MOSFET boost, no sound (voltages)

spargo · July 28, 2011, 07:57:45 PM

I breadboarded a MOSFET boost. Worked awesomely. So I boxed it up, and now it...doesn't work at all.

The voltages on my 2N7000 WERE:

D: 9V
G: 0V
S: 0V

So I figured I may have reversed it...flipped the transistor around and my voltages are now:

D: 3.6V
G: 0V
S: 4.2V

Voltage divider reads 5.6V at Vr.

No audio. Any ideas? I've checked everything else 10 times, and even started over from scratch.

PRR · July 29, 2011, 01:31:44 AM

Why is G always 0V? Might be a short there?

spargo · July 29, 2011, 02:16:16 AM

I've omitted the protection diode as I believe the 2N7000 has one built in. So my Gate is connected directly to the Source. Upon looking at the schematic and datasheet for the correct manufacturer again I believe I had the MOSFET positioned correctly in the first place, which produced these voltages:

D: 9V
G: 0V
S: 0V

The 9V on the Drain is very confusing to me...I will keep checking again for shorts, etc.

spargo · July 29, 2011, 02:26:39 AM

Nevermind on this one...

spargo · July 29, 2011, 06:51:29 PM

So, as mentioned I've removed the zener diode. When removing this, should you fill the gap and connect the gate to the source? Or should you simply pull the diode, leaving the Gate connected to R3, and the Source connected to R5?

Kearns892 · July 29, 2011, 07:10:06 PM

Gate and Source should not be shorted together. The internal diode in a 2N7000 connects Drain to Source not Gate to Source.

Think about it though, even if there was hypothetically an internal diode connecting Gate to Source, "filling the gap" and connecting Gate to Source would create a short. Imagine you place a diode between Gate and Source of a MosFET on a breadboard, then proceed to place a jumper in parallel with the diode also between Gate and Source; such a jumper would bypass the diode completely.

In this case you still need to add the protection diode between Gate and Source. I'm not sure, but MosFETs are finicky creatures and you may have fried yours in the process (hopefully you socketed this one and have another you can try just for debugging purposes). If you don't have a 9.1v zener as called for in the schematic, an LED should work fine (same orientation as the zener) as per the peppermill schematic from runoffgroove http://www.runoffgroove.com/peppermill.html.

Try those changes then post back.

spargo · July 29, 2011, 08:37:35 PM

That was it. Accidentally connected Gate to Source on the PCB, but probably didn't have it that way on the bread board. Doh!

I just cut a couple traces and used a couple jumpers to verify.

Kearns892 · July 30, 2011, 11:23:05 AM

Great! Glad to hear you got it working. Just out of curiosity, did you put an LED or diode in? It may work without one, but I think they are there for protection purposes. By leaving it out, you may be opening yourself up to reliability issues. (I may be wrong here so someone else feel free to chime in and correct me).

spargo · July 30, 2011, 01:12:43 PM

Quote from: Kearns892 on July 30, 2011, 11:23:05 AM
Great! Glad to hear you got it working. Just out of curiosity, did you put an LED or diode in? It may work without one, but I think they are there for protection purposes. By leaving it out, you may be opening yourself up to reliability issues. (I may be wrong here so someone else feel free to chime in and correct me).

Nope, I used a 2N7000 which has an internal protection diode. The BS170 on the schematic does not.

Kearns892 · July 30, 2011, 01:45:59 PM

Hmmm, I dug a little more on the 2n7000 and it looks like you you don't need the diode. Oops

. At least I could help you diagnose the problem!

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MOSFET boost, no sound (voltages)