4 wire speed control computer fans

"Mauried"

Or read this or both.

..... Phil

It's a 5V 25khz PWM signal. You need to let it spin up first. I have one motherboard that always cycles them at 50% duty right from power on, only problem is it does this straight from bootup, so some fans don't even spin up and the bios just cuts out with CPU fan fail error for under 800rpm... the only way I found around it was to hold reset until the fans spin up. Because of all the dodgy brands of fans you will find that some need way longer than 30% PWM to keep spinning and some won't even start spinning unless they have 100%.

Thanks guys. Looks like I have a dodgy fan. Buzzes a lot when powered then turns slowly then stops completely. If the 4th wire is left unconnected it spins at full speed. This one is CPU fan, so better to have it run flat out that not at all.

Excellent thanks for that