DC bias in transformer - Page 1
Forum section: electronics / beginners

author: Stefan
author_rank: Contributor
timestamp: September 06, 2013, 12:47:13 pm
content: Hi!I'm currently trying to better understand transformers, and there is something I don't get.We use AC for transformers because if you put DC though a winding, the magnetic flux will build up until the core saturates.But what if there is a small DC bias on, say, the mains? Shouldn't that, after a while, run the core into saturation as well? Ruining the efficiency? As far as I can tell transformers run on mains directly without any coupling caps or anything.If, for example, I drive a transformer from an H-bridge, and my FETs are not all equally fast, a small DC Bias can easily occur. Do I have to do anything about this?Thanks,Stefan

author: Simon
author_rank: Unknown
timestamp: September 06, 2013, 12:52:49 pm
content: no it won't be a problem, you don't need a perfect balance. you can infact send DC pulses to a transformer and you will get AC out the other side although I guess it is not the best method

author: Andy Watson
author_rank: Super Contributor
timestamp: September 06, 2013, 12:55:57 pm
content: The current "builds up" to the point at which it is limited by the DC resistance of the coils. Hopefully, the DC offset, and therefore the DC current will be insignificant compared to operating current of the inductor/transformer - so saturation is not a problem.

author: Stefan
author_rank: Contributor
timestamp: September 06, 2013, 01:15:39 pm
content: Ahh, I think I see it now. The voltage bias gives a time-increasing offset to the magnetic flux, which creates a DC current propotional to it. That in turn produces a DC voltage over the wire resistance which counters the DC bias. A negative feedback loop.The loss will be dc_offset2/ wire_resistance + additional core losses due to the higher magnetic flux density.Thanks for the answers!

author: c4757p
author_rank: Super Contributor
timestamp: September 06, 2013, 01:25:35 pm
content: Quote from: Stefan on September 06, 2013, 12:47:13 pmThe mains is unlikely to have a significant DC bias, because your transformer is run through a couplingtransformer- i.e., the one outside your house (or down the street, depending on where you live).Also, you seem to have confused a bit of the function of a transformer with thememristor. The transformer doesn't store more and more flux as charge moves, andkeepit when the current is gone. The flux through a transformer winding isphi=Li- directly proportional to the instantaneous current. The flux is higher during the half of the wave that is biased up, but still drops to zero* at the zero points, it doesn't just build and build.*Realtransformer cores have hysteresis due to the ferromagnetic core, but the basic idea still stands. The hysteresis will cause abitof this type of behavior, but unless the DC bias is large, it will be insignificant - the core will not become heavily magnetized.

author: Stefan
author_rank: Contributor
timestamp: September 06, 2013, 01:44:17 pm
content: Maybe I was unclear, Simon and Andy give me the hints I needed. Just to avoid confusion:di/dt =U/L(assuming zero current on the secondary)so a DCvoltagecomponentUwould create a DCcurrentthat increases towards infinity with time, assuming an ideal transformer. In a real transformer this does not happen because the DC current produces a DC voltage over the ohmic resistance of the winding that will balance out the DC voltage. Since the DC bias is small, the DC current never gets significantly high.For once the non-ideal behaviour works for our advantage, yey!

author: Jeff1946
author_rank: Regular Contributor
timestamp: September 06, 2013, 03:06:18 pm
content: As an aside, this is what happens with a superconducting magnet. In this case the current will build up until the magnetic field is strong enough to destroy the superconductivity.

author: IanB
author_rank: Super Contributor
timestamp: September 06, 2013, 03:21:51 pm
content: Although not discussed much above, it is my understanding that a DC bias on a transformer (for example due to unsymmetrical loads) will reduce its efficiency and in extreme cases may cause damage from overheating. In the case of big, expensive substation transformers the power company really tries to avoid this by, for example, penalizing bad power factors at consumers.

author: sub
author_rank: Regular Contributor
timestamp: September 07, 2013, 12:29:45 am
content: Quote from: IanB on September 06, 2013, 03:21:51 pmIndeed. The example of this that I love (which I originally discovered from EEVBlog) is the fluxgate magnetometer, which measures static magnetic fields by applying an AC field to an inductor and measuring the level at which it saturates.

author: c4757p
author_rank: Super Contributor
timestamp: September 07, 2013, 01:37:24 am
content: Quote from: sub on September 07, 2013, 12:29:45 am"Fluxgate magnetometer" is the sort of name you expect somebody to make up to make their job sound cool

author: johansen
author_rank: Super Contributor
timestamp: September 07, 2013, 02:46:25 am
content: Quote from: Simon on September 06, 2013, 12:52:49 pmyou do need a perfect balance or the dc current will flow according to the dc content of the signal, saturating the transformer, and you won't get much ac out the other side at all.one part in a thousand is enough to notice. that's only half a volt on a 500 volt inverter, enough to cause major issues.lower voltages don't help.. 30 milliohm mosfets and a 10 milliohm transformer running from 12 volts will have .17 amps flowing.. probably not enough to notice, because most people don't use ferrite emi beads as transformers.. but sometimes they are handy.. you must have a dc blocking capacitor most of the time.

author: Electrocow
author_rank: Newbie
timestamp: October 11, 2019, 02:06:37 pm
content: I'm sorry to wake up an old thread, but this thread was the top proposal when searching the topic on Google, and I have to leave a comment.As DC bias in mains is usually extremely low, unbalanced loads in a big network (such as in your neighborhood) may cause a substantial bias. As mentioned earlier in this thread, this DC bias will go straight through any transformer primary winding only limited by the winding resistance. Depending on the DC current, this can partially or fully saturate the transformer core. Aside from the heating caused by the DC current, this can also manifest itself as humming in the transformer. As the bias is dependent on the network load, this humming can come and go during the day (such as when your metal-head neighbor turns on their 10 kW stereo).This is not something mortal people encounter during our day-to-day activities, but this issue is highly prevalent among audio-engineering aficionados. A common solution is to put two huge electrolytic capacitors in series with the mains in the power supply for example an audio amplifier.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 11, 2019, 03:29:33 pm
content: A known source for some DC bias on the mains grid are some cheap hair dryers on the low power setting. Some use a diode to draw only half the current, which is not really nice to the grid. Normally even such an current in the 8 A range does not cause too much DC voltage.

author: BravoV
author_rank: Super Contributor
timestamp: October 11, 2019, 03:41:59 pm
content: Quote from: Kleinstein on October 11, 2019, 03:29:33 pmYep, I did a test using a hairdryer and measure the generated DC across the mains line (220VAC), details with video ->HERE

author: Simon
author_rank: Unknown
timestamp: October 11, 2019, 05:41:41 pm
content: Quote from: Electrocow on October 11, 2019, 02:06:37 pmSo you felt the need to resurect a 6 year old thread....

author: AVGresponding
author_rank: Super Contributor
timestamp: October 12, 2019, 04:05:35 pm
content: Quote from: Simon on October 11, 2019, 05:41:41 pmI've seen worse necros...*almost on topic* a colleague once upon a time (in the late 1990s) said he'd heard 'they' (I'm assuming he meant the 'Powers That Be') were thinking of banning SMPS's due to their noisy switching having a deleterious effect on the mains line.He went on to say "but transformer/rectifier PSU's have noise too, when they pass the zcp and the diodes in the bridge are switching from on to off to on, there's noise!".I seem to remember saying something along the lines of "yes, that's a good point", while thinking "yes, but the noise frequency is directly related to the transmission line frequency, and so can be filtered easily...".Given the number of SMPS's running at any given time, and the random frequency and phase, it's likely the noise sum is effectively near enough zero as makes no difference, but it does make you wonder, what would happen if they all synced into phase at the same time...

author: Simon
author_rank: Unknown
timestamp: October 12, 2019, 04:15:29 pm
content: rectifiers and smoothing capacitors create plenty of distortion due to poor power factor. SMPS are usually power factor corrected which mens they have to appear as near linear loads. if they are near linear I expect that anf noise has been minimised already.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 12, 2019, 04:30:35 pm
content: Have you seen the stuff coming out of China recently?Hell, forget China (in terms of cheap Chinesium crudites anyway), have you seen the power factor on premium consumer goods like a PS4 for example? On standby I see a power factor of 0.02! And yes, I did get the decimal in the right place!

author: Simon
author_rank: Unknown
timestamp: October 12, 2019, 04:34:12 pm
content: Quote from: ThickPhilM on October 12, 2019, 04:30:35 pmhow much does it use on standby? what is the power factor during use? Have a look at Dave's latest pair of videos. A dirt cheap LED driver that has 100% ripple to save money but a power factor of over 0.9, the only thing that lets it down is the fact that you can see the flicker caused by the 100% ripple when using a video camera.....

author: AVGresponding
author_rank: Super Contributor
timestamp: October 12, 2019, 06:24:22 pm
content: Quote from: Simon on October 12, 2019, 04:34:12 pmUnderlined relevant sections...That LED driver will have the same affect on the line as a linear PSU; 100Hz.Think about it.In point of fact, all the SMPS's I have encountered rectify line current first, therefore their main interference is going to be line f or 2f. Their switching frequency will be less of an effect due to components sitting between the switching transistor(s) and the line, and as I've also pointed out, they are all going to be running at slightly (or more) different frequencies and phases, so the net effect is likely to be effectively zero.The only consistent factor is the line frequency, insofar as the noise generated by diodes switching on at their thresholds will be at the line f (half wave) and 2f (full wave).Most of the modern electronics I see has a pretty shit pf, probably a conspiracy to charge us more for our elastic trickery by charging us for apparent power instead of real power (I worked with ferro-resonant transformers for UPSs, so I know a bit about tinkering with power factors).If it makes you happy, I'll measure the pf of my PS4 next time I turn it on, later tonight most likely.

author: Simon
author_rank: Unknown
timestamp: October 12, 2019, 06:29:41 pm
content: There are regulations now. sub 1kW stuff you may get away with but everything is becoming power factor corrected to improve infrastucture and power usage. It's not a conspiracy. If your power factor is 0.5 for every 1W you use they have to generate 2W, you will appear to use 2W but then shove 1W back to them when they don't want it. Why should you only pay for what you use when your equipment is so badly designed that it requires more power to be generated than used.

author: Zero999
author_rank: Super Contributor
timestamp: October 12, 2019, 06:39:52 pm
content: They actually don't have to generate double the power to power loads with a power factor of 0.5. It just results in double the transmission losses.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 12, 2019, 07:18:43 pm
content: My apologies; since your flag indicates you're British, I thought you'd understand sarcasm and satire...And let's look at your argument from a different angle:Why should I pay for badly designed consumer goods that have piss-poor power factors?Answer: Because I have no choice, I have to buy what they will sell (or buy nothing and go completely without).LED lamps are a good example.We were told to buy compact fluorescents instead of incandescents, to save power and the environment, then LEDs instead of compact fluorescents.Each step along this road has brought us poorer power factors. It should also have brought us greater lamp life; the manufacturers are wise to that one, they have introduced lower and lower quality standards to offset the natural longevity of the newer technologies.They have also introduced more waste, as now it's harder to replace just a lamp; now it's far more common to have to replace an entire luminaire.Ehhh... thanks to my own arguments, I now have to withdraw the 'sarcasm' comment; it's a real conspiracy after all...EDIT: My PS4 has a pf of 0.61-0.68 when running, settling at 0.64 on menu screen. Pretty damn crappy for such an expensive and pervasive piece of consumer electronics.

author: dom0
author_rank: Super Contributor
timestamp: October 12, 2019, 07:32:15 pm
content: Quote from: Electrocow on October 11, 2019, 02:06:37 pmYou can increase tolerance to DC currents of a transformer by reducing the flux density in the core.

author: Simon
author_rank: Unknown
timestamp: October 12, 2019, 10:50:50 pm
content: there has always been a conspiracy, it's called the majority of consumers want dirt cheap goods and the majority of manufacturers don't care how they make their money. Thats why we have to have standards.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 13, 2019, 07:13:38 am
content: Indeed, and yet it's clearly apparent that the majority of cheap goods (some may come from places other than China I guess) don't meet any of the standards they claim to meet.Seen loads of bigclivedotcom videos demonstrating complete lack of isolation in any number of line powered "low voltage" stuff, seen it myself in plenty of things too.Then there's things like poor termination, undersized conductors, non-certified line fuses in plugtops, lack of sufficient insulation, and all that's just the safety stuff, not even got to the poor build quality, poor quality out of tolerance components, poor and inappropriate designs.And THAT'S before we even get to the deliberately engineered to fail, deliberately disposable stuff!I'm not naive enough to think these are new problems, but I would argue the magnitude of the problem is far greater now than ever before.The CE mark and others are a complete joke, and make me wonder if anything ever gets checked for safety by the relevant authorities.

author: Simon
author_rank: Unknown
timestamp: October 13, 2019, 08:12:42 am
content: CE marks are a bit of a joke. The standards are not but a manufacturer is allowed to self certify. I blame the authorities on not carrying out random checks. Ultimately it is the responsability of the last person that made the sale. If i buy something from china that is CE marked and i sell it and there is a problem I am liable. Sure I can sue the chinese seller but good luck with that. On the other hand getting something CE marked can be daunting. Dave's µCurrent was being rejected by over zeleaous german customs because it was not CE marked when such a marking is hardly neccessary and Dave may be safe to just put one on and hope for the best.Ultimately i blame consumers not demanding better and being willing to pay for better.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 13, 2019, 02:22:54 pm
content: I prefer to blame the shoddy education systems that allow people to think "shiny = good" and "cheap = best".The lack of critical thinking these days is a huge problem, its effects are extremely visible everywhere you look.I know otherwise moderately intelligent people that believe things purely on the basis they saw it on Farcebook or read it in a newspaper, without taking the slightest time to think logically about it, or even do the most basic fact checks.If something is ridiculously cheap, there's going to be a good reason for that, which will be one or more of the following:1. Cheap, substandard materials2. Cheap, substandard design3. Cheap, exploitative labourIt can be hard even for a well informed person to avoid falling into the trap of "ooh, shiny, ooh, cheap, me want!", but I personally do at least try...It's sadly ironic that even premium consumer products can employ these elements, anyone who watches Louis Rossmann's YT channel will know what I mean.It can't be a good thing when a major manufacturer has to install anti-suicide nets on their factories.Anyway, back on-topic, if you're worried about dc bias on your transformer, stick a big-arsed cap in series with the primary. Problem solved

author: Simon
author_rank: Unknown
timestamp: October 13, 2019, 02:45:40 pm
content: that is why we have standards because people will always buy the cheapest so you legislate on quality (warranty period) labour conditions and safety and these days enviromental standards. It's not a foolproof system but it's the best we have.

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 13, 2019, 02:49:17 pm
content: Quote from: ThickPhilM on October 13, 2019, 02:22:54 pmNo, that just prevents the mains from seeing the saturated transformer as an abnormally high load; put the capacitor in series with the secondary and restore balanced operation completely. In fact, pretty much all microwave ovens with a mains transformer instead of an inverter use this trick for the half-wave doubler.Also note that most distribution transformers located closest to the point of use have a small gap in the core to prevent hard saturation from a little DC bias in the first place (at the expense of drawing a higher magnetizing current from having a reduced magnetizing inductance).But at this point I bet Simon is wishing he'd locked the thread rather than responded to it...

author: Simon
author_rank: Unknown
timestamp: October 13, 2019, 03:09:17 pm
content: Quote from: MagicSmoker on October 13, 2019, 02:49:17 pmI don't mind a genuinely useful discussion and the thread was not that long so fine to carry on. the problem is people reviving threads that are pages and pages long and have not been posted in for years. The initial revival was for a near pointless post but if something comes of it all the better.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 13, 2019, 05:21:57 pm
content: Quote from: MagicSmoker on October 13, 2019, 02:49:17 pmI would argue that putting a big cap on the primary sidepreventsthe transformer from becoming saturated and therefore an abnormal load, and that putting one on the secondary side is a compromise as you can use a smaller, and hence cheaper, cap...Until the "Your momma" jokes start, I see no reason to close the thread