Repair or adapt 125V SMPS wall wart for 230V - Page 1
Forum section: electronics / projects

author: soldar
author_rank: Super Contributor
timestamp: September 15, 2023, 07:41:03 pm
content: I have some Ryobi tools which use some Ryobi batteries which I charge using a Ryobi P118B charger. All these things are bought in the USA because, due to mysterious reasons, they are much, much cheaper than in Europe. In Europe they are like three to four times more expensive which is just ridiculous.The charger has two parts: the base where the battery is charged and which actually controls the charging process and a separate SMPS which shows 18V nominally but which I have measured as actually 21V. The SMPS wall wart is connected directly to the charging base with a permanent cable with no connectors.The SMPS wall wart is only good for 120V. The only reason I can imagine for not making it 100-240V is to prevent it from being sold outside the USA. No matter. I have been using it with a transformer and no problems.Until some days ago when, I do not know WTF I was thinking, I actually looked for a flat to round plug adapter and plugged the thing directly into 230 V. It took a fraction of a second to blow a cloud of smoke and really bad smell. Oh well.After some ventilation and reflection on my own stupidity I split open the wall wart and, sure enough, there was a 200V electrolytic capacitor exploded and made a mess. A simple diode bridge followed by the exploding capacitor.After replacing it with another one cap (which does not fit in place so connected with wires), apparently the SMPS works again at 120V. I have not tested it for long but I want to be optimistic that the cap is all that was damaged and the rest is OK.I wonder if replacing the cap with a 400V cap the rest of the SMPS would work fine but it is an experiment I do not think I should do directly. Maybe someone has an idea of how I could test this safely but the SMPS circuit is complicated and I am afraid it is too risky.My options right now are1- Go back to using an autotransformer to lower 230 V to 120 V. And pay attention or install and wire permanently on a common base to avoid mistakes.2- Use a different 21V SMPS which can use 230V. I don't have one but I could find one.3- Instead of an autotransformer use some electronic circuit to lower/limit the voltage. This is what I want to explore here. It would have to be simple enough to make it worth while.I am thinking of a switch which would conduct when the input voltage is under 175 V and not conduct when the input voltage is over 175V, as in the attached graphic. That way the input capacitor would charge only to 175V even though the 230Vac peak goes up to over 300V.I have plenty experience with triac dimmer circuits but these will not work for obvious reasons.Is there a very simple circuit which will do this? A series transistor which blocks voltages over 175V?Obviously this is more for fun and learning than a practical project.

author: SeanB
author_rank: Super Contributor
timestamp: September 15, 2023, 09:41:56 pm
content: Grab an old laptop power brick, most deliver 19VDC output, and enough current to charge the drill. then do a wire transplant to get the socket the charger uses, and you have a working unit. Easier than fixing the original, and now also multi voltage, and very cheap as well.

author: Shonky
author_rank: Frequent Contributor
timestamp: September 15, 2023, 10:04:40 pm
content: There are specific isolated step down transformers for exactly what you're doing. A bit better than a (variable) auto transformer.Chopping the AC may work using a trailing edge dimmer but you'd have to be very careful to ensure it doesn't run any more than about 25%.I expect the circuit is specifically designed around the much lower DC rail on 115VAC so I doubt just putting a 400V cap in will be enough. It may work initially but will probably stress whatever switching its using.

author: soldar
author_rank: Super Contributor
timestamp: September 16, 2023, 09:46:14 am
content: Quote from: Shonky on September 15, 2023, 10:04:40 pmCould be. Or maybe not. These days it seems most SMPS are designed for wide input voltage range. It could be that they took one of those and just put a lower voltage capacitor and labeled it accordingly. But since we do not know that it is best to not assume anything and not put more voltage on the thing than we know it can reliably withstand.Quote from: Shonky on September 15, 2023, 10:04:40 pmI have several crates full of old transformers with dual 125/230 V input. I can use any one of them as autotransformer and that is what I have been doing until now and can continue to do. I posed the question, not to solve a practical problem, but to investigate the possibility of a simple electronic way of doing it.Quote from: Shonky on September 15, 2023, 10:04:40 pmI have quite a bit of experience with triac dimmers and they are not suited for this at all because the RC delay triggering circuit is very unstable with regards to line voltage condition and a tiny difference in time triggering results in a large difference in peak output voltage. I suppose this type of triac dimmer might work if fitted with a totally different trigger circuit. Maybe measuring voltage directly or maybe measuring a precise time delay after the zero crossing.As I say, this is just an exercise for fun and learning. If this were a low voltage application I could design a circuit easily but I am not sure I want to scale it up to over 300V. I am very used to working with low voltage transistors but not with high voltage.And I suppose I should install a crowbar just in case...

author: soldar
author_rank: Super Contributor
timestamp: September 16, 2023, 12:56:36 pm
content: Here is the kind of thing I have in mind. Left upper rail has rectified sine with 325 peak. R1/R2 cause T1 to conduct over, say 100 V. R3/R4 cause T2 to conduct over 175V which, in turn, blocks T1.T1 needs to block the full peak voltage; T2 does not.If I can find in my junk some transistor with that kind of high voltage I might experiment.

author: Zero999
author_rank: Super Contributor
timestamp: September 16, 2023, 02:24:48 pm
content: Quote from: soldar on September 16, 2023, 12:56:36 pmThe problem with that is the inrush current from the capacitor blowing up the transistor.Relying on VBEis a bad idea. A zener could be connected in series with the base to provide a more reliable reference. Some hysteresis is also a good idea.

author: soldar
author_rank: Super Contributor
timestamp: September 16, 2023, 02:47:16 pm
content: Quote from: Zero999 on September 16, 2023, 02:24:48 pmIf the tiny diodes can take it then it should not be difficult to specify a transistor which can take it. I can put a resistor in series, an NTC, whatever. This should not be a problem.Quote from: Zero999 on September 16, 2023, 02:24:48 pmWell, I was thinking in terms of the transistor entering and leaving conduction a bit gradually, transitioning through the active region, so it would smooth the current peak although that has the inconvenience of heat generation.Yes, I suppose the circuit can be improved but I am looking for simplicity rather than perfection. The main thing it needs to do well is block voltage greater than 175 V. As long as it does that well it really does not matter whether it does the rest well or not so well.

author: Zero999
author_rank: Super Contributor
timestamp: September 16, 2023, 03:30:03 pm
content: Diodes tend to be more robust, compared to transistors.The voltage drop of the base-emitter junction is highly temperature dependant

author: soldar
author_rank: Super Contributor
timestamp: September 16, 2023, 03:53:52 pm
content: Quote from: Zero999 on September 16, 2023, 03:30:03 pmI was merely showing the basic idea (and secretly hoping someone could point me to an existing and tried and reliable circuit). It is possible that transistor T1 would need to be a darlington pair and other changes would be required.If I can find in my junk pile a high voltage transistor then I can start experimenting and designing around that.Another solution could be something I have used in the past with the purpose of generating 115V/60Hz using an H topology with one side using capacitors to divide the voltage. This produces AC rather than the pulsating DC of my previous idea. It worked well in the application I used it for but it seem more complex than strictly necessary for this application where the frequency and wave shape are unimportant.

author: ConKbot
author_rank: Super Contributor
timestamp: September 16, 2023, 04:43:03 pm
content: Quote from: SeanB on September 15, 2023, 09:41:56 pmFor normal situations, definitely. For a 5S 18V nominal, 21V peak battery charger, being fed by a 21v supply, I'd expect the charger to just be a buck converter. Maybe a 24v supply, maybe a 24 supply with a few diodes inline or an actual regulator, or the feedback tweaked a bit if you feel like it.Quote from: soldar on September 16, 2023, 09:46:14 amIf it was a company beside Royobi, I would agree, but with Royobi there is a chance it could be a Japanese domestic market design with a new sticker on it and packaged for the US.If a global company is going to cost optimize for US 120v I would have expected them to penny pinch everything, not just swap the cap and call it good, because the logistics of tracking yet another manufacturing line has its own cost. So check some transistor voltages diode voltage ratings if you can.

author: Zero999
author_rank: Super Contributor
timestamp: September 17, 2023, 09:45:24 am
content: Quote from: soldar on September 16, 2023, 03:53:52 pmThat's effectively a charge pump, which reduces the voltage by half, rather than boosting it and is probably your best bet.