Making a safe bench supply from a salvaged transformer

[jj5]

I found an old digital clock radio and took it home and took it apart. It had a transformer with primary connected to 240 VAC in and then there were four wires out from the secondary. I took out the transformer and put it in a box. The thing is, I haven't taken any safety precautions at all. I imagine that if I carelessly dropped a wire and shorted a pair of wires on the secondary that I would have an explosion followed by a fire. It's a shame it is so unsafe, because I would like to have a low voltage AC power source on my bench. Is this the sort of thing that it's wise to completely leave alone? Or with enough fuses and snubbers and such could I actually design a bench AC supply that was sufficiently safe, even in the presence of a short circuit?

This is the video of me taking the clock radio apart:


This is the video of me putting the transformer in a project box:

[Doctorandus_P]

I'm not going to watch an hour of video for some hidden details.

With a salvage (low voltage output) transformer, just make sure the primary section is OK.

• Mains cord stress relief or plug.
• Mains switch
• Fuse, filter maybe
• Earth connection, maybe your transformer has an earth shield.

And make sure you can't touch it, even accidentally, such as dropping a screw driver on top of it. Shrink tube helps, or even putting a temporary box around it from some cardboard and tape already helps. This just takes a few minutes.

After that, everything on the secondary side is quite safe. If you short out the transformer, the transformer will overload, draw a high primary current and the fuse blows. Shorting out a big electrolytic cap can lead to noise, sparks and ruined DMM probes.

When you get further in the project, stuff may heat up, especially in an enclosed box. Even the transformer may heat up a lot during prolonged use and little airflow.

If you want more specific advise, or have some doubts, then write a question in text, and possibly add a screenshot for clarification.

[golden_labels]

Just like Doctorandus_P, I didn’t watch the videos. To don’t give wrong advice and be sure we’re talking about the same kind of a transformer, I skimmed over one to find a picture of said transformer. But that’s all.

Transformers can’t pass arbitrary power. Even as an ideal components, much less as real devices. Even if the primary can swallow 3 kW, which for small transformers is unlikely, the secondary wouldn’t be able to produce it. Unless designed to do so. Look up photos of welding transformers or high-power isolation transformers, to get the idea what “designed to do so” means.

Without knowing the exact specs of the transformer, nobody can tell anything for sure. But — generally speaking — a small, low-power, cheap transformer can usually supply an ampere or two. After that the output voltage drops towards 0 V. So does output power. There is no risk of a sudden, catastrophic event. Of course fire hazard is always there, but it’s exactly the same as for any other low power source.

The primary side isn’t going to draw much more power. Since it’s at 240 V, which is an order of magnitude more than the secondary voltage, the current drawn is negligible even for a shorted transformer. Of course shorting primary directly is a different beast, but I assume this is not what you’re concerned about. If yes, Doctorandus_P gave the answer.

There are two main risks I can see now: connecting it wrong and using a wrong transformer.

Make sure you know which side is primary and which is secondary. Connecting 240 V to secondary is very likely to cause a short. And that one may be spectacular. For salvaged components it’s best to mark sides before you even remove the transformer from the original device.

The second mistake is using a wrong type of a transformer. Transformers are designed to work on a particular frequency. Those made for being driven directly from power cord (50–60 Hz) are different than SMPS components (a dozen kHz range). Again, when salvaging a component make sure you mark what kind of a circuit it was working for.

The one you have seems to be the type connected directly to mains. Both by its size compared to power, the general look, using wires, and the device it came from.

[jj5]

Thanks for your help. With regard to your points:

1. there is a knot in the mains power lead for stress relief
2. there is a mains switch DPDT
3. I'm not sure what sort of fuse I should install? I'm thinking I will install a fuse on each mains wire because I can't be sure which is active and which is neutral. can you suggest which type of fuse I should install? maybe 250 mA fast blow?
4. there is no earth connection, just two wires for mains

I have attached a photo of the inside of the box. You can see the white cable running in from the left is the mains power, then it runs into the DPDT, then the four secondary wires are run over to banana sockets.

[chilternview]

It would appear your transformer has 3 windings in series :

Red-Blue : 3.2v
Blue-Black : 8.4v
Black-Yellow : 8.4v

Essentially there is a centre tapped winding (the 8.4-0-8.4) with an extra tap on one end.

You should know this else you might short out the secondary e.g. if you connect red to black etc.

A knot in the mains lead is NOT a good idea, its a failure point. Use a proper stress relief.
I'd also earth the transformer case, and fuse the live mains input.

[golden_labels]

Nice connections, neat box.

Output fuse: in this scenario makes little sense.

Input fuse: depends.

Fuse’s job is to protect your home installation and power cord from overheating, if the device fails. I believe in Australia standard outlets are 10 A. If you used a cord with a lower current rating, choose the fuse with the same value.⁽¹⁾ Your home already has circuit breakers installed (I hope so! ), so don’t be worried too much. The fuse is only a backup for a very reliable system.

A fuse may also limit the risk of a fire or smoldering inside the device. I used word “limit” intentionally. Yet, of course, may still be worth doing. The minimum value is determined by how much power you wish to draw from that transformer. We still don’t know, how much power it can deliver. Let’s guess — based on the look of it, what it came from, and voltages used — that it’s 10–20 W top. 20 W / 240 V ≈ 84 mA. So a 100 mA or 250 mA fuse should be fine, given the above guess is correct. Don’t try to match the value exactly: it is going to be too low, while providing no additional protection. This risk is, in your case, minimal: transformers are hard to set on fire.^{(citation needed)} So, again, don’t be too worried you did something wrong.

I agree with chilternview, that proper strain relief is preferred over a knot. But a knot works better than nothing, so don’t undo it until you can mount a proper stress relief.

The taps indeed seem to be as chilternview described them. How the wires come out of the transformer is in agreement with that. One thing to be aware of is the polarity. Both black-to-blue nad black-to-yellow (note the order!) are 8.4 V, but 180° out of phase.

⁽¹⁾ I don’t know about Australia, but down here we have things like eurplug. Rated for a fraction of the normal value and in many situations the wiring in the device or in the cord is capable of even less.

[themadhippy]

I'm thinking I will install a fuse on each mains wire because I can't be sure which is active and which is neutral.

NO,only fuse 1. I  believe australia uses  polarised plugs and sockets and brown is live ,so put your fuse in that wire

[jj5]

I'm thinking I will install a fuse on each mains wire because I can't be sure which is active and which is neutral.

NO,only fuse 1. I  believe australia uses  polarised plugs and sockets and brown is live ,so put your fuse in that wire

The problem is that the mains power cable is for IEC 60320 C7/C8 which can be plugged in either way around.

[themadhippy]

The problem is that the mains power cable is for IEC 60320 C7/C8 which can be plugged in either way around.

In that case just put it in 1 line. As your plugs aren't fuse you've the added advantage of not being able to accidentally have a fuse on both lines

[calzap]

I won't watch the videos.   Maybe this was covered, but use a DMM and make sure the secondary windings are isolated from the primary.  If I'm unable to determine brand/model and check specs on used transformers, they're trash.  Mike