LED strip/chip driver from AC, what more than a diode bridge - Page 1
Forum section: electronics / beginners

author: Unknown
author_rank: Unknown
timestamp: April 20, 2014, 10:17:43 pm
content: Hi,I have some spare parts arround:-> a 100W AC220V-AC12V coil transformer, output frequency 50Hz in Europe-> several 60W AC220V-AC12V halogen transformers, output frequency ~50khzI am building some light sources for photography with LED strips and chips.Though not sure for the coil transformer a diode bridge would make it to "raw" DC.What about 50khz. Would the bridge work well.Even for further 12V led supplies. Old halogen transformers are so cheap now, wonder if I should buy some more for further 12V transformers.A 12V/60W transformer is >15$. Such a halogen transformer is less than 3$ on ebay, a shotky diode less than 10c. (BTW SS12 would be good for this I assume). Or the poor LED's need some more care in terms of current etc?

author: Simon
author_rank: Unknown
timestamp: April 21, 2014, 06:50:10 am
content: if the LED strips have built in resistors should be ok but really a roughly regulated 12VDC would be better

author: Simon
author_rank: Unknown
timestamp: April 21, 2014, 10:27:11 am
content: Well a smoothing capacitor may be in order as else for most of the rectified cycle the LED's will surely be off as they will want around 3V each to work and will be very bright at every peak with 12x1.414 = 17V, in fact as the peak is V*2sqrt you are looking at twice the rated power during peak voltage (2sqrt*2sqrt = 2)

author: Kjelt
author_rank: Super Contributor
timestamp: April 21, 2014, 10:28:58 am
content: Most ledstrips are 12VDC. So their max. current is limited by a resistor for 12VDC operating voltage.If you use a 12VAC transformer and rectify it and use a large capacitor to stabilize it a bit then you end up with 12V*SQRT(2)= 17VDC.In that case the leds will get 40% over current and won't live long.So you have some options:1) change all the resistors to match with 17VDC and the wanted current.2) use a cheap switching DC-DC converter (can be had on ebay for $2) to get 12VDC.

author: Simon
author_rank: Unknown
timestamp: April 21, 2014, 10:30:50 am
content: if a reasonable value smoothing capacitor is used it won't hold too much charge and will just cope with the loss of voltage at under 12V but yes it's not ideal.

author: Simon
author_rank: Unknown
timestamp: April 21, 2014, 10:31:25 am
content: I had a similar dilemma and just bought a PC power supply to hack

author: Kjelt
author_rank: Super Contributor
timestamp: April 21, 2014, 10:33:56 am
content: Quote from: Simon on April 21, 2014, 10:31:25 am+1 for my own RS485 indoor controlled led project i needed 200W 12VDC power so used a very good high efficiency PC power supply and forced it always on (powersense pin if I remember correctly), they don't cost that much more AND you have some helper voltages at the same time to use for other sensors/uC's etc. (5V, 3,3V)

author: Simon
author_rank: Unknown
timestamp: April 21, 2014, 10:55:46 am
content: yep that is what I did, had the metal shop make me a case with holes for an atx supply so I have 100's of W for my LED's my control board under it and the psu fan will cool my board too, all in one neat case (slight smaller than a shuttle sized pc case) and the 5V can supply the ATmega

author: Kjelt
author_rank: Super Contributor
timestamp: April 21, 2014, 03:20:20 pm
content: If you don't need 100's of W's I usually use a TDK-Lambda PSU, they cost from about €14 for 25W, €19 for 50W and so on upto €35 for 150W in 25W increments.If I had to do it again I would probably buy two of those TDK-Lambda 100W PSU's for my project, if one fails still half of my lights would work

author: Simon
author_rank: Unknown
timestamp: April 21, 2014, 03:28:40 pm
content: well second hand computer PSU's will have plenty of power too. In my case I'm running 21 channels

author: Simon
author_rank: Unknown
timestamp: April 21, 2014, 04:15:20 pm
content: To start a computer PSU connect the green wire to any black one., you should also have some small load on the 5V and 12V lines