of dealing with 500ma)
in series with the main 12v into the board at P1 pin 1 (which also protects the Raduino).
We can afford a slight voltage drop there, and this is much easier to understand.

The 12v into PA-PWR on P1 pin 2 for the IRF510's can be a direct connection, since a reverse voltage
on PA-PWR will cause the IRF510's to conduct through their intrinsic reverse diode.
If there is no fuse then some easily repaired board traces (and/or L8,, L9) will blow,
but not your IRF510's.  Diode protection here is ok if you insist, but you will need
a fairly hefty diode, and the diode drop will reduce transmit power by a bit.

A 3 Amp fuse between the power supply and uBitx is highly recommended.

Without reverse protection into the main board, the Raduino can fry because
the ancient LM7805 on the Raduino does not block a negative voltage.
There are newer regulators such as the LM2940 that do.
Replacing the protection diode recommended above with an LM2940T-12,
gives protection for reverse battery voltage, it will also protect the rig
from excessive battery voltage.  (PA-PWR should not go through an LM2940T,
as that will draw too much current.) 

Jerry, KE7ER


But many people who use don't understand how it works, and often circuits found on the web show the FET backwards.

for this scheme show the FET in backwards.

On Wed, Mar 27, 2019 at 04:30 AM, Tom, wb6b wrote:

Here is an interesting link to a mosfet reverse polarity protection circuit. I found this mentioned in a post on this group from about a year ago.

https://kc9on.com/ham-radio/station-accessories/reverse-polarity-protection/

Tom, wb6b


Virus-free. www.avg.com

--

…_. _._


Jerry Gaffke
 

Tastes vary.
A series diode into P1 pin 1 plus a fuse at the power supply seems plenty elegant.
If worried about overvoltage, use an LM2940T-12 instead.

Not being a mechanical engineer, relays not so much.
But if building a 1kW linear with high currents involved, I'd consider the relay.

Jerry


On Wed, Mar 27, 2019 at 10:43 AM, MadRadioModder wrote:
More elegant solution.


kj5wi@...
 

MadRadioModder presents a universal—no messing around inside—safer for all solution. Both work but his leaves any equipment unmodified. 


Kevin Luxford
 

MRM, I like your approach, which looks pretty foolproof to me.  I used a different approach - SS resettable fuse, PNP FET reverse polarity protection followed by 5V regulator to supply just the nextion.  You have spurred me to re-examine the PNP FET connections.  A job for later on.
73 Kevin VK3DAP / ZL2DAP


MadRadioModder
 

Excellent!  We can burn up stuff together!

MRM

 


On May 1, 2019, at 1:05 PM, Kevin Luxford <kbgluxford@...> wrote:

MRM, I like your approach, which looks pretty foolproof to me.  I used a different approach - SS resettable fuse, PNP FET reverse polarity protection followed by 5V regulator to supply just the nextion.  You have spurred me to re-examine the PNP FET connections.  A job for later on.
73 Kevin VK3DAP / ZL2DAP

--

…_. _._