Isolated lamp limiter supply

[TrevorM6TSE]

Hi All,

I don't know whether this is worth sharing, but I've made an isolated supply for testing mains driven appliances such as radios, TV's etc with a bit of a twist. I can draw up if you really need to, but here are the basics:

I have a 3amp 240>12V transformer connected to the mains and then another nearly identical transformer wired to the output to raise the 12VAC back to 240VAC at a limited current and isolated from the main supply.

To give me an indication if I'm running near to the limit of the 12V windings, or if there's a fault, I have a large (about 150W) 12v bulb wired in series with the 12v windings so the power runs through the bulb.

The result, if I connect a black and white TV with load of about 30W to the output of the second transformer it works fine and gets about 220v. The 150w bulb glows dimly. If I short the output, the bulb glows brightly and no harm is done.

I've tied one side of the output to an earth strap also to give a realistic output supply too.

I hope this all makes sense and is of use, or you have any comments. I can draw up if you want.

All the best,

Trevor

[ppppenguin]

Originally Posted by trickytrev

I've tied one side of the output to an earth strap also to give a realistic output supply too.

Which removes the protection given by an isolating transformer. A simple 240V lamp in series with the mains would do exactly the same job.

Using a pair of transformers like this is a perfectly good way to improvise an isolating transformer when none is available.

[TrevorM6TSE]

Originally Posted by ppppenguin

Which removes the protection given by an isolating transformer. A simple 240V lamp in series with the mains would do exactly the same job.

Using a pair of transformers like this is a perfectly good way to improvise an isolating transformer when none is available.

I not convinced of that I'm afraid. The lamp limiter is a good idea and does a similar job, however it does not isolate the user from the mains, should the worst happen.

I would have thought that connecting one side of the output of the second transformer to the Earth of the mains supply should not compromise isolation from the current carrying side of the mains as the 240V at the output has no direct connection to the mains supply.

Assuming of course that your mains supply is safe.

[Station X]

I would have thought that connecting one side of the output of the second transformer to the Earth of the mains supply should not compromise isolation from the current carrying side of the mains as the 240V at the output has no direct connection to the mains supply.

Try this experiment:-

With one side of the output connected to mains earth measure the voltage from the other side of the output to mains earth, a radiator or a cold water pipe.

Repeat with the output disconnected from earth.

[ppppenguin]

Originally Posted by trickytrev

I would have thought that connecting one side of the output of the second transformer to the Earth of the mains supply should not compromise isolation from the current carrying side of the mains as the 240V at the output has no direct connection to the mains supply.

We've covered this before in the forum in electrical safety discussions.

One side of the mains (neutral) is at approximately earth potential and is safe to touch. The other side (live) isn't. One side of the output of your arrangment is at earth potential and is safe to touch. The other side isn't.

In either case if you manage to touch the live side and earth at the same time a harmful current will flow through you.

PS: StationX got there before me. He's quite correct except that if you do the measurement with a high impedance meter you will get spuriously high voltages due to capacitative coupling. Suggest using a low wattage mains light bulb for the test.

[TrevorM6TSE]

Ok, I stand corrected. I will remove the earth strap.

[Nickthedentist]

Hi Trevor,

That's a clever way of doing things.

But the others are right about the wisdom of tying one side of the OP to earth. The other downside with your current arrangement is that if you touch the non-earthed output terminal, not only could you get a nasty shock, but any RCD that you may have in your consumer unit wouldn't trip either - so more dodgy than using "raw" mains directly from a 13A socket

So cut that earth connection, then enjoy your useful gadget.

Nick.

[TrevorM6TSE]

Originally Posted by Nickthedentist

So cut that earth connection, then enjoy your useful gadget.

Nick.

Thanks Nick. I've done it as we speak. I must admit I have found it quite useful.