Re: Yet another use for a curve tracer


 

Hi Sean,
Lucky you to own a 577D1!

One clever thing you can do with the 577D1 that no other Tek curve tracer can do is find the perfect bias point for a junction FET. You should bias a FET right at the point where the temp coefficient is zero but how do you find that point except by trial and error? FETs are very temperature sensitive little critters with a very unusual temperature coefficient. It is positive up to a certain point and then it turns negative. For your circuit's best temperature stability you need to bias a FET right at the point where the temp coefficient is zero but how do you find that point except by trial and error?
The 577D1 can find that exact zero temperature coefficient point to bias the FET in less than 30 seconds.

Here is how to do it: Display a set of typical FET curves on the 577D1 with storage turned on. Heat the FET up with your finger, or with anything hot for just a second. As the FET die warms up the gate voltage curves will begin to move up above a certain bias voltage and they will move down below that voltage.

The result on the CRT will be that one gate voltage curve will not move so it will remain sharp. All the other bias voltage curves will smear as they move away from the perfect point to bias the FET.

You can also use something cold like an ice cube touched to the FET. The results will be the same except the smears will go in the opposite direction. But one bias curve right in the middle of the smeared ones will be sharp because it didn't move at all.

Give it a try. It is fascinating to watch and it only takes a minute to do this and it is a perfect example of yet another use for a curve tracer. But in this case it is hard to do without storage so that is another reason why my favorite curve tracer is the 577D1.

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of sdturne@q.com
Sent: Tuesday, December 08, 2020 6:38 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Yet another use for a curve tracer

Dennis,

This is most definitely true. I got me a very nice 577D1 on the same trip I picked up my 519 on, and suffice to say, I am discovering something new it can do every day. Now to collect all the handy test fixtures Tek made...

Sean

On Tue, Dec 8, 2020 at 10:53 AM, Dennis Tillman W7pF wrote:


Hi Ed,
Curve tracers are very versatile instruments and the only real
limitation they have is the person at the controls.
Nearly everyone I know who owns a curve tracer thinks they are only
good for checking transistors.

What you did was think outside the box, and as you said in the
subject, you found another use for a curve tracer.
I hadn't thought of using it to test lamp dimmers myself so I learned
something new.
But I have used it to test capacitors, inductors, transformers, pots,
SCRs, TRIACs, DIACs, MOVs, OpAmps, Voltage Regulators ICs, Spectrum
Tubes, light bulbs, etc.
Anything with two or three leads is fair game for a curve tracer.

Anyone who owns a curve tracer is extremely fortunate. You can learn a
great deal about how electronic parts of all kinds respond when you
connect them to a curve tracer.

Thanks for sharing your clever idea.

Dennis Tillman W7pF






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Dennis Tillman W7pF
TekScopes Moderator

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