Hi - has anybody seen any board mountable 5V DC supplies? Like something that would take a 120VAC signal and output 5VDC. It wouldn't need to supply very much power - probabaly somewhere along the lines of a .1W. I'd rather not use a wall wart, as I already have AC ground, neutral, and live on the board. I'm also hoping for a ready made solution - as I really don't know much about making DC power supplies.
Small PSUs are very easy to build. Start with a small PCB mount transformer, a bridge rectifier, and a filter capacitor. Use a 7805 regulator to regulate the voltage.
http://www.eleinmec.com/article.asp?16 - Here's a good article on building on 5v regulated power supply (it uses 230v mains, but the transformer is easily changed to a 120v model) with explanations of components, schematics and a stripboard diagram.
you can actualy do it without a transfomer... if i can find the PCB somewhere... it was a digital humidifyer with a 120v fan and 5v logic and there wasnt a transformer in the power supply, pretty cool if you ask me. a edit: i found it, and it is an AWSOME board because everything is labled and it is only one sided. ill have it traced out in 10 minutes. standby
alright, sorry for the low-quality schematic, but i was feeling MSPainty today. MC=Mylar Cap, EC = Electrolitic Cap, and CC= Ceramic cap. it is traced out how it is on the board... the most important part is the power resistor and the zener diode. it outputs a rock solid 5v from 120 ac.
am i sure what isnt a cap? the varistor? true, there is no isolation. but it was running an MCU in a production unit, so it cant be that bad.
I would get a switching cellphone wall adapter. If you can't get a 5V one (Siemens A55, A60 and some more) you could grab a Nokia 3110 or some others that give you 7.5-12V and 400-800mA, and just use a linear regulator like 7805. The board inside that switching regulator is smaller than most transformers and leave a pretty good voltage. The one in the Siemens charger is extremely small and can push 5V 400mA
The part marked "125k/250v". 125k is a resistor measurement. BTW, your circuit seems to have a short. Did you really mean to draw the line between the fuse and the varistor? As drawn, your circuit would output -5v. Turn around the diodes and capacitors to make it output +5v.
the markings came directly off the PCB, that isnt what i decided to call the parts. the silkscreen reads "MC1 125k / 250v" and a quick search shows it is ( http://www.mouser.com/index.cfm?han...uctid=307743&e_categoryid=497&e_pcodeid=59809 ) "Sub-Miniature Metallized Polyester Film Capacitors" type .125uF picture edited apropreatly.
I'm trying to avoid the whole wallwart thing, for various reasons. I really want a through hole device that I can mount on a PCB. Hmmm... This seems like a fairly viable option I suppose. Might somebody be able to reccomend a fairly inexpensive, board mount transformer? I'm looking at this one - http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=229524 - my understanding is that it can do 300ma at 12V, which is wayyy more than I need - but it's cheaper than the 120ma version, and 60ma might be pushing it a bit. (I'm talking about the three similar transformers listed on that catalog page: http://www.mouser.com/catalog/622/1335.pdf). But also - I don't need 12V - I'd really like to find one more in the 6-8V range... I expect normal current will be around 10ma or so - but I have a relay on board, and it will get switched occasionally, which I think will cause a bit of a spike. Is it ok to momentarily go above a transformer's current rating? Well OK - I should explain the project more - this device will be switching AC wall power. It will be embedded in a wall right under a switch box, so I don't expect it will get much ventilation. Thus - I want this thing to be as efficient as possible. Would a transformer rated for higher current be more or less efficient than one rated for a lower amount of current? Argh this is confusing. I wish I could just find a magic black box with 4 pins - AC live, AC neutral, DC 0V, DC +5V. edit: forgot to ask - is AC ground not used at all?
dude D2 = short... Strange circuit, any idea on the power output? I have seen just a 10W resistor + bridge rectifier used before if only a few Ics need power, bit like the LED calculator thing.
I thought you were a "professional engineer" as you once called yourself. This is a pretty standard circuit for providing low currents (usually less than 50mA) to small electronic equipment. The varistor should be across the live and neutral as it is only conducting when the voltage across it is higher than it should be to help suppress surges. The capacitor is there to drop the voltage in the same way as a resistor would. It's far more efficient (heat) than just using a resistor. The zener clamps the voltage to 5.1V. It is more usual to see a bridge rectifier however. The +5V will output +5V not the opposite, the drawing is correct. 125k is often used to represent 0.125uF. I'd have disconnected the neutral from D2 and disconnected D1 from the 0.125uF capacitor, and the connected the neutral to D1. At the moment D2 is just conducting during the unused half cycle.
Check out the VB408 and VB409. They should be able to provide a 40mA supply from a rectified/smothed mains supply.
America well, it draws less then 1.5a nope, R2 = 200k... that is FAR from a short probably limited by the power resistor to 2 watts... but im sure you could scale it up you = i knew i wasnt crazy! yeah, they could have even worked in a bridge rectifier... but if you just cut D2 out then the negative voltage cycle has to go through the zener diode the 'right' way and i guess they didn’t want to mess with that. who needs efficiency anyway
I'll add that (a) if anything goes slightly wrong, like the zener going open-circuit, you've the potential for a current-limited mains voltage on the output, and (b) the capacitor size needed depends on mains voltage, frequency and load current - values shown are for an unknown load (though you can work it out from cap reactance) and an 110V input. Don't try this on our perky 230V supply without l33t skillz. A 3VA transformer, bridge rectifier, caps & 7805 will set you back under £5 and be safe.
yes, but nleahcim sais that he is from Oregon (USA) and if the zener did blow, the fuse would pop. thats what its there for
The only time the fuse will blow is if you drop it all in the bath. Work out the max current possible. Reactance = 1/2PifC ignoring j
point taken. why is it there then? and IIRC a 16v cap turns into a resistor at 120v... drawing the current away from any electroncs, no?
