LM*** grid bias PSU

[Diabolical Artificer]

A while back I built a homebrew valve tester and used a LM317 to adjust the positive DC voltage. Whilst making some improvements today to the VT it occured to me that perhaps I'd be better regulating the negative rail as the positive is grounded to chassis/cathode.

I've used two 1N**** diodes reversed on the output of the bias PSU as protection.

HT/g2/g1 PSU's are all fed by one transformer, but seperate PCB's.

Any thoughts/advice most welcome.

Andy.

[Restoration73]

There is a negative equivalent LM337 which should make life easier;
http://www.electroniq.net/files/surs...owersupply.jpg

[Diabolical Artificer]

I forgot to mention using the 337. thinking about this a bit more though, as the negative side is tied to earth won't this mean that effectively it will be at 0v, so there won't be any voltage to regulate?

Using a LM317 to adjust the bias has worked so far, but again this rail is tied to earth.

Having a bit of trouble getting my head round this.

Andy.

[MrBungle]

Have a look at chapter 9 of The Art of Electronics. It covers linear regulators from this series and how they work. This chapter is conveniently available as a free sample from their web site: http://artofelectronics.net/wp-conte...3_chapter9.pdf

Simply put the positive output to 'ground' and then the negative output will go negative.

[ionburn]

It did make me think though as to what I did with the bias supply. On checking I used an inverter as my unit is supplied by 12V so I needed to generate all supplies by readily available transformers. A complex (or at least component rich) arrangement but IC regulation switched by mosfets gives a stable voltage.

For a simpler setup from a dedicated transformer winding I would use a negative regulator.

[Diabolical Artificer]

I should have mentioned that the transformer has just two two wires, I think this is where the problem might be.

I tried a LM337 yesterday, but here's the problem; if we have 0v and +30v and we tie +30v to cathode/ground/earth, and we try and regulate the 0v side, where do you ground the regulator? The LM337/317 regulator datasheet says 0v, so if our +30v is now ground then we should ground it there. Did that and the circuit didn't work, but with the 337 grounded to the circuit 0v, it did. But how? It doesnt make sense, as the negative rail is ground to itself.

So I found a little 14 0 14v tfmr which I'll try and leave the + rail floating, that way the AC part of the circuit will be correct as will the DC part too I hope.

I have the Art of Electronics which skims is hard reading sometimes but I'll have a read later.

That's what happens MM with the circuit, and it's worked, but the 220 ohm fixed resistor on the regulator gets very hot.

Thanks for your input, Andy.

[ionburn]

Another approach I have recently tried and had success with is parallel fed triplers. This essentially feeds a series of bridge rectifiers through capacitors so a two wire supply can be used (the capacitors providing isolation). It takes a bit of thinking about but can be used for both negative and positive supplies from the same winding. The parallel feed, I think, limits the voltage drop as found with the more common tripler design. On redrawing the circuit I found it appeared like stacked power supplies.

[julie_m]

If the grid supply has a separate transformer (or at least, its own secondary on the main transformer) and rectifier, then you just need to connect its "+Vout" to your circuit's 0V rail, and use the regulator's "0V" line as your negative supply. The regulator will need a load when used this way; using it to feed a power-on LED will satisfy this.

You would need to use a proper negative regulator in order to use the same transformer for both positive and negative supplies; because you want the positive sides of the input and output sides of the regulator to be commoned. But if you are dead set on using a positive regulator, you might be able to add an overwinding to an existing transformer just by poking the wire through the gaps between bobbin and laminations. Wind on 10 turns, measure the voltage and adjust accordingly. While you still have enough wire to go for a few more turns, do an on-load measurement and again adjust accordingly. If there is only room to fit the new winding on top of the primary half of the bobbin, be sure to add at least three thicknesses of high-temperature tape first to maintain good insulation. I'm sure Ed_Dinning or someone else will be along shortly with more to say

[Diabolical Artificer]

After a bit of experimentation using a 14 0 14 v tfmr with negative regulaton I went back to using a LM317 as a fixed regulator with the OP across a 10k 10 turn pot, wiper/+ to cathode earth, 0v/negative to g1, as using the LM317 in adjustable mode, IE pot in place of R2 had a few issues. Obviously it didn't go down to 0v because of the 1.25v ref and also the adjustment was all at one end of the pot and not as sensitive as altering the output. I fiddled about with diodes to lower ground on the adjust pin to -1.2v etc, but in the end kept things simple.

Still can't quite understand how the circuit works though as I would have thought tyeing the positive to ground/earth would cause issue's with regulation and issues with the AC part of the circuit. But in the end this was the most stable way of doing things.

I read what the AOE had to say, it doesn't really cover the 317 in great detail, but I did learn a lot about PSU regulation and protection. IC regulation is something I havnt used, but I have a better idea of how to use an opamp in such circuits. I'd also like to experiment a bit with the 723 regulator in future as this seems a very versatile IC.

Andy.