Hi there,
A friend bought a Taiwanese made switchmode power supply that delivers 0 - 24 V at several amps. Problem is that it triggers his RCD (Residual Current Device, Core Balance Relay) in normal use. He asked me to take a look, since the nuisance tripping is major annoyance.
Seems the RCD is rectifying the 40kHz ripple from the PS to produce what it thinks is a fault current.
Unlike PC power supplies I've opened up, this one solidly grounds the output and does not have the usual large clearance between mains side and load side on the PCD, that I'd expect to see in switching power supplies. Other PS units I've seen usually have a large gap with an obvious break between mains side and load side.
I made up a power lead with a 1.0 Ohm, 3W resistor (6 x 1R5,
1/2W, 1%, with some bypass diodes for fault current) in the ground lead to measure earth current from the PS. It's bad.The RCD triggers at around 50mA p-p of 40kHz noise from the PS.
I've been asked if I can stop the nuisance RCD tripping.
Because the DC side is grounded, isolating the DC side from mains earth is non-trivial as I can't just cut the earth trace and isolate the DC, as that trace is needed for safety. Need to isolate the DC side then add tiny caps to ground to stop RF interference.
Got me beat why this PS doesn't have the usual clearances?
I put photos of the waveform, measurement setup, RCD and the offending PS PCB up on
What I'm seeking is an easier way to stop the PS dumping that
40kHz switching noise current down the ground lead? I have no circuit for the things.This is second one I've looked at, the first one triggers the RCD too, so it's not a unit fault, it appears to be a design fault.
In the worst case, we may need to dump this brand PS and get a better ones for the task, I don't like chucking stuff if it can be made to work.
The PS claims a 'C tick' approval mark, I didn't think lack of trace clearance and mains separation like I see here was allowed any more? But I haven't worked (professionally) with consumer gear since the '70s.
Thanks, Grant.