On my boat I have a 12v central heating pump. It's a car one. Speed is controlled by a pwm doodaddy. Think that's it - pulse width modulation? Normally I'm plugged in the mains with a 120w charger topping up the batteries. I ran out of electric credit over the weekend so it's been running purely on the batteries. The speed controller has a pot and a digital readout. Plugged in the mains I set it to 60. Without the mains charger in the power supply it goes the same speed set at 30. Why? If it helps, before I changed all my lighting to LED, the more lights I switched on, the faster the pump ran. Weird eh?
It is slightly broken or rubbish. The lower the supply voltage the more time it spends reaching some fixed voltage comparison level before switching off each pulse, so the wider the output pulse becomes. Sort of thing you get with a circuit made by Fred-in-a-shed and sold by Maplins.. .. before Arduinos let you make a fully programmable feedback speed controller.. Uses about 2 or 3 transistors where the pulse width threshold is based on turning on a transistor, comparing a fixed 0.7 volts (approx) against the supply voltage. It probably goes slower/produces a lower output when it's hot too.. Replace it with an adjustable buck converter off eBay rated for the current. Then it produces a fixed voltage that should balance against the back EMF of the motor and the motor's resistive voltage drop to keep the pump running at a constant speed if the heating circuit is a constant load on the pump.
Will 8 hours of sleep, regular bowel movements and bouts of vigorous burpies allow @Zed to produce his own arduinos?
