Variac voltage regulation under load - Page 1
Forum section: electronics / beginners

author: belzrebuth
author_rank: Frequent Contributor
timestamp: June 07, 2020, 09:27:38 pm
content: Hi all,I've got a used variac from someone who said it belonged to his father.(he had no clue about anything else despite the fact that it's "working")It's rated for 2A output current.I found out after buying it that the fuse, which is on the input side, had a wire wound around it so that's definitely a red flag.I then went on to check the windings in order to verify the transformer is okay, from what I've seen the enamel coating seems okay except for a few spots where it's darker so it's probably had a few rough ones but I don't think it's seriously abused despite the owner's ignorance.I then proceed to load test it and found out that the voltage isn't very stable under load.It kind of swings around maybe half a volt or so and then maybe stabilises a bit but it's not stable like most transformers I've seenI've got a toroid here that's swinging about 10mV or so (maybe even less) when loaded.Anyway that's issue #1.The second issue is that unloaded voltage is significantly greater than the loaded voltage.This may be normal for a couple of volts max from what I've seen in general, but I'm not sure how it is with variacs since I've never used one before.I would expect the output to be a little better regulated than with a regular transformer but I really don't know.So here's what I did:I've used a 10ohm 100w power resistor as a test load.I upped my voltage up to around 15V with the load connected (with a 2A fuse in series with the output) in order to draw about 1.5A.I did this to ensure the tranformer is able to give some current near to its max rating.When I disconnected the load I got about 18V which is thought might be okay.But then I realised that the difference is also present (and even greater ) if I even get down to 1V with load.The unloaded voltage then (with the load disconnected) is about 4.5V which I think is too much considering I'm asking a lot less current than I did before.It is to my understanding that the greater the stress (current asked) the greater the voltage drop should be..My questions are:Are these things normal somehow or I got a defective unit?How should a variac behave under load?Does the voltage vary so much under it?Is it worse at voltage regulation than a regular transformer?Should I always have the load connected to it before powering it up and have a multimeter in parallel to slowly raise the voltage to match the circuits' needs?I was hoping to use the variac in order to eliminate the pile of wall plug of AC transformers I've got laying around in order to test things quickly and I say I'm a bit dissapointed..

author: james_s
author_rank: Super Contributor
timestamp: June 07, 2020, 10:22:45 pm
content: There is no regulation at all, the output will vary depending on input voltage and load. What you describe sounds completely normal, if the brush is in good shape, the dial rotates smoothly and there isn't something like a shorted turn causing it to heat up with no load then there's nothing at all wrong with it. Just put a properly rated fuse in there and use it, it's a variable autotransformer, not a voltage regulator. You have to manually adjust the dial to regulate the voltage.Also the output of a variac is NOT isolated, so it's not a substitute for a power adapter. You can plug a large (iron transformer type) power adapter into the output of a variac and you'll get a variable power supply of sorts, still unregulated though. You seem to be misunderstanding what a variac is and what it is for.

author: ArthurDent
author_rank: Super Contributor
timestamp: June 07, 2020, 10:44:30 pm
content: "I've got a toroid here that's swinging about 10mV or so (maybe even less) when loaded."First off the variac IS a toroid. I can't visualize under any conditions where a transformer of any type would have an output that doesn't vary more than 10mV, 10%+ might be more reasonable from zero to full load.As mentioned above, a variac is an autotransformer and trying to use it to replace a wallwart transformer which does have isolation is asking for trouble. You're likely to get shocked, or worse, and destroy your circuit by having an unintended loop circuit with your mains supply. A variac is also least efficient at the low end of the winding when you're trying to get low voltage out.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: June 07, 2020, 10:52:38 pm
content: Nice tip about putting another transformer after the variac.I don't know it still seems pretty weird that with the same load the voltage swings around without anything else changing.I'm not talking mV here though..More likely 200-300mV or even half a volt at times.I mean I can set the voltage at 9v go for a cup of tea and find it at 8.4v ( all these under load)I'm glad it's normal just didn't think it'd be so different than a regular transformer.And I get that's it's not isolated although I do have an isolation transformer so that wouldn't be an issue at least safety wise.I think that at some positions it's better so maybe the carbon ring or the pot wiper needs some carbon to help as a lubing factor?Can I maybe apply a bit of graphite powder on the top in order for the pot to turn smoother?Also, what could be the correct value for the input fuse?Is there some way to calculate that?The transformer is rated at 500VA.

author: bdunham7
author_rank: Super Contributor
timestamp: June 07, 2020, 11:49:48 pm
content: Quote from: belzrebuth on June 07, 2020, 10:52:38 pmYour original idea is not a good way to use a variac for many reasons, one being that the load sensitivity at the lower ranges and high load is, as you've observed, not good. A small part of the variac is heating up when you do that. You can inspect the brush to make sure it's OK (post a good pic and we'll tell you) but no lubing! If it is dirty, just turning it up and down a few dozen times should clear it.Standard practice for a variac of 2A would be an input fuse of 2 or 2.5A and output of 2.5 or 3A, both slo-blo. You can go a bit less if you don't need the power. As for substituting for wallwarts, I have two small low-voltage lighting transformers 12VAC @ 50VA and 24VAC @30VA that I use instead of a direct connection. You could use larger transformers if you wish, especially if you want less load sag. I can easily draw 2A out of the 12V model without much sag--it doesn't droop with time--and I can vary the voltage from 0 to 15 or so volts at that current. It works pretty well and you can basically just scrounge a transformer from anywhere.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: June 08, 2020, 12:13:27 am
content: Great! Thanks for the tips.I have a lot of spare bare transformers laying around so I could use one with a lot of taps and the variac as the input in order to have any ac supply needed in a pinch.I will post a photo of the transformer brush tomorrow.

author: bob91343
author_rank: Super Contributor
timestamp: June 08, 2020, 12:26:52 am
content: If you feel the voltage is both unstable and varies too much with load, check the brushes. They should have decent pressure, not be worn out, and the winding on which they bear should be clean.The connection from the brushes should be inspected to see if it's faulty.Incidentally, regulation doesn't necessarily imply a regulating circuit. Regulation is defined as the change in voltage divided by the loaded voltage, often in percentile.

author: james_s
author_rank: Super Contributor
timestamp: June 08, 2020, 03:28:19 am
content: Quote from: belzrebuth on June 07, 2020, 10:52:38 pmThis is perfectly normal, your expectations are unrealistic. Have you monitored the input voltage? My line voltage typically wanders around by a few volts throughout the day and the output of a variac will wander around similarly. Don't try to lubricate it with anything, especially not with something conductive. It's working fine, there's nothing wrong with it and nothing to fix. It's not regulated, that isn't the purpose of a variac. If you want a regulated supply to sub for adapters you should look into getting a bench power supply. Variacs are typically used for servicing equipment so you can bring the voltage up slowly and test the behavior under low and high line voltages.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: June 08, 2020, 10:57:11 pm
content: I have a DC bench supply.What I don't have is an AC variable supply.So..From what I understand in order to make a variable AC supply I only need to connect any "ordinary" transformer after the variac for isolation purposes.This essentially gives me a variable isolated AC power supply with the voltage varying from 0- whatever the rated voltage of the transformer I put after the variac.I've gone a step further and tried it and it works as expected but I've been very conservative when load testing it.I'm not sure what would be the max current I can draw from the transformer which I put after the variac since all the "fixed voltage" transformers have a certain rating that applies to that voltage only and VA rating that from what I gather serves as a more general current indicator.Now, I've read that the variac amperage rating is the absolute limit regardless of the voltage output so we should respect that at all times.But this isn't true for a regular transformer, correct?I mean the max Ampere rating is valid for its voltage rating but if I make it variable would the max current be calculated by its VA rating, correct?I have a lot of step-down (220 to 110) transformers I don't use so I though I could put such a transformer which rated for 1A after the variac so I can have a 0 - 110 volts AC supply.My question is: If I need to output say 20Vac from said transformer will I be able to draw something like 2A@20V from it?Is the VA rating the figure I need to take into account?Meaning a 200VA transformer (regardless if its rated for an output voltage of say 48V) can supply about 8A of current @20V if I vary its input voltage?And what could be the current draw on its input side? (from the variac I mean).I can blindly put a 1.6A fuse after my variac and be on the safe side but it sure is best to know exactly what I'm doing.

author: Cliff Matthews
author_rank: Unknown
timestamp: June 08, 2020, 11:41:03 pm
content: Don't know how well this would work, but I wonder if PWM could be a viable option with some filtering on the output.

author: bdunham7
author_rank: Super Contributor
timestamp: June 09, 2020, 12:32:01 am
content: Quote from: belzrebuth on June 08, 2020, 10:57:11 pmNo, the transformer will be limited by the size of the wire in the secondary, among other things. Putting more than the rated current through it will overheat it. The VA rating is good for the specified voltage, current and frequency and has to be derated if any of those change.