Hi everybody,
My next project involves switching a flip-dots matrix. This should be done 'pixel-wise'. Therefor I need a switch-able current-limited source which provides a define pulse which is then switched towards the correct flip-dot with some transistor arrays. The question is how to design this switch-able current-limited source with the following requirements:
- Should work with an input voltage of ~12 to ~24 V.
- Pulses need to be between 0.5 and 1 ms. This should be controllable by a µC, e.g. by having an enable input.
- Current should be limited to ~350 mA.
- High-Side
The first idea was to use e.g. a LM317 in current-limiting mode with an extra switching FET or a device like the FPF2702MX (http://www.onsemi.com/pub/Collateral/FPF2702-D.pdf) like done by https://github.com/545ch4/flippie. However my flip-dots have a coil resistance of ~18 ? which would mean that up to P=U*I=(24V-(18?*.35A))*.35A=17.7V*.35A=6.2W are dissipated in the current source. Therefor I think a switching current source would be much better.
The number of switching controllers/device is insane and it is hard for me to find a good candidate. Does any has a good suggestion where to start?
How often is the dot flipped? I presume they're latching and remain in the same state, without consuming additonal power. If the duty cycle is low, then the average power dissipated by a linear regulator, could be quite low. The problem with an LM317 is it requires about 4V of headroom, when used as a current regulator.
The coils are inductive, so it will take time for the current to build up. All that's needed is a monostable circuit which uses the inductance of the coil as the timing element: wait for a trigger pulse, apply power to the coil, monitor the current, until it exceeds a certain threshold (high enough to flip the dot), then cut the power to the coil.