Variable HT Power Unit

[Variometer]

I mainly mess about with battery valves these days, either the old B4/B5 types or the small 1950s types such as DK91 DAF96 etc. As one can no longer buy high voltage batteries at reasonable cost, for some time I have used small 9V batteries clipped back to back, but even this can be expensive & they don't last very long either. Last birthday, my wife asked what I wanted & I opted for a varibale mains transformer (Maplin). When I got it, I found that it was an auto transformer with no isolation from the mains. I built a variable HT supply with it. The mains input was fed into an isolation transformer with two output windings, 6.3 V & 250V. I put a switch & fuse in the input circuit. The 250 V winding was then connected to the input of the auto transformer & the variable output fed into a silicon bridge rectifier followed by a standard smoothing circuit of a 32-32 uF electrolytic & a smoothing choke. The output was also fused & a bleed resistor & indicator lamp included. I found that I had a wonderfully smooth controlled output of between zero vots & about 300 DC. Becuase of the construction of the variable transformer, it is quite easy to put an insulating "stop" on it for safety reasons if you only require voltages up to 90 or 120 V. This is a picture of it before I put all the labels on it. Works fine on battery radios with practically zero hum. The volatage hardly falls at all on load, but I usually only require 12mA or so & it can be re-adjusted anyway. I also put a 6.3 V socket on the front panel as well rather than just leave the extra winding unused. Thought you might be interested.
Bob

[Robert Darwent]

Hello Bob

That looks just great, very well produced. Well done!

Is it possible for you to put up a schematic of the circuit as well? Your description is easy enough to follow but I would be interested in seeing exactly how you've connected things, a simple pencil drawing would be good enough.

I have an unused variac sitting around and you've just wetted my appetite into making something along similar lines.

Regards

[Nickthedentist]

Originally Posted by Variometer

I Last birthday, my wife asked what I wanted & I opted for a varibale mains transformer...

If only everyone was so easy to buy for

[Variometer]

Thanks or replies. I will post circuit late tomorrow afternoon (going out to local shipmodelling club during morning & early afternoon. Circuit is simple enough, but very effective. The auto transformer is very smooth in its operation. I had toyed with the idea of getting one for some time. When birthday came up, it was the only thing I could think of!
Regards
Bob

[Mike Phelan]

Originally Posted by rrdmpb

Hello Bob
[ ... ] and you've just wetted my appetite into making something along similar lines.

Regards

Take it that's a Freudian slip, Robert. Better having a glass after the electronics ....

Originally Posted by Variometer

Thanks or replies. I will post circuit late tomorrow afternoon (going out to local shipmodelling club during morning & early afternoon. Circuit is simple enough, but very effective.Regards
Bob

An excellent job, Bob - will look forward to seeing the circuit.

[Variometer]

Here are the details of the variable power supply. Not much explanation required.
The mains transformer is simply to isolate the unit from the mains & make it safer.
The auto transformer was obtained from Maplin (UK). It looks like a rather large wirewound potentiometer, but the windings are transformer windings on an iron core & the slider arm moves directly across them. Smoothing choke & bridge rectifier from Maplin. Fuses inserted in mains input, 6.3V AC output & variable HT output.
Bleed resistor pretty important as it reduces capacitor voltage rapidly to zero when unit is switched off, value 47,000 Ohms (47K). Indicator lamp, neon (with limiting resistor included, connected across mains transformer input winding.

The unit is shown before wiring for simplicity.
Bob

PS. Will answer any questions tomorrow - going for a glass of vino now!

[GrimJosef]

This looks like A Very Handy Thing . If you want "a quieter life" then for your next birthday you could ask for a Constant Voltage Transformer. Replace your existing isolation tranny with this and you can seriously reduce the irritating voltage changes which your electricity supplier will currently (no pun intended) be subjecting you to. I know this because I've just done it. Unfortunately you'll trade random noise for a load-dependent voltage (roughly 7 volts per amp from my recently-acquired 250 watt CVT). But this is where the Variac comes in. When the volts drop under a large load you can just wind them back up again ! I have to admit to an almost child-like glee now I can finally have the volts I want rather than the ones that Southern Electric, apparently on a whim, choose for me. I should mention a couple of other "features" of CVTs. It seems they alway buzz noisily (mine certainly does). And for what they do they are large and heavy. If you only need 12mA you might get a small one to fit inside your box. But the 250 watt one, plus 2A Variac, was a real challenge to screw to the wall !

Cheers,

GJ

[Robert Darwent]

Originally Posted by Mike Phelan

Take it that's a Freudian slip, Robert. Better having a glass after the electronics ....

Hello Mike,

Yes I think I must have had my mind on another activity when I wrote that! But don't worry I always follow your advice and partake after I've unplugged the soldering iron.

Hello Bob,

Thanks for the schematic and further photograph. I must say its certainly an handy unit you've put together there. I will definitely experiment along similar lines!

Regards

[Variometer]

Thanks for replies. I find it very useful when wanting odd values of HT & the control is very smooth indeed. I don't really need a perfectly constant voltage as with the small battery receivers I tend to play around with, a very volt here or there doesn't make any difference.

I have just successfully developed & built a 4 valve superhet using DK91,1T4, DAF91 & DL92. It works fine, but is a mess to look at at the moment. Next task is to build it properly combining my model-making skills to make it very neat. But, as I am sure most of you have discovered, tidying them up often stops them working. Will not be re-building it for a while yet as my interest in radio comes & goes, re-surfacing every few weeks.
Bob

[Mike Phelan]

Bob
That's a nice, simple and straightforward job - well done!

[Tim]

Looks a good one so far!
I have often thought of doing this, and it makes sense if you can find a small variac. A truly variable HT supply!

[Colin]

Is the Variac rated at 0.75A ? (Trying hard to read the lettering on the picture).

[Variometer]

Hi Colin,
Yes, it is 0.75 Amps. If you just put "Variac" in Google, the Maplin one that I used comes up towards the top of the list. It was about £30. Not very big, but quite heavy!
Bob