Mini Power Supply - Page 1
Forum section: electronics / projects

author: markus1234
author_rank: Newbie
timestamp: September 01, 2015, 07:16:12 am
content: Hi,im kinda new to thiss stuff, so please don't mind any dump questionsI'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

author: Zero999
author_rank: Super Contributor
timestamp: September 01, 2015, 07:58:50 am
content: 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 01, 2015, 08:01:03 am
content: 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.

author: jeroen79
author_rank: Frequent Contributor
timestamp: September 01, 2015, 08:24:42 am
content: 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.Quote from: Hero999 on September 01, 2015, 07:58:50 amIf you're in a country where mains plugs can be plugged in either way then even the 'ground' may be connected to live mains.

author: markus1234
author_rank: Newbie
timestamp: September 01, 2015, 08:38:16 am
content: 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=54They can be bought pretty cheap (like ~ 1,2$ each), but i don't know whats in there...could be even worse than my curcuit.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 01, 2015, 09:05:52 am
content: 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.

author: markus1234
author_rank: Newbie
timestamp: September 01, 2015, 09:26:32 am
content: I will order one just for the sake of trying it. But its a brick compared to the < 10 components above :/

author: LukeW
author_rank: Frequent Contributor
timestamp: September 01, 2015, 10:49:06 am
content: 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