ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Soft Start question ?

:

6/27/2000 4:21 AM

Soft Start question ?

I recently saw a post where Doc suggested using a 100k resistor in the power supply between the solid state rectifier an first filter cap. This could be switched in and out with a time delay circuit relay. a small relay with 400 volt contacts seems to be unavailable. I was wondering if an N-channel mosfet could be used with an RC circuit for this application. It seems like you could heat sink it and ramp it up to short out the resistor.

Tracy

6/27/2000 12:01 PM

MOSFET Follies

I keep pestering people to use high voltage MOSFETs for "utility" functions like this. See my web page (GEO) on "MOSFET Follies".

It's a little more convenient to use an N channel MOSFET like the IRF820 and put it in the transformer CT return, as it is now down around ground instead of floating up around B+, so you don't have to do floating drive circuitry. You can use it for standby and automated protection as well.

6/27/2000 7:52 PM
Stephen Conner	R.G. Surely when the PT centre tap is disconnected it would try and go negative? In this case you would need a high-voltage P-channel power MOSFET - good luck finding one Steve C.

6/27/2000 9:02 PM
Tracy	Re: OK NO P CHANNEL SO ? Hi So, how do i do floating drive with an N channel mosfet across the 50K or 100K resistor in the power supply. it seems like the resistor could share the disipation with the mosfet. and once it's on it would stay on keeping the resistor out of circuit. I like the idea of using a seperate mosfet for cathode standby switching. I just need a little help. Thanks Tracy

6/27/2000 10:02 PM
R.G.	Don't do that yet. Let us thrash it out for a bit.

6/27/2000 9:45 PM

Re: MOSFET Follies

quote:
"Surely when the PT centre tap is disconnected it would try and go negative?"

Could, but then if you do your gate drive correctly, the N-channel MOSFET still blocks current flow. Not a problem, I don't think.

quote:
"P-channel power MOSFET - good luck finding one"

I don't think this would be necessary, but yep, they are rare.

6/28/2000 9:13 PM

quote:
"if you do your gate drive correctly, the N-channel MOSFET still blocks current flow."

Remember every power MOSFET has a built-in diode between drain and source. As soon as you take the drain more negative (for N-channel) than the source it will conduct.

Steve C.

Page 1 of 2