H bridge constant power

I ran into an issue the other day and wanted to know if it was actually an issue - for a project I am using 2 seperate power sources - one to power the arduino, and a second one to power 2 motors through an h bridge…

the 9v holding up my arduino is switched on/off by a standard switch… but my motor power source is connected by a jumper (for now)… So, my problem was, if I leave the power source for the motors plugged into the h bridge as it would during operation, would the power drain, or would the battery overheat (is there a short circuit occuring?) or is it safe to have the motor power source connected as long as the arduino is off (and thus the enable pin on the h bridge is also LOW)?

Is the H Bridge a discrete H Bridge made from transistor or a monolithic IC Bridge.

Generally H Bridges are just switches, which switch power the load. If you have no input signal the switches are ideally off. However due to leakage currents etc you find in reality there will be a slight current drain.

The IC based H Bridges also have logic circuitry inside which will draw a little bit of power even under quescient (no input signal) conditions. The current drain won’t be a lot, but it will be there and can run your battery down if left for a long time.

The battery shouldn’t overheat when your Arduino is off, but i migt get warm if the quiescent current is too high. The best is to check the datasheets if you are using a monolithic H bridge.

It not always a good idea to power up different parts of a circuit independantly, although it should be ok in this case. If possible rather turn everything off.

although it should be ok in this case.

I would disagree, you will be powering the output side of the circuit but not the logic side driving it. In effect this is floating and thus subject to all sorts of pickup which could accidentally trigger the motors. It could also lead to a latch up problem that means the chip would not work unless all the power was removed and then re applied. However this tends to happen only on more complex chips.

My advice would be to disconnect this supply as well.