Mini Power Supply

[markus1234]

Hi,

im kinda new to thiss stuff, so please don't mind any dump questions

I'm currently designing a product which is extremely limited by size (max 35 x 35 x 18 mm. The logic stuff and the product itself will take like 22 x 22 mm.

So there is not that much place for an 230 VAC to 5V DC power supply, which should be able to deliver ~ 20 mA of current.

Since im limited in size i thought about an transformerless design since transformers are normally relativ bulky and the thing itself will be fully enclosed in thick plastic.

What about this minimal transformerless (capacitive) design?



Maybe i can du a full bridge rectifier with 4 diodes.

What do you think?

Thanks in advance,
markus

[Zero999]

That will work but it'll give one diode drop less than 5.1V due to D1. If you want 5V, use a 5.6V zener.

There should be a 1M bleeder resistor across C1 so it discharges, after the power is disconnected, that way no one gets a shock if they touch the plug.

C1 should be rated to the peak mains voltage (350V at least, preferably 400V) so check that the 250V rating for RMS not the DC/peak value.

The circuit does not provide any isolation from the mains, so everything connected to the 5V rail needs to be treated with the same precautions as the mains.

[Kleinstein]

The full bridge is likely better, as it uses both half-waves and thus needs only half the AC current (though about the same power). So the capacitor can be half the size. The input capacitor likely needs to be a X-type for safety reasons (fire hazzard). Similar a fusing rsistor is a good idea.

If the circuit is really isolated, this type of power supply is OK.

[jeroen79]

How about a 5V wallwart?
That would require only a DC connector in your product.

There are also small PCB mounted switching supplies the size of a small transformer.

The circuit does not provide any isolation from the mains, so everything connected to the 5V rail needs to be treated with the same precautions as the mains.

If you're in a country where mains plugs can be plugged in either way then even the 'ground' may be connected to live mains.

[markus1234]

I also thought about this, but it will be an in-wall installation. Wired by an electrian (not an end user). So i guess a visible power adapter wouldn't be the best for an "invisble" product.

btw: what about putting an MOV between the N und L of the mains?

I also found these modules:

http://www.hlktech.net/product_detail.php?ProId=54

They can be bought pretty cheap (like ~ 1,2$ each), but i don't know whats in there...could be even worse than my curcuit.

[Kleinstein]

These modules have about the same inside as cheap USB Chargers: As little filter as needed to about meet EMI requirements and a flyback converter.  The module linkes even looks comparativly powerfull. There might be even smaller lower power versions.

Having a rating of <2000 m altitude somewhat makes me think of small magins in thermal design or isolation distances. However many products have that limitation.

[markus1234]

I will order one just for the sake of trying it. But its a brick compared to the < 10 components above :/

[LukeW]

If you only need 20mA and you understand and accept what the lack of galvanic isolation in a transformerless power supply means, then it's OK.

The full-bridge version is more efficient, but you can't use it if you need the hot ground (or +Vcc) referenced to the mains line, which is quite common in these sorts of circuits, for example if you want to do power metering with a shunt or you want to control a triac.

Microchip AN954 is a good primer on the basics of these circuits: http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf