Variac's accuracy - UK Vintage Radio Repair and Restoration Discussion Forum

oldticktock · 3rd Feb 2010, 2:01 pm

Hi is it normal for variacs to be out.

The reason I ask is I thought a recent AVO minor I had was showing AC voltage incorrectly I set the variac @ 60V it looked about 5 volts out. I then tested with an AVO 7 and that was 5 volts out. Hmmm checked with Bench DMM and that gave a reading of 66.5V

OTT

dominicbeesley · 3rd Feb 2010, 2:07 pm

Variacs just divide down the mains voltage by a fraction so there are two things:

What nominal mains voltage is the variac designed for (240 or 230 or 220)?
What mains voltage do you actually have - this can vary throughout the day as well as be always out by a large margin depending on many factors...

Add into the mix the fact that your mains probably isn't a very pure sine wave too so different meters may give different results depending on how they perform the measurement!

Dom

KeithsTV · 3rd Feb 2010, 2:07 pm

Don't forget the AC input from the mains will vary. I use a variac at work and in the early morning can usually get up to 270v with the variac st to maximum but later in the day when the load is greater I may only get 260v maximum.

I'm not sure what the tolerance on the mains voltage is (I'm sure someone will know) but it's only a nominal 240v or is it 234v these days.

Keith

EDit Dom beat me to it. AVOs are usually calibrated on the AC ranges for a pure sinewave so a miss-shapen ac waveform will give a different reading.

kalee20 · 3rd Feb 2010, 6:29 pm

Another complication - often, Variacs have a tap at about 90% of the winding as well as a connection to the extreme (and a centre tap too).

If you connect the supply to the extremes, you get an output variable from 0 - 100% of the input. But if you connect the input to the 90% tap, you can achieve an output spanning 0 - 111% of the input. Clearly, the scale isn't going to be right for both. So, if you unscrew the scale plate, you may find an alternative set of numbers on the other side.

McMurdo · 3rd Feb 2010, 6:35 pm

5 volts out is very good. My Claude Lyons 'regulac' is a good 15-20 volts out on the dial and varies on load of course.

Steve_P · 3rd Feb 2010, 6:40 pm

Just put a meter across the variac when in use and use that to determine the voltage.

Cheers,

Steve P.

XTC · 6th Feb 2010, 8:55 pm

I've got two variacs and they are both out by something like 10%. I recall the % out varies across the scale. I use them with a meter. So I'd say the answer to your question is that your variac isn't unusual and they don't seem to be high accuracy devices.

The disc with the scale on sometimes has another scale on the back.

With things like this, with a scale that should be calibrated, it's worth checking what they are actually doing, rather than assume everything is as it seems and end up with confusing results or damaging something.

Pete.

kalee20 · 6th Feb 2010, 10:36 pm

Quote:

Originally Posted by XTC

With things like this, with a scale that should be calibrated, it's worth checking what they are actually doing, rather than assume everything is as it seems and end up with confusing results or damaging something.

Definitely!

Always view anything which hasn't been calibrated as suspect (including test meters).

And if you don't have a calibrated meter, use two different uncalibrated ones. If they give similar readings, it's likely they are right. If they give different readings, view them as both suspect.

Skywave · 6th Feb 2010, 11:15 pm

Indeed, Kalee20, that is a very valid point: independent verification of measured quantities. Mr. Scroggie goes to great pains to point out the importance of this and various methods to achieve same in his excellent book "The Radio Laboratory Handbook". Mr. Terman does much the same in "Measurements in Radio Engineering". Both authors highlight the importance of allowing for 'invisible components' - usually reactances - that can upset measurements, even if the calibrations of the test equipments are known to be good - and devising methods to minimise (or eliminate) them.

Al. / Skywave.

wireful3 · 7th Feb 2010, 10:19 am

The earlier posts have summed up the situation. I would just add that the construction of a Variac is also a factor. A 5v difference from the dial setting is not really surprising. So if the output voltage is important put a meter on it. If you are using it to check a meter there are problems.

From the way a Variac is constructed expecting the scale to give the output voltage accurately is a bit optimistic. It only needs the pointer or scale to be in the wrong position to give an error. I wonder how repeatable the output setting is when the scale is used to set it to the same value several times? The mains voltage is also another source of error that can be large.

The big advantage of the Variac for us is that it allows us to increase voltage from a low value to the working voltage smoothly. It can also be used to adjust for variations in supply voltage but this needs a meter on the output.

Sideband · 7th Feb 2010, 11:07 am

I use a variac nearly every day when safety-testing medical equipment. It has to be connected to a callibrated voltmeter because of the inherent inaccuracy of these devices.

For hobby work, a few volts either way doesn't make any difference and the one I use at home is a good 10 volts out compared with the dial.

Rich.

3rd Feb 2010, 2:01 pm	#1
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Variac's accuracy Hi is it normal for variacs to be out. The reason I ask is I thought a recent AVO minor I had was showing AC voltage incorrectly I set the variac @ 60V it looked about 5 volts out. I then tested with an AVO 7 and that was 5 volts out. Hmmm checked with Bench DMM and that gave a reading of 66.5V OTT

3rd Feb 2010, 2:07 pm	#2
dominicbeesley Octode Join Date: Nov 2004 Location: Hebden Bridge, West Yorkshire, UK. Posts: 1,885	Re: Variacs accuracy Variacs just divide down the mains voltage by a fraction so there are two things: What nominal mains voltage is the variac designed for (240 or 230 or 220)? What mains voltage do you actually have - this can vary throughout the day as well as be always out by a large margin depending on many factors... Add into the mix the fact that your mains probably isn't a very pure sine wave too so different meters may give different results depending on how they perform the measurement! Dom

3rd Feb 2010, 2:07 pm	#3
KeithsTV Nonode Join Date: Aug 2007 Location: South Bradford, West Yorkshire, UK. Posts: 2,573	Re: Variacs accuracy Don't forget the AC input from the mains will vary. I use a variac at work and in the early morning can usually get up to 270v with the variac st to maximum but later in the day when the load is greater I may only get 260v maximum. I'm not sure what the tolerance on the mains voltage is (I'm sure someone will know) but it's only a nominal 240v or is it 234v these days. Keith EDit Dom beat me to it. AVOs are usually calibrated on the AC ranges for a pure sinewave so a miss-shapen ac waveform will give a different reading.

3rd Feb 2010, 6:29 pm	#4
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: Variacs accuracy Another complication - often, Variacs have a tap at about 90% of the winding as well as a connection to the extreme (and a centre tap too). If you connect the supply to the extremes, you get an output variable from 0 - 100% of the input. But if you connect the input to the 90% tap, you can achieve an output spanning 0 - 111% of the input. Clearly, the scale isn't going to be right for both. So, if you unscrew the scale plate, you may find an alternative set of numbers on the other side.

3rd Feb 2010, 6:35 pm	#5
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,274	Re: Variacs accuracy 5 volts out is very good. My Claude Lyons 'regulac' is a good 15-20 volts out on the dial and varies on load of course. __________________ Kevin

3rd Feb 2010, 6:40 pm	#6
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Variacs accuracy Just put a meter across the variac when in use and use that to determine the voltage. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

6th Feb 2010, 8:55 pm	#7
XTC Retired Dormant Member Join Date: Oct 2004 Location: Bristol, UK. Posts: 837	Re: Variacs accuracy I've got two variacs and they are both out by something like 10%. I recall the % out varies across the scale. I use them with a meter. So I'd say the answer to your question is that your variac isn't unusual and they don't seem to be high accuracy devices. The disc with the scale on sometimes has another scale on the back. With things like this, with a scale that should be calibrated, it's worth checking what they are actually doing, rather than assume everything is as it seems and end up with confusing results or damaging something. Pete.

6th Feb 2010, 11:15 pm	#9
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Variac's accuracy Indeed, Kalee20, that is a very valid point: independent verification of measured quantities. Mr. Scroggie goes to great pains to point out the importance of this and various methods to achieve same in his excellent book "The Radio Laboratory Handbook". Mr. Terman does much the same in "Measurements in Radio Engineering". Both authors highlight the importance of allowing for 'invisible components' - usually reactances - that can upset measurements, even if the calibrations of the test equipments are known to be good - and devising methods to minimise (or eliminate) them. Al. / Skywave.

7th Feb 2010, 10:19 am	#10
wireful3 Retired Dormant Member Join Date: Sep 2003 Location: Bristol, UK. Posts: 808	Re: Variac's accuracy The earlier posts have summed up the situation. I would just add that the construction of a Variac is also a factor. A 5v difference from the dial setting is not really surprising. So if the output voltage is important put a meter on it. If you are using it to check a meter there are problems. From the way a Variac is constructed expecting the scale to give the output voltage accurately is a bit optimistic. It only needs the pointer or scale to be in the wrong position to give an error. I wonder how repeatable the output setting is when the scale is used to set it to the same value several times? The mains voltage is also another source of error that can be large. The big advantage of the Variac for us is that it allows us to increase voltage from a low value to the working voltage smoothly. It can also be used to adjust for variations in supply voltage but this needs a meter on the output.

7th Feb 2010, 11:07 am	#11
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,580	Re: Variac's accuracy I use a variac nearly every day when safety-testing medical equipment. It has to be connected to a callibrated voltmeter because of the inherent inaccuracy of these devices. For hobby work, a few volts either way doesn't make any difference and the one I use at home is a good 10 volts out compared with the dial. Rich. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....