Pots in Series

[Phrazemaster]

Yes it is suspictios!

I just checked - yep - over 4V DC coming out of the signal generator! It's a kit, an Elenco. I did hack in another pot for fine-tuning the output, so it's possible my hack may have inadvertently bypassed some DC blocking caps.

I'm installing a 1uF cap on the signal generator output.  I'll let you guys know what happens. I already tested the cap; it does indeed block DC as expected.

The real question is, is this what has been screwing up my power problem with the FET tester?

I'll let ya know.

Mike

 

[Phrazemaster]

Well I'm afraid I'm at blubbery stupid now. I managed to doctor the signal generator to removed the DC offset, but this did nothing to change anything.

I got a larger power supply - 19v regulated - I know, I know, way overkill. But something's just really wrong. I beeped out the circuit with my DMM to make sure I had connected everything correctly, and it appears I have.

Without a FET, the gate voltage is -17.76. When I drop in the FET it drops to -9.

I'm terrified to try flipping the FET around because even in the battery version when I did that, magic smoke came out of the pots along with a bright flare.

Just to make sure I've got this right, I'm attaching the circuit diagram with my understanding of which lead is which for the FET.

I'm sure this is eye-rollingly stupid, but could someone verify my labels for Collector, Base, and Emitter?

Thanks kindly,

Mike

PS I'm just testing some NPN 2N3708's.

 

[Phrazemaster]

Actually let me state an additional factoid. With no FET, if I trim the gate voltage to -9.00 v, and then drop in the FET, the gate voltage drops to -7.67.

If I then raise the voltage in the power supply to get it to -9.00 V at the gate, wtih the FET installed, removing the FET shows the gate voltage was -17.67V .

That seems excessive; where's the current going? The power supply does not get hot. The FET gets hot.

Could I have something backwards; see previous post?

Thanks in advance,

Mike

 

[PRR]

Do you know the difference between a BJT and a FET?

 

[Phrazemaster]

Do you know the difference between a BJT and a FET?

I know that the 2N3708 and 5088 transistors are BJT's, PNP configuration, and that BJT's are current controlled and FETs are voltage controlled. They have different internal construction, and many subtypes. Since you asked I found a good reference article here: https://www.westfloridacomponents.com/blog/transistors-what-is-the-difference-between-bjt-fet-and-mosfet/

Are you saying the test circuit will not work for BJT's as well as FETs?

I know your circuit works; in the simplest breadboard form it was fine. Only reason I went more elaborate is because the damn 9V battery kept falling in current over time and that made the tests less reliable. So I built the PSU. Which led to a box, and the rest is history.

But I've just gotten some strange results and I'm wondering if I've either misinterpreted something, hence the post with markings for what I believe are the ECB positions, or if I've miswired something, although I've checked and double checked.

I appreciate any thoughts. Tomorrow I will probably tear it all up and rewire from scratch unless anyone can spot an obvious reason I'm missing several volts and the BJT gets hot (checked and rechecked the ECB orientation on that 1000 times however).

Thank-you.

 

[bluebird]

With a transistor your going to have current from the base to the emitter along with the current from the collector to the emitter.  I'm pretty sure your creating a short for the power with a BJT transistor.

Fets work different than a BJT
FET's are labeled Gate, Drain, Source. In an FET current only flows between the Drain and the Source. The Gate is isolated from the Drain and the Source and only creates a "field" that allows current to flow between the Drain and the Source. The gate is kind of like a magic wand... You can literally touch a Gate wire not attached to anything with your finger and it will let current flow from Source to Drain. Or stop the flow.

Thats why they sound good for instrument input devices. The Gate has a very high impedance, and wont load another high impedance source like a guitar. A BJT's base to emitter is like a diode and current can flow. The base is actually touching the collector and emitter. Its P type silicon touching the collector and emitter's N type silicon. (NPN) transistor.

Hope that helps.

 

[Phrazemaster]

Hi BlueBird, yes that does help. So I will be able to use this test circuit for FETs only, not BJTs.

Well it's still been a great romp through learning about pots, and power supplies, and it looks like I can still use this for FETs specifically. I'll need another way to test BJTs for matching where necessary.

Thanks for sharing. I feel really stupid, but then again, I'm learning. Thanks for being kind about it.

Any other thoughts are welcome. Thanks.

 

[MagnetoSound]

Ha! Good one ...

At least you fixed the offset at your signal generator! 

 

[ruffrecords]

Thanks for sharing. I feel really stupid, but then again, I'm learning. Thanks for being kind about it.

As a wise man said, the only people who don't make mistakes are those not doing anything.

Cheers

Ian

 

[Phrazemaster]

Thanks guys - you really are the best.

I’ve noticed that truly learning seems to happen after frustration.

I’ve learned a lot. Thx for not making me feel dumber than I already do!

Best days to you pals.

Mike

Ps I’m going to find some actual FETs and put the FET Tester 9000 through her paces!

 

[Phrazemaster]

I had a go this afternoon matching some actual FETs with PRR's circuit - it worked absolutely flawlessly as described. The FETs didn't heat up, I used a much lower power supply, and it was a pleasant and easy experience.

What a dumb way to learn the difference between transistors, but I had fun along the way and you guys were gold.

I am going to post back here again after I analyze what may have been  happening with a BJT in the circuit instead of the FET. I'm still "in it to win it" and understand what the heck happened even with the "wrong" component in place.

In the meantime, here's a link to cool little  JFET tester based on R. G. Kean's circuit: http://stompville.co.uk/?p=112

You can DIY the circuit or he sells a little kit for 22 pounds.

Also, if anyone would like to borrow the FET TESTER 9000 from me for some serious FET matching, let me know and I'll send it out so you can try it. It's really a cool little box! Not mention, you guys poured yourself into my silliness so you deserve a thank-you.

Thanks PRR, BlueBird, Magneto, Ian, JR, and if I'm forgetting anyone sorry and thanks!

 

[PRR]

....the gate {Base!!} voltage drops to -7.67.  ......

Something which will not come up in your superficial survey of BJTs.... the Base-Emitter Reverse Breakdown Voltage is typically 7V, more or less.

 

[Phrazemaster]

Something which will not come up in your superficial survey of BJTs.... the Base-Emitter Reverse Breakdown Voltage is typically 7V, more or less.

Thank-you PRR - I suspected as much actually, but that's a great confirmation. So much to learn; thanks for your kindness and patience.

I was especially suspicious when almost burning myself on the BJT's and nothing else in the circuit got hot - not the pots, not the Power Supply, just the BJT!

Lessons learned in pain are the ones we remember best.

Happy Sunday!!

Mike