pre-charge circuit

Next up in Super Mega Microbot 2’s existence is being able to run untethered.  Before that can happen, I need to be able to plug in a battery, and hopefully not have everything explode.  As seen with the IMU junction board, even minor inductive links can result in chips getting toasted.  I had thought that just adding sufficient capacitance to each of the point-of-load converters would resolve the issue, but in fact that almost made it worse.

Thus, I built a simple pre-charge board that I could put in line with the main power.  It has two big FETs, one power resistor, an ATTiny44, and the random regulators and glue necessary to make it work.  The microcontroller has one job.  On power on, it waits a bit, energizes the “pre-charge” FET which has the power resistor in line.  Then, a short while later, it energizes the main FET through which all power will flow.

The pre-charge board as back from MacroFab
Wired up for testing

I did some minimal qualification testing first with a single motor which went fine.  Then I tested it against the whole quadruped, where I scoped the output ground line.  Here, you can see that the output ground line initially rises linearly with the ramp up rate of the lab supply I was using to test.  Then, about 80ms later after the ATTiny has powered on, it energizes the pre-charge FET and the output ground asymptotically approaches the resistance of the power resistor.  Then again, at 100ms after that, the main FET is engaged and the output ground voltage drops all the way to 0 (or close enough modulo the FET on-resistance).


After that, all that was left was to try it with a real battery:


Spoiler alert: It didn’t smoke.


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