Author Topic: Very simple, possibly transformer-less AC to 5v dc power supply  (Read 1298 times)

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Offline xtv

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Hi everyone,

I've been looking around for a low cost, low current, likely transformerless power supply to convert mains (AC 240/127V) to a stable, relatively low noise 5v DC to power a around 1mA load microcontroller.

I have seen designs like this before, but I assume their efficiency is very low, using a somewhat high-value resistor in parallel to drop the intake current.



Since mains operates in AC, for a short period of time the tension is assumed to be close to 5 volts. Would it be possible (or smart) a design that would allow current to flow and charge an output capacitor only when AC input is close to 5v?

Advise, discouragements or other recommendations?

Many thanks!
 

Offline dmills

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Re: Very simple, possibly transformer-less AC to 5v dc power supply
« Reply #1 on: July 13, 2021, 06:50:20 pm »
That is a capacitor dropper, and while they have purely CRAP power factor, the efficiency is actually not horrible, most of the voltage is dropped across the cap.

HOWEVER, they are non isolated, which limits the use case to things that have no accessible user connections apart for the power input, EVERYTHING on that board (and on any boards connected to it) must be assumed to be live at full mains voltage (Which makes plugging in the debugger a pain).

Anything you come up with that is better is likely to be more complex and less reliable.
 

Online tunk

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Re: Very simple, possibly transformer-less AC to 5v dc power supply
« Reply #2 on: July 13, 2021, 06:52:11 pm »
Do a web-search with this: capacitive dropper
 

Online TimFox

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Re: Very simple, possibly transformer-less AC to 5v dc power supply
« Reply #3 on: July 13, 2021, 06:56:37 pm »
I have used capacitive droppers to drive a carefully-insulated LED from mains without a transformer.  Note that a good (high-Q) series capacitor should have no loss or heating, but it's a good idea that the load current is sinusoidal, such as a medium-size (resistance and power) resistor in series with a LED in parallel with a reverse diode, or in series with a bridge as in your drawing.  It's probably not politically correct to have such a high power factor, and insulation safety is paramount, as dmills stated.
 

Online BrokenYugo

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Re: Very simple, possibly transformer-less AC to 5v dc power supply
« Reply #4 on: July 13, 2021, 09:19:40 pm »
You can get small isolated SMPS modules/boards just for this sort of thing. Or build your own linear supply, though the dual voltage requirement complicates that.
 

Offline Faringdon

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Re: Very simple, possibly transformer-less AC to 5v dc power supply
« Reply #5 on: July 16, 2021, 04:58:23 pm »
Here are some ideas with LTspice sims (LTspice is free)

You can also use a linkswitch or a tinyswitch,.....and power integrations website designs it for you if you want.
 

Online mariush

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Re: Very simple, possibly transformer-less AC to 5v dc power supply
« Reply #6 on: July 16, 2021, 05:51:29 pm »
If you want non isolated power supply, you could just get a diode for half wave rectification, a small input cap, a hv linear regulator, a couple resistors to set the output voltage and an output capacitor

See for example LR8 or LR12 (max 100v dc) regulators:

LR8 https://www.digikey.com/en/products/filter/pmic-voltage-regulators-linear/699?s=N4IgTCBcDaIDYCcAcACEBdAvkA

Datasheet : https://ww1.microchip.com/downloads/en/DeviceDoc/20005399B.pdf

Even with half-wave rectification, the regulator will work from 13v dc to 450v or whatever and fill the output caps.
If you add a small input cap, it will probably also work for most of the dead period discharging the input cap.
 
 

Offline jonpaul

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Re: Very simple, possibly transformer-less AC to 5v dc power supply
« Reply #7 on: July 19, 2021, 01:29:20 pm »
dangerous and cap will fail shorted if exposed to line transients.

Jon
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Offline NiHaoMike

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Re: Very simple, possibly transformer-less AC to 5v dc power supply
« Reply #8 on: July 20, 2021, 03:10:06 am »
dangerous and cap will fail shorted if exposed to line transients.
The capacitor will need to be X rated, specifically designed to handle such transients.

If you use a Y rated capacitor and return it to ground, the output will be touch safe but then the available current will be very small indeed.
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Offline voltsandjolts

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Re: Very simple, possibly transformer-less AC to 5v dc power supply
« Reply #9 on: August 03, 2021, 10:13:33 am »
When you can buy a suitable offline, isolated, safety qualified psu for comparable cost to the parts shown in the OP ($2), why bother making your own more risky equivalent?
These are in Farnell, probably made by MeanWell?
 

Online Alti

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Re: Very simple, possibly transformer-less AC to 5v dc power supply
« Reply #10 on: August 03, 2021, 11:34:21 am »
Would it be possible (or smart) a design that would allow current to flow and charge an output capacitor only when AC input is close to 5v?
This is possible of course but I have never seen such design in practice.
It seems your design has to use some kind of active switch that clamps to 5V capacitor just when mains is passing +5V (which lasts for super short time on 230V mains). And this switch has to be capable to withstand full mains voltage + transients, but it is idling most of the time. So instead of charging 5V cap with huge currents for super short time (1mA continuous out means you need ~1A if charge lasts 1/1000th of the time) when crossing 5V, why not charge and discharge some inductor during all the cycle? You already have the active switch for the rated voltage, also evenly loaded switch is much smaller then.

And that is how we got to an offline buck converter.

These mains buck converters are super simple and cheap but not suitable for such low powers, I think there is no market for 5mW mains switchers. For 1mA just use capacitive dropper which has higher efficiency than any mains buck switcher, and is also super cheap.

inrush resistor
cap rated for transients
bridge rectifier
output capacitor
zener


 


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