School me on the use of a variac

crouse

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People often recommend using a variac to slowly provide power to an older piece of equipment when testing it. My question is why?
 
The main reason is to slowly charge the capacitors so they can reform and support the necessary voltages without overheating and failing. They need some time to do that, not an instant hit of high voltage. The other use is in troubleshooting to prevent damage by testing at much lower voltages. Many solid state circuits will function just enough at a low voltage to determine if components are defective. Not quite the same as a dim bulb tester, which is current dependent, but some of us prefer the Variac.
 
Good advice Conrad, though I'd like to add some clarifications. Reforming of capacitors only affects electrolytic capacitors; small-signal capacitors using paper, ceramic, film, etc. as dielectric do not benefit from the reforming process, nor do tantalum capacitors AFAIK. Also, reforming electrolytics using a variac is only effective in devices using a solid-state rectifier; a tube rectifier won't really begin to conduct until input voltage reaches 80-90V, so instead of the power supply feeding said capacitors going from 0-300V or so as the variac goes from 0-120V, the capacitor(s) would suddenly be hit with 250V or so as the tube reached its conduction point, which wouldn't do a whole lot of good with regards to reforming.
-Adam
 
The biggest thing I used mine for was to monitor current consumption after making repairs to a piece of gear. I have a pair of banana jacks that I added to mine that allow the simple insertion of a series ammeter (fluke 87 is what I use). Slowly ramp up voltage while monitoring current - that way you can shut it down if current is excessive long before you smoke a bunch of new parts.
 
Also, reforming electrolytics using a variac is only effective in devices using a solid-state rectifier; a tube rectifier won't really begin to conduct until input voltage reaches 80-90V
Good point, and I didn't know that about tube rectifiers. Same in SS gear: a voltage regulator may not allow any voltage to flow to anything, until it reaches 75V or so. Result: everything is hit by 75V. Defeats the purpose of gradual increase.
 
I use mine connect to an amp meter. This way I can watch the amps consumption on power up as
The biggest thing I used mine for was to monitor current consumption after making repairs to a piece of gear. I have a pair of banana jacks that I added to mine that allow the simple insertion of a series ammeter (fluke 87 is what I use). Slowly ramp up voltage while monitoring current - that way you can shut it down if current is excessive long before you smoke a bunch of new parts.


I use mine the same way. It works very well to not smoke parts.
 
Yes, an AC ammeter is an essential accessory when it comes to bringing up the voltage to something using a variac. Unfortunately, stand-alone AC ammeters are a bit hard to come by, for some reason, though there are work-arounds. One way is to use a multimeter with an AC amp clamp, either built-in or as an add-on; you can't just put the clamp over the power cord, but AC line splitters make things fairly easy. As an alternative, you could wire a 1 ohm power resistor in series with the hot lead going to the outlet the device being tested is plugged into, and then you'll be able to read the current drawn using a simple AC voltmeter. A schematic of a variac device I built using this principle can be seen here:
miniwieck_schem.gif

Oh, and one further tip with regards to variacs: Only use them to bring up the voltage to devices which use a standard linear power supply. Switching power supplies as used in computers and some stereo equipment don't take kindly to low line voltage levels.
-Adam
 
Good advice Conrad, though I'd like to add some clarifications. Reforming of capacitors only affects electrolytic capacitors; small-signal capacitors using paper, ceramic, film, etc. as dielectric do not benefit from the reforming process, nor do tantalum capacitors AFAIK. Also, reforming electrolytics using a variac is only effective in devices using a solid-state rectifier; a tube rectifier won't really begin to conduct until input voltage reaches 80-90V, so instead of the power supply feeding said capacitors going from 0-300V or so as the variac goes from 0-120V, the capacitor(s) would suddenly be hit with 250V or so as the tube reached its conduction point, which wouldn't do a whole lot of good with regards to reforming.
-Adam
9

Interesting point which I never considered.
It might pay to keep a plug-in SS rectifier on hand for this purpose (I know you can cheaply rig something up. I was thinking from a point of convenience)
 
All good points. Also, never use a Variac on anything with a switching power supply unless the service info specifically says you can/should do so. Even then it might be risky. My Tektronix 7000 mainframe says to use a Variac during service, but the power supply failed the minute I did. Tube equipment is a whole different thing, but on average, not even a rule of thumb, I've found old caps have the most trouble when they get near the rated voltage. An older piece might run fine and reform at 100 VAC, but fail quickly at 120 VAC because the leakage current of the caps rises so quickly with increasing voltage. Thus, even with tube equipment, it might be beneficial to run at a lower voltage, rest it for a while, and slowly step up to full line voltage over several hours. Really though, when caps are that bad, it's time for replacement. I've never had an old reformed cap work like a new one, and they tend to "unform" very quickly.
 
Good advice Conrad, though I'd like to add some clarifications. Reforming of capacitors only affects electrolytic capacitors;...as dielectric do not benefit from the reforming process, nor do tantalum capacitors AFAIK.
-Adam

There are wet tantalums that might respond to reforming. Not used much as they are not cheap and the ones in the Crown reel to reel electronics get replaced, not reformed.
 
Interesting point which I never considered.
It might pay to keep a plug-in SS rectifier on hand for this purpose (I know you can cheaply rig something up. I was thinking from a point of convenience)
That would help, though I would have a meter monitoring the B+ going to said capacitors to make sure that their ratings aren't exceeded during the process as the variac is turned towards 120V, since there's far less voltage drop with a solid-state rectifier vs. the original tube rectifier. Also, as I said above, the current draw of the DUT needs to be monitored as well since some old electrolytics just plain aren't going to respond well to the attempt at reforming them.
There are wet tantalums that might respond to reforming. Not used much as they are not cheap and the ones in the Crown reel to reel electronics get replaced, not reformed.
I wasn't aware of that. Frankly, however, I would tend towards replacing all vintage tantalum capacitors outright due to some of their less-desirable qualities, not least of which their nasty tendency to go up in flames.......... :eek:
-Adam
 
I wasn't aware of that. Frankly, however, I would tend towards replacing all vintage tantalum capacitors outright due to some of their less-desirable qualities, not least of which their nasty tendency to go up in flames.......... :eek:
-Adam

Tants have their place. I'm not a cap guru and don't know what that is but I'm sure there are designers using them for something. But they are very sensitive to over or reverse voltage.

Now back to the variac. I like having one and use it to control a fan at lower speeds than on offer with the three position switch. This keeps it conveniently at hand when needed in the shop but in a different room as the shop is jammed with stuff.
 
I also use one on vacuum tube equipment to lower the input voltage to something that is more appropriate for tone in which the equipment was manufactured.
 
Tants have their place. I'm not a cap guru and don't know what that is but I'm sure there are designers using them for something. But they are very sensitive to over or reverse voltage.

Now back to the variac. I like having one and use it to control a fan at lower speeds than on offer with the three position switch. This keeps it conveniently at hand when needed in the shop but in a different room as the shop is jammed with stuff.
Tantalums are generally prized for their lower ESR vs. aluminum electrolytics, and (more in the old days than now) small size in terms of capacitance rating. However, in my opinion, the risks overweigh the benefits for most applications, at least where older-style parts are concerned. According to Kemet, modern polymer-type tantalum capacitors aren't prone to ignition like older wet and dry tantalums, but I have yet to see this demonstrated. Anyway, another issue with older wet tantalum capacitors is their tendency to physically leak; since said capacitors sometimes used acid inside, this can potentially damage the board it's sitting on, and/or any nearby components.
I also use one on vacuum tube equipment to lower the input voltage to something that is more appropriate for tone in which the equipment was manufactured.
That's a decent use for a variac, though I'd prefer to not reserve one for that sort of use, since variacs can do much more than just constantly lower voltage by a small amount. I'd probably tend towards using a sufficiently sized bucking transformer for this sort of purpose, at least if the equipment in question doesn't have a 120 or 125V input tap on the power transformer.
-Adam
 
Tantalums are generally prized for their lower ESR vs. aluminum electrolytics, and (more in the old days than now) small size in terms of capacitance rating. However, in my opinion, the risks overweigh the benefits for most applications, at least where older-style parts are concerned. According to Kemet, modern polymer-type tantalum capacitors aren't prone to ignition like older wet and dry tantalums, but I have yet to see this demonstrated. Anyway, another issue with older wet tantalum capacitors is their tendency to physically leak; since said capacitors sometimes used acid inside, this can potentially damage the board it's sitting on, and/or any nearby components.

That's a decent use for a variac, though I'd prefer to not reserve one for that sort of use, since variacs can do much more than just constantly lower voltage by a small amount. I'd probably tend towards using a sufficiently sized bucking transformer for this sort of purpose, at least if the equipment in question doesn't have a 120 or 125V input tap on the power transformer.
-Adam


I got two of them for free. One of them is for the power transformers in my Dynakit 70 and PAS3 are happier on lower input voltage, especially the PAS.

The other is for my work bench, and various vintage radio equipment.
 
Yes, an AC ammeter is an essential accessory when it comes to bringing up the voltage to something using a variac. Unfortunately, stand-alone AC ammeters are a bit hard to come by, for some reason, though there are work-arounds. One way is to use a multimeter with an AC amp clamp, either built-in or as an add-on; you can't just put the clamp over the power cord, but AC line splitters make things fairly easy. As an alternative, you could wire a 1 ohm power resistor in series with the hot lead going to the outlet the device being tested is plugged into, and then you'll be able to read the current drawn using a simple AC voltmeter. A schematic of a variac device I built using this principle can be seen here:
miniwieck_schem.gif

Oh, and one further tip with regards to variacs: Only use them to bring up the voltage to devices which use a standard linear power supply. Switching power supplies as used in computers and some stereo equipment don't take kindly to low line voltage levels.
-Adam

Great post. Just so I’m clear is the meter M1 here being used to measure the current? Looks like it would read Voltage AC.

I have an old tube preamp I’m restoring and when I have it back together I want to slowly bring it up and monitor the current for potential
Issues.
 
I got two of them for free. One of them is for the power transformers in my Dynakit 70 and PAS3 are happier on lower input voltage, especially the PAS.

The other is for my work bench, and various vintage radio equipment.
Whatever works. I think I'd prefer to use a bucking transformer myself, at least if I could find one large enough to support the device in question. The line voltage around here tends to run around 115-120V, so I don't see a great need to use one, though I do have a couple of OneAC CP1110 power conditioners which should do the trick if needed.
Great post. Just so I’m clear is the meter M1 here being used to measure the current? Looks like it would read Voltage AC.

I have an old tube preamp I’m restoring and when I have it back together I want to slowly bring it up and monitor the current for potential
Issues.
The unit takes advantage of Ohm's Law to allow a voltmeter to read current draw. One volt read across a one ohm resistor represents one amp.
-Adam
 
Whatever works. I think I'd prefer to use a bucking transformer myself, at least if I could find one large enough to support the device in question. The line voltage around here tends to run around 115-120V, so I don't see a great need to use one, though I do have a couple of OneAC CP1110 power conditioners which should do the trick if needed.


Line voltage around here is a year round 123vac.
 
The unit takes advantage of Ohm's Law to allow a voltmeter to read current draw. One volt read across a one ohm resistor represents one amp.
-Adam

Very simple and clever. If I have a DMM with current reading I assume I can simple connect that in series with the live lead?

Don’t want to fry my meter.
 
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