I'm new to electronics, I'm guessing I did something stupid but I don't know what yet. I bought a second hand 12V transformer today from an electronics scrapyard, the guy selling it tested it in front of me with his multimeter and it output 12V.

I was excited to test it out myself when I got home, so I connected a power cord to the two blue wires and connected to two green wires to my multimeters probes, then I went to plug in the power cord and a bolt of lightning came out of the electrical outlet, the multimeter lit up then died. Some smoke came from something but I couldnt see what.

What exactly happened? I thought multimeters can handle high voltages so how could 12V output destroy it like that? Its possible the probes might have been too close together, would that explain the lightning bolt from the outlet? Does this mean the transformer is likely damaged now too?

An additional question:

Do AC transformers have a direction in which you need to hook them up? In other words, could it be I should have hooked the power supply up to the green wires instead of the blue wires?

 

Until you get more experience, you should not connect anything to line voltage without a fuse.

What exactly happened? I thought multimeters can handle high voltages so how could 12V output destroy it like that?

We can't tell you exactly unless you can tell us exactly what you did.

What setting did you have your meter? If you had it on DC or AC volts, the meter wouldn't be damaged (unless you had it on the wrong scale). If you had it on Amps, that would blow the fuse, but the meter should be otherwise undamaged. If you had it on Resistance, that scale is dead and may have taken others with it.

Its possible the probes might have been too close together, would that explain the lightning bolt from the outlet?

If you shorted the transformer secondary and didn't have a fuse on the primary, sparks might be possible.

Does this mean the transformer is likely damaged now too?

It depends. Did you have a fuse on the primary? Was the short removed quickly? Did any smoke come out of the transformer?

Do AC transformers have a direction in which you need to hook them up? In other words, could it be I should have hooked the power supply up to the green wires instead of the blue wires?

You don't mention your region or line voltage. With a 12VAC secondary, it's either a 10:1 or 20:1 stepdown. You can't swap the primary and secondary wires. If you do, you'll generate some lethal voltages and might break down the insulation on the wires.

If you're connecting multiple secondaries in series or parallel, there is a polarity you need to be mindful of.

 

What setting did you have your meter? If you had it on DC or AC volts, the meter wouldn't be damaged (unless you had it on the wrong scale). If you had it on Amps, that would blow the fuse, but the meter should be otherwise undamaged. If you had it on Resistance, that scale is dead and may have taken others with it.
If you shorted the transformer secondary and didn't have a fuse on the primary, sparks might be possible.

It was on 200V AC. The mains is 220V. Some smoke came out of something, not sure if it was the transformer or the multimeter. The short was removed really quickly, I didn't actually plug it in, the sparks came when I got close so I instinctively backed away. I didn't put any fuse in no. I need to learn how to do that.

So its possible I hooked it up the wrong way and turned it into a step up transformer. How can you tell which wires on a transformer are for input and which are for output? I see that this is potentially very dangerous so Im going to take your advice and never experiment with lines voltage without a fuse.

 

You likely did one of two things or both:

1) Your meter was set to read amps instead of volts

2) You connected the transformer backwards. Instead of stepping down 220VAC to 12VAC, you stepped up 220VAC to 4000VAC.

ay caramba! tenga cuidado la próxima vez!

 

It looks like I hooked it up the wrong way and stepped up the voltage. Do some of these transformers come with fuses built in and some don't? I have a brand new 220V to 30V transformer which doesn't seem to work at all, would mean it has a fuse thats blown? I attached a picture of this transformer.

Is there a way to easily open these kinds of transformers and see if it has a fuse?

 

No, but if your meter still worked you could check the winding for resistance to see if they had shorted or opened. Glad you didn't get hurt. Next time check the xfmr with a battery...

 

Do this with an external fuse in an insulated fuse holder:



Technically, transformers have a polarity but that is "erased and re-written" every power cycle, and in the case of small transformers like the ones we are discussing the memory (hysteresis) should have little effect.

 

No, but if your meter still worked you could check the winding for resistance to see if they had shorted or opened. Glad you didn't get hurt. Next time check the xfmr with a battery...

How do you check a transformer with a battery?

 

You can use a battery to check the conductivity of a winding to see if it is open, but it takes AC to check the primary to secondary turns ratio. Feeding 12VAC into the primary of a 120VAC to 12VAC transformer will put ~1V on the secondary for example. Your meter actually uses a battery to measure resistance.

 

Do some of these transformers come with fuses built in and some don't?

Some transformers have a safety fuse buried in the windings. It's not intended to be replaced and not usually accessible without taking the transformer apart.

 

Do some of these transformers come with fuses built in and some don't?

I think I recall reading about or seeing a transformer with an integral fuse. But they would be the exceptional case, not the norm. Manufacturers expect anyone using a transformer to know how to do so safely. You don't have enough experience to be working with line voltage and transformers.

If I ever have the slightest doubt about which winding is the primary, I connect it to the secondary of a low voltage transformer (like 6.3VAC) so things don't get out of hand.

Is there a way to easily open these kinds of transformers and see if it has a fuse?

Use a meter to check winding resistance. If it's opened, either the winding opened or there's a fuse. If you take the transformer apart, be prepared for the possibility that it will be unusable if reassembled.

 

The primary of s step down transformer will generally use thinner wire than the secondary. If you connected the one with thicker wire to 220V it was backwards and disastrous.

Bob

 

Also, you can measure the DC resistance of the windings.
Generally, the primary winding will be about 100-500 ohms. The secondary for a LV step-down transformer will show very low resistance, less than 10 ohms. The secondary winding uses lower number of turns of thicker wire.

 

The primary of s step down transformer will generally use thinner wire than the secondary. If you connected the one with thicker wire to 220V it was backwards and disastrous.

Bob

I thought this too but I asked the lady at the shop to test my new 30V transformer and she connected 220V the two bigger wires and connected the smaller wires to her multimeter and it output a low voltage.

 

How do you check a transformer with a battery?

To check the turns ratio you would need an oscilloscope.
You momentarily connect a small battery (e.g. 1.5V AA alkaline) across one of the windings and observe the peak output of the other winding(s).
The relative voltage compared to the battery voltage will tell you the approximate turns ratio.

 

To check the turns ratio you would need an oscilloscope.
You momentarily connect a small battery (e.g. 1.5V AA alkaline) across one of the windings and observe the peak output of the other winding(s).
The relative voltage compared to the battery voltage will tell you the approximate turns ratio.

I suspect that someone who doesn't know there's a difference between input and output of a transformer will not have a handy o'scope, nor understand this method.

 

Post a picture of the 12v transformer. There won't be a fuse in that transformer, its a low-end product. On the 30v transformer you have, the green wires are definitely the output, but it doesn't follow that the green wires on the 12v one are the output, the colours are not representative.

The transformer is almost certainly fried! I'm guessing the blue wires are (were) the output - that winding is now probably open-circuit. The primary winding almost certainly has shorted turns where the 4000v AC generated has punctured the insulation.

Carry on like that and you'll win a Darwin award!

 

Post a picture of the 12v transformer. There won't be a fuse in that transformer, its a low-end product. On the 30v transformer you have, the green wires are definitely the output, but it doesn't follow that the green wires on the 12v one are the output, the colours are not representative.

The transformer is almost certainly fried! I'm guessing the blue wires are (were) the output - that winding is now probably open-circuit. The primary winding almost certainly has shorted turns where the 4000v AC generated has punctured the insulation.

Carry on like that and you'll win a Darwin award!

How did you know that the green wires are the output? Has it something to do with being on the same side as the brown wire? What is the brown wire for BTW?

 

The thicker solid-core wires are invariably the secondary, the brown wire is almost always a centre tap, so a 30v secondary is actually 15v-0-15v and can be used to give 20v DC with 2 diodes rather than a bridge rectifier.