Variable HT regulator circuit.

[SurreyNick]

Even better. BTW the seller accepted an offer of $15, so all-in including postage it was less than £13.00!

[Diabolical Artificer]

Hope it does the job Nick, but at an OP power of 20-30mA / 9W ,you might it be severely constrained to what valves you can test.

Andy.

[Al (astral highway)]

Andy, Nick's measuring small signal valves and a low power output valve, not line-output or transmitting valves. He'll be fine. If not, just one MOSFET has to be replaced - 2 mins work and a quid's outlay.

[jim_jobe]

Back again. I'm assembling the bits to build one of these. Is there any need to have an AF choke/capacitor smoothing circuit following the rectifiers?
Jim

[Al (astral highway)]

Originally Posted by jim_jobe

Is there any need to have an AF choke/capacitor smoothing circuit following the rectifiers?

Hi Jim, if you're talking about the circuit that was posted early in this thread, no, you don't need a choke, but you do need the big electrolytics shown.

[jim_jobe]

Hi Al,
This is the circuit I eventually ended up with,
Jim

Good result, I would have it variable down to zero (you never know, a ten turn pot is good here) and put a switch and a fuse (FETs go short on failure) on the output (after the meters). I was going to mention an output bleed but the volt meter will do that, talking of meters, a protection diode across the ammeter is a good idea. I like the gate series and protection too, well thought out and simple as all good designs are.

[jim_jobe]

Cheers Merlin,
I do intend adding a fuse but hadn't thought of the diode across the meter. The LED and its resistor will bleed the 220uF capacitor. I'm thinking I might publish this* as I'm sure there are others out there who would like to build one.
Jim
*With due acknowledgements to the Forum of course!

The LED and its resistor will bleed the 220uF capacitor

True, but I was thinking about the output voltage, assuming it is at 0V (from the knob) and having even a 100nF cap at 300V can give an unwelcome zap. Anyway it's sorted in your design.

One thing I like to use on high voltage stuff is a neon (even inside), very reliable. Rather guiding the lilly in this case though.

[jim_jobe]

Yes, I realised you meant the output Merlin and I'll do that as well I think. I considered using a Neon indicator too as even taking just 5mA extra from the transformer for the LED is quite a big percentage of the 100mA available with the transformer I'm using and Neons take less than that.

[stuarth]

Could you replace the string of Zenners with small wire-ended neons (with shunt resistors to help striking...)

Stuart

[KeithsTV]

Rather than put the LED, and its series resistor, across the supply put it in series with R1. You'll still get 5mA through the LED and save a resistor and get an extra 5mA available at the output.

Keith

[jim_jobe]

Originally Posted by stuarth

Could you replace the string of Zenners with small wire-ended neons (with shunt resistors to help striking

Yes, I've used that trick before Stuart!

[jim_jobe]

Originally Posted by KeithsTV

Rather than put the LED, and its series resistor, across the supply put it in series with R1

Now why didn't I think of that! Thanks Keith, I'll do that.

Now why didn't I think of that!

And me, there are always small improvements one can make to a design.

[Al (astral highway)]

Originally Posted by jim_jobe

This is the circuit I eventually ended up with,

Nice one Jim.

This may just be the icing on the cake, but that Mosfet can source a lot of current..would it be an idea to add some simple crowbar s/c protection?

[jim_jobe]

Yes Al, I've been mulling that over. I'll see what I can come up with tomorrow.

[Bazz4CQJ]

I know that this has been devised as a VV supply, but would it also be a good way of dealing with limiting the very high off-load HT voltage which occurs during the period when a valve receiver is warming up? If the power supply uses silicon diodes, the HT volts can be surprisingly high - and close to the limits on various caps - prior to the time when the valves start conducting. Usually, I've used some crude combination of resistors/zeners/neons to keep things under some degree of control, but these can be a drain on the current available when the valves are on.

B

[Al (astral highway)]

Originally Posted by Bazz4CQJ

... would it also be a good way of dealing with limiting the very high off-load HT voltage which occurs during the period when a valve receiver is warming up?

I looked into this recently, by coincidence. Here's one simple way of doing it with a low component count - comparator, opto-isolator and MOSFET, hardly any passives. The MOSFET switches on the HT+ after a delay set by an RC time constant. Here it's just under 50 seconds.

My sketch is a bit rough but I didn't tag the original circuit.

Cheers

[jim_jobe]

Nice one Al. I remember my boss having problems with the electrics on his boat many years ago. The solution there was to have a built in delay so I designed and built a unit based on a 555 which solved the problem for him. I never got a trip on his boat though!