Minimising Idle Power in a Transformer Based Supply - Page 1
Forum section: electronics / projects

author: gmr1981
author_rank: Contributor
timestamp: December 23, 2023, 04:08:36 pm
content: Hi All,I'm trying to design a board that runs all the time from 240V and provides power to a microprocessor which occasionally wakes up and does stuff (the project is a lighting controller).My aim is to minimise the quiescent power used by the device - the PIC24F I'm using draws only about 10mA @3.3V when its running, and most of the time it is asleep, with an LP oscillator keeping track of the time. Sometimes though, much more current is needed, when the processor wakes up to toggle the state of the latching relays that are connected to the lights.The architecture I am thinking of would use a triac to turn the transformer on every so often, let it charge a capacitor big enough to keep the PIC alive for a while. When the PIC wakes up to do its housekeeping, it can turn pulse the triac to recharge and can also keep the transformer fully on while the peak power is needed. However, I can't think of a way to bootstrap this circuit so that when 240V power is first connected, the system will get a charge.Has anyone got any thoughts? Are there some common circuits for doing this kind of thing? Any help greatly appreciated.Cheers,Greg

author: Andy Chee
author_rank: Super Contributor
timestamp: December 23, 2023, 05:12:19 pm
content: Is a transformer even required in your application? Could you go transformerless?https://ww1.microchip.com/downloads/en/appnotes/00954a.pdf

author: Benta
author_rank: Super Contributor
timestamp: December 23, 2023, 07:19:45 pm
content: The question is why the quiescent current is an issue for you?It's mainly magnetizing current for the transformer, which is inductive and thus free of charge.Small transformers do have iron and copper losses, but we're talking sub-watt range here.

author: gmr1981
author_rank: Contributor
timestamp: December 23, 2023, 07:25:10 pm
content: Thanks Andy - some good info in that Application Note.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 23, 2023, 07:33:05 pm
content: Don't reinvent the wheel - get a 5V USB phone charger. Legislators have done their best to enforce excellently low idle power consumption. If it's legal and has CE marking - it's probably much better than what you can engineer yourself.Do you have any numbers, how little power consumption you need?

author: gmr1981
author_rank: Contributor
timestamp: December 23, 2023, 07:43:44 pm
content: Hi Benta,I have some small transformers (2VA) (Myrra 44121), with no load, they dissipate about 0.5W. Does that matter? Not really, but as this a project that is just for my edification, I thought it would be nice to try and build something that doesn't register on the power meter, especially considering how little work it will do almost all of the time.Greg

author: Siwastaja
author_rank: Super Contributor
timestamp: December 23, 2023, 07:47:51 pm
content: Quote from: gmr1981 on December 23, 2023, 07:43:44 pmWikipedia says (https://en.wikipedia.org/wiki/No_load_power) EU limit for phone chargers is 0.15W since 2011, but you may want to double-check that. Pretty small IMHO. I don't think you can easily do any better. Put USB-C connector on your thing and call it a day.

author: gmr1981
author_rank: Contributor
timestamp: December 23, 2023, 07:50:35 pm
content: Interesting idea - I just plugged my Samsung one into my power meter - it registers 0.00W when not loaded, which is exactly the kind of result I was looking for. Wouldn't integrate nicely into my project, but it might be a good circuit to reverse engineer.

author: pqass
author_rank: Super Contributor
timestamp: December 23, 2023, 08:09:51 pm
content: Quote from: gmr1981 on December 23, 2023, 04:08:36 pmFor a real-world implementation seethis thread; the schematic in the top post and reply #8 (4th paragraph) for an explanation. But you could probably simplify with a triac and [zero-crossing] opto-triac on the mains-side to switch the transformer. You could then use a momentary button to short the opto-triac to bypass turning it on initially where from then on (after button release) the PIC can kick the opto-triac on every time the low voltage side falls below a threshold (resulting in the transformer seeing periodic half-cycles spaced many cycles apart on the high voltage side). And remove the signal completely to turn your device off.

author: pqass
author_rank: Super Contributor
timestamp: December 23, 2023, 09:25:36 pm
content: Instead of using a momentary push button to turn on your device (as described in my previous reply), you'll have to use a more complex circuit consisting of a cap dropper to give that initial opto-triac turn-on pulse. Then use another opto to give it periodic turn-on kicks.See thissimulationand attached schematic.Notice, upon power on (via main switch), the 10uF cap appears like a short turning on the NPN, LED, and opto-triac. After it charges up fully, the NPN is turned off. Periodically, the other opto can discharge the 10uF cap and start the cycle again. You'll have to experiment with the cap to configure the timing such that only a few cycles are gated via opto-triac.Unfortunately, this cap dropper may consume more than you're saving in switching the transformer. Maybe you can take this further.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 23, 2023, 09:47:01 pm
content: Small transformers are pretty low in efficiency. One way to get a little less low load loss is using the transformer with a lower magenetization, which means winddings for a higher voltage. The used magnetization is what sets appart the transformers made for low standby loss. So the 0.5 W loss is already quite good for a classical iron transformer.So maybe have 2 of the transformers in series on the primary (and secodndary), as higher voltage ones are hard to get. AFAIR the loss for the transformer shoudl go down about like votlage to a power of 2 to 2.5. So the 2 transformers in series should have a little less then half the loss.A ready made SMPS is likely the way to go. A capacitove dropper or solar and a super cap could be an option too, if the system us more efficient (e.g. get away with some 1 mW).

author: oz2cpu
author_rank: Frequent Contributor
timestamp: December 23, 2023, 10:50:36 pm
content: Hi GregI see you are from UK a classic 230-240 land :-)so my first suggestion can not be used, I would have said use a 230V transformer in usa, that will give you super low idle usage.Kleinstein is on the right track with his suggestion, to use two 230V transformers in series,this way your idle power is now very low, ofcourse you also need the two sec windings to be in series to get the output voltage back up where you want it,please explain : WHY idle power losss is so important ?why you want a conventional mains frequency transformer solution ?

author: soldar
author_rank: Super Contributor
timestamp: December 24, 2023, 05:07:59 am
content: Quote from: Benta on December 23, 2023, 07:19:45 pmTransformers lose quite a bit of heat.

author: gmr1981
author_rank: Contributor
timestamp: December 24, 2023, 10:48:25 am
content: Many thanks to everyone for your replies - lots of good ideas. I'll go away and think about it in light of these suggestions.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 24, 2023, 12:50:13 pm
content: What about a sensitive gate triac with a series RC circuit providing the gate signal and a small MOSFET SSR bypassing it? Would be normally on until the SSR bypasses it.Or use a normally closed SSR, they exist but are hard to find.