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AC/DC power supply w/ variac question

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LAN_403

Forty

Thu May 26 2011, 10:17PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

So i've got a couple nice variacs, but they are just the transformer itself, no housing or wiring.
I turned an old wooden speaker box into a housing for one of them (knob coming out where the speaker was.)
I'd like to incorporate a dpdt switch onto the unit that switches the output from AC to DC using a bridge rectifier chip.
however, i think that if i have the + and - from the bridge rectifier connected to the output terminals, then i'll have some problems when i switch to AC (the AC signal would be fed into the + and - of the rectifier.)
can anyone think of a simple solution so that i can have just two output posts and a switch that changes from AC to DC?
or should i just use more binding posts for the output?

Also, since these variacs have no extra components with them, is there anything i should add to make them more safe to just plug into the wall? (fuses, ripple caps, inductors, etc)

Nah

Fri May 27 2011, 12:12AM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

DPDT switch

Jrz126

Fri May 27 2011, 02:11AM

Registered Member #242 Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210

I'd use separate binding posts for the AC. You'd have bigger problems if you accidentally flipped the switch to AC when you had a DC load connected.

Putting some fuses and smoothing caps in there too would make a pretty handy powersupply. Should also include some 100K or 1Mohm resistors across the caps to keep them discharged.

Sulaiman

Fri May 27 2011, 10:12AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I would have the ac line (mains, netz) go via;
- ac line input socket (for 'kettle lead' mains cable)
- ac line filter
- input fuse (or miniature circuit breaker)
- on/off dpco switch (2p2w)
- POWER ON indicator (eg panel mounted neon lamp)
- Variac input.

I use a line input socket with inbuilt emi filter and a miniature circuit breaker on/off switch.
Depends upon what salvaged stock you have.
You could put a 'power factor correction' capacitor across the variac input but I never bother.

IF space/style allow then a mains filament lamp (or a high wattage resistor) in series with the fuse (or circuit breaker) acts as a nice 'inrush' protector when charging capacitors or just a 'protector' when the output terminals get accidentally shorted etc. A 'bypass' switch allows full power.

A voltmeter across and ammeter in series with the Variac output would be nice but expensive.

The Variac output requires a fuse rated to protect the variac wiper.

I would wire via the output fuse to a pair of yellow terminals for ac output

and via a bridge rectifier rated much higher than the variac output fuse
(due to volume etc. here in uk a 400V 35A bridge rectifier is quite cheaper than a 300V 15A)
to black(-) and Red(+) output terminals.

Often I put a transformer on the Variac output to get a (usually lower) variable ISOLATED output.
The most important thing to remember about a variac is that it's not isolating
adding an isolation transformer to the output greatly increases safety and convenience
and can be very useful at low output voltages.
(e.g. my 240 Vac input, 1A output variac via a 240:12 transformer gives an output limited to 240VA
(about 0 to12 V @ 10 A dc after rectification etc.) or whatever your transformer is rated for.
Building-in an isolating transformer, bridge rectifier and large electrolytic gives a really good general purpose variable dc supply.
MUCH more rugged and repairable than an smps, and none of the high-frequency switching noise.
If you can get a 120:240 isolating transformer for the variac output
it would allow you to test circuits from 120 and 240 volt regions.

Ideally a well ventilated, well earthed metal box would enclose all high voltage parts but wood and plastic are ok.
For a cheap/quick version use a 'chocolate block' connector (e.g.

) for all inputs and outputs and just the fuse for the output wiper.

Well, a few ideas for you....

P.S. it is quite common for a high power variac to trip the mains at switch on
(depends on the phase of the ac at switch on)
even with no output - just try again.
I would use 'time delay'/'anti-surge' fuses as 'fast'/'quick blow' fuses need frequent replacing.
P.P.S. and check out the 'Check out my new PSU! (fixed variac)' thread as that is roughly what you are aiming for

Forty

Fri May 27 2011, 05:41PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

thank you very much Sulaiman, that is everything i needed to know and then some. I've got all the bits and pieces you mentioned and plenty of space in the enclosure so i'll get to work on it and we'll see how it turns out.

Sulaiman

Fri May 27 2011, 08:09PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Enjoy the project.
The thing that you end up with will find many uses in future.

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