Transformer question - Page 1
Forum section: electronics / beginners

author: PinheadBE
author_rank: Regular Contributor
timestamp: February 02, 2019, 04:10:08 pm
content: Hi,I just bought a transformer whose datasheet (https://www.mouser.be/datasheet/2/410/media-1067921.pdf) states that:"Primary and secondary windings are designed to be connected in series or parallel. Winding are notintended to be used independently"Does that mean that I may not use it to build a +12V/-12V where the currents in each winding would not be perfectly balanced ?Why is that ?Thanks

author: soldar
author_rank: Super Contributor
timestamp: February 02, 2019, 04:21:17 pm
content: I think that refers to the connection jumpers and I cannot see how it would matter that the windings have equal currents.

author: Benta
author_rank: Super Contributor
timestamp: February 02, 2019, 05:31:42 pm
content: Very unfortunate phrasing. I think it means you can't use the transformer with two independent 115 V inputs.Probably added by a lawyer.It's a perfectly normal transformer.

author: djacobow
author_rank: Super Contributor
timestamp: February 02, 2019, 05:45:06 pm
content: I think they are trying to say that their 4kV isolation guarantee does not exist between the two secondaries, or between the two primaries. So, on the secondary, +12/-12 would be fine; they have the same common, but maybe not ok to generate two totally separate outputs expected to be completely independent.However, I'd be interested to know how they managed to achieve this limitation!

author: rcbuck
author_rank: Frequent Contributor
timestamp: February 02, 2019, 05:57:21 pm
content: I believe they are trying to say you must use both primary windings. In other words, if you want to use the transformer with a 115 volt input, you cannot use just 1 and 2 or 5 and 6. You probably actually could do that but the output current would only be half of the transformer rating.You can easily use it for +/-12 volt supply. However, if currents are different for the + and - circuits, the voltage would be slightly more on the circuit that has less current draw. Also, with a light load on the 12 volt windings, you will see voltages closer to 17 volts. If you need +/- voltages to be equal, you will need regulators.I'm curious what the application is that requires such a large current? Maybe a power amplifier? If so, the current draw should be equal on both + and - sides.

author: djacobow
author_rank: Super Contributor
timestamp: February 02, 2019, 06:17:51 pm
content: Quote from: rcbuck on February 02, 2019, 05:57:21 pmThis makes the most sense! But man, they could have said that better.

author: Audioguru
author_rank: Super Contributor
timestamp: February 02, 2019, 06:39:02 pm
content: The instructions are perfectly clear. Its max rated power is 130VA when it has a 25 degrees C air conditioned room. Less allowed power outdoors in summer.Primary: Series 230V, Parallel 115V.Secondary: Series 24V 5.4A, Parallel 12V 10.8A.You are using the secondaries in series so 24VAC which is 34V peak before rectification and filtering. Then your max allowed current (when the transformer has air conditioning) is 130VA/34V= 3.82A.The outputs will be unregulated +16V and -16V and about +17V and -17V when not loaded to the max of 3.82A.

author: Benta
author_rank: Super Contributor
timestamp: February 02, 2019, 07:02:30 pm
content: Quote from: Audioguru on February 02, 2019, 06:39:02 pmWhat on earth are you talking about? Mains transformer voltages and currents are always RMS. Your calculations might make sense if the OP wanted to rectify and filter, but this is not stated. And even then they are questionable, as crest factor plays a major role.

author: ArthurDent
author_rank: Super Contributor
timestamp: February 02, 2019, 09:34:26 pm
content: Benta – “Your calculations might make sense if the OP wanted to rectify and filter, but this is not stated.”Actually I believe that is what they meant when they said:PinheadBE – “Does that mean that I may not use it to build a +12V/-12V where the currents in each winding would not be perfectly balanced ?”AudioGuru and rcbuck had good answers. Here is a handy reference to explain it all.http://www.hammondmfg.com/pdf/5c007.pdf

author: Brumby
author_rank: Unknown
timestamp: February 02, 2019, 10:17:44 pm
content: Quote from: rcbuck on February 02, 2019, 05:57:21 pmI would have added: "To get the rated output current"QuoteYes, just this. I am lost as to why they would add the secondaries in this statement. It only makes sense for the primaries.Quote from: djacobow on February 02, 2019, 06:17:51 pmAgreed. They could have simply left the secondaries out of the statement for it to become less confusing - and then added my suggested comment for it to be pretty clear.Then again, the work that's gone into the engineering of a product is usually more thorough than the documentation. The only thing I can say is that following the instruction as written will result in proper operation of the transformer.Quote from: djacobow on February 02, 2019, 05:45:06 pmWhile I tend to agree with your conclusion, I'm not too sure they are being explicit enough in the datasheet. In the statement on Safety, they say:"These units are designed with 4000VAC isolation between the primary and secondary, and also, between each winding and the core."which leaves as much unsaid as said. In some applications, knowing the rated isolation between the two secondaries would be useful. The isolation between the two primaries - not so much.While this datasheet will provide sufficient information fortypicalapplications, it's a bit lacking as far as providing full engineering specifications - IMHO.

author: Benta
author_rank: Super Contributor
timestamp: February 02, 2019, 10:31:19 pm
content: Quote from: ArthurDent on February 02, 2019, 09:34:26 pmThat "handy reference" doesn't explain anything at all. Quite the oppositePlease explain to me why "FULL WAVE BRIDGE Resistive Load" tells me to multiply voltage and current by 0.90?I don't even want to comment on the other configurations, that would be a total waste of time.Apparently "Hammond Manufacturing" needs any excuse to derate their transformers, leading me to doubt their products.

author: Nusa
author_rank: Super Contributor
timestamp: February 02, 2019, 10:32:00 pm
content: QuoteThe CT presumably stands for center-tap, so clearly they're perfectly fine with that configuration. That's what you want anyway, right?

author: soldar
author_rank: Super Contributor
timestamp: February 02, 2019, 11:19:59 pm
content: Quote from: Benta on February 02, 2019, 10:31:19 pmVA or Amps are given but they are for a power factor of 1. If power factor is less then Amps should be reduced accordingly. Full bridge rectifiers have a very poor power factor. Thus the 0.90 reduction which I would probably reduce even more.

author: ArthurDent
author_rank: Super Contributor
timestamp: February 03, 2019, 02:36:13 am
content: Benta – “That "handy reference" doesn't explain anything at all. Quite the oppositePlease explain to me why "FULL WAVE BRIDGE Resistive Load" tells me to multiply voltage and current by 0.90?I don't even want to comment on the other configurations, that would be a total waste of time.”So far you’ve questioned Triad and Hammond (who have been making transformers for ages) about their transformer application knowledge. Perhaps reading other explanations about reducing transformer ratings for use with different rectifier setups might help you understand. The ratios may vary depending on the transformer design used but the principles are the same.https://www.powervolt.com/how-to-determine-page-25https://www.nutsvolts.com/questions-and-answers/transformer-ratings

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: February 03, 2019, 06:51:24 am
content: Quote from: PinheadBE on February 02, 2019, 04:10:08 pmWow. Here I thought it was pretty obvious what the manufacturer was referring to. I never thought a bunch of people like you guys could go off on so many tangents.The only explanation for that quote that makes sense to me is that to obtain the rated maximum VA rating of the transformer one must use all of the windings. In parallel or series as needed. That trying to get that much power through the transformer by using only two windings and leaving the other unconnected would not work so well as the wire gauge would be inadequate for it. Yes, I'm making an assumption here but it is the only way this makes sense.

author: xavier60
author_rank: Super Contributor
timestamp: February 03, 2019, 07:16:44 am
content: Quote from: Benta on February 02, 2019, 10:31:19 pmThe "0.90" is used to multiply by the RMS voltage to get the Average voltage, 0.9 x 0.707 = 0.636.I have some doubts about some of the information in the Hammond guide.

author: Ian.M
author_rank: Super Contributor
timestamp: February 03, 2019, 07:55:12 am
content: Quote from: xavier60 on February 03, 2019, 07:16:44 amWhich specific circuit(s)? Every time I've set up a LTspice sim for one of them, the worst case derating factors come out very close to the Hammond figures. I must admit I haven't tried them all though.

author: xavier60
author_rank: Super Contributor
timestamp: February 03, 2019, 08:38:07 am
content: For the FULL WAVE BRIDGE Capacitor Input Load.V (Avg) D.C. = 0.90 X Sec. V A.C.With a 30VAC secondary, I wouldn't expect the average DC to be loaded back to 27V at the recommended load derating factor. That's a long way down from the unloaded DC of 42V.I have started a linear bench supply project so I guess I'll have a better idea when it's fully tested.For the FULL WAVE BRIDGE Choke Input Load."V (Avg) D.C. = 0.90 X Sec. V A.C" mathematically agrees with the expected DC voltage at some light load that causes continuous inductor current flow, assuming that the inductor is large enough.At full rated load, the DC voltage would be something less due to expected resistive losses.

author: Ian.M
author_rank: Super Contributor
timestamp: February 03, 2019, 09:24:06 am
content: I hadn't noticed that.A somewhat simplistic LTspice sim of the FULL WAVE BRIDGE Capacitor Input Load circuit that assumes all the transformer losses are resistive is attached. Its got a lot of .measure commands so view the spice error log for results.With ideal diodes, a transformer with 10% regulation and 1000uF per A of load current, I get 38.6V average, and 35.8V ripple trough.It also shows the Hammond 0.62 current derating factor may be insufficient for 24/7 duty if the transformer has good regulation, and the diodes and reservoir cap are large for the load current.

author: soldar
author_rank: Super Contributor
timestamp: February 03, 2019, 09:54:35 am
content: If you put the mains voltage on the screen of your scope the first thing you will notice is that it is not a sine at all but the tops are flattened and that is due to all the rectifiers getting shots of current only at the top of the wave. The output resistance of the network makes the voltage fall.If you put the current on the scope you will see short bursts of current and zero current most of the cycle. Power factor is very poor.The behavior depends an many factors. Higher resistance and lower capacity mean a longer diode conductive duty cycle, a better power factor but worse regulation. OTOH, big capacitor and low impedance means more stable output regulation, lower ripple, but worse power factor, bigger transformer and diodes needed, etc.Calculations are good but there is a lot of rule of thumb and pure educated estimation. If you are building a single unit the best way is to start building and testing behavior under different loads.But, yeah, DC current out of the rectifier + capacitor is nowhere near what the transformer can supply with unity power factor on, say, a purely resistive load.

author: xavier60
author_rank: Super Contributor
timestamp: February 03, 2019, 07:31:33 pm
content: The "V (Avg) D.C. = 0.90 X Sec. V A.C." isn't really a mistake.It is stating that full wave rectified AC voltage averages 0.9 x RMS voltage.It isn't referring to the capacitor voltage at all, so it's just irrelevant to the FULL WAVE BRIDGE Capacitor Input Load. And misleading.

author: Benta
author_rank: Super Contributor
timestamp: February 03, 2019, 07:45:44 pm
content: Quote from: ArthurDent on February 03, 2019, 02:36:13 amApparently you completely missed the "Resistive Load" part. The transformer is specified for resistive load (for complex loads you need reduction factors, of course).Please explain to me why voltage and current need to be multiplied by 0.9 when inserting a rectifier bridge between transformer andresistiveload.

author: soldar
author_rank: Super Contributor
timestamp: February 03, 2019, 07:51:48 pm
content: Maybe I did not explain it well. Yes, we have all understood by now that in a sine signal RMS value is 0.707 of peak and average value is .637 of peak and therefore average is 0.9 of RMS. It has been explained, understood and accepted by all (I think).My point is that, further to that, most uses are for rectified and capacitor-filtered power and many people do not understand that a transformer specified as 10 A secondary, that means 10 A RMS, 9 A average, into a pure resistive load but much less than that through a rectifier bridge and capacitor. Many people seem to think that if they need 10 A dc out of the unregulated power supply all they need to order is a "10 A" transformer.

author: Benta
author_rank: Super Contributor
timestamp: February 03, 2019, 08:28:32 pm
content: Soldar, we understand perfectly.But really, no one uses average for anything at all, it's a useless measure.If you want to calculate transformer power loss, RMS is the variable, and yes, with capacitive storage after the rectifier, you need to take crest factor into account, which is considerable when calculating power dissipation in the transformer.That still doesn't explain why Hammond uses a 0.90 factor for a resistive load.I'm still waiting for an explanation from Hammond or ArthurDent.

author: soldar
author_rank: Super Contributor
timestamp: February 03, 2019, 08:49:27 pm
content: Well, I can't say as I have not seen the original or the context. It could just be a plain mistake and I wouldn't give it much importance. Mistakes creep in in any work with more than a few pages. It is inevitable. I go over anything I write several times and later I still find mistakes. It happens in the best regulated families.Some fifteen years ago I downloaded a document from Texas Instruments and it had so many mistakes I felt compelled to email the author who quickly replied with a tirade telling me I got it for free and had no right to complain. I emailed TI and they quickly replied with an apology and I guess they told the author of the document to STFU because that was the end of it.

author: Nusa
author_rank: Super Contributor
timestamp: February 03, 2019, 09:10:36 pm
content: Actually, the first question with a resistive load is whether you need a rectifier at all. Often the answer is that the load is perfectly happy with AC, but will work on DC if you insist. Soldering irons, for example.

author: Brumby
author_rank: Unknown
timestamp: February 04, 2019, 01:17:01 am
content: Quote from: soldar on February 03, 2019, 08:49:27 pmArrogant twat. Doesn't care about anybody or anything outside of themselves. If I was his manager, he'd be pulled up rather sharply for (a) producing sub-standard work and (b) sticking it to the customer. If he wasreallygood at design and crap on documentation, I'd look at ways of addressing the quality of documentation - but if he was so-so on design as well, I'd be putting him in a seat near the door.It's unfortunate, but becoming more prevalent that challenging sub-standard work gets an aggressive response - as if that's supposed to make the sub-standard work "acceptable". People that try that with me may have the last word in the opening shouting match, but they will never win.Yes, documentation is a biatch, but when I do it, I want to make it a good effort so I don't have to come back and work on it any more, if possible.

author: PinheadBE
author_rank: Regular Contributor
timestamp: February 04, 2019, 08:14:03 pm
content: Sorry that my question fired up so much... That was not my intention.Anyway, thanks for the useful answers

author: Brumby
author_rank: Unknown
timestamp: February 04, 2019, 08:34:07 pm
content: No need to apologise .This is the sort of thing that happens all the time. Don't let that put you off asking questions.