EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: watchmaker on March 01, 2024, 12:18:37 pm
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In my pursuit to perfect the ThaiKit Curve tracer, I added a function generator driven VI board on the outside of the case to have variable voltage for testing zeners. Works good, but my AWG only puts out 10V. I realized I have a 25 V AC secondary inside the case.
In my simplistic world view, I want to run that 25 volts across a switched 10k ohm pot. Take the center tap and the side that feeds the negative 9just seems right, but...) voltage supply up to my board. This is so I can test zeners up to 25 volts.
I also need to find a plastic box bottom to protect the ckt board which will reside on top of my curve tracer box. All will be inside cabinet except where the variable voltage comes up underneath the test board.
Because of the low currents used by the boards, I am going to fuse the secondary since the primary side of the transformer would need a fuse that is too small (1/5 that needed to protect the secondary side boards).
Thanks.
Why is this bad?
Thanks!
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Not 100% sure what you mean. Are you using an Active VI tracer or an Octopus? If you transformer is 25VAC then drawing voltage from centre tap would only give you 12.5VAC. Or is the transformer 25V-0V-25V. What ever the case may be, adding a voltage divider wouldn't pose any problem.
A 1/2W potentiometer should be plenty for the few mA needed to test zeners
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Because of the low currents used by the boards, I am going to fuse the secondary since the primary side of the transformer would need a fuse that is too small (1/5 that needed to protect the secondary side boards).
A fuse on the secondary doesn't help if the transformer develops a short circuit within the windings.
Also, the purpose of a fuse isn't to protect your electronics. It can't and won't do that. It's there to prevent your wiring from starting a fire. There is no need to size your fuse to the actual current drawn if that is extremely low. It just needs to be sized to protect the wiring. A 400 mA fuse (which is quite common) is often fine even if normal current draw is only 10 mA.
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The pots may want some extra protection, so that a short on the output side would not destroy it.
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Thank you all!
Yes, I will fuse the pot.
JWillis, I am using my scope see the trace of the Zener for which a variable ac voltage is useful. And it is correct that all I have across the full winding is 15 vac.
I was dissatisfied with the lack of means for safely checking Zeners on the ThaiKit tracer. And I like having things as compact as possible. Just look at the space I have to work with! My watch shop is 3 times larger and that does not include my machine room.
The purpose of the probe pins and the separate zener socket is to allow testing several zeners without messing with the probes. The cover is a watch band case (what else you expect?).
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