Transitor Advise - Page 1
Forum section: electronics / beginners

author: paul23
author_rank: Regular Contributor
timestamp: April 11, 2012, 06:24:42 pm
content: I am working on an LED project and I am designing the power supply part of it. I need to have 3 voltages available, Mains 220Vac (for a couple of relays), 12DV for LEDS and 5vd for the controls (AVR, sensors, etc).My initial plan was to supply the circuit with mains, step that down with a transformer to say 15V( or whatever the nearest transformer available puts out) then step down again to 12V with LM7812 and then again to 5v with a LM7805. This gives me my three voltages and all is good.I have measured the LEDS and they pull 1.64A @ 11.89V, so I have calculated the power supply to be to able to supply 2.5A @ 12V. I think that should give plenty of headroom available.The problem is I think I need a transformer rated at 30VA, which doesn´t seem right to me, I don´t know why, it just doesn´t seem right. Have I calculated it right?

author: chimera_786
author_rank: Regular Contributor
timestamp: April 11, 2012, 06:42:39 pm
content: yeah 30 VA is correct. But, the thing is that this power supply is going to be massive in size caz a 30VA transformer is not going to be physically small in size, neither will the rectification part and the SMOOTHING cap.You will be better off buying a pre-made charger that can supply you 2A @ 12V (ebay, farnell, mouser, digikey, jameco etc). It this way, you can modify it with (or add) a 7805 to give you 5V out for your digital circuitry. I promise that it'll a lot cheaper on you in terms of part count, availability, shipping etcRemember that when the LEDs will be drawing (according to your info) 1.64A, then the output voltage is gonna get loaded pretty heavily so it will drop down to around 11.9ish volts (as you mentioned in your post). This is where the the 7805 kicks in and still provides a smooth 5V digital logic. This way, your part count will be low, which means low cost.Think about it and post back if you need further suggestions/clarifications. Best

author: paul23
author_rank: Regular Contributor
timestamp: April 11, 2012, 07:00:33 pm
content: That´s pretty much what I thought, I wanted to check I hadn´t made a mess of the calculations as it seemed massive. I think the way I will do it is like you said. The only difference is to have a 12V supply for the LEDS and control circuitry (stepped down to 5v) and a mains supply for the relays.Thanks.P.S. Don´t know why I wrote transistor advise for as the subject. It´s been a long day

author: SeanB
author_rank: Super Contributor
timestamp: April 11, 2012, 07:23:18 pm
content: Buy a cheap HDD interface, which has a USB dongle you plug the HDD into, as well as a PSU to provide 12V and 5V via a molec connector. Self contained power supply, cheap and nasty, but does the job. They normally give 12V at 2A or so and 5V at 2A as well. Costs around $10.

author: T4P
author_rank: Super Contributor
timestamp: April 11, 2012, 07:35:26 pm
content: A 3.5" USB/eSATA external harddisk's power supply usually gives 5V at 2amps and 12V at nearly 3 amps .

author: chimera_786
author_rank: Regular Contributor
timestamp: April 11, 2012, 07:51:50 pm
content: Quote from: paul23 on April 11, 2012, 07:00:33 pmQuote from: Dave.S on April 11, 2012, 07:35:26 pmExactly my point.Just buy one of these and then modify to your hearts content till you have what you need for your LEDs/relays/digital. Just remember, be careful when working with mains voltage (220VAC) and relays (use a 'free wheeling' diode, in case you end using a relay that gets driven by DC voltage).