# Power supply fails when PWM an halogen lamp

In this video I'm trying to light a 50W 12V halogen lamp using PWM, and switching from 0/500 to 500/500 duty cycle within 15 seconds (31000Hz frequency). I'm using an ATX power supply (276W spec on 12V rail), a MOSFET (IRF540N, "33A 100V 0.040 Ohm N-Channel UltraFET Power MOSFET"), and an Arduino Mega controller (wire going directly from chip PWM pin to MOSFET gate).

• System works a treat when below ~300/500 duty cycle or above ~400/500.
• System works a treat with led strips attached (instead of halogen lamp), I suspect they draw about 15 W (no specs available).
• Power supply turns off when around ~350/500 duty cycle (problem I'm asking your help for).
• I haven't got to measure the lamp resistance when cold (Fluke 106 multi-meter shows same reading for both the halogen lamp resistance and the multi-meter probes touching each other directly).
• I haven't got to measure power draw/current (I'm afraid of blowing the 10A multi-meter fuse).
• I haven't got an oscilloscope.

1. I was wondering how could the power supply withstand the load of a colder halogen lamp when around 200/500 duty cycle or a fully powered lamp and not a 350/500 duty cycle (if power draw was the one causing the power supply to fail).

2. I was wondering if there were any other plausible explanations for my problem that I should be checking for.

3. I was planning to buy a new ATX power supply to connect 6 of this lamps in parallel and needed to solve this problem first/learn the required specs of the future power supply.

4. I liked the idea of using PWM and not a dimmer (not messing with 220V, avoiding lamp transformers magnetostriction noise when dimming the lights, learning of the problems I'm encountering ATM).

5. I wanted to thank you all for your patience and hope somebody else finds this thread useful!

• How are you achieving the $\frac{1}{500}$ step PWM? There's a built in on the Arduino that uses $\frac{1}{256}$ steps. Is there a reason for why you are not using the one that's built in? – Harry Svensson Aug 8 '17 at 0:02
• TCCR3A = B10100010; TCCR3B = B00011001; // Use ICRn as counter top and set frecuency to 31000Hz ICR3 = 500; // ICRn set to 500 - I wanted to be able to dim the lights a bit more than 1/256 ! - Thanks for the reply ! – Juan Manuel López Manzano Aug 8 '17 at 0:11
• Draw a schematic, it's very hard to know how you've wired these components together unless you tell us. – Finbarr Aug 8 '17 at 0:19
• I'll draw a schematic right away ! Thank you for your time ! – Juan Manuel López Manzano Aug 8 '17 at 0:20
• Is the drawing helpful in any way? I know the MOSFET pins are correctly wired because they've managed to control led stripes to light my room for the last year or so. The drawing might not be representative in that way. But pin 5 is connected directly to a MOSFET placed between the lamp and GND as shown. If necessary, I'm willing to remake the schematic ! – Juan Manuel López Manzano Aug 8 '17 at 0:34