Usage UPS transformer "backwards" (step-down) - Page 1
Forum section: electronics / beginners

author: npelov
author_rank: Frequent Contributor
timestamp: February 08, 2018, 11:03:34 am
content: Can I use UPS transformer for step-down by connecting the secondary to the power line?

author: abraxa
author_rank: Frequent Contributor
timestamp: February 08, 2018, 09:12:30 pm
content: Yes, a transformer is a passive device and will work just fine in reverse. However, UPS transformers are designed only to be used for as long as the batteries last, as far as I understand. This means that if the UPS is an 800VA UPS, you can push 800VA through the transformer only for 20 minutes or whatever, not 24/7. I'd say using it for a load that's 50% of the UPS load is a good safety margin.

author: Cliff Matthews
author_rank: Unknown
timestamp: February 08, 2018, 11:10:51 pm
content: Efficiency may not be great either since the primary is now the outer winding. For duty and heat concerns as already stated, this could be similar to microwave oven transformers (MOTs) where a manufacturer may use as few lamination's as required (built down to a price as Dave would say). Power it up with a light load and check heat levels over time and post some results.

author: KrudyZ
author_rank: Frequent Contributor
timestamp: February 08, 2018, 11:58:48 pm
content: Quote from: npelov on February 08, 2018, 11:03:34 amI think you need to clarify what you are talking about.Do you have a UPS and want to take it apart, find the transformer and then use it?Or do you plan on making a power cord with plugs on either side and connect the UPS outputs to the wall?Neither one will work, but the second one will be a lot worse for you and your UPS.A UPS takes in power and uses it to charge a battery.The power from the battery is then used to drive an inverter. The output of the inverter looks similar to the wall power. Depending on how fancy your UPS is it might even be sinusoidal.Driving this output by connecting it to an outlet on the wall will be a very bad idea.

author: npelov
author_rank: Frequent Contributor
timestamp: February 10, 2018, 07:58:00 pm
content: Quote from: KrudyZ on February 08, 2018, 11:58:48 pmI have two very dead UPSes. I took them apart for parts - transistors and capacitors. I wonder if I can use the transformer (alone, no other electronics from the ups) by connecting it to the line. After all doesn't the ups use the same transformer for charging the batteries? Why would it be a bad idea to connect the secondary (the one with more turns) to the wall? Except the efficiency as Cliff Matthews said. Maybe using it for lab power supply and not for something that will be connected 24/7. I think it's 1000 V/A I won't push it to the limit (the cheaper one is probably overspec'ed).I could get 12V few amps.Honestly I don't even have an idea how I want to use them. I'm just looking for a reason to save them from the trash.

author: KrudyZ
author_rank: Frequent Contributor
timestamp: February 10, 2018, 08:24:37 pm
content: Quote from: npelov on February 10, 2018, 07:58:00 pmUnless these are VERY old UPSes the transformer will not be made for 50 / 60 Hz operation, but rather be a part of a switching power supply.A 1000 VA rated 60 Hz transformer would be about 6" cubed and weigh about 30 lbs.If yours is significantly smaller / lighter then it will be made to operate at much higher frequencies and just fry when directly plugged into a mains outlet.Just noticed you're in Europe. Sorry for the non-metric units...

author: Cliff Matthews
author_rank: Unknown
timestamp: February 10, 2018, 08:53:46 pm
content: I've never seen a UPS output transformer used to recharge the batteries. What model was it taken from?*edit - this one's an old APC Smart-UPS from 2000 (note the 2 PCB transformers above chassis mount output)

author: SeanB
author_rank: Super Contributor
timestamp: February 11, 2018, 01:49:44 pm
content: Most of the cheap ones use the main transformer to charge the batteries, using a tap on the transformer mains side to select a voltage on the mosfet switches of around 8VAC, and relying on another winding that gives 15VAC with the normal mains input. When switched to UPS mode the input mains is disconnected by a relay contact, and the mosfets are switched at 50/60Hz, applying 12V to the centre tapped battery side winding. At the same time another relay switches the tapping on the mains side to a tapping that will supply 115/220VAC to the load, so the output is approximately the incoming mains voltage.Some better models also include an extra 2 relays to do tap changing on the transformer so that the UPS can buck or boost the incoming mains in case of brown out or slight surge so the output is reasonably constant, and not switch to battery operation for the duration of short brown out events. This is to increase battery life, as the loading on the battery when running damages the battery, though of course with the cheap units the battery is in any case both under rated and over drawn, along with being placed in a nice toasty cooking oven ( the transformer and such in a small space and poor ventilation) so they typically only last a little over a year and a half even unused but powered, and less when you have regular power cuts.

author: Cliff Matthews
author_rank: Unknown
timestamp: February 11, 2018, 04:35:09 pm
content: Quote from: SeanB on February 11, 2018, 01:49:44 pmI 'd consider that APC I'm holding cheap.. I've looked at the schematic (could be similar to ops) and must be missing it. The charge circuit must be spread across at least 3 of 5 pages. I see the transfo output gets rectified by puny 1N4148's and heads off to the ADC on page 2, but that's just for output sense.. definitely there's more magic here than normally meets the eyes of some guy used to audio amps and small gear.

author: KrudyZ
author_rank: Frequent Contributor
timestamp: February 11, 2018, 07:47:00 pm
content: QuoteThe best way to find out is to see if the secondary of the transformer is directly connected to the power outlets of the UPS, which means that the transformer would be made for line frequency.In that case, there should be no problem running the transformer in reverse and you can disregard my dire warnings.****************edited to show original quote

author: MarkL
author_rank: Unknown
timestamp: February 11, 2018, 08:13:14 pm
content: Quote from: Cliff Matthews on February 11, 2018, 04:35:09 pmThe main power transformer does not seem to be on the schematic. The AC line side w/taps connects to J5 (sheet 1) and the low voltage side to a couple of connections directly on the heatsinks of the power MOSFETs (sheet 3).

author: Cliff Matthews
author_rank: Unknown
timestamp: February 11, 2018, 09:14:35 pm
content: Having it on schematic would have been too easy.. you're right J5 on PCB runs to transfo output, but it's two input wires are bolted directly on the heatsink. Mine can be a guinea-pig for the op.. I'll connect my variac to J5 2 and 4, while measuring V secondary and J5 1 and 3 (tap).

author: mcinque
author_rank: Unknown
timestamp: February 11, 2018, 09:16:04 pm
content: Quote from: npelov on February 08, 2018, 11:03:34 amNO!!Secondary has too few Ohms and this value affect impedance.Quote from: npelov on February 10, 2018, 07:58:00 pmBecause of IMPEDANCE. This video explains very well and in a very fun way the details (I LOVE the ElectroBOOM style!)https://youtu.be/sFTjFD84Q1c

author: Stray Electron
author_rank: Super Contributor
timestamp: February 11, 2018, 09:26:40 pm
content: Quote from: KrudyZ on February 10, 2018, 08:24:37 pmThat's what I'm thinking. UPSs haven't used a 50/60 Hz transformer in the output in a LONG time. The modern ones have a very high frequency output that is coupled through a high frequency toroid transformer and they simulate a 50/60 Hz frequency by what amounts to modulating the output. If your's has a 50/60 Hz transformer there will be NO doubt of it, It will be MASSIVE.

author: MarkL
author_rank: Unknown
timestamp: February 11, 2018, 09:33:16 pm
content: Quote from: Cliff Matthews on February 11, 2018, 09:14:35 pmJ5-1 and J5-3 appear to be an isolated extra winding for buck or boost (as SeanB mentioned) on the AC side. It can be switched in series with the AC line in forward or reverse phase by RY2 and RY3.You'll see the buck/boost voltage on J5-1 to J5-3, but it's not going to be the high current secondary you want. The high current secondary would be the winding connected to the heatsinks.Also, being on the AC side, the buck/boost winding may not have the same safety rating for the insulation as that on the high current secondary. Without specifically knowing its rating, I wouldn't use it for low voltage circuits that people might touch.EDIT: I agree that J5-2 and J5-4 would be the primary.

author: Cliff Matthews
author_rank: Unknown
timestamp: February 11, 2018, 09:56:38 pm
content: No panic please (and I did say variac..) Rechecking then, J5 pin 2 to 4 measures 1.0 ohm (expected), but what I'm not sure of is the 964H inductance. I'll have to look it up.. J5-1 to 3 measures 0.1ohm so that's the boost, and would trip the breaker on my variac even at 20v input.

author: Cliff Matthews
author_rank: Unknown
timestamp: February 11, 2018, 10:10:32 pm
content: Started 60vac in and got 7.5v out (doubling @ 120v). Clamp-meter shows 0.1 amp at 120v. I put a 2 ohm 225watt resistor on the LV winding and got 14.7vac out at 7.3 amps and 0.9 amps on the 120v side with barely audible hum. These numbers do not seem unusual.I can put a fan on the load and let it sit for an hour at @110w if anyone's interested I'll do a temp reading on the core.

author: MarkL
author_rank: Unknown
timestamp: February 11, 2018, 10:26:46 pm
content: Just to be clear on the semantics, I'm referring to primary and secondary as the OP wants to use the old UPS transformer, namely as a step-down transformer. What I'm calling the primary is the AC line in, and the secondary is the high current, low voltage output. If you were looking at it as an inverter, the references would be reversed.In the unit you have on the bench, the AC line in would be J5-2 and J5-4, and low voltage AC out would be on the heatsink connections. I'm not sure why you'd want to use your variac to put current into the buck/boost winding.Quote from: Cliff Matthews on February 11, 2018, 10:10:32 pmOn what connections are you seeing those voltages?A little bit of no-load current draw is expected.

author: Cliff Matthews
author_rank: Unknown
timestamp: February 11, 2018, 10:43:11 pm
content: I am testing the transformer alone (no PCB). The alleged boost winding 1 and 3 is electrically separate from 2 and 4 on the J5 connector. Under load it has 14.9v on it, so maybe it's used for charging only. I could in-phase add-it but the output on the LV side would drop down.I doubt this APC 1000 ever had brown-out boost, but then again, I've already shown my dodgy schematics skills..FWIW, yes I do know the semantics. I still remember a thing or two, it's over 4 decades since DeVry training (wa wa wa wah..)

author: ovnr
author_rank: Frequent Contributor
timestamp: February 11, 2018, 10:57:37 pm
content: Quote from: Stray Electron on February 11, 2018, 09:26:40 pmEh, I have a APC smart-UPS 1000XLi which uses a single giant 50 Hz transformer for both charging and output. I expect any true sinewave line-interactive UPS uses a boring lump of metal like mine. Online UPSes will use a more modern topology, and modified sinewave UPSes will use a small HF output stage.

author: Cliff Matthews
author_rank: Unknown
timestamp: February 11, 2018, 11:13:58 pm
content: Quote from: ovnr on February 11, 2018, 10:57:37 pmAgree, I have a Smart-UPS SC1500 with 60% less iron but it cranks out 50% more power. Maybe most of the suspicion here is "how do we know if a poster really knows what big iron really is?" If I were to use this one I'm testing now at 100% duty cycle, I'd never expect any more than 250VA.Speaking of iron, my brother's been trying to sell this:https://www.kijiji.ca/v-buy-sell-other/city-of-toronto/1500-va-hammond-transformer/1332764512(the price is right, but the shipping fee = cardiac arrest...)

author: MarkL
author_rank: Unknown
timestamp: February 11, 2018, 11:14:53 pm
content: Quote from: Cliff Matthews on February 11, 2018, 10:43:11 pmYeah, the buck/boost winding should be isolated, given how it's wired up to the relays. That winding is always connected to some part of the AC side. It's not used as an output to charge the battery.Connected in one phase or the other, it will increase or decrease the low voltage output. But its main purpose is to increase or decrease the voltage across J5-2 and J5-4, which goes to the UPS protected equipment (J2, J6). This is how it would compensate for under or over voltage conditions on the incoming AC line.QuoteYou could always use your variac to test that theory...QuoteI only posted that because I noticed there was some potentially confusing posts from others about which winding connects to the AC line.EDIT: Removed unneeded sentence.

author: james_s
author_rank: Super Contributor
timestamp: February 12, 2018, 12:38:33 am
content: It's pretty easy to tell, if it's a big heavy iron core transformer then it's almost certainly designed for 50/60Hz use. If it's a small ferrite core transformer then it's designed for high frequency.From my limited set of samples, inexpensive "modified sine" (square wave with dead time) UPSs use ferrite transformers in a high frequency inverter, while most of the better true sine UPSs use a big bulky iron transformer which doubles as the output filter.

author: Cliff Matthews
author_rank: Unknown
timestamp: February 12, 2018, 01:30:37 am
content: That's is all the testing I'm gonna do. At 141 watts of load, it's 79.5F after 45 mins (16.67v @ 8.5a with variac set to 135v). I might be inclined to add the other winding but I'll call it day. Summary: I'd trust it encased and fused at 250va all day (provided it could pass some Hi-pot tests).

author: Cliff Matthews
author_rank: Unknown
timestamp: February 12, 2018, 12:46:59 pm
content: FWIW, here's 30mins w/ boost winding in series with 135v on the mains. That's the biggest resistor I have and at 177w that's enough tests. In summary, I don't see too many uses for this outside of maybe a really big 13.8v regulated ham supply.Question:Schematic has 2 rows of 5 FET's on each half... Is each half of the waveform 3 steps? (ie: 7 steps total including dead time)Question 2: With 2 cells (24v) I'd say 22v p-p is the primary voltage. So would they PWM the primary for light loads? (ie: my tests would indicate a solid stepped waveform would put at least 150v p-p on the secondary or 106Vrms...) is that enough for most loads?

author: MarkL
author_rank: Unknown
timestamp: February 12, 2018, 02:33:25 pm
content: The two rows of MOSFETs are implementing a half bridge. They connect one side of the winding to either battery+ or ground. The other set does the same thing to implement a full bridge to drive the transformer winding. The winding sees battery+, 0V, or battery-.If you have a dual channel scope you can look at each of side of the MOSFET chain and the timing relationship to eachother. And you can use the subtract function on the scope to see what the transformer is seeing across the winding.Many (all?) cheap UPSes have a "modified sine wave" output. Below is a screen shot of one. Those are +/-150V peaks, 0V in the middle marked by the "+".

author: Cliff Matthews
author_rank: Unknown
timestamp: February 12, 2018, 03:02:43 pm
content: OK, I got it.. At first, for each x-former lead I saw two rows of 5 FET's (page 3) each run by dissimilar drive circuits, and well... the amp guy in my brain went linear (don't know why, but I did see 2 invertors driving one and 1 on the other, but the old elevator didn't go up to the top..)

author: npelov
author_rank: Frequent Contributor
timestamp: February 27, 2018, 10:20:04 pm
content: It's good that Cliff Matthews did the tests, but every transformer is going to be different. And mine is from an unknown brand. The video that mcinque posted made me think about measuring the inductance and calculating the impedance at 50Hz to see the current at no load. Yes, he probably thought I would connect high voltage at the low voltage side (which is secondary in a regular transformer). No, I wanted to connect line voltage at the HV side.And yes, it's probably a low frequency transformer. I've seen high frequency transformers from big power supplies, that's not one of them. It's quite heavy.Many mentioned that usually the transformer is used backwards to charge the battery. That's probably true. I didn't see second power supply. But I don't have the PCB... it's long gone.Well the transformer is berried in the basement (but not lost - it's described in a catalog) until I have a purpose for it. Usually you would want linear power supply if you wantlow noise.But these days it'll be hard to get anything clean from the power line. So to get low noise out of a transformer + linear power supply you need to filter the power line. To me it seams the same as putting few LC filters on a switching power supply. I've tried to filter SMPS and one LC filter is enough to bring it down to 1-3 mV (rms). Putting one more makes it so low that I can't reliably measure it even with short ground spring on the probe.But I can't think of a use for it. Why would I want to have higher currents from a line transformer. It's inefficient for today's standards and this one is probably even worse. But I couldn't just throw it away. Maybe I can get the copper wire from the low voltage side and use it for some power inductors.