Rx for Millenium 2 Bypass

[ampman50]

Rx for Millenium Bypass.

I've used the mil 2 bypass on all of my pedals and it worked without a fault, however this weekend was the exception. I finally have the change to box up my rebot delay (PT 2399) which I built right after R.G. posted the schematic. The mil 2 bypass I used was right out of R.G.'s site, it would not turn the LED off. After cutting parts out, checking voltages etc. I socketed the FET. I noticed that I could pull the gate out of the socket and the LED would still light, but grounding the gate had no effect, the LED was still on.
I then looked at one on my other pedals to see what was different. The other pedal had the current limiting resistor and LED between the source leg of the FET and ground. After thinking about it and taking some voltages measurements I found that all of the voltage drop in the circuit was taken up by the limiting resistor and the LED so that the voltage at the drain was in effect ZERO. Since it was ZERO grounding the gate had no effect.  By placing the current limiting resistor and LED in the source to ground leg, the voltage at the drain was 9 volts and gounding the gate now shuts the LED off.

If I have copied someones post, I appologize.

[brett]

it would not turn the LED off.

In my experience, this is because you've blown the FET.  There's now no control of the gate over the current through the FET.  If I read your post correctly, you've already worked that out.  The FET simply acts as a switch to turn the LED on or off.  With the gate grounded, the FET is normally off, so no current flows.  With a "blown" FET, current always flows.

My 2 suggestions are;
1. replace the MOSFET (taking care with static electricity)
2. use a Darlington transistor instead of the MOSFET.  (drain=collector, gate=base and source=emitter)  It works well, reduces the cost and is more rugged.

cheers

[brett]

Oops.  Forgot to mention - if you replace the MOSFET with a Darlington transistor (MPSA13 or MPSA14 are good), the LED might be a bit dim.  This can be fixed by adding extra "leaky diodes" eg (1n914, 1N4148) in parallel with the one in the standard M2.  I find that 2 diodes work well if I'm using a Darlington and a low-brightness LED (8mcd).  Some diodes are leakier than others, so the optimum number might vary from 1 to 3.

[ampman50]

The FET was OK.  What I'm getting at is the voltage at the drain and the voltage at the gate were the same, namely ZERO.  This being the case then you can't turn the FET off because you haven't reached Vgsoff since the gate voltage has to be lower then the voltage at the drain/source to shut the FET off.  By placing the limiting resistor and LED on the source side of the FET the drain/source voltage is now 9 volts and grounding the gate will shut the FET off. You could also put either the limiting resistor or the LED in the source leg of the FET, as this would bring the drain/source voltage to somewhere between ZERO and 9 volts.

[R.G.]

I ran into some ugly truths today on the net. I was putting a BS170 design into a board and decided to look up the pinput. I wound up on the Vishay site and the datasheet showed me that  - yep, you guessed it - they make the same chip in a to-92 package with the gate in the middle and drain source *in both possible variations*.

Before you go too far, check to see that the MOSFET is inserted correctly, drain and source. Unfortunately, "correctly" seems to vary. If the drain and source are reversed, the substrate diode conducts all the time and makes the thing look shorted.

If that's not it, could well be a blown MOSFET.

Funny - I know that's a possibility, but most brands of BS170's have internal zeners on the gate that make them pretty much invulnerable. I've never seen a really bona fide killed one that wasn't overheated to death. Gotta happen sometime, I guess.

Using a darlington like the original Rat bypass works, but as Brett says, you're likely to need more leakage current to turn it on, which shows up as additional pop when switching if the tolerances add up against you.