What Do You Think of This Circuit?

[bernardgreen]

Number 8 is plain incorrect.

How is it incorrect ?

 

[aptsys]

How is it incorrect ?

IF2 waveform is not correct at the motor end

 

[aptsys]

But if one of the diodes in the bridge is open-circuit then no current can flow during one of the two half-cycles. This will mean that all the power the SMPSU needs will be drawn during the other half-cycle and the input current will have a substantial DC component. Depending on how much margin the design has this may or may not be noticeable to the user.

Ah, I read your comment as plain DC not pulsed DC.

 

[winston1]

Any DC bias on the AC line is a very unusual circumstance caused by a fault - A properly working piece of equipment should not be injecting DC current.

Most TVs and radios up to and including the 80s did this. A half wave rectifier and live chassis was very common.

 

[ericmark]

I used many ½ wave rectified appliances in the 80's however in the 80's we did not have plastic water and gas pipes and due to bonding even with a TT supply there was enough current with a line to earth fault to rupture a 30 amp fuse wire, I actually managed to do this in my fathers house, however by 2004 the water and gas pipes were plastic in the street and when I did an earth loop impedance test without removing any earth wires, I found it would not have ruptured even a 1 amp fuse.

I could find no sign of the supply every having an earth from the DNO, and this was not the only case, also found cases of RCD being fitted to a supply with a ELCB-v at the DNO input, on a TT supply which failed to operate, once the ELCB-v was removed it worked OK, however the owner would not have fitted the RCD himself, he would have used an electrician and I can't see any electrician walking away after fitting a RCD when even the test button failed to work. I would assume again some bonded metalwork likely a water pipe has been removed, and with it the premises earth connection.

So clearly because it was OK in 1980 does not mean it is OK today, and it may end up that way without the owner or occupant being aware of the changes, we can't rely on a TT earth without having a RCD any more, but in 1980 we could, with a TN system the RCD is an addition, and as such not so important, but as @plugwash said caravans, boats, and fuel filling stations can't use a TN-C-S system and for the same reason we need to be very careful with electric car chargers, I will not say EV as clearly a mobility scooter, electric bike, fork lift, and milk float are all EV (electric vehicles) but they are normally charged either in doors or with an isolating transformer, i.e. the charger is not built into the vehicle, and there is no mains connection to the vehicle, but with the electric car we have a mains connection to the vehicle, and in the main no idea of what is included in the vehicle.

The switched mode power supply can work simply on the extra low voltage from a transformer, but other than the circuits shown to us in university to explain how a SMPS works, I have not seen this in practice, in the main AC is rectified and then turned into high frequency AC to reduce the size of the transformer, and the mark/space ratio feeding the transformer is modified to ensure correct output, so a faulty diode as already said can go unnoticed.

So we can get a DC component going through the RCD, there is really no question about that, however as said this is a pulsed DC at 50 Hz not a smoothed DC as used in the test rig.

So in conclusion we simply don't know if SMPS or any other device can cause the RCD to fail, however we have been told for years to test the RCD will all load removed, so we are unlikely to find a situation when the RCD fails, I can't see any IT guy being in favour of testing the RCD before logging off all PC's.

What we need is some one like Alan Ward to do a test using a failed diode in a switch mode power supply, rather than borrow the same rig used by Joe Robinson training, however to make a video which could potentially put people in danger by ignoring the warning is not some thing I would expect Alan Ward to do.

So we know there is a risk when using ½ wave rectifiers, so simple thing is don't use them. We know we can drive a diesel wheel driven car at 350 MPH and live to tell the tail, JCB has proved that, but we also know what happens when things go wrong, even when TV programs health and safety has done all they can to make it safe, Richard Hammond has shown that, so we know there is a risk, and some times we have to do things involving risk, but if that risk can be reduced as a reasonable cost, then we should do it.