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I'm trying to find out the wattage of some bulbs supplied to a company I'm working for.

The product is a set of garden lights (Cheap Chinese) which is 10x small bulbs in parallel, powered by a 12Vac 5A 60VA transformer. The issue is we seem to be receiving a lot of burned out transformers back, and looking at the bulbs, i think they are more than the rated 5w.

I can remember back to my electronics days with V=IR etc, but all i am able to measure is the un-loaded voltage of the transformer (12.6Vac) and the voltage measured across the bulb is 10.1v. I cant measure the AC current, or the ON resistance of the bulb.

Is it at all possible to calculate the bulb wattage?

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To determine the power you need to measure both the current and voltage. You have already measured the voltage, so what is left is the current.

Yes, you can measure the current. What you need is a ammeter, but you don't necessarily need to specficially get a ammeter. Ammeter capability within the range you need is included in most off the shelf multi-meters. These are cheap and available. Break the circuit somewhere, like right at the secondary of the transformer, and put the ammeter in line. This will tell you how many Amps are flowing. That times the voltage measured accross the secondary of the transformer at the same time (can be measured with a second multi-meter, for example), will tell you the power.

Technically, RMS voltage times RMS current only gives you the VA rating, not actual power. However, VA is probably more relevant to determine stress on the transformer. And, the load is mostly resistive, so VA and W should be close to the same anyway.

It seems the transformer is dropping significant voltage when the bulbs are on. You say the open circuit (unloaded) transformer output is 12.6 V, and that it is 10.1 V at the bulbs when they are on. What is it at the tranformer when the bulbs are on? That will tell you if you are dropping significant voltage in the wires between the transformer and the lights.

Or, you could forget all that and put a 5 A fuse in series with the transformer secondary. Of course if the bulbs draw more current then they are supposed to, then the fuse will pop often, but then you have a problem anyway.

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What Olin suggested is probably the best way for determining the true load on the transformer.

However, doing a quick calculation:

Let's assume the bulbs do output P=5W at steady state on. That means:

Ion = 10 * 5W / 10.1V = 4.95A

This is rather close to the current rating of the transformer to begin with, and keep in mind that incandescent bulbs increase their resistance as they warm up. That means it's possible that peak start-up current could easily exceed the 5A rating, or any slight tolerance variations in the bulbs would exceed the rating (say, bulbs with actual 5.06W output). Additionally, the transformer's actual current rating may not be exactly 5A or better, it could be slightly less than 5A resulting in failure (especially over prolonged use).

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A less exciting solution is to buy a Kill-A-Watt for $18:

photo of kill-a-watt

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