Electronic – Max size smoothing capacitor without melting the rectifier

capacitorinrush-currentpower supplyrectifiervacuum-tube

I am currently making a tube amplifier for a school project. I started out with a finished design, but have changed a lot and need some guidance as a result.

I am currently using a full wave rectifier bridge I made with 1N5404 diodes, and I will be rectifying 50Hz 230V AC into 325V DC. I have a 470uF capacitor I intend to use to smooth out the voltage, and I am wondering if it's even safe to put it directly after the bridge without resistors in series to limit the surge. The diodes should withstand a 200A surge, but I’m not sure how big the surge will get. Is the isolating transformer going to get saturated and limit the surge enough? I haven't bought one yet, but it will be a 30-50VA transformer. Also, would it be safe to test the circuit without an isolating transformer? I would like to test it out before I get it!

Best Answer

Also, would it be safe to test the circuit without an isolating transformer?

You can burn your finger on a hot resistor or tear your skin on a sharp edge of a PCB. Nothing is safe; it's all a matter of reducing risk so I'd use an isolating transformer and there's no reason to believe it will saturate due to the load in fact, it'll saturate less on full load than on no-load (watch this space for anyone disagreeing!!).

The leakage inductance of the transformer windings will tend to offer a little bit of current limiting for each time the diodes re-charge the capacitor.

I will say this about the diodes - they are rated at 400V and I believe you might expect diode failure when other circuits switch in and out around where you plug this thing in - I'd go for something a bit bigger like 1N5405 (rated at 500V in reverse). I'd also ensure the cap is 450V rated for the same reasons of reliability.