Hi all,

I've been thinking about a way to acquire a usable Variac for my growing pile of valve electronics and bits. Since new/second-hand variacs seem to command premium prices, I've been thinking about building my own crude version?

My idea was to build a device incorporating a main potentiometer or series of switches to step down the mains voltage in increments (most likely 20 or 30V), using resistors. I would have to, obviously, incorporate a fuse and one of those Clipsal Portable RCD power outlets.

Would this be a feasable idea, or just go up in a shower of sparks?

Chris

If it was me, I'd just buy one. They are readily available from Jaycar and Ebay.

Portable RCD units aren't required if your switchboard is already fitted with hard wired ones.

Since new/second-hand variacs seem to command

I have two variacs both of which I picked up at hamfest type days. One cost me $30 the other $40. I use one on the bench and the other one is portable.

Your local radio club will have such days.

Burnt fingers likely here: Variac is an auto transformer, RCD will not save you after a rectifier on a hot chassis set.

Also a 30Ma RCD will not trip at around 130V.

I actually bought a Jaycar Variac. Be careful running valves at low volts you can poison the cathode in some by doing that.

Marc

Chris
please dont use a plain old carbon pot to vary mains voltage.
They are not meant to carry any sort of current.

Keep your eyes open, a cheap one will turn up sooner or later.

Ben

I have one for sale, contact me via my user name.
I actually have a complete test panel with variac and meters to go.
Peter

Ben, I've been thinking about using a mains-rated wafer switch or a series of SPCO toggle switches. I don't think a carbon pot would have enough range for an application like this.

Peter, I'll send you an email now.

Chris

If you are considering an incremental switched solution rather than a continuous arbitrary range, you might be better off looking at one or more multi-tap transformers with a rotary switch selecting the taps. I have one like that feeding a rectifier for benchtop DC supply. Assuming you are not going to need more than about 2amps, the transformers should be affordable or obtainable from salvage.

Maven

If you load mains with a potentiometer,
it might not just fuse nicely,
but pop or explode, potentially causing injury.

Same with a simple 8 Ohm speaker from transistor radio.
You'd think the tiny guage wire would just fuse, but that's not what happens.

I opened up the foot-pedal speed controller from an old electric sewing machine some years ago, and it appears to be nothing but a large variable resistor across the mains supply with coils of resistance wire strung about an insulating frame. There's no switching or transformer involved, so I wonder if it is using resistance to cut voltage or just limiting current?

I must dig it out and see if it can be calibrated with something like a C-clamp on the pedal and AC voltmeter across the output plug.

Maven

Dug out my sewing-machine footpedal, and found it contains a set of 5 resistors (wire coils, as in old radiators) in series with one side of the AC 240volt mains current. The connector is a standard earthed "appliance plug" as used for toasters etc

As the foot-pedal is depressed, a set of brass leaves make contact progressively with an angled row of contacts, shorting out the resistance loops one by one, until at full depression all resistance is shorted out.

Each resistance loop measures ~100ohms (I thought it would be more), but this is enough to cut the AC voltage by ~20VAC per loop.

I measured the voltage increments as the pedal is depressed:

~140,~162, ~182, ~198, ~218, 240.

That's far from the precision of a proper Variac, but gives a reasonable range of voltages for some purposes, say to bring up some valve gear that has been long out of service. I'd use a clamp instead of a foot to adjust the spring-loaded pedal.

As a bonus to vintage collectors, this device comes in genuine bakelite!

For the purpose of bringing out of service gear back into service I have a light socket screwed to a piece of particle board with the light in the active line wired to male & female GP plug and socket, neutral and earth lines going straight through.

Then with a range of incandescent globes eg 15W, 25W, 40W, 60W,75W,100W can bring things into service by watching the brightness of the globe. As they dim down as the caps reform, put in the next globe up in wattage. If they don't dim, time to stop.

That's sneaky,
I thought of a bunch of bulb sockets with different Wattages
selected with a switch, but the cost would end up
outweighing a proper variac, so didn't mention it.
Never thought of switching the bulbs in the same socket!

The other one is finding some way to slowly insert the
core into a power transformer that is temporarily running
the thing under test, but again, too much modification,
and potentially unsafe...

An idea that I haven't followed up on is connecting two or three of the auto-transformers used for pedestal fan speed control in such a way as to provide a series of voltages.

Comes with the fan speed control switch so might be able to make a switchable set-up. These fans usually rated 50-60 Watts so would have to watch that a winding somewhere didn't get overloaded.

Only have one such autotransformer and its switch so haven't followed it up.....

I wonder if the caps would like PWM even if the radio does not.

I have learned about the vibrator powered radios,
and that is essentially a very dirty supply until something is done with it...
but it's the caps you are trying to reform that are
supposed to be doing something with it!