General > General Technical Chat
Class 2 SMPS - Removing the Y capacitor - Good design solution wanted !
<< < (4/8) > >>
Zero999:
I can assure you shaver sockets are completely floating. The one in my house has both a 240V and a 120V output. The point is that touching one side of the output can't give you a shock. I think you're confused with building site transformers, which do have a centre tapped to earth output.

Regarding the resistor vs capacitor debate, they each have their up/down sides. I've seen both. Bench top power supplies and lab equipment tend to use capacitors rated to a few hundred volts, so the power supply can be floated with respect to earth at reasonable voltages. That 11k resistor would get very hot if if had  a couple of hundred volts across it. Laptop power supplies and IT equipment often have a resistor connected to earth. If it were me, I'd do both: a capacitor with a 1M or so discharge resistor across it
Johnny B Good:

--- Quote from: Zero999 on January 15, 2019, 09:50:08 am ---I can assure you shaver sockets are completely floating. The one in my house has both a 240V and a 120V output. The point is that touching one side of the output can't give you a shock. I think you're confused with building site transformers, which do have a centre tapped to earth output.

Regarding the resistor vs capacitor debate, they each have their up/down sides. I've seen both. Bench top power supplies and lab equipment tend to use capacitors rated to a few hundred volts, so the power supply can be floated with respect to earth at reasonable voltages. That 11k resistor would get very hot if if had  a couple of hundred volts across it. Laptop power supplies and IT equipment often have a resistor connected to earth. If it were me, I'd do both: a capacitor with a 1M or so discharge resistor across it

--- End quote ---

 Thanks for the info regarding shaver socket outlets being completely floating. My only knowledge of such transformer safety measures comes from what I've read about those building site transformers. Considering the very light electrical load and the use of potted split bobbin isolating transformer construction, I guess the risk of one side of the secondary winding 'silently' developing an earth contact fault must be considered astronomically low enough to make it a safer option than the 55-0-55 bi-phase system used with building site transformers to power 110v power tools.

 I can see why you were giving this option serious consideration.  :) BTW, do you have any idea what sort of VA rating these shaver socket transformers have? In the case of my modified FY6600-60M signal generator, this can demand almost a full 10W from its little smpsu board (the rating label states "10W max") so I'd be looking at an isolating transformer with at least a 10VA rating, preferably 20VA  if you wish to preserve the full 90 to 265 volt universal voltage rating without adding a 120/240 volt selector switch (you'd use it as a 1:1 isolating transformer using the 240v windings to simply pass on whatever the mains voltage happens to be, hence the need, in this case for ideally, a 24VA rated transformer to cater for the higher I squared losses in the windings when operating at the low end of the voltage range in this mode).

 Obviously, if you're prepared to fit a voltage selector switch, you can get away with a 12VA transformer and use the 110v secondary to feed the smpsu with a floating mains supply voltage - just how often would anyone ever need to adjust between mains voltage standards to the point where this becomes an onerous task?  :)

 Regarding bench supplies, they're normally class I earthed with the outputs floating wrt to the case metalwork so they can simply tie the 'zero volt' rail and all output terminals to the earthed case via 100nF caps with a static bleed resistor to eliminate potentially damaging static build up under conditions of extremely low humidity. My cheap AWG, otoh, was never blessed with an earthed case to begin with, relying on the class II rating of its integrated smpsu to make the complete unit a class II double insulated device.

 Unfortunately, the half mains live 'touch voltage' that results is of some concern in such T&M equipment where a moment of forgetfulness over linking the BNC shield connection to the DUT via a bnc cable or a separate ground linking cable could result in damage to the item being tested. The concern may be more imagined than real but, nevertheless, the fact that such a scenario could play out given half a chance for Sod's Law to be invoked by a moment of carelessness, does legitimise the concern. Since I don't plan on stacking a bunch of these up in synch to generate 80v p-p signals, the issue of burning out the 11K earthing resistor won't normally arise, Since I don't wish to create unwanted earth looping paths for HF harmonics, the resistor is all I require to effect a cure for the half live 'touch voltage' issue. Obviously, that won't suit everyone's needs but it suits mine just fine.  :)

 If the original question is in regard of wallwart smpsus, the Y capacitor can only be relocated to connect to a screening foil between primary and secondary windings in the transformer instead to the 0v point of the output rails. That being the case, the practical solution is limited to spending extra cash on a "Medical Grade" replacement wallwart.

 Regards, Johnny B Good.
Zero999:
The issue with the resistor overheating is due to the device under test being at a high voltage, relative to earth. The main advantage of the capacitor is the device being tested can float at any voltage, up to the capacitor's maximum rating.

Shaver transformers are normally rated at about 20VA, although they won't be rated to that continuously. Dual voltage operation is standard. The one in my house has separate sockets and a mechanism which ensures only one can be used at a time. I haven't taken it apart, but I'd expect it has two 120V secondary windings and a switch to select between series and parallel configuration, depending on whether the 240V or 120V socket is used.

I had a quick search for 25VA isolation transformers.
Triad VPS230-110
Available from Mouser and Farnell. Lookst good, double bobbin and low capacitance.
https://uk.farnell.com/triad-magnetics/vps230-110/transformer-2-x-115v-25va/dp/1610413
https://www.mouser.co.uk/ProductDetail/Triad-Magnetics/VPS230-110?qs=%252bfhPKvh1fqM%2fIMKduQ8CRw==

Triad VPT230-110
Toroidal. Probably higher capacitance, but will be more efficient. US stock so available from Farnell at high cost or Mouser, with a long lead time.
https://uk.farnell.com/triad-magnetics/vpt230-110/transformer-toroidal-2-x-115v/dp/1785731?gclid=EAIaIQobChMIlZq7ypjy3wIVqrftCh2WqQLMEAAYAiAAEgL2sPD_BwE&mckv=s4rYYLzOH_dc|pcrid|99299485688|kword|vpt230-110|match|p|plid||slid||product||pgrid|5285736368|ptaid|kwd-24784252128|&CMP=KNC-GUK-GEN-SKU-MDC
https://www.mouser.co.uk/ProductDetail/Triad-Magnetics/VPT230-110?qs=wkKrz7WmEgNNNgyT8w4YqA%3D%3D&gclid=EAIaIQobChMIyMfwz5Hy3wIVo7XtCh3yIQbZEAAYASAAEgJLFPD_BwE

Bel Signal A41-25-230
Similar to the first one. Available from Digikey and Mouser.
https://www.mouser.co.uk/ProductDetail/Bel-Signal-Transformer/A41-25-230?qs=sGAEpiMZZMvwUzoUXIIvyZ8riR3Pta28%252b8Y6KbSbVRk%252bKxx4UgnHTw%3d%3d
https://www.digikey.co.uk/product-detail/en/signal-transformer/A41-25-230/595-1303-ND/953173

Another possibility is to buy a cheap shaver socket or salvage one from a skip and take the transformer out of it. Some are cheaper than buying an isolation transformer. The only issue is whether it will be able to power the SMPS continuously, without overheating.

https://www.tlc-direct.co.uk/Products/PU8900.html?source=adwords&ad_position=&ad_id=315107931576&placement=&kw=&network=u&matchtype=&ad_type=&product_id=PU8900&product_partition_id=297072075353&version=finalurl_v3&gclid=EAIaIQobChMI7dGoqZry3wIVDJztCh1ErAEjEAQYBCABEgKNE_D_BwE
https://www.directtradesupplies.co.uk/product.php/193005/ansell-dual-voltage-shaver-socket--white-?gclid=EAIaIQobChMIwImru5vy3wIVRofVCh0nzwF8EAkYASABEgJrgfD_BwE
https://cpc.farnell.com/unbranded/701875/shaver-socket-dual-voltage-white/dp/PL15991?mckv=sKmngMpNY_dc|pcrid|224681503328|kword||match||plid||slid||product|PL15991|pgrid|47203068935|ptaid|pla-559841591210|&CMP=KNC-GUK-CPC-SHOPPING&gclid=EAIaIQobChMIiejRrZvy3wIVSrftCh18vghaEAkYDiABEgJ95fD_BwE

Regarding dual voltage operation: I don't see any issue with having a jumper or switch. How often are you planning to take it abroad? A 1:1 230V transformer will work, but bear in might the current draw on 120V will be greater, so a 230V transformer running on 240V will only have half the VA rating. The power factor of the SMPS might also be poor, so even if it only uses 10W, it still might need a 20VA transformer.
Johnny B Good:
 I'll address the several points of concern that you've raised, one by one.

 Firstly, the resistor overheating risk in this case doesn't (or damn well shouldn't!) apply since only an idiot would elevate the signal generator's ground rail above the 50v dc level typical of telecoms kit. In general connecting a signal generator's ground via the BNC connector shield to a device is always only done on a device which has its common rail floating or at zero dc/ac potential.

 Accessing test points within the DUT (old valved gear for instance) that have elevated potentials (the more likely scenario) are normally dealt with by using either capacitive coupling or else an isolation transformer. I certainly won't be taking such risks when using test gear on equipment containing destructively high voltage potentials which may also pose an electrocution hazard just as any EE  worth his salt would not. I would have much greater concerns than that of overheating or burning out a half mains live suppression grounding resistor in such cases.

 If you need to power a 10W rated class II smpsu via a 20VA peak rated shaver isolation transformer, it makes a lot of sense to use a voltage selector switch to prevent the smpsu compensating for the halving of the voltage by a doubling of the current draw. You can either wire the secondary windings in series to supply 240v or else in parallel to supply 120v to the smpsu and add a voltage selector to switch the split primary into either a 240v series  or a 120v parallel configuration to match the local supply voltage. As we've both observed, "How often would anyone need to switch between mains voltage standards as to make this an onerous task?"

 As for the examples you linked to, the only viable contenders were the split bobbin types. Their voltage regulation (an indicator of their efficiency) might be poor compared to the toroidal type but the leakage capacitance stands a better chance of being 50pF or less whereas the toroidal type may well have 500pF or worse leakage capacitance making them no solution to the problem caused by the accursed 1000pF Y cap in the typical 10W rated class II smpsu.

 I happen to have a 30 year old example of the 24VA split bobbin breed which is still in manufacture today in precisely the same PCB mount form right down the the plastic shell having been produced by the same injection moulding machinery. The difference being that it has a pair of 15.4 volt 350mA and a a pair of 10.3 volt 700mA secondary windings which would nicely supply, after rectification and smoothing, a set of switching regulator modules to provide the +/-15v and the +5v rails, obviating the horrendous losses of the analogue regulators used by others who'd replaced the original smpsu board inside their FY6600 AWGs with analogue PSUs of their own design.

 However, having tracked down the current manufacturer of that transformer, I was able to download their data sheet on this family of mains voltage isolating transformers and spotted the 71% efficiency reference which rather put me off any further consideration of its use to eliminate the class Y cap leakage issue. I'd originally guestimated a 12VA figure some 30 years ago when I first acquired it but, having had access to the data sheet, I've been able to identify it as a 24VA unit and could estimate the current ratings of the secondary windings with some confidence.

 It may yet see use in a future psu project. :-) I might even wire it up to a mains connector to test its leakage capacitance if curiosity ever gets the better of me when I've nothing better to do but, for now, there's no burning need for that information now that I've found a simpler solution to the mains leakage issue.

 Regarding your concern over the poor power factor of such class of smpsus that aren't required to meet the PFC requirements that apply to smpsus with 75W or higher wattage ratings, I wouldn't worry. The Power Supply Utility (and the smpsu for that matter) will thank you for thoughtfully adding a measure of PFC in the form of a high leakage inductance transformer with its circa 100 ohm per 120v winding to further take the sting out of the smoothing capacitor induced narrow angle rectifier conduction current spikes. In view of the obvious use with such rectifier/smoothing circuits on the 15 and 10v secondary windings version in my possession, I should think the same conditions apply wrt such a rectification and smoothing technique even at mains voltage levels. The losses in the transformer will be slightly increased but not enough to raise any concerns imo.

 Regards, Johnny B Good.
Briain:
Hi Zero999

A while back, I bought myself a Feeltech FY-3224s signal generator and it had the Y cap issue (from memory, unloaded it measured 85 Volts on the BNC screen, WTR mains earth) so I bought a Triad Magnetics N-68X (50 VA isolation transformer) and with me being in the UK, I used the primaries in series to give me a 230 V to 110 V step down isolation transformer (I also fitted a switch whereby I could leave the outputs floating, or mains earth reference them). With the switch open the BNC screen now shows as having about 5 Volts present, WTR mains earth (though before building it, I'd noted that it was significantly more than 5V when reversing the 110V connections from the transformer to the SMPS). This was pretty 'cool' in that with it weighting 0.77 Kg, it gave the tester some weight (so it didn't slide back when pressing a button, though more weight would be even better); attached photo shows a picture of the tester, with a piece of hardboard added below the transformer to spread the load, such that the plastic doesn't so easily shatter if it's dropped a small distance).

So, it all appeared to be working very well (I bought the transformer back in 2017, but I only got around to fitting it a couple of days ago) but after it had been on for a few hours, I noted that the top of the plastic case was quite warm, so I opened it up and discovered that steel band surrounding the transformer core was sitting at well over 50 deg C (and the Feeltech only draws about 2.5 Watts) so that was not quite so cool (sorry about the pun). I thought about adding a vent gauze to the case, but with the case lid completely removed, it still slowly rises to quite a high temperature (I gave up the test when it crept beyond 40 deg C; the probe was not well coupled to the metalwork so I suspect both temperatures were probably even higher than was indicated) so I cannot say that I'm too impressed with that particular Triad transformer. Indicative measurements - just using one of these cheap and cheerful domestic power meters, so I am not sure how accurate the readings are - showed that when sitting off load, the 50 VA Triad N-68X transformer consumed 4.9 W, whereas my big, yellow 750 VA 'site transformer' consumes just 5 W when it is sitting off-load.

During my Internet dredge for a suitable replacement (there are surprisingly few available and most are all too tall to fit into my case) I stumbled across this thread and the 25 VA Bel (A41-25-230 ) that you identified in your post looks to be well suited to my own requirements (so thank you for that). I have just ordered one and I will post back in a few days to let everybody know whether it runs at a more sensible temperature than that toasty Triad.

Briain
(GM8PKL)

NB The new transformer will only weigh 0.57 Kg, so I'll maybe add about 400g of lead to kelp keep the tester stationary when pressing the buttons on the front panel (there is another thread about these units - and the Y capacitor issue - and someone else has also added lead). :-)
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod