Relay supply fuse

[lowell]

My amps relays draw 150ma when all on. I put my meter in series with my relay transformer secondary and the rectifier and my meter reads only 3ma. Can someone suggest how to rate my relay transformer and supply fuse? I was initially gonna put a 500ma between the relay supply cap and the relay coils but then realized that this will not protect in the event of a shorted relay transformer.

[loudthud]

I put my meter in series with my relay transformer secondary and the rectifier and my meter reads only 3ma.

Was the meter set to measure DC current? On the AC side of the rectifier, set the meter to AC current.

[lowell]

Sometimes I don't know what I'm thinking... thanks LT. So secondary of the relay transformer measures 175maAC. I put a fast blo 500ma fuse there. Fuses are really there just to keep the amp from lighting on fire right? Seems like a reasonable value, using the "double it and round up" rule.

[R.G.]

Yes, what he said, check to see if you measured AC or DC. I think that was a measurement aberration.

The trick to protecting things is to realize that a capacitor will run down eventually, and fuse the transformer secondary on the AC side of the rectifiers. How to rate the fuse depends on the rectifier set up. Is this a full wave bridge?

Rectifiers charge the caps in capacitor input filters in short, sharp pulses. These are quite large, as the amp-seconds going into the caps from the rectifiers has to equal the continuous amp-seconds going out of the caps to the load. The pulses have to be lotsa amps because they don't get many seconds. A high-peak pulse waveform has a higher heating (RMS) value than the average current, which is what charges the caps. So the RMS/heating current in the rectifiers and transformer secondary is bigger than the DC out. The math is complicated, but you can usually nail it by estimating 1.6 to 1.8 times the DC average current out of the caps to the load.

So if you have to supply 150ma continuously to the relays, you could carry that load with some small safety factor with a 300ma fuse in the transformer secondary feeding the rectifiers. Make it slow blow and maybe even a bit bigger to protect against the odd surge.

Other style rectifiers - half wave, full wave center tapped, etc. have different heating/rms values of current. FWB is the worst.

[loudthud]

Here's a link to a thread where I detail how to get Duncan's PSU designer to calculate the RMS transformer current for a given DC power supply and load.

http://music-electronics-forum.com/t20730/

[lowell]

Oh ok so I should use slo blo?

[loudthud]

Slow blow is best for most power supplys because of the turn on surge. Set the delay in Duncan's program to zero and you can see the first pulse is huge. Note that the program may not be that accurate because the models are somewhat simplified for the sake of speed. If you have heaters connected, they draw a surge roughly 2x the normal current.