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Variacs - hoping for some background info

twocoaster

twocoaster

Well-Known Member
Hi folks, I was hoping to get some info regarding use of a variac. My understanding thus far is that these take 110-120v from the wall and allow you to vary the voltage sent to their outlet. I also read that it is recommended to use a variac when first powering up "new to you" vintage gear, to help mitigate any potential damage to the gear. How does this help? I figure if you go all the way up to 110-120, it's the same as just plugging the gear into the wall.

Also, would a rheostat, as used for dimming lamps, work the same way? (Obviously there is no scale showing output, and no meter either.)

Thanks! :)
 
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a variac lets you raise the AC to the unit under test slowly so that if anything is haywire you can catch it before doing mucho smoke. Excess current draw on the input side of a pwr supply for instance. If you just plug into 110V and turn the thing on you mite get sparks and smoke before pulling the plug. When I was a tek at HP we always used a variac on units coming from assembly before doing any testing.
 
I have not tried a light dimmer because I like to jump in with both feet and fix what got burned. But I have high confidence that it will work as good as a variac on stereo stuff. Need someone with a don't care unit to test the concept.
 
At roughly $40 for a new, small one, why mess around? I picked mine up last year and it has paid for itself 10 times over in peace of mind.
 
Would not recommend a dimmer for audio ramp-up work

Variac use is basically an autoformer - think of it like a volume control except for it is used for AC voltages at high current levels. That is, its output can be control for voltage but the AC sine wave remains intact.

This is not necessarily the case for dimmers, which tend use SCRs or other devices that chop up the AC sine wave. This is why you may hear one buzzing at times in operation. The outputs from dimmers are not necessarily a sine wave or, at least, only part of a sine wave. Since light bulbs don't care about the quality of the AC wave, dimmers are appropriate in that application.

Cheers,

David
 
The use of a variac, which operates way different than a dimmer, is to allow electrolytic capacitors to "reform" before being slammed with full voltage.

Image how you yourself would like to be awakened from a long restful slumber... a gentle loving touch, a light caress, then the brush of a kiss... well, it ain't like that for a capacitor, but you can dream all you want! :D

Seriously, the word from Those Who Know is that you're much less likely to have to replace a filter cap (and perhaps even a power transformer as a result) if you bring the system up slowly (over a time of a few hours) on a variac.

You can get variacs for like $40, and filter caps can be like $30, so the economics are definitely favorable.

Why does a regular household dimmer not work? As I'm not an electrical engineer, I don't know every last detail, but I'd wager that the harmonics resulting from the distorted waveform you get out of a regular household dimmer is extremely likely to croak your power transformer, much to your disappointment.

Don
 
In addition to everthing above--- A rheostat is a POWER variable resistor. They work ok in low voltage applications, but the power (heat) that they would have to dissipate to run 110V would make them huge. In old cars they were used as the dimmer for the dash lights. In newer cars they now use PWM for eficiancy.
What makes the variac so useful is that most amplifiers will begin to work with a line voltage of about 25V. At that voltage if there is a short in the amp there isn't enough current to do heavy damage.
 
I kind of see it both ways, I still Variac most peices I get in, but if the caps are old enough to fry at full power, I'd rather them just go now then later (no variac, plug it in, turn it on).

Is there any logic to that statement? I mean, if the caps are old enough to be varaic'd, the smart money says they probably should be replaced anyway, no?

Cheers,
Russ
 
That would be fine if you replace 'em before plugging the thing in, but if the caps go, sometimes you can risk a transformer that would have been good otherwise. Also, sometimes the caps are still OK once they're reformed, but will go only when they're quickly brought up. Basically you can replace the caps ahead of time if you want, but using a variac can save some money replacing ones that don't necessarily need to be replaced. And it can also help you find any other shorts and such in the circuit before frying other components...
 
I repaired an old Marantz receiver that had many shorted transistors in it. I powered it up fully and saw a wisp of smoke. Immediately shut it off and bought a variac on ebay. I was able to partially power it up and leave it that way for hours while I troubleshot the problems at my leisure. Without the variac I would have been replacing parts for months. I think I only paid around $20 for a raw one. (not boxed just the transformer) I built a particle board box for it with a fuse, on/off switch and wall outlet.
 
Not much chance any of us getting a piece with no juice applied for years. So I figure if it does not work, somebody else had fired it up recently already. Babying it with a variac is a waste of time.

Now a variac is the first choice. But if you are thrifty, then the light bub and light dimmer are alternatives to explore. However, if the dimmer works you have very fine control over the babying of gear-- as much as the variac.

I am pretty sure the dimmer is a triac with a duty cycle control on every 90 degrees of the 60 Hz. For audio gears with a power transformer, the dimmer is a good way to control power supply volt. Should know that dimmers are advertised for resistive loads, so a small chance (very very smal) it will died running a stereo. But it is cheap so it is hardly a loss if it dies. Don't worry about damage to gear. You start with low power anyway. Transformer can hack it. You do have higher frequency harmonic to the transformer, but they have not much effect unless you work up to the max output of the amp. You just have a bit more core loss and 'former may get slightly or hardly warmer. But I don't think you can feel the diff. The higher harmonic gets filtered out by the power supply anyway-- it is a low pass filter in the first place. Besides you are not marrying the dimmer to the gear.
 
I dissagree on using the average dimmer, if only because its not designed to handle higher wattage loads. A lot of gear runs more than a couple of hundred watts. lights are rarely that high. few dimmers are rated at say 500W... :thumbsdn:
 
Light Bulb Variac

As an alternative to a variac or dimmer, use an ordinary light bulb wired in series with your 110v input. This will drop the voltage applied to the unit and help protect it from a full voltage surge. The brightness of the bulb will indicate the relative current draw. The brighter the light, the higher the draw. It is actually kind of cool to watch. As you turn up the volume the bulb gets brighter.

The rig is easy to make, just a pair of junction boxes, a cheap ceiling lamp “barebulb” socket and a 3 prong electrical socket. Wire one side of a power cord to one side of the electrical socket and the other side to the bulb socket. Connect the other side of the bulb socket to the other side of the electrical socket and you are done. $5 in parts and 10 minutes work.

Also the lower the wattage of the bulb, the lower the applied voltage to the unit being tested. So you can reform capacitors by simply changing from a 40w to a 60w to a 75w to a 100w.

With all this said, I am still lusting after a variac.

- Pete
 
Thanks to all for the info, gang

Much appreciated. :) This site is a great resource, especially for the raw amateur like me.
 
"Not much chance any of us getting a piece with no juice applied for years. So I figure if it does not work, somebody else had fired it up recently already. Babying it with a variac is a waste of time."

I guess that depends on whether you want to repair something or blast it with a fire extinquisher and throw it out.
 
I don't believe the variac provides adequate protection when first applying the power. I use a 100 watt bulb for all amps when I first bring them up. The least of my concerns is the power supply caps. I'm more concerned about limiting current into the amplifier circuits and avoiding further damage. A light bulb limits the current becuase it's used as a series resistor. I use the variac after a repair to verify the amp will run at high and low AC line voltages. It's an accepted practice that electronic equipment should operate at +/- 10% (108-132 vac).

A light dimmer is known as a phase angle controller. Each half cycle it turns on a triac at some phase angle between 0-180 degrees. to lower the voltage to a bulb the triac is fired later in the half cycle. Once fired the triac conducts until the next zero crossing. This ocurs on both the positive and negative half cycle. Light dimmers can be responsible for trashing the power line because they draw a pulses of current. The problem can be reduced somewhat with a snubber (R/C network) across the triac.

A light dimmer could have a potential problem when used with an inductvive load such as the transformer of an amplifier. If the positive and negative half cycles are fired at slightly different phase angles, the result would be a DC bias on the xfmr. This causes the xfmr to saturate and at best run very hot.
 
The light dimmer is easy and cheap to get. It has the control range that is nice like a variac. We just need a volunteer to test the idea. :D

It most likely does not have a snubber and may give up it's holy smoke with an inductive load. A snubber is easy to add on if you know what you are doing. The speed control in your electric drill is a sister to the ordinary dimmer. That is some what of an inductive load.

I think putting the dimmer in series with several 100w bulbs in parallel then the unit under test (UUT) is the way to go. A cap from the juction of the bulbs and UUT to the power line return is a good start for a snubber. There are other ways to electricfy the cat of course.

When you are in the service mode, you don't need to use a lot of power. Start the dimmer at the low setting and turn it up as need. You don't need to crank out 50 watts to know it's fixed. Get the thing running to the point of putting out a few watts then go on without the dimmer. Because by then it's unlikely the thing is broke anymore. :scratch2:
 
RuSsMaN said:
I kind of see it both ways, I still Variac most peices I get in, but if the caps are old enough to fry at full power, I'd rather them just go now then later (no variac, plug it in, turn it on).

Is there any logic to that statement? I mean, if the caps are old enough to be varaic'd, the smart money says they probably should be replaced anyway, no?

Cheers,
Russ
Click to expand...

If you run the old capacitors for a while at lower voltage, they will reform. My understanding is that the current will decrese over time due to regeneration of the insulating film on the foil inside the caps. So using the variac can eliminate the need to replace the old caps.
 
OK, for those that don't want to spring for a variac, (I bought a small 3A one, but its not built into a box as yet...) I have been reforming the caps in a Motorola tube amp, and used my Dim Bulb Tester to do it.

I also did some documentation on actual voltages achived with various bulbs, and there measured resistance just for grins.

Keep in mind. all this tester cost me was a 240 watt bulb, sisnce I didn't have one of those laying around. All else I already had...

Dim bulb tester - Bulb ratings and results.

Load Motorola Mono Tube amp, .62A @ 117VAC (72W)

bulb wattage - ohms resistance - resulting Voltage @121VAC

40 - 26 - 67
60 - 19 - 81
100 - 10 - 91
150 - 7.5 - 100
240 - 5 - 110

The collums spacing may suffer, But you get the idea...

I have a post on the tester and how I built it somewhere in DIY. I'll add to it if I can find it...
 
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