This past week the parts for my project have shown up. I got a surplus 10A solid-state relay for about $8 on eBay (I’m perplexed why SSRs are so expensive – there are like $2 worth of parts in a $20 part), 12 20W lights, and a substantial 600W transformer from intermatic.
The transformer is actually 2 300W transformers rather than a single 600W one. I wanted to verify that the SSR that I got was working, so I got out one of the lights, carefully wired up the primary of the transformer to an extension cord and the 12V output to the light, and plugged it in.
Flash Snap!
The light got really really bright for a really short period of time, and the snap was circuit breaker tripping.
I’m surprised. It’s pretty hard to mess up hooking up a transformer – there are two AC supply wires, two wires on the transformer, and polarity doesn’t matter. I check my connections and make sure that there are no shorts, and try it again.
Snap!
No flash this time because the light got toasted the first time. I pull out my Fluke Multimeter, set it to resistance, and put it across the primary, where it reads 0.1 Ohms. Same reading across the primary of the second transformer. Let’s see, 120 Volts across .1 ohm = 1200 amps, which makes it pretty obvious why the circuit tripped.
It’s really not as simple as measuring the resistance, since transformers are inductors, and resistance isn’t the same thing as impedance, so I go upstairs to make a sandwich, and do a little thinking. What I need is a way to do some measurements of the transformer with a low-voltage AC current across it. I remember there’s an old trick where you put a bulb inline with the load – that limits the current to what the bulb will pass.
I hook up a 7watt nightlight bulb I use to debug my light projects in line with the transformer, and plug it in. At 7 watts, it pulls 7 / 120 = 58mA of current, which means that (V = IR), it has a resistance of 120 / 0.058 = about 2100 ohms.
A voltmeter on the transformer shows 0.25 volts. That’s pretty low. If 0.25 volts results in 58mA through the transformer, putting a full 120 volts on it would give us a current of 480 * 0.058A = 28 Amps, or about 3300 watts. That’s a lower-end result – I actually think it’s quite a bit worse since 3300 watts would only be a mild overage on the circuit, and that level of current takes a fair bit of time (say, half a second) to trip a breaker. I think it’s more likely that we’re looking at 100 amps or so.
And guess what – the second transformer is exactly the same as the first. Must be a manufacturing defect with some of the windings shorted together.
So, what do you think ?