It's a long story. Please bear with me as I try to describe what I intend to accomplish.
I've got a Raspberry-PI controlled Lego Technics (pre-Mindstorms era stuff) construction.
It uses a couple of 1980's Lego motors, that are normally be powered by 3 C-size 1.5V batteries in series: Effectively some 4V to 4.5V.
The motors appear to be just that: No fancy electronics, overload protection, rectifier circuity, at all. (There may be an internal capacitor in parallel, like in the newer 9V Lego motors, but that would be just about it.) The battery-box only contains a mechanical switch with neutral, left and right positions. Single 2 wire lead from battery-box to motor. (Polarity gets reversed to change direction of motor.)
I'm not directly switching the power to the motors from the PiFace. I found a relay with 2 inputs (cutoff and polarity) which mimics the behavior from the mechanical switch on the battery-box and which completely isolates the "controlling" circuitry from the "controlled" circuitry. (I know I'm not using the proper terminology for relays, but I hope I'm clear.)
It made sense for me to use that, to make sure the rather delicate Pi/PiFace doesn't get sprayed with electric noise from the motors, when the polarity reverses.
The entire setup is now working, using the original battery-boxes, to power the motors.
As they eat batteries at an alarming rate I'm interested in powering these motors with a 5V feed from the power-supply that currently feeds the Raspberry-Pi and PiFace that control the Lego motors.
The PSU is actually a normal 450W computer PSU, where one of the 5V leads is brought outside the case to feed the hobby-project. The computer itself is always on (drawing abotu 250W), so there is no issue with using an unloaded or severely under-loaded switching PSU.
My days of analog electronics are 25 years behind me. I was never any good at it either. (I'm a digital guy.)
Even so I'm quite certain I better not wire this up "as is". (I do know enough to know when I need a bit of help.)
- Will 5V straight from the PSU fry the motor? I guess some sort of voltage (and maybe current) limiting circuit is desired. (I don't have any spare motors, so this really worries me.)
- Will the induction of the motor-coils cause stability issues with the PSU when suddenly reversing polarity to the motor. I can, in software, introduce a "not-powered" period, between reversing the polarity if needed, but how long should that period be?
- If a limiting circuit (1st point) is used how will that affect point 2?
Has someone tried something like this before? Any guidance will be appreciated.