| General > General Technical Chat |
| Class 2 SMPS - Removing the Y capacitor - Good design solution wanted ! |
| << < (3/8) > >> |
| Zero999:
Johnny B Good, I don't see how your anti-phase earth leakage idea can work. Please post a schematic. Wouldn't it be easier and safer to simply power the SMPS off a small isolation transformer? The type used in a shaver wall socket will probably do the job. |
| Johnny B Good:
--- Quote from: Zero999 on January 10, 2019, 09:45:42 am ---Johnny B Good, I don't see how your anti-phase earth leakage idea can work. Please post a schematic. Wouldn't it be easier and safer to simply power the SMPS off a small isolation transformer? The type used in a shaver wall socket will probably do the job. --- End quote --- Hi Zero999, That's an excellent question. Rather sadly for me, this idea of using a tiny transformer to provide a Hi Z anti-phase voltage to buck the nasty half live leakage current, also falls foul of the direct fullwave rectification that dooms my (not so) clever idea of combining the the mains supply connections of a second smpsu in "anti-phase" so that the leakage currents null out at the commoned ground connections on the LV side. I now have some inkling of what it's like to be Baldrick[1]. :-[ Your reply, btw, materialised in this thread during the 30 second window between refreshing the page to check for any replies and my actually starting to post a follow up. I got a warning message in the send box, advising me that I might want to review my posts. Unfortunately, there seems to be no cancellation/abort mechanism so I clicked the back button and refreshed the page. For all I know, there's a post reply window now sat in Limbo on the server. Anyhow, I'll forget that imponderable and cut back to the chase... I should have realised from my test with a second smpsu that the buck transformer idea would also fail for exactly the same reason. I've mulled the results of my transformer testing over and it's finally dawned on me just what the real problem is. In essence, if you want to reduce this leakage voltage to a safe level (circa 5v rms) without the benefit of a safety earth connection, you need some means of identifying which way round the mains live and neutral connections are so that you can link the 0v rail to the actual neutral via a 100K resistor (1nF Y cap case, a 2n2F cap would need a 47K resistor). It's possible to detect which is the live (hot) and which is the neutral in the absence of an actual safety earth connection by using a very high input impedance detector referenced to a small probe antenna (a few square inches of foil placed clear of any high voltage influence within the case of the double insulated smpsu powered gadget - in my case, a small AWG) to sniff the ambient electric field in lieu of an earth connection. This can then switch the 100K safety resistor to the identified neutral connection to load down the 80 volt half live (as measured with a 10Mohm DMM), typical of such kit plugged into a UK 240v mains outlet, reducing it to around the 5vrms mark for the sake of any "At risk of esd" device under test. The virtue of using such a high value resistor being that should the automatic routing of the resistor become confused to the point of connecting to the live instead of the neutral, the touch current is limited to a mere 2.4mA, eliminating the risk of electrocution. The user might get an unpleasant shock but they'll at least survive the experience to investigate the failure. However, I've now reconsidered the idea of using a polarized three core mains lead with safety earth connection after checking out a couple of my spare ex- laptop charging brick trefoil plug ended mains leads. Whilst not all such leads use thinner and more flexible three core flexes than those typically found sporting the bulkier C12/13 connectors used by desktop PCs and the like (including my Siglent SDS1202X-E DSO), I do seem to have at least two such leads flexible enough to not 'wag the dog' quite so much as I'd feared. Whilst it's possible to solve the 'Touch Voltage" issue without adding a safety ground wire by using some clever electronic add-on, the upgrade to an earthed solution now has a lot more going for it than I'd originally thought (largely on account of its pragmatic nature versus all the other possible and much more difficult to implement solutions I've given serious consideration to). So, all I have to do now is check out my junk pile collection of salvaged 'useful bits', recovered from scrapped ATX PSUs and laptop charging bricks, to see if I happen to possess any already blessed with a plug in flylead to match the existing plug in headers on the PSU board. If I'm lucky, I might be able to save a bit of time on the soldering work. I'm planning on using a 10 or 15 K resistor to link the 0v rail to the safety earth. This will reduce the leakage voltage to less than a volt which is more than sufficiently low enough to eliminate the ESD risk and neatly avoid potentially troublesome 'earth loop issue'. At the end of the day, there is no easy way to eliminate the undesired effects of the class Y EMI suppression capacitor in class II smpsus without the benefit of a transformer which includes the electrostatic shield layer between the HV and LV windings to allow it to truly float free of an earthing connection[2]. In my case (Feeltech FY6600-60M AWG), I have the extra complication of having to mount a supporting bracket to strengthen the rather flimsy rear panel against the much higher insertion/extraction forces mandated by the trefoil plug and socket upgrade. It's all extra effort I was hoping to avoid by a "More Brain, less Brawn" solution I'd imagined I'd be able to come up with, hence my delay in dealing with the problem. The answer to the question posed in the thread title is this: You can't really eliminate the Y capacitor unless you're prepared to accept the resulting increase in RFI (and even then, there'll still be some residual 'touch voltage' via the 50 to 100pF leakage capacitance between the primary and secondary windings). Short of finding a screened version of the transformer to replace the unscreened original or else the capability to take the transformer apart in order to rewind it so as to add a screening foil yourself[3], the pragmatic solution is to modify it to use a 3 pin plug in mains lead so as to ground out the touch voltage on the low voltage output terminals (with or without a 10 to 15 k resistor to eliminate concerns over adding an earth loop risk). It turns out that the pragmatic approach is the best solution after all in this case. Who knew? Well, I'm a little older and wiser after all that faffing around with back to back transformers so I certainly know it now! >:( ][1] A "Black Adder" reference. [2] Seemingly the preserve of the much more costly "Medical Grade" wallwarts. [3] Yes Sir! I reckon I must have considered every conceivable option there was to consider, including that one but, after stripping apart a few spare 12 and 15 VA smpsus for spare bits and some inspiration, it became obvious all too soon that this was a step too far, even for me. I'd had visions of unwinding a dozen or so turns of secondary windings (doable without splitting the core), laying a screening foil down and rewinding the secondary windings. Unfortunately, it seems the common winding practice is to lay down the LV secondaries first and then wind the HT primary windings on top. It's one thing to rewind a dozen or so turns of relatively robust enamelled wire but dealing with a hundred or so turns of very fine enamelled wire was another world of pain I felt best not to visit (I know my limitations). Harking back to your comment regarding using a shaver wall socket isolating transformer, I'm not entirely sure such a commodity item would necessarily provide enough isolation at the microamp leakage level required to sufficiently reduce the ESD risk. Even if you use a high grade 10VA or so transformer (and this is something else I gave plenty of thought to), there'll still be some leakage to deal with unless you sacrifice some efficiency to improve the isolation performance using split bobbin construction to separate the primary and secondary windings more effectively (71% efficiency was a figure I saw for one of these transformers), so a significant heat load if it's installed into something like that FY6600 AWG, especially if still reliant solely upon the original, ill thought out, passive ventilation (the fan upgrade was not so much a 'luxury' as it was an 'essential' item that really should have been fitted as standard to be begin with). Regards, Johnny B Good |
| Johnny B Good:
I'm following up my own post simply to report that I finished the class II to class I earthed psu modification on my FY6600 late Saturday evening. I could only find the one C6 socket, an ex-laptop charging brick type that slides into location rather than the easier to deal with panel mount screw on styled type. This just meant I spent a lot more time manufacturing the reinforcing/mounting bracket from a piece of 1mm thick scrap of aluminium sheet. I'm quite pleased with the result (non of that flexing of the rear panel when inserting and removing the original 2 pin C8 mains lead connector). Also, the 90v touch voltage has been loaded right down to just 500mV by the 11K earth drain resistor (3x33k in parallel for a bit of redundency) which all but eliminates the risk of ESD damage to sensitive devices under test. A useful but predictable side benefit was elimination of the loss of output from the generator from ESD events created whenever connecting any earthed equipment to the generator at just the wrong part of the mains cycle. It may also have cured another, slightly less annoying issue with the left and right arrow digit select buttons but I won't know without further testing or extended use. Suffice to say that this pragmatic cure for the earth leakage touch voltage seems to have been the best way forward despite my initial misgivings about this type of modification. Even if you can source or create a mains leakage free class II PSU, there'll still likely be a 50 to 100 Megohm half live leakage path simply to prevent destructive static charge build up on the ELV side so don't get your hopes up of getting a perfectly isolated class II psu, regardless of type, any time soon. Regards, Johnny B Good |
| Zero999:
I thought shaver transformers are wound on double bobbins for safety reasons? In any case how lower leakage current are you after? How about using a capacitor, rather than resistors to earth? That would remove any DC path to earth, yet shunt the AC leakage current and has the advantage of lower impedance, at higher frequencies. |
| Johnny B Good:
--- Quote from: Zero999 on January 14, 2019, 09:33:01 am ---I thought shaver transformers are wound on double bobbins for safety reasons? In any case how lower leakage current are you after? How about using a capacitor, rather than resistors to earth? That would remove any DC path to earth, yet shunt the AC leakage current and has the advantage of lower impedance, at higher frequencies. --- End quote --- A dedicated 110v shaver socket transformer probably will use a split bobbin construction for safety reasons in the damp/wet environment of a bathroom. The extra losses, maybe 3 or 4 watts, aren't any problem in that situation and actually a blessing in the circumstances to help keep damp at bay. Unless I'm very much mistaken, the 110v output winding will be centre tapped to connect to the safety earth so as to create a bi-phase 55v supply with no neutral return (both wires in the shaver plug will be 55v 'lives' to reduce the risk of electrocution through body contact with earthed objects to meet the safety standards required in a wet room environment. If you're going to use such a transformer to power a smpsu designed to work over the mains voltage range from 90 to 265vac here in the UK, the reduced 110v will reduce the half live mains leakage voltage by a factor of two. Whether the balanced nature of the supply will provide another 50% reduction should you choose to connect the centre tap earthing point to an actual protective earth or not is not something I've given any consideration to. It might or might not help but I Have a feeling, in the light of recent experience, it won't help. Obviously, this is something you can test for yourself. You'll probably get the most benefit in this case by allowing the secondary to float free of any earth connection, assuming you're just looking to avoid the need for a three core cable connection to the PSU and all but eliminate the troublesome half mains voltage leakage without introducing troublesome hum loops or unwanted conduction of the switching harmonics into the mains wiring, perhaps to radiate and interfere with portable LW/MW/SW AM radios you (or your immediate neighbours) might have in regular use around your(their) home(s). As for my choice of an 11K earthing resistor, rather than, say a 100nF capacitor as others have used, let's just say I know my own mind on this. If the smpsu board had used a shielded transformer with a 2n2F Y cap connected to the inter-winding shield instead of to the common end of the secondary windings (effectively the zero volt common ground return for the + 5 and 12 and -12 volt rails), the worryingly high touch voltage would have been reduced to a vanishingly small value with less destructive capacitive charge (circa 50pF with perhaps 10v peak versus 1nF and 170v peak) potentially able to damage the more fragile parts of any kit through ESD transients when having test signals injected into it, then there wouldn't have been any perceived need for such mitigation measures. As it happens, Feeltech elected to go with a commodity three rail smpsu board rather than order one with a screened transformer costing just a penny or two more to manufacture, so we have a potentially serious problem in the event that anyone using such a signal generator needs to inject test signals into random locations inside a possibly very expensive piece of kit sat on the repair bench. The absence of a ground loop is a handy feature but not when it comes with a hefty half live mains voltage riding on the BNC ground return. My aim with upgrading from a 2 pole C8 connector to a 3 pole C6 connector was simply to provide access to an earth point by which to link the BNC grounds via a carefully chosen 11K resistor so as to knock the half live leakage back from 120vrms right down to just half a volt rms without creating a low impedance ground loop circuit to complicate the business of injecting test signals into various electronic devices, including audio gear. In this regard, the 11K resistor is virtually an open circuit compared to the one ohm or so resistance of the signal generator test lead ground connection to the ground or common return point of the device under test. A capacitive link to the safety earth connection would be of no benefit whatsoever in this usage case - it would actually be a hindrance and very probably inject unwanted switching harmonics into the mains wiring as I've already pointed out. I'd still like to test the odd bit of radio kit from time to time, so needlessly polluting the local environment with RFI is an activity best avoided in this circumstance. In short, I chose to use a resistor instead of 100nF capacitor because the capacitor, to my mind, is not the best solution in this case. Regards, Johnny B Good |
| Navigation |
| Message Index |
| Next page |
| Previous page |