Variac: what should I add when putting it in an enclosure?

I have a big naked Powerstat Q116U and a nice Bud enclosure that I want to mount it in. I haven't used a Variac before so I'm not sure what kinds of features would be useful. Anyone have input?

So far, I'm planning on adding:
-a fuse (of course)
-on/off switch (switch both live and neutral)
-single 3 prong outlet

Would a pair of binding posts be useful? I suppose these could be wired in parallel with the outlet, but binding posts also seem a little unsafe.

Probably want an indicator lamp, maybe a 120V neon?

I have the ring and knob for the variac, so I'll know about what voltage is being delivered, but would a gauge or two be actually useful?

Should there be a switch to the output in addition to the on/off?

I'm building mostly tube gear, if that makes a difference.
 

mcandmar

Member
2012-02-23 7:02 pm
Dublin
I'm about to build a similar unit as soon as i source an enclosure. My parts list is a dual pole toggle switch for the output, a neon indicator,two panel meters for voltage and current, and a fuse.

I am also pondering the idea of being able to switch in a current limiter, but i already have an inline lamp contraption for that.
 
It is well worth fitting meters and fusing on the output but don't worry too much about the input.
The neon indicator is on the isolating transformer this being the best safety feature you can add to a Variac.
The bench needed tidying when the photo was taken:eek:
 

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Without a current meter, a variac is crippled,, in my view.

The dial is close enough, but it only shows what comes out if it is calibrated, AND the input is exactly 120v (or whatever your nominal mains voltage is) A voltmeter showing output voltage is important to me.When I test a circuit, I watch the current as I dial up the voltage. If I see the current ramp on tghe meter, I back off immediately. WIth current and voltage meters side by side, I can see both.

For me having the variable outlet is a necessity, but I also like having a non-adjustable convenience outlet as well. For example, I could power a heater supply on the convenience outlet, and vary only a DC supply with the variac. Both would turn on and off with the variac power switch, but only the one is adjustable. That is just me.
 

mcandmar

Member
2012-02-23 7:02 pm
Dublin

infinia

Member
2005-05-15 9:51 am
SoCal
Without a current meter, a variac is crippled,, in my view.

The dial is close enough, but it only shows what comes out if it is calibrated, AND the input is exactly 120v (or whatever your nominal mains voltage is) A voltmeter showing output voltage is important to me.When I test a circuit, I watch the current as I dial up the voltage. If I see the current ramp on tghe meter, I back off immediately. WIth current and voltage meters side by side, I can see both.

For me having the variable outlet is a necessity, but I also like having a non-adjustable convenience outlet as well. For example, I could power a heater supply on the convenience outlet, and vary only a DC supply with the variac. Both would turn on and off with the variac power switch, but only the one is adjustable. That is just me.


+1
an AC analog current meter is essential * gives fast response in case of faults
voltage meters are useless i usually put a couple of tape marks ( infact you just calibrated it ) on the dial for voltage.
also a resettable breaker isn't that expensive about the cost of a 1/2 box of cheap fuses.
 
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I have a big naked Powerstat Q116U and a nice Bud enclosure that I want to mount it in. I haven't used a Variac before so I'm not sure what kinds of features would be useful. Anyone have input?

So far, I'm planning on adding:
-a fuse (of course)
-on/off switch (switch both live and neutral)
-single 3 prong outlet

Would a pair of binding posts be useful? I suppose these could be wired in parallel with the outlet, but binding posts also seem a little unsafe.

Probably want an indicator lamp, maybe a 120V neon?

I have the ring and knob for the variac, so I'll know about what voltage is being delivered, but would a gauge or two be actually useful?

Should there be a switch to the output in addition to the on/off?

I'm building mostly tube gear, if that makes a difference.

An alternative view:

I have had an old 15 Amp 120 volt Variac in a vendor-supplied case for decades that has none of that. It came with no cord or outlet so I cut up a power cord for a large appliance and wired it up using that and a few cable clamps for strain relief. Pressed it immediately into service and as they say, the rest is history.

(1) Switch? It's a Variac. If you want no power, turn it all the way down. If you want a really safe Off position, unplug it!

(2) Power indicator light? It hums loud enough so that there is no question about whether its powered on or not.

(3) Current meter? The hum increases proportional to current drain.

(4) Voltage meter? The dial markings are not all that accurate so such a thing would seem to be in order. However, the meters that are monitoring the operation of the UUT can do the job.

(5) Binding posts? Safety concerns, and besides what can truly improve on a 3 prong cord-mounted outlet that came with that HD extension cord that I cut up and soldered on way back when?

(6) Fuse or circuit breaker? That's what the house breakers are for. Every once in a while mother nature says you need to take a break and walk down to the basement breaker panel.
 
Ability to switch OFF, or pull out the plug.
Mains fuse, or plug top fuse (UK).
Socket outlet, with switch.

I also have a isolation low voltage transformer fed from the Variac.
230:15+15Vac 100VA, gives a variable dual output from 0Vac to 16Vac.
four 4mm banana sockets that can be used separately or series connected for dual polarity.
I can plug in a rectifier/smoothing cap PSU that gives an isolated 0Vdc to ±21Vdc

Inside the same box (an old speaker box) I also have a Mains Bulb Tester with a single socket outlet, but fed from the mains power, not from the Variac.
 
Arnyk, I don't know...

My variac is plugged into an outlet not very coinvenient for unplugging and plugging, as is it under the bench. A mains power switch onb the unit not only depowers it, but also prevents me getting unwanted voltage that would occur if I bumped the dial turned to zero.

My bench variac is fused at 8 amps. I'd hate to rely on the 30 amp breaker down in the basement to protect the unit.

I can use my bench meter to monitor the mains voltage I am working with, but then I have tied up that meter. Same with a current meter. Why tie up my test gear when simple panel meters on the variac cover those common readings?
 
Personally I use the arnyk method, but any sane person would want a power switch (OK, I have that), a current meter so you can spot defective gear before vaporizing it, especially if the music is up too high to hear the increase in hum (don't laugh, it's a good method), a voltage meter so you can make decent amplifier power measurements at a true 120 VAC, a fuse because good Variacs (and Powerstats) have gotten expensive (Ok, I have that too). An isolation transformer if you work on gear that needs it. Maybe an input and output power light. Finally, if the unit has a tap just below full output, you can add a toggle switch to change the range so the unit can compensate for low input voltage, usually 0-130 VAC for 120 VAC input.
 

infinia

Member
2005-05-15 9:51 am
SoCal
my main purpose of using a Variac is for testing the ranges of AC high line and low line specs (roughly marked on the dial with tape pointers). Monitoring the Variacs output current gives good estimate of input power AND it has saved me many times when things start to go south. I could see the AC current dial movement out of the corner of my eye, I quickly flip the breaker saving repair time. It's true you can learn to feel the change in hum vibrations on the knob into a dead short or low ohm input on failed gear besides pegging the current meter at 10-20V. so for me large faced analog AC current meter and circuit beaker on the front panel are must haves, lights and voltage monitors are icing on the cake.
@Conrad IMO adding a range switch is defeating the whole purpose of the thing or you have reached a limit somewhere else.;)
to me it would be like adding a confusion factor on the dial!
 
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infinia

Member
2005-05-15 9:51 am
SoCal
ebay surplus online shops?
I don't have much experience with newer China units but they cant be all that rugged for the price.
you can spend anywhere from 8 bucks up for small panel AC ammeter.

size, ruggedness, and then accuracy?


id put most of my budget on a single analog meter and get the largest one you can fit.
my voltage doesn't fluctuate much, so like I say I use the Variacs dial for my rough readings and just attach a DMM when accuracy is called for.
 
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infinia

Member
2005-05-15 9:51 am
SoCal
brown-out conditions! It's a beast, .


yea sweet> compensating for poor power
yet another of NJ many industrial blessings err I meant disasters:D
the only brownouts we've had were caused by Enron* while GWB washed his hands
Involvement of Enron yeah for the free market, they turned CA into a third world country overnight.
 
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Arnyk, I don't know...

My variac is plugged into an outlet not very coinvenient for unplugging and plugging, as is it under the bench. A mains power switch onb the unit not only depowers it, but also prevents me getting unwanted voltage that would occur if I bumped the dial turned to zero.

If you haven't noticed there's a second plug where you plug the UUT into the output of the Variac. That is the one I use.

My bench variac is fused at 8 amps. I'd hate to rely on the 30 amp breaker down in the basement to protect the unit.

My bench Variac is rated at 15 amps and it has survived a goodly number of house circuit trips.

I can use my bench meter to monitor the mains voltage I am working with, but then I have tied up that meter. Same with a current meter. Why tie up my test gear when simple panel meters on the variac cover those common readings?

I have at least 5 general purpose DVMs that ran me from $20 to $300. It means not having to worry about not having one when you need it.

These days it seems like you can obtain a really pretty good DVM for less money than a meter with comparable function, accuracy and quality that you would build into something.