Electronics > Projects, Designs, and Technical Stuff
Driving a small coil directly with a microcontroller pin atmega or stm32
ale500:
I would like to drive a small coil (magnetic actuator), small in size, current say ca. 10 mA with the pin of a microcontroller. I would like to keep part count to a minimum, say microcontroller and coil(s), and battery :) Do I need extra diodes ?
Any thoughts ?
Circlotron:
Some older CMOS logic had a parasitic SCR in the totem pole output stage that could be triggered if the output pin was pulled outside the supply rails. The SCR would switch on and short the supply rails and perhaps destroy the device. When turning off your inductive load it might do the same thing to your micro. Others may care to comment on this possibility.
T3sl4co1l:
CMOS latchup these days is usually >100mA, and you'll be hard pressed to draw more than 10mA from a coil that idles at 10mA. But YMMV. A lot of MCUs don't spec it.
Could even wire it between two pins (H-bridge drive), to speed turn-off. You'll have to know the inductance though.
Schottky clamp diodes won't hurt; a BAT54S per pin will certainly do.
If you need the assurance of a proper spec'd logic family, 74HC3G14 or the like would be a good idea; wire all three sections in parallel to get extra-strong drive. (Or use a 2G or 1G part and only as many as you need. One should be enough, after all, but you can also overkill if you like. :P ) The output transistors will handle that current with very little voltage drop indeed, and the CMOS latchup rating is given either in the datasheet or the logic family sheet.
Tim
ahbushnell:
What is the inrush current on the coil. It takes more current to pull in the contact compared to the DC current. I would put a diode across the coil. Use an open collector transistor to buffer. 10 kohm gate resistor. 2n3904 transistor. and use a diode on the coil would be the safest way.
rvalente:
Yes, can do...
Some analog gauges based on small stepper motors uses just the uC IO + some diodes
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