Power Supply Options for 127v AC to DC

Hi All… Quick question. Does anyone has an opinion on the best option/design/schematic to power the core and a couple of servos using 127v AC… In short I am looking at transformer less designs and want to know your opinion and how to get the formulas to compute components to get 5VDC unregulated…

I’ve seen a lot of design with bridge diodes and some without, and I don’t understand if this is a good choice and how to calculate the zener diode and the multiple resistors and capacitors.

Any suggestions would be greatly welcomed!

if this is mains powered ? If so, are you taking isolation into account ?

I am talking about powering the Spark using 127V AC as the power source. I see some fire and smoke alarms use transformer less power supplies. However I am not familiar with this very much. I am newbie… I guess isolation would require a transformer?

They do indeed, but you will notice anything that does this also takes extraordinary measures to make sure no-one can touch any of the electronics. 127AC from the mains can kill, so I’d like to understand more about your intended use case before I give detailed advice out that can get people hurt if you don’t understand what you’re doing.

There are plenty of circuits that can do what you want, most of the simple ones will not supply enough current for a core, so I would recommend something like the VIper family from ST.

However, I’ll repeat - you should not even consider doing this unless you are confident you know what you are doing and no-one will get hurt or property damaged.



The design for this would require ALOT of power resistors and zener diodes. Think of it this way, all the power you are transforming with the xformer has to go somewhere, In this case it is sucked up in to huge power dissipating resistors. this is not the best design not to mention all the filtering required cap wise. They do make chips that can handle the voltages you are talking about but that too take alot of careful planning and design consideration. In short, more research and maybe collaboration with someone who has the experience is the way to go. there are plenty of people here who can help.

When searching for similar solution, I found these. But for reasons AndyW mentioned, I abandoned the idea. Don’t want to have my house burned down, mains power is no fun. If you never designed anything involving mains power, I strongly suggest looking into other options. I am sure people around this forum help you.

12V, 200mA (should be enough for Spark), simple components:

Even simpler, but requires 220V and not enough power for Spark anyway (5V, 100mA)

Well, Knowing what I know now, I think I will go to more traditional way’s of isolation power supply with transformer.

Any references for a simple power supply design with transformer?

I do need to move some motors, so maybe a 500ma to 1 A power supply would be awesome.

Thanks for the input!

I misunderstand, perhaps, but now you don’t mind if there is a transformer involved, any of the standard micro-USB phone chargers will do.

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there are ways to do this that don’t involve simply burning the excess voltage as heat, but it sounds like the OP is heading down a safer path now, thankfully.

Any standard 5V 1A wall wart should work for you - look first at USB chargers, as @psb777 suggested, you’ll probably find the best value there.

I’m relieved you’re choosing to take the isolated path, it is orders of magnitude less hassle.

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Im looking for a way to include the PS in the PCB… Is there anything I can use for that?

There are many like these. They don’t tend to be cheap through the likes of digikey, but you can often find them on the surplus market. There are lots of vendors, I’m just posting the CUI link as it was near the top of the list of google results.

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@AndyW The Viper family seem like they are power hungry when not in use. 30mW in standby mode. Now - I’m not familiar with HV switching PWS, but shouldn’t they be more efficient?

How does Apple do it with their small wall wart? I’ve never opened it up but I would be surprised to find a transformer in there.

I have seen some transformers integrated into IC’s for isolation. Probably more for signal passing, not any power production.

I would be very interested in learning about low cost efficient onboard PWD solutions. Do they exist - or should we just look for the USB adaptors and use them for powering our circuits?

I think 30mW quiescent is great for a transformerless, non-isolated line in regulator but YMMV.

The small wall warts are all standard isolated switched mode power supplies, using transformers - just diminutive ones.

My 2 cents that will hopefully help. I also struggled badly on how to power my project without a battery or a wall wart. I do not have a final solution I LOVE yet but I do have one that works.

You can get a step down transformer very easily from just about any local wholesaler they are used in lots of things. In my case I am using a transformer to step down to 24volts ( Size at this point does not matter for me the hardware I am controlling is the size of a fridge )

I then purchased some super simple Bridge Rectifiers that convert the 24volts to the DC voltage I need. I bought 8 of them for under $20.00.

This is great. I will just need to add a transformer to isolate and to step down from 127VDC…


¿Schematics somewhere?

Frankly if you have no experience dealing with line voltage DONT. Its fine to tinker on small projects but messing with line voltage can be lethal to you and those who happen to stumble across your project ( in my case my kids ).

Ask a physical friend who has experience to teach you how to do it safely

I understand… I am using a wall transformer that was purchased off the shelf… So no worries l😜 Im sticking to the safe side.

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Good good :slight_smile: I would not sleep well at night if I heard you hurt yourself.

Using a standard wall wart is how I started my project