# If the wire of the secondary circut is thicker than that of the primary in a transformer, what type of transformer is this and why?

There is this question in my physics book, and two teachers (a private teacher of a friend of mine and the school teacher) say that it's a step down transformer, while two other teachers say that it's niether of them, since a transformer's type is only determined by the number of turns.

I dont really know which one is correct and why, so if someone could explain id appreciate it.

• someone correct me if i'm wrong: but transformers depend on the amount of turns in the primary and secondary coils, not thickness? – Alex Robinson Mar 6 '18 at 10:39
• Well that's what my book says and what two of the teachers say, but I looked it up online and it says that it depends on thickness as well since it's related to resistance which in turn is related to the current passing through it for a given voltage – Dahen Mar 6 '18 at 10:45
• I'm not going to answer this because it smells like homework but: why do wires need to be thick? What does this tell you about the difference between the primary and secondary? – tfb Mar 6 '18 at 10:46
• Hold on a minute, you just said in your comment "if no change in voltage occurs" -- what exactly do you believe a transformer is? The purpose of those questions is to see if you understand the concepts, and if you think that transformers typically keep voltage constant, then you have a badly mistaken belief. So: what are your beliefs about the purpose of transformers? – Eric Lippert Mar 6 '18 at 15:10
• I think you've all misunderstood my point, One of the teachers wasnt saying that this is a step up transformer, he was saying that the information given was not enough to determine it (ie you cant tell just from having thicker wire around the secondary) and so the answer would be none since it isnt determined. And I was trying to express the flaw in this arguement when I said "if no voltage change occurs" since that was the arguement the other teacher gave me – Dahen Mar 7 '18 at 19:57

The thickness of the wire determines the maximum current the wire can carry without overheating. Thicker wire means a greater current.

With a transformer the power coming into the primary, $P_p = V_pI_p$, is the same as the power coming out of the secondary, $P_s = V_sI_s$, (less a few resistive losses) and this means $VI$ is constant.

• For a step down transformer $V_p \gt V_s$ so $I_s \gt I_p$ - the current in the secondary is higher than the current in the primary so the secondary needs to be wound with thicker wire.

• For a step up transformer $V_s \gt V_p$ so $I_p \gt I_s$ - the current in the primary is higher than the current in the secondary so the primary needs to be wound with thicker wire.

• There's a difference between "the wire thickness needed in the secondary is greater than the wire thickness needed in the primary" and "the secondary needs to be wound with thicker wire". – Acccumulation Mar 6 '18 at 20:13
• If the wire is thicker, the length of the secondary might be larger, and thus have less inductance, no? – Scott Seidman Mar 7 '18 at 12:08

Practically speaking yes, this will almost certainly be a step-down transformer, but I would agree with those other teachers: it can't really be concluded from the wire thickness. It's perfectly possible to build a transformer in which the secondary coil has more windings, but nevertheless use thicker wire.

As John Rennie explained, this doesn't normally make sense, because then there would be more AC current in the thinner primary wire than in the secondary, though that could cope with more. However, that's not necessarily the only current that's relevant: there are many applications where the voltage stepping is not even the reason to use a transformer but to decouple voltages and/or currents. In particular, you could have a high DC current or low-frequency AC flowing on the secondary side, and use the transformer to modulate a much weaker, high-frequency control signal on top of it. In this case, you will need a thicker wire on the secondary side regardless of the winding ratio.

• Honestly I can't think of a single application where it would make sense to feed DC into the secondary winding of a transformer. – Dmitry Grigoryev Mar 6 '18 at 15:08
• @DmitryGrigoryev one common use case is phantom power. I think power grids use such transformers to send control information over the lines. And I'm pretty sure most physics experiments that use strong, time-variable magnetic fields use this idea too, i.e. particle accelerators, MRT etc. (also in medical applications). – leftaroundabout Mar 6 '18 at 15:19
• Also the series injection transformers used to ignite large arc lamps, the run current is several tens of amps, at maybe 20-100V or so across the arc, but for ignition the things need hundreds of kV to initially ionise the gas. The solution is a transformer with a massive secondary (VERY well insulated) and a primary of only a few turns of much smaller wire (Also very well insulated), a capacitor discharge arrangement with a spark gap pulses the primary repeatedly during lamp ignition. EG&G are one manufacturer. Cinema projection and Lasers are two users for xenon and krypton arc lamps. – Dan Mills Mar 6 '18 at 15:24
• While the turns ratio may be 1:1, having a larger secondary wire diameter will mean that the coil's reactance will be different. It could be in some sort of isolation or impedance matching scheme, tuned-tank, or something completely different. The secondary may also have a DC component passed through it in an isolated (from the primary) ground scenario. – Mad Myche Mar 6 '18 at 23:15
• I doubt that could be the logic the question was trying to go with, since what we've covered of transformers is pretty surface-level. Another issue is that it doesnt even specify if the number of turns is equal to make a comparrasion on that base either. – Dahen Mar 7 '18 at 20:02

In a transformer the thicker wire is usually the one that carries a larger current. According to the simple formula relating the voltages and currents in a transformer $V_1 I_1=V_2 I_2$, you have the larger current at the lower voltage terminal. Thus if the primary voltage of this transformer is the mains voltage, then this is a step down transformer to a lower voltage. The ratio of the voltages of a transformer is given by the ratio of the number of turns of the primary and secondary coil.

As several others have correctly responded, the ratio of primary to secondary turns determines the ratio, NOT the cross-section (size) of the wire. As a side note, (aside from core losses) the POWER in the primary and secondary portions of a transformer will be equal. For example, in a 2-to-1 stepdown transformer: : Secondary 12V, 10A = Primary, 120V, 1A = 120W. On both the primary AND secondary. (Requiring MUCH 'heavier' wire on the secondary...) So; fair to infer/assume that the transformer of interest is a step-down. (Assuming manufacturer built it w/ suitable, lowest cost materials.))

A heavier secondary gives us a very strong clue that a higher-voltage, lower-current primary is coupled with a lower-voltage, higher-current secondary.

It's a clue, not a definite though. We can imagine a situation where only a low current is required from the secondary of a step-down transformer, hence heavy wire would be unnecessary and wasteful. It might be convenient to use the same light gauge wire for both.