Re: message repost

John Lee wrote:
>
> Zonn wrote:
> >
> > At 04:10 PM 9/4/97 -0800, Clay Cowgill wrote:
> > >>With a very slight modification to this design you can use the original WG
> > >>PNP pass transistor to boost the current on the positive rail, and the NPN
> > >>transistor for the negative rail (the opposite of the way WG currently uses
> > >>them). Assuming only a current gain of 10, for the power transistors, this
> > >>allows the regulators to only dissapate 500ma for 5 amps of current.
> > >>
> > >>I don't have the schematic here, (it's hand drawn) but I can scan it in this
> > >>weekend and send to to Al to post.
> > >
> > >Mmmmm. Good point, I'd like to see your modifications there.
> > >
> > >One other approach would be to get the regulators in a TO-3 package and
> > >mount them on the chassis instead of the NPN/PNP pair for the old LV
> > >supply. I think the TO-3 versions are rated for something like 7A
> > >continuous?
> >
> > I couldn't find them, if I remember the TO-3 were the same ratings as the
> > Tab parts, the TO-3 have better coupling onto the heatsink allowing slightly
> > higher power dissipation. It wasn't all that different though, and the
> > current limiting was the same.
> >
> > I have seen some TO-3, 5 volt fixed parts that can regulate up to 10 amp (in
> > a surplus store). They could be used as an adjustable part. I've only seen
> > positive regulators that do this, and the pins won't fit in a standard
> > socket -- too fat, and you'd still have to do something for the negative side.
> >
> > The design of an external pass transistor was taken right out of a Motorola
> > data book (though I had to grab a little here and a little there to get the
> > whole design).
> >
> > It's hooked up like this (Pardon the ASCII art):
> >
> > Vin -------*--------\ /---------*-------- Vout
> > | v / |
> > | ----- |
> > | | |
> > | | |
> > | | |
> > | | |
> > *--/\/\/----*---[LM317]--*
> > R1 |
> > |
> >
> > Adjust circuitry
> >
> > Use the NPN for the LM337 side.
> >
> > As the LM317 draws current through the PASS transistor's BE junction the
> > transistor it turns on.
> >
> > R1 is used to substain regulation at low current levels and is calculated to
> > have a .6v drop across it when you want the transistor to turn on. I picked
> > somewhere between 50ma and 100ma. Anything below this threshold will use
> > only the regulator. Above that the current is shared between the regulator
> > and the pass transistor based on the gain of the transistor. It's probably
> > not needed here since the HV supply makes sure you're never uses that little
> > current.
> >
> > The output of the pass transistor shares the output of the regulator so full
> > regulation is maintained. A scope on the output of this shows a flat line
> > regardless of what is being displayed. The regulator is faster than the
> > noise generated by the HV, so all the HV noise goes away.
> >
> > There is one problem with this circuit, it is no longer short circuit
> > protected (the WG design was never short circuit protected!). The regulator
> > protects itself but has no way of turning off the pass transistor. It takes
> > a second transistor and a current sense resistor to do this, it all depends
> > on the amount of bullet proofing you want to do.
> >
> > -Zonn
>
> How about a crowbar circuit like this added to the reg circuit. You can
> adjust the amount of current limit with VR1. The scr will force the fuse
> to blow if it goes
> beyond the set limit.
>
> ---------------------------------------------------------------
> [Image]

Ooooops forgot a ground.

Received on Thu Sep 4 20:05:16 1997