Offboard power switch

#1

I am developing an off board power switch that will use a mosfet to switch power to not only the magni but other custom boards we use. I have noticed that when plugging in batteries directly to the magni there is sometimes a spark that I believe would be from inrush currents. Is there any documentation on whether or not there is inrush limiting on this board. I note Q903 looks to be the main power switch mosfet for the MCB but I am unsure if it does any inrush limiting. I have measured many hundreds of amps during plugin using a current sense resistor and oscilloscope. I plan to have soft start on my power switch and want to make sure it is specc-ed correctly to not be overwhelmed by the magni’s power draw on turn on.

Any information regarding it’s turn on characteristics are appreciated.

#2

Hello.
Q1904 as of rev 5.3 boards is the ‘main power’ switch and is controlled by the black button. Q1904 is a rear mounted large power mosfet. Q903 is also a power switcher but is only for motor power circuits.

The comment on ‘many hundreds of amps’ inrush is a surprise however since several large capacitors are charging up you may be describing an extremely short term spike in current which is a very tricky thing to measure as it is more or less an impulse.

Both of the power switch mosfets are controlled by tiny power control chips that make up our ECB circuits which are mostly there to prevent and shutoff power for overcurrent situations. How those react in active situations had been studied a great deal however what they do for initial powerup has not been studied by myself or anyone for years.

There is no intentional usage of hundreds of amps on power connection unless perhaps you have left the power switches on the magni in the ON position in which case there could be many amps running to the motor coils.

In your current sense tests be aware that the negative lead of the battery does not connect to the MCB ground. The negative lead of the battery passes through a polarity protect MOSFET on the back of the board very near the black negative battery connect lead bolt.

There has been no testing to model what you are wanting to implement. I know of no reason that inrush limiting would be bad for the Magni as long as you do not do your own current limiting a second or more after powerup of the magni.