1. Oct 30, 2011

### rhlee

Hi physics forum,

I'm going to use a transformer to step down 240V AC mains to 24V AC.

I remember my physics teacher telling me about matching the load between the power supply and the load.

Regards,

Richard

2. Oct 30, 2011

### vk6kro

No.

The transformer will have some internal resistance which will limit the current that can be supplied to any load you put on the transformer.

Manufacturers make this easy by telling you how much current the transformer can supply without overheating.

So, if it is a 2 amp transformer and supplies 24 volts, then the maximum load you can take from the transformer is 2 amps and this would be drawn by a load of 12 ohms.
This is Ohm's Law. Resistance = 24 volts / 2 amps = 12 ohms.

Any less resistance than this for more than a short time may result in the transformer overheating and possibly being destroyed. If you don't know what your load is going to be, you can include a fuse in series with the output to avoid overheating problems with the transformer.

3. Oct 31, 2011

### rhlee

Thansk for the info and safety advice.

4. Nov 1, 2011

### sophiecentaur

When you're dealing with 'power supplies' you really want to avoid matching the load to the source. This would. of course, involve maximum power transfer but it will also involve producing twice as much power as you wanted and then wasting half of it!

Imagine - the series resistance of the 'mains' will be just a few Ohms. Put a load of this on the mains and you will produce a lot of smoke and sparks, drawing a huge amount of current and blowing the fuses. What you need, when supplying power, is as low source resistance as possible, so that as much of the generated power as possible is transferred to the load.

Matching is used in different circumstances - such as radio receivers and transmitting antennae.