How to fix PS conducted interference triggers RCD?
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From: Grant on 24 Jul 2010 20:42 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 http://grrr.id.au/ps-earth-current/ for the curious. With summary of this problem. 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.
From: cbarn24050 on 24 Jul 2010 21:18 On Jul 25, 1:42 am, Grant <o...(a)grrr.id.au> wrote: > 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 onhttp://grrr.id.au/ps-earth-current/ > for the curious. With summary of this problem. > > 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. Use a line filter
From: Tim Williams on 24 Jul 2010 22:28 Board top view? I'm guessing it has neither X nor Y-type capacitors on the input? The way it's supposed to be laid out, Y-types RF-ground the circuit side, and a common mode choke limits ground loop current. X type caps provide differential filtering, bypassing line and circuit sides; leakage inductance in the CM choke provides a crude lowpass, but this is sometimes enhanced with a series choke of 10-100uH to provide more differential filtering. The maximum value for Y-type caps is somewhere around 4700pF; more and the 60Hz leakage can cause problems (564kohm at 60Hz, or 0.4mA at 240V, enough for a minor shock hazard I guess). However, this sounds substantially better than the ~18mA you measured. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms "Grant" <omg(a)grrr.id.au> wrote in message news:jgtm4656qbdd525sp81ennlkoct8lb7pmj(a)4ax.com... > 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 http://grrr.id.au/ps-earth-current/ > for the curious. With summary of this problem. > > > 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. > > >
From: Grant on 25 Jul 2010 01:39 On Sat, 24 Jul 2010 21:28:56 -0500, "Tim Williams" <tmoranwms(a)charter.net> wrote: >Board top view? Yup, there now, with commentary :) " Those three resistors standing up behind the fuse are part of the output current shunt resistor, another three resistors are further back. Next trace to left is mains neutral, next trace to the left of that is mains active, going to the fuse's right hand terminal. Next trace goes under the fuse is mains earth! There's none of the usual large separation of secondary side and mains side traces. Left side of fuse goes offboard to power switch, back on board to the right of the yellow X2 cap onto the tab also marked switch. Behind the green core common-mode filter there's bridge rectifier just out of sight. It feeds the two 330/200 electrolytics connected in series in foreground with the classic voltage doubler option tabs east and northeast of the left electrolytic cap. A couple of 150k 1/2W resistors balance the caps. On the left side just behind the mains entry, and to the left of the right hand side electrolytic capacitor you see the two small blue Y caps. In particular, note how the Y cap on left goes from neutral to earth, which is also the DC output negative line. The blue cap to the left rear of the green cored CM filter is centre-tap series capacitor for half bridge drive to the power transformer. Half bridge power transistors are K3469, controlled by a signal transformer. No sign of opto, but there's a second signal transformer likely used for feedback. Output is rectified by TO247 U30D20C with snubbers, feeding 2200/35 with 200 Ohm 2W across it, through a filter toroid to another 2200/35. Secondary power centre tap goes to the right hand side of the vertical current sense resistors mentioned above. There's more control circuitry in a tinplate shielded box not in frame, control signal in front panel is a triple(!) gang pot, two sections control voltage while a third section in opposite sense controls max current. Output adjustable 0 - 24V and output current depends on voltage and loading. " Hope that helps? > >I'm guessing it has neither X nor Y-type capacitors on the input? Got them, just doesn't follow the common layouts I'm used to seeing :( >The way it's supposed to be laid out, Y-types RF-ground the circuit side, and a common mode choke limits ground loop current. X type caps provide differential filtering, bypassing line and circuit sides; leakage inductance in the CM choke provides a crude lowpass, but this is sometimes enhanced with a series choke of 10-100uH to provide more differential filtering. Yeah, that I know :) X2 cap is 100n, Y2 rated caps are 2200pF. No extra series chokes, there's a couple more Y caps on other side of CM choke, snubber cap 4700pF plus 5W resistor across power transformer primary, that's about it for the mains side. > >The maximum value for Y-type caps is somewhere around 4700pF; more and the 60Hz leakage can cause problems (564kohm at 60Hz, or 0.4mA at 240V, enough for a minor shock hazard I guess). However, this sounds substantially better than the ~18mA you measured. > >Tim As stated in the text, that signal on DPO is twice current reading for voltage shown, as I'm feeding 50R into coax with 50 Ohm termination on input (divide by two from one Ohm shunt). So you're seeing about 40mA, trip point is about 50mA (+/- 2 1/2 divs at 20mA/div). Oh, and the PCB reference designators show component values, so those four Y caps are all marked CF222, the two 150Ks across mains filter electros: R150K, I see an R39, the colours are orange white black gold. This is one very strange layout! I'll put the thing back together and measure the ground noise with high load (200W of lamps) connected and post results later, might need to remove the diodes -- I put the diodes in, in case of fault. After all, the idea of cutting the earth lead is not appealing, I need to check for DPO ground interference to the measurement too. That the RCD triggers on this noise is real. Mains is 240V, 50Hz in the land downunder. BTW: to the poster suggesting line filter, I've yet to meet a line filter that does anything for interference conducted along the ground lead ;) Grant.
From: Grant on 25 Jul 2010 03:06
On Sun, 25 Jul 2010 15:39:36 +1000, Grant <omg(a)grrr.id.au> wrote: >On Sat, 24 Jul 2010 21:28:56 -0500, "Tim Williams" <tmoranwms(a)charter.net> wrote: > >>Board top view? > >Yup, there now, with commentary :) Now added new measurements, without that confusing x2 factor :) http://grrr.id.au/ps-earth-current/#new_measurements DPO now shows earth 1R shunt rms and p-p voltage, as well as PS' DC output voltage. Grant. |