# Transformer

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kBillingsley
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27 Feb 2012, 10:11 pm

I was wondering if you account for the voltage drop across a resistor when hooking up a step-down or step-up transformer to an alternating current power source (say, a wall plug). For example, let's say I have a step-up transformer with five turns on the primary side, and 500 turns on the secondary side. The primary side turns have a resistance of two ohms, but the breaker on my wall plug gives out at 10 amps, and the voltage from peak to peak on my plug is 120v. I will need to add on at least a ten ohm resistor to stop the breaker from giving out, so I what want to know is: is the voltage across my primary side going to be 120v or 20v (because of the resistor). If there is a voltage drop, how do I account for it if I want 12000v across the secondary? Should I Just add more turns to the secondary or what? Thanks in advance for tolerating my nescience.

mcg
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27 Feb 2012, 11:37 pm

You don't need a resistor on your primary side. Just hook it straight up to the power source. The currents through transformer windings are related by the reciprocal ratio as the voltages (so that power is conserved). In other words, Np/Ns = Is/Ip. So if you have a transformer with 5 turns on the primary and 500 turns on the secondary, then you would need to keep the current through the secondary coil at .1 amp or below to avoid blowing your breaker. With nothing hooked up to the secondary winding, the primary coil of a transformer is just an inductor. Ideally, it doesn't have a resistance, just a reactance (that is to say, the impedance of an inductor has only an imaginary component). An ideal inductor doesn't dissipate any power when hooked up to a power source because the voltage and current are 90 degrees out of phase (and this is necessarily so since the inductor just stores energy). A real transformer of course has a resistive component and will dissipate some heat, but not much.

Hopefully this answers you question. To build circuits with transformers you should be fine if you just remember the power relationship between the two coils.

RazorEddie
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28 Feb 2012, 5:25 am

Is this a transformer you are building yourself? If so, a low turns count like 5 turns will not work. Getting the primary turns count right is quite important. Too few and the transformer's impedance will be too low and you'll blow it up as soon as you plug it in. Too high and you'll saturate the core and blow it up as soon as you plug it in. At the sort of voltages you are talking about you also need to be careful how you wind the secondary to prevent flashover.

As mcg says you don't want a resistor but you may want to put a reasonably high powered light (say a 500W floodlight) in series first time you turn it on. If the light comes on you know something is wrong.

Be VERY careful playing with high voltages like 12kV. If you get careless you could very well end up dead. High voltages do some very strange things and can easily catch you out. I've had ~25kV track along nearly 6" of insulating tape and get me. Luckily it was low power so it only gave me a solid kick. It turned out that while the tape was a good insulator the glue wasn't so good at high voltages. Another time I got hit by a ~5kV high frequency arc. You don't feel high frequency until it starts burning. It is slightly disconcerting to see your own skin smoking

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Fogman
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28 Feb 2012, 12:27 pm

I don't know what you need the transformer for, but it might be easier to rape an old neon type sign for the PSU, as these have secondaries in the 10-15Kv range.

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Oodain
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28 Feb 2012, 1:24 pm

hmm what do you need the 12kv for?

have you worked with HV before?
if not a neon transformer might be a better bet than building one from scratch.

then again sometimes we do stuff simply because we want to learn or simply because.
my first jacobs ladder and van de graaf came about in that fashion, (van de graafs by the way are extremely fun and easy to build, can even be made from legos)

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Cornflake
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28 Feb 2012, 10:58 pm

Oodain wrote:
Ah, happy - if lethal - days.
I had an 807 valve rigged with an old TV LOPT in a self-oscillating setup, which managed a spark about 4" wide at the top of the ladder - and a nice cherry-red anode, before it all fried itself to death.

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kVArc
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05 Mar 2012, 8:52 am

Making a transformer for 50/60Hz input frequency takes lots of time and money:
Those transformers usually have about 2 to 5 turns per volt. Therefore, a 120V primary already has about 300 turns; and the 12kV secondary would have 30000 turns.
The transformator cores also are quite expensive.

The in my opinion best HV source for beginners are the flyback transformers from TVs or CRT monitors. They deliver a maximum voltage of about 25...35kV; and by limiting the input power it's easy to limit the output curent to relatively safe values.
To drive the transformer, a squarewave signal can be used; another possibily is a self-oscillating driver with a power transistor. There are lots of those circuits on the internet.

But please don't use the resolant "ZVS" driver unless you exactly know what you're doing: The power of chis circuit is damn high; it can easily push more than 500 watts through a standard flyback transformer .

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Oodain
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05 Mar 2012, 10:14 am

Cornflake wrote:
Oodain wrote: