PWM high voltage low curent - Page 1
Forum section: electronics / beginners

author: locoo
author_rank: Contributor
timestamp: September 13, 2015, 06:40:54 am
content: I have a stupid Q?I want to put 100w@24vDC lights in my garter but would want for the power supply to be indoors.Only problem is that i need 30m of cable to connect them.So i would like to know is it possible to use PWM running on 240v DC and then run it on 10% duty circle.Since pwm is average voltage that will give me 24v dc.And will it decrease current load on the wire so i can use a small wire?Thanks for your reply's.

author: cs.dk
author_rank: Unknown
timestamp: September 13, 2015, 06:44:54 am
content: 100W @ 24V is only about 4 amps, that will not be a problem.. 2,5 mm2 cable would be enough.240V @ 10% DC would be a problem however - PWM is modulating the on/off time, the peak voltage is the same at 10% and 100%.

author: locoo
author_rank: Contributor
timestamp: September 13, 2015, 06:49:50 am
content: Yes peak will be 240 but it will average a lower voltage .But i wold to use small wire for visual appearance so it looks like it is power from air

author: Chris C
author_rank: Frequent Contributor
timestamp: September 13, 2015, 07:19:31 am
content: Doesn't quite work that way.Assuming these are incandescent lights at 100W and 24VDC: We can calculate that they're drawing 100/24 = 4.17 amps. From the voltage and the amps, we can determine the filament resistance as 24/4.17 = 5.76ohms. Given that, we can calculate the amps it will draw at 240V as 240/5.76 = 41.67A. And the watts, 41.67*240 = 10,000W! Even at 10% PWM, it's still going to draw an average of 1,000W at 240V. With a fixed resistance, power goes up exponentially with the voltage; not going up 10x as you expected, but 100x. You would actually need a voltage of 72VDC, at 10% PWM, to maintain an average power of 100W.Assuming these are any other kind of lighting: They will burn out instantaneously from the overvoltage, regardless of PWM. Unless you have some downconverter directly at the light to convert the higher voltage back down to 24VDC.

author: locoo
author_rank: Contributor
timestamp: September 13, 2015, 07:27:26 am
content: Ups , they are LED strips but that does answer my question .So i have to put big fat cablesThanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 13, 2015, 07:49:31 am
content: Power in a resistive load goes as sqrt(D) when PWM'd at duty cycle D. So you need 1%, not 10%.Since they're LED, this would be very stressful on them. Incandescent wouldn't mind as much, but the EMI would still be awful (you'd have to use coax cable to even begin to get away with it).Tim

author: matseng
author_rank: Frequent Contributor
timestamp: September 13, 2015, 09:03:47 am
content: Quote from: locoo on September 13, 2015, 07:27:26 amWhy not just install thinner cables and then compensate for the drop over the cable by increasing the 24dc to whatever voltage that gives you the desired current? Sure, you'd be pissing away a lot of power in the cable, but that might be of less relevance than the aesthetics of the installation.

author: Chris C
author_rank: Frequent Contributor
timestamp: September 13, 2015, 09:44:52 pm
content: Quote from: matseng on September 13, 2015, 09:03:47 amCould work. Or some variation, like having most of the cable large gauge, and buried. At the point where it exits the soil to go to the strips, switch to fine gauge there. If that's only required for a few feet, the loss may be small enough that 24V will still work.