Variac protections - Page 1
Forum section: electronics / beginners

author: DisasterNow
author_rank: Regular Contributor
timestamp: June 06, 2024, 11:16:12 pm
content: I bought a variac (Vevor TDGC2-3KVA-10A), to power circuits under test with a gradual mains voltage.The variac has a 15A input fuse and has a button to reset any overload. The ground of the home electrical system is connected to the chassis of the variac, and also to the output socket.In your opinion, are the protections sufficient, or would you add other precautions when using a variac?Thank you

author: David Hess
author_rank: Super Contributor
timestamp: June 07, 2024, 12:13:45 am
content: I would put a fuse or circuit breaker on the output tap of the variac. That is the point where a short will damage it. The fuse on the input is not enough.When the variac is set to 50% output, acting as a step down transformer means that the output can now supply twice the current of the input, so if the output is shorted, it will damage the variable tap before the input fuse blows.

author: Whales
author_rank: Super Contributor
timestamp: June 07, 2024, 01:31:56 am
content: QuoteFuses won't protect you from electrocution. They only reduce the risk of fire.I added an RCD to my variac. That way I don't have to trust the house's RCD. Safety is better in layers.Covering exposed terminals on your variac (if it doesn't already have a cover) is also a good idea.Make sure to check the input active & neutral wires are the right way around. This will be a minor (but still desirable) safety improvement.Always remember that a variac is NOT a safety device, the output will still kill you.

author: DisasterNow
author_rank: Regular Contributor
timestamp: June 07, 2024, 02:26:23 am
content: Quote from: David Hess on June 07, 2024, 12:13:45 amI opened the variac to better check where the fuse was: I confirm, it is on the AC input cable (conductor L). So I should put an additional fuse (for example 12A, between the variac and the dut. ThanksQuote from: Whales on June 07, 2024, 01:31:56 amSo always between variac and dut, I could add a differential switch (30mA). As you can see in the photo, the outlet is a Schuco socket. I opened the variac on purpose to check which was the line conductor and the neutral one, I put labels on the input plug and also on the output socket so as not to make mistakes.Obviously when I do work with the variac, I will always use electrician's gloves. Thank you

author: Whales
author_rank: Super Contributor
timestamp: June 07, 2024, 02:43:26 am
content: Quote from: DisasterNow on June 07, 2024, 02:26:23 amHmm, I added it before the variac, not after. I wonder which configuration is more sensitive? My variac has exposed terminals everywhere so I thought it would be better to protect everything. Also Isuspectan RCD can't trip its mechanism if fed lower voltages (eg 80VAC), making it useless. I might be wrong.Thankyou for the photo. The way they've wired the grounding isn't quite to safety spec, but it could still work. Could you please test with your multimeter to make sure all parts of the metal case connect to GND? The yellow wire that goes to a screw at the bottom of your photo looks suspicious.

author: DisasterNow
author_rank: Regular Contributor
timestamp: June 07, 2024, 03:05:35 am
content: Quote from: Whales on June 07, 2024, 02:43:26 amI don't know if a differential switch works correctly at voltages of 80v.For earth, it comes from the cable of the wall socket to the schuco and starts again with that yellow cable on the screw of the metal protection. Yes, I checked, the ground is correctly connected to the metal parts of the variac.

author: TizianoHV
author_rank: Regular Contributor
timestamp: June 07, 2024, 08:32:47 am
content: Quote from: Whales on June 07, 2024, 02:43:26 amActually, if the variac is stepping down, it will be less sensitive depending on the position of the cursor!For example, if you have a 30mA leakage to ground on the output, and you have set the variac to half the nominal voltage the differential on the input will measure only 15mA (and[<--Wrong!, see Zero999's comment below.] here in italy a 30mA RCD won't trip at 15mA).Classic RCDs measure only the differential current with a current transformer (without any electronics), so they shouldn't be affected by the input voltage.

author: DisasterNow
author_rank: Regular Contributor
timestamp: June 07, 2024, 09:37:13 am
content: ThanksSo a good solution could be a differential circuit breaker between the variac output and the DUT. I should choose it with a current lower than 15A (so it also saves the variac fuse) and as a differential I could also use a value lower than 30mA. Could that be enough for security? (especially to save the variac, I will however use suitable gloves and be very careful).

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: June 08, 2024, 01:05:49 am
content: I have found a variac most helpful if it has both voltage and current meters that can be read at the same time.

author: DisasterNow
author_rank: Regular Contributor
timestamp: June 08, 2024, 02:17:29 am
content: Quote from: Jeff eelcr on June 08, 2024, 01:05:49 amOf course, I keep the voltage monitored on the variac display (it varies by a few volts from that detected by the multimeter), while for the current I will use a multimeter in series between the output and the DUT.----I checked the quality of the output sinusoidal signal with the oscilloscope, I must say that it is quite good.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 08, 2024, 02:47:45 am
content: Quote from: TizianoHV on June 07, 2024, 08:32:47 amThe measurement is done with a current transformer but most RCD/GFIs I have seen use an electronic trip circuit. That circuit is pwored from the input by a capacitive dropper. If the voltage is too low it's possible the driver won't operate.It's easy to check by just pushing the test button on the RCD.

author: Electro Fan
author_rank: Super Contributor
timestamp: June 08, 2024, 03:29:32 am
content: Quote from: Jeff eelcr on June 08, 2024, 01:05:49 am+1

author: Zero999
author_rank: Super Contributor
timestamp: June 08, 2024, 08:31:30 pm
content: Quote from: TizianoHV on June 07, 2024, 08:32:47 amNo, that's not true. The leakage flows to earth and will be picked up by the RCD, regardless of the wiper position. Incidentally, this can lead to a fire hazard, if the Variac can output more current than the earth conductor can take and there's no RCD, because the over current protection is always in series with the phase conductor.

author: Laval
author_rank: Regular Contributor
timestamp: June 08, 2024, 09:19:52 pm
content: Quote from: Jeff eelcr on June 08, 2024, 01:05:49 amThat's interesting, I have a Vevor TDGC2-2KM quite similar to yours and the integrated voltmeter is fairly inaccurate. If the output voltage is 100 V or lower, the measured voltage is off by 15-20 %. Above 100 V things get better and the error is about 5%. I use one of these cheap plug in power meter to monitor output voltage, current and power simultaneously with a better accuracy then the integrated voltmeter.As for protection, I use a cable with a fuse socket which allows me to put an HRC fuse of the calibre I want if needed (rarely).----I checked the quality of the output sinusoidal signal with the oscilloscope, I must say that it is quite good.

author: TizianoHV
author_rank: Regular Contributor
timestamp: June 09, 2024, 01:03:27 pm
content: Quote from: Zero999 on June 08, 2024, 08:31:30 pmThankyou for pointing this out! I got distracted by the ratio. In any case I prefer to have some overcurrent protection at the output since the fuse on the input won't trip for most of the overloads (when the variac is set for low voltages).Another useful feature is to add a ballast resistor in series to the output with a parallel switch (to exclude the ballast resistor). When testing dubious things you leave the resistor in series so that, in the event of a sudden short circuit, it will limit the current.

author: DisasterNow
author_rank: Regular Contributor
timestamp: June 09, 2024, 03:08:53 pm
content: Quote from: TizianoHV on June 09, 2024, 01:03:27 pmTypically (before the variac), I used an incandescent bulb, bulb trick (it is immediately obvious if there is an overvoltage).Quote from: Laval on June 08, 2024, 09:19:52 pmYes, the display indicates less than the real voltage value: from 110v upwards it fluctuates by 3V, at 60V it fluctuates by 5v, at 15v it fluctuates by 6v.For the current I intended to use a current clamp before the dut. I also thought about a device like the one in your photo, it would be more convenient.Quote from: Zero999 on June 08, 2024, 08:31:30 pmThis variac 'should' still have protection in the case of high temperatures (above 100°). Thanks for the explanation and the graphs.

author: Zero999
author_rank: Super Contributor
timestamp: June 09, 2024, 04:07:16 pm
content: Quote from: TizianoHV on June 09, 2024, 01:03:27 pmThere should be over current protection on the secondary, because the secondary current can be much higher than the primary current, at low voltage settings. My comment was purely referring to an RCD.

author: DisasterNow
author_rank: Regular Contributor
timestamp: June 10, 2024, 03:58:22 pm
content: I found a 0.03A differential and a 10A MT in the cellar. I put them in a box with a DIN rail, I would say in sequence as in the image. The MT only protects the phase and not the neutral from a short circuit. I hope it can be adequate protection.

author: DisasterNow
author_rank: Regular Contributor
timestamp: June 12, 2024, 02:19:31 am
content: With the variac output as in the image above, I verified the intervention of the differential: above 75 V it intervenes, below it it does not trip (check by pressing the appropriate test button).

author: Zero999
author_rank: Super Contributor
timestamp: June 12, 2024, 07:19:12 am
content: Quote from: DisasterNow on June 12, 2024, 02:19:31 amThat's what you should expect to happen, with the RCD on the secondary side. The test button connects a resistor between the phase on the output side of the RCD, to the neutral on the input. If the supply volage is too low, insufficient current will flow through the test resistor, so it won't trip. Some RCDs are more than a transformer and contain active circuitry, which requires a high enough voltage to work. The RCD must be on the primary side of the Variac.

author: DisasterNow
author_rank: Regular Contributor
timestamp: June 12, 2024, 04:05:53 pm
content: Quote from: Zero999 on June 12, 2024, 07:19:12 amThanks Zero.But on the primary side of the variac (i.e. the alternating mains voltage), I already have the differential in the apartment control unit. At this point why should I add a further differential before the prmary? Or ok I'll leave it on the variac output, knowing that up to 75V it won't do its job, but above that it will.

author: TizianoHV
author_rank: Regular Contributor
timestamp: June 12, 2024, 04:44:37 pm
content: Just out of curiosity, could you do an additional test?At low voltages the test button won't work because the "test ressistance" is too high.But you can still connect a lower value resistor (220ohms...) between the output and the neutral before the RCD and see if it trips at 30mA*220r = 6.6V.*I didn't put any differential on my variac. It would be really usesful if it was faster and more sensible than the one on your appartment (so that you don't trip the whole house).

author: DisasterNow
author_rank: Regular Contributor
timestamp: June 13, 2024, 03:22:12 am
content: Quote from: TizianoHV on June 12, 2024, 04:44:37 pmRCD works correctly even at low voltage (I checked with 9.5v), excellent. This RCD disarms when there is a difference of 0.025A.The idea was to put a 10mA RCD anyway, so it would avoid tripping the main apartment RCD.Thanks

author: TizianoHV
author_rank: Regular Contributor
timestamp: June 13, 2024, 08:19:29 am
content: Good. Just remember that if you use a more sensitive RCB the chance of it being electronic are higher.

author: DisasterNow
author_rank: Regular Contributor
timestamp: June 13, 2024, 11:42:30 am
content: Quote from: TizianoHV on June 13, 2024, 08:19:29 amHowever, having done the test to trigger the differential (I tried both with 1kohm at around 24v, and then with 390ohm at around 9.6v), both times the main apartment differential (even the 30mA one) is not intervened. So I believe that this Siemens which switches off at around 22/24mA can allow me selectivity.Thanks

author: TizianoHV
author_rank: Regular Contributor
timestamp: June 13, 2024, 03:06:54 pm
content: The resistor was connected to ground or to the neutral like in the schematic? With the connection in my schematic the main differential won't trip. It will trip only if the leakage goes to ground.*edit: "main differential" = differential protecting the whole house.RCDs must trip at Id > Idn but they could trip also if I> Idn/2 (so the mains differential could trip also at 15mA...)

author: DisasterNow
author_rank: Regular Contributor
timestamp: June 13, 2024, 03:39:52 pm
content: Quote from: TizianoHV on June 13, 2024, 03:06:54 pmThe resistor was connected between F and N. At this point I will wait for it to intervene for the first time during a test of some dut, to see if only it will trip, or both, or just the main apartment one. Thank you

author: Zero999
author_rank: Super Contributor
timestamp: June 13, 2024, 09:04:52 pm
content: Quote from: TizianoHV on June 13, 2024, 03:06:54 pmIt will trip if the resistor is connected between the live on the output and neutral on the input of the RCD, as shown on the schematic he posted.https://www.eevblog.com/forum/beginners/variac-protections/msg5539915/#msg5539915

author: DisasterNow
author_rank: Regular Contributor
timestamp: June 13, 2024, 10:18:43 pm
content: Quote from: Zero999 on June 13, 2024, 09:04:52 pmIf you mean that the main differential I have at the entrance to the apartment will trip, it hasn't happened; only this secondary differential tripped (the resistor was precisely between the RCD output live and the input N).

author: themadhippy
author_rank: Super Contributor
timestamp: June 13, 2024, 10:29:39 pm
content: Quoteand it shouldnt as its not seeing an imbalance

author: DisasterNow
author_rank: Regular Contributor
timestamp: June 14, 2024, 12:02:54 am
content: Quote from: themadhippy on June 13, 2024, 10:29:39 pmIn fact the imbalance was only in the RCD where I added the resistor.

author: BillyO
author_rank: Super Contributor
timestamp: June 14, 2024, 03:43:19 am
content: Quote from: David Hess on June 07, 2024, 12:13:45 amIt will actually supply twice the current in this circumstance without issue as both parts of the coil supply power.For general use, if you are not going to take advantage of the extra current you get at intermediary voltages between 100% and 0%, fuse the output at ~ 1.3x the nominal current capability it has at 100%. For example, if it is a 1000VA 120V variac it can supply 1000/120 A @ 120V (8.3A). So put a 10A fast blow fuse on the output.This protects the variac, not you.

author: Zero999
author_rank: Super Contributor
timestamp: June 14, 2024, 07:05:51 am
content: Quote from: BillyO on June 14, 2024, 03:43:19 amThat depends on whether the brush and wiring to it are appropriately rated or not.

author: DisasterNow
author_rank: Regular Contributor
timestamp: June 14, 2024, 12:57:53 pm
content: Quote from: BillyO on June 14, 2024, 03:43:19 amI placed a 10A thermal magnetic switch between the variac and the dut.

author: BillyO
author_rank: Super Contributor
timestamp: June 14, 2024, 01:45:45 pm
content: Quote from: Zero999 on June 14, 2024, 07:05:51 amTrue. Most reputable ones I have seen are so built but I can't vouch for those cheap red Chinese ones. I have a small 300VA one that is wired using 16ga wire and the brush looks more than willing to pass 6 amps, but if they used the same wire and brush in the 2000VA or 3000VA units, all bets are off.

author: DisasterNow
author_rank: Regular Contributor
timestamp: June 14, 2024, 02:31:26 pm
content: Quote from: BillyO on June 14, 2024, 01:45:45 pmThis by specification should deliver 10A, and support well over 15A (they have set the fuse at 15A). I put a photo above of the internal cables, let's hope that the Chinese have calculated any overcurrents well (the special switch they positioned should also intervene, in addition to the fuse).

author: TizianoHV
author_rank: Regular Contributor
timestamp: June 14, 2024, 03:33:11 pm
content: One of the many nice things about variacs is their overloading capability.General Radio, Belotti... give an overload curve that shows how they can supply up to 10 times their nominal current for multiple seconds!Even "crappy" ones are capable of some overloading.So i wouldn't be that worried about breaking it.

author: DisasterNow
author_rank: Regular Contributor
timestamp: June 15, 2024, 06:24:06 am
content: Quote from: TizianoHV on June 14, 2024, 03:33:11 pmThanks, I also believe that it will not be easy to damage the variac. However, I am satisfied with the purchase.