can you pass dc through a transformer together with 0 resistance? - Page 1
Forum section: electronics / beginners

author: robowaffe
author_rank: Regular Contributor
timestamp: July 11, 2023, 08:15:19 am
content: Say you have 0 resistance, will a transformer (even low henry) pass dc forever this way?

author: Jeroen3
author_rank: Super Contributor
timestamp: July 11, 2023, 08:23:12 am
content: For DC the windings of a transformer are a "short circuit", or just "a piece of wire with resistance".DC will not pass to the secondary, because only changes in current become a magnetic field induced by the primary.

author: wasedadoc
author_rank: Super Contributor
timestamp: July 11, 2023, 08:42:16 am
content: Quote from: Jeroen3 on July 11, 2023, 08:23:12 amFirst part of that sentence is true but the reason in the second part is at best unlear and can be interpreted as false.Even a steady current in the primary produces a magnetic field in the core. But such field will be constant. Only a changing magnetic field has an effect on the secondary.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 11, 2023, 09:30:14 am
content: Quote from: robowaffe on July 11, 2023, 08:15:19 amIn theory, yes, if the voltage source in the primary is also ideal (constant voltage, 0 internal resistance, and capable to supply any current).In practice, no, because nothing is forever.

author: EPAIII
author_rank: Super Contributor
timestamp: July 11, 2023, 09:56:12 am
content: What do you mean by "pass DC"? In the general sense of passing it from one winding to another (primary to secondary), NO, it definitely can not unless it is defective (has an internal short).A transformer will have two (or more) windings. These windings will each have two connections. Each winding has NO electrical connection to any of the other windings. It operates when a CHANGING current in one winding INDUCES a Voltage in the other winding(s). When I say a CHANGING current, that implies AC, not DC. DC is a constant, NON-CHANGING current and it does not induce anything in the other windings of a transformer except at the instant when it is switched on or off because those instants are when it is actually changing. And at those instantaneous times you will only see a instantaneous spike in the Voltage on the other windings. This is the total opposite of "forever".Now, if you are talking about running a DC current in just one winding of a transformer, you certainly can do that. But there is no reason for it because, as I said above, it will only produce any effect in the other windings for those two instants when it is turned on or off.You need to study electricity and magnetism some more. Your question is meaningless or, at the very least, very poorly expressed.And by the way, there is no such thing as zero resistance except in super-conductors that are chilled to very low temperatures.Quote from: robowaffe on July 11, 2023, 08:15:19 am

author: EPAIII
author_rank: Super Contributor
timestamp: July 11, 2023, 10:02:49 am
content: And ANY, ABSOLUTELY ANY conductor, even one with a very high resistance, will pass a Direct Current for as long a time as a constant or DC Voltage is applied across it.That's why we have switches to turn such circuits off.And that works just as well for AC also.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: July 11, 2023, 10:18:10 am
content: Depends on the type of transformer. There are common mode transformers which can pass DC.

author: ArdWar
author_rank: Frequent Contributor
timestamp: July 11, 2023, 10:34:14 am
content: Quote from: EEEnthusiast on July 11, 2023, 10:18:10 amWould you actually call that thing a transformer, or a coupled choke instead, or even coupled inductor?And then there's autotransformer which does pass DC. It does not provide galvanic isolation, nor have defined primary/secondary distinction for that matter. But I'm sure this is definitely besides the point asked by OP

author: RoGeorge
author_rank: Super Contributor
timestamp: July 11, 2023, 10:55:13 am
content: In theory, when a constant voltage is applied to a coil, the current will start to increase linearly. That increasing current means a variable magnetic flux, which will induce a voltage in the secondary.So if all resistances are 0, when a constant DC voltage is applied at the primary, a constant DC voltage will be seen at the secondary output. Seems strange, I know, because in practice we never have zero resistances and ideal voltage sources.For example, in this LTspice simulation, for the series Rs = 1 ohm, we see the Vout spike (in red) we expect from practice.In the next plot (in yellow), Rs is only 1 miliohm, and the Vout is almost constant in all the 100 seconds plotted. Then, for Rs = 1 microohm (the green plot), the Vout is even closer to a constant 10V output.If Rs were to be precisely 0 ohms, then the Vout will stay at precisely 10V forever.

author: Berni
author_rank: Super Contributor
timestamp: July 11, 2023, 11:27:20 am
content: In theory yes, as explained above the current would keep on rising. to 10A 100A 1000A 10000A 1000000A 10000000A...etcIf you actually built such a transformer in real life it would be pretty exiting.The transformer would have to be made from superconducting coils in order to get that 0 resistance. As you hook it up to your magical ideal voltage source (don't worry i wont ask where you managed to find one of those) you will indeed get a stable DC output on your transformer. However careful not to leave it turned on for too long. The current is steadily rising higher and higher, rising the magnetic field with it.Soon enough you would find any stray screwdriver on your bench flying towards your transformer and sticking to it. as the magnetic field gets stronger more and more ferromagnetic objects would fly towards the magnet (Hopefully you ran away by then). Depending on your houses construction things might start getting much worse. If your drywall has metal framing prepare your walls to suddenly implode, if you have a sheet metal roof that might also start coming down.Hopefully by now the superconducting material of the coils has went trough saturation by now, this would cause the gas used to cool the superconducting coils to suddenly evaporate, making a mini explosion of gas that fills your whole yard. The interruption of such a current likely causing an EMP blast that destroys a fair bit of electronics around the area, so be prepared for some very upset neighbors.If you also somehow made your superconducting coils handle an infinite amount of current then things might get even worse, not sure how badly it would go, maybe someone familiar with high energy physics can chime in for that. But i am pretty sure people all over earth would not be happy about your experiment making the whole planet uninhabitable, so please don't make such a transformer.Tho i am sure those physicists would be having a lot of fun while it lasts.

author: xavier60
author_rank: Super Contributor
timestamp: July 11, 2023, 11:34:09 am
content: Also, passing a constant current through the primary of the super conducting transformer will cause a constant current flow in a short circuited secondary.Maybe this is what robowaffe is looking for?

author: radiolistener
author_rank: Super Contributor
timestamp: July 11, 2023, 11:58:30 am
content: Quote from: robowaffe on July 11, 2023, 08:15:19 amif you put DC through transformer coil, the current can flow for a very long time, but not forever, just because lifetime of transformer is limited. At some time the Sun will explode and transformer coil with DC source will be destroyed, or a Black Hole flying nearby can destroy it... If you're talking about "forever" any thing can happens...Another issue here is that it's impossible to put DC through transformer coil, because such a coil has inductance and DC current connected to it turns into a pulse with almost infinite period (limited with a coil and DC source lifetime)...After all, there is no DC source in our Universe, actually all signals are AC, the difference is just frequency...

author: Berni
author_rank: Super Contributor
timestamp: July 11, 2023, 12:37:17 pm
content: Quote from: xavier60 on July 11, 2023, 11:34:09 amAh yeah that could also work.If you feed constant current DC into such a transformer and the output is shorted there will indeed be a constant current flowing trough the output.Then again you don't even need an transformer to do this. All shorted superconducting inductors have the property of keeping a DC current flowing trough them forever. If you had a coil of superconducting wire, place a permanent magnet inside it, short the coils terminals then remove the magnet, you will get DC flowing trough the coil forever. At the moment you removed the magnet you caused a change in magnetic flux and this induced a current into the coil, since there is no resistance to stop the current, it keeps flowing. The only way to stop the current is to open the terminals of the coil, or put the permanent magnet back (this induced a reverse current that cancels it out back to 0)Here where we have a transformer, we just replace the permanent magnet with an electromagnet and get the same effect.This principle is why superconductors magically float on top of magnets. The eddy currents in superconductors act as a shorted superconducting coils, effectively freezing the magnetic field in place, any movement would cause a change in the magnetic flux, so the superconductor wants to stay in place as any movement would oppose the outside field and creates a restoring force.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 11, 2023, 01:41:25 pm
content: Quote from: robowaffe on July 11, 2023, 08:15:19 amYes this is a superconducting flux transformer and works down to DC. It's a pretty common technique used in things like SQUID bias circuits.

author: xavier60
author_rank: Super Contributor
timestamp: July 13, 2023, 04:14:14 am
content: Quote from: xavier60 on July 11, 2023, 11:34:09 amFound this simulation by robowaffe. Shows the shorted secondary with current applied through the primary.https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWcMBMcUHYMGZIA4UA2ATmIxAUgoqoQFMBaMMAKADcQ09OAWKlPryrCQgpCJgIWAJyFzMKOVVYAVThkUCq2BJsFUe4UdFLFdkBHiwkUYMyAY8TBFMTS6eO4mHIxTLAHd1TQ1OfGCoQLDuBWilKJ09bV0IyBYgA

author: MrAl
author_rank: Super Contributor
timestamp: July 13, 2023, 07:55:32 pm
content: Quote from: robowaffe on July 11, 2023, 08:15:19 amHello,In the simplest case if you apply a DC voltage to the primary of a regular transformer you will see a short blip on the secondary and then it will go to zero and the input DC current will go very high.The best way to pass a DC voltage through a regular transformer is to chop it up into a square wave, then rectify the output from the secondary. By chopping it up the DC becomes a sort of AC, and that transfers to the secondary, then the rectifiers turn it back into DC. You also would need some filtering.There are some limitations and some parameters that need to be right to get it to work though due to the way magnetic coupling in a transformer works.One prime example of this technology comes in the form of wireless cell phone chargers. The primary winding gets a chopped DC signal, and that is transferred to the remote secondary, and the secondary voltage is then rectified, filtered, and regulated to provide the correct power for the cell phone or other device.[LATER NOTE]Some wireless chargers use resonance that creates sinusoidal waves that transfers energy from primary to secondary.Also, some DC-to-DC power converters use the chopping idea combined with Pulse Width Modulation. Almost all modern wall warts use this method after rectifying and filtering the line voltage. The transformer is a higher frequency type.