PWM Dimmer / Voltage reg question - Page 1
Forum section: electronics / projects

author: mickc
author_rank: Contributor
timestamp: May 26, 2014, 05:33:13 am
content: Hi,I have several 12v PWM dimmers that run LED strips, 12vdc supply in, and 12v DC PWM on negative side out.I have some 5v LED strips i want to run off the same dimmer, so they are all controlled by the same potentiometer.Do you think if I ran them through a 7805 voltage regulator that they would work ok?Thanks

author: theatrus
author_rank: Frequent Contributor
timestamp: May 26, 2014, 05:58:31 am
content: Unless we know more details on these dimmers/drivers it's impossible to say.

author: Asim
author_rank: Regular Contributor
timestamp: May 26, 2014, 06:09:46 am
content: 7805 will give you a constant output voltage if the input voltage is 7 or more ( depends on the load) . So while changing the pot from 12-7ish the led strip will receive 5v ( same brightness £but then the output voltage will start to drop. I can't consider this a working dimmer.

author: sleemanj
author_rank: Super Contributor
timestamp: May 26, 2014, 06:19:00 am
content: Use the PWM signal to drive a transistor (bjt or mosfet) through which the 5v strips are powered from a 5v output dc-dc converter off the 12v supply.Using a 7805 unless the current demands are very small, is not a good idea.NB: If your dimmers provide current limiting, that's not going to work, naturally

author: Asim
author_rank: Regular Contributor
timestamp: May 26, 2014, 06:20:08 am
content: To add to the last post, if you meant to put the 7805 in the input of the dimmer, then this depends on your dimmer itself if it works in such voltages( more info required )

author: mickc
author_rank: Contributor
timestamp: May 26, 2014, 06:21:24 am
content: Thanks for the input.The dimmers I have put out 12v all the time, just at varying duty cycles depending on the dimming level.I guess my question should have been, will a 7805 work with an input voltage that is pulse width modulated?

author: Asim
author_rank: Regular Contributor
timestamp: May 26, 2014, 06:33:50 am
content: It will if the average voltage is greater than7( you will need filtering at the input). As you may know, these dimmers outputs different duty cycles which will correspond to a a different average voltage for example if you have a 50% duty cycle with a 12v pulses you will get 6v average voltage. Would this work for you as a dimmer for the 5v led strips? I would say no.

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 26, 2014, 06:48:21 am
content: Those 3-terminal regulators are NOT designed to regulate PWM. Now, it might work, but I wouldn't bet on it.The most straightforward method is probably to just put a big dropping resistor on each 5V LED strip to allow equivalent current operating point with the 12V PWM instead of a proper 5V PWM source.

author: rob77
author_rank: Super Contributor
timestamp: May 26, 2014, 08:43:20 am
content: actually it's a interresting idea to put a 7805 therebut i doubt the 7805 is fast enough handle even a low frequency PWM - check the datasheet for the load transient figures ... but it's almost certain it won't work. and even if it would work, it would be ringing like a good old doorbell from the 80'solution: just add the big fat resistor as others suggested or go for a 5V dimmer.

author: ajb
author_rank: Super Contributor
timestamp: May 26, 2014, 03:17:18 pm
content: If the dimmer uses low-side switching, then you can use an external power supply to run the LED strip at whatever voltage you want up to the maximum voltage on the dimmer's switching transistors. You can identify if your dimmer uses low side switches by cracking it open, or by measuring the voltages at the output terminals with the dimmer powered on but the output off. If you get 12V at the positive output terminal then you have low side switches. To connect an external 5V supply, join the 5V supply's negative terminal to the negative input on the dimmer, wire the negative side the the LED strip to the negative output of the dimmer as normal and wire the positive side of the LED tape to the 5V supply.

author: rob77
author_rank: Super Contributor
timestamp: May 30, 2014, 08:05:47 am
content: Hi Guys,actually i was curious about the behaviour of an 7805 under PWM ... and hooked up a small circuit - despite the fact that i was convinced it won't work.and i was suprised... it worked !PWM was 490Hz , L7805CV by ST , a P-MOSFET high side switch for PWM and a 4R7 dummy load on the output of the 7805.there was some ringing on the 12V PWM side, but nothing we couldn't live with.I'll post a video later today (need to upload to youtube first - and via my ADSL it'll take some time).

author: rob77
author_rank: Super Contributor
timestamp: May 30, 2014, 11:58:43 am
content: Quote from: rob77 on May 30, 2014, 08:05:47 amso here is the proof<iframe width="560" height="315" src="//www.youtube.com/embed/RXZE3AKgeZQ" frameborder="0" allowfullscreen></iframe>embed code not wrking obviously

author: SeanB
author_rank: Super Contributor
timestamp: May 31, 2014, 01:14:05 pm
content: Very nice, and I have the same power supply as well. Just note it may not work with all 78xx regulators, as some are different internally, and some may oscillate at power edges. You might need a 100n capacitor at input and output still. Might not work well with LDO regulators as they will oscillate.

author: rob77
author_rank: Super Contributor
timestamp: May 31, 2014, 05:37:34 pm
content: i know all those possible problems, that's why i was convinced it will not work.the 100nF caps were there during the PWM test. if i'll have time i'll do a second test with higher frequencies in 1,2,5 steps (1kHz,2k,5k,10k....) untill it will stop working or the ringing will be unacceptable.