Driving high voltage coils (and connecting to 7.2V

All, For my project I need to be able to drive several electro magnets (just large hand wound coils). I'm planning on using darlington arrays to sink the current through the coils.

I will also power my arduino pro (from sparkfun) with the 7.2V nicd battery pack.

I know that I need to place a 500uf capacitor across the battery and a diod to stop backflow so that the draw from the coils do not drop the logic supply too far. And I also know that the Arduino pro has a 5v regulator built in, but what I'm worried about is the current.

The question is this. What precautions in general do I need to take when connecting a 7.2V nicd pack (like the kind for remote cars) to the arduino? Do I need a current limiting resistor? I do not want to overheat the regulator. Can I connect it to the JST battery connector on the Arduino Pro's board? I want to make sure that I do this correctly. If you have any experience with this I would greatly appreciate any help.

Thanks in advance.

I know that I need to place a 500uf capacitor across the battery and a diod to stop backflow so that the draw from the coils do not drop the logic supply too far.

Well sorry this is wrong. Don't connect a capacitor directly across the battery. The diode will not do anything here. You need a diode across the coil to stop the back voltage generated when you turn it off.

Power the arduino through the Vin and the coils direct from the battery, switched from the darlington. However you only need a current limiter if the coil draws too much for the darlington and the battery. It is best if you put more turns on the coil rahter than limit the current.

The capacitor across the battery supplies voltage during heavy load on the battery. The diode prevents current from heading back towards the battery. Here is an example:

http://letsmakerobots.com/node/3880

In fact, this circuit already exists on the pro board, but the capacitor is insufficient. Please see the schematic:

http://arduino.cc/en/uploads/Main/Arduino-Pro-schematic.pdf

The darlington arrays already have backflow diods built in for the coils.

To ask my original question, do I need to take any precautions attaching the battery to this board? From what I'm seeing I do not. One site recommended an inline 1 amp slow burn fuse. Not sure that is absolutely necessary because the pro board has a resettable fuse already onboard.

The diode prevents current from heading back towards the battery.

Backwards current supplied to a battery is called charging the battery. So unless you plan backwards currents to such a time frame and magnitude level as to overcharge the battery I wouldn't be too concerned ;)

The voltage drop loss caused by the series diode robs some energy away from the total capacity also.

Lefty

The capacitor across the battery supplies voltage during heavy load on the battery.

The amount of power you are going to get from a 500uF capacitor is tiny compared to what the battery can supply. Even so the battery has to supply that power to the capacitor anyway. All it can do is supply extra current for a very short time, in the order of milliseconds.

While a large capacitor on a PSU output is not going to do much harm (except for a low drop out regulator where it can cause instability) it is not going to do you much good either. You would be just as well off with a 47uF.

The diode prevents current from heading back towards the battery

Why would it do that unless there was another voltage source in the circuit, connected to it, that was higher than your battery voltage? The example you posted the link to had two diodes in series to drop the voltage from the 7.2V battery to make an alleged 6V supply rail, it was nothing to do with backwards current flow.