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Old 23rd Jan 2013, 11:02 am   #1
mark pirate
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Default Variable AC/DC power supply.

I was pleased to find this interesting and useful unit for only £5
It is obviously a homebrew, but well constructed. It contains a Zenith variac, two quality transformers and some substantial capacitors on the DC supply. It was missing its Bulgin mains lead, but luckily I had one in the workshop.

According to the Dymo labels it can put out 0-240V AC at 0.8A (variac), 6.3 and 9.0VAC @ 2 amp (fixed) and 0-350VDC @ 150ma.

It achieves the variable DC by driving a transformer via the variac. When tested, it does what it says on the labels. I will modify it further, with the inclusion of some panel meters for both volts and amps.

I tested it on a battery valve set, with the DC set to 90V, and the LT from a battery, it works fine and the voltage is stable.

I would be interested in any comments to improve its features. I would like to add a regulated DC output of 1.5 and 7.0VDC (for battery radio LT), there is room inside to add this.

This is really going to be a useful addition to the workshop.

Mark
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Old 23rd Jan 2013, 12:11 pm   #2
TrevorG3VLF
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Default Re: Variable ac/dc power supply

Looks like a very good buy.
I would add a 2V supply (LM317T) and either another 317 to give 1.4V or just use a power transistor with base and collector joined together to drop the 2V to 1.4V.
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Old 23rd Jan 2013, 9:15 pm   #3
Ed_Dinning
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Default Re: Variable AC/DC power supply.

Hi Mark, I'm just in the process of building a unit like this for a forum member. It has the addition of a variable regulated (807's) HT supply based on the Farnell circuit (0-350v @200mA). The variac side is capable of being used direct or through a 120v transformer for isolation. The DC is smoothed by 4700uF. This is connected through an output switch that cuts primary supply to the transformer and applies a 10R "dump" resistor to the cap. This is a safety mod you may like to incorporate.

Ed
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Old 23rd Jan 2013, 10:18 pm   #4
Nickthedentist
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Default Re: Variable AC/DC power supply.

Don't forget that Variacs are autotransformers, i.e. do NOT provide any mains isolation. It's possible that there is a separate isolating 1:1 transformer in your box of tricks though, but you need to sketch out a circuit diagram if you haven't already done so.

Sorry if I'm teaching granny to suck eggs (which I probably am, of course).

Nice find.

Nick.
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Old 24th Jan 2013, 12:26 am   #5
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Default Re: Variable AC/DC power supply.

The way I do it is to have two sockets on the output of my isolating transformer and one has the Variac plugged in all the time.

The transformer is powerful enough for this.
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Old 25th Jan 2013, 9:24 am   #6
mark pirate
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Default Re: Variable AC/DC power supply.

Thanks for the input. I think it is running through an isolating transformer, the variac is a 2 amp model, but the Dymo label says 0.8 amps max so this must be due to the rating of the transformer. Both the 6.3 and 9V AC terminals are also supplied from this transformer.

The other transformer is used for the HT, the primary being fed from the variac. There are other unused tappings on the secondary of this transformer but I have yet to test the voltages on these.

The HT output is run into a small board containing some diodes and what looks like a regulator. This then feeds two large caps, one of which has a bleed resistor fitted.

This is as far as I have got with this unit, as there are too many projects which need my attention first. I will reverse engineer it when I get the time.

Trevor, I like your ideas on the LT output. 1.4V regulated output is the main addition I would like to add. I have a couple of LM317T in the workshop, so will use these as suggested. I have also got a nice 0-2 ammeter, but nothing suitable for a voltmeter although I do have a small analogue multimeter that could be used.

Another idea would be to incorporate a capacitor reformer, this would make this unit even more of a boon.


Mark
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