Let's Make Robots!

Powering up DAGU 4 channel motor controller and logic at same time - Problem?

I have wired up my Dagu motor controller to the same switch that gives power to the logic (ie Arduino board) - they are on different circuits though...

The DAGU instructions say to power up the logic first:

"The motor power supply should not be connected without first connecting the +5V for logic."

Is this (ie powering them both at the same time, or the logic slightly delayed - it goes via the Arduino board regulator) going to be a problem?   Why does the logic need to be powered first?

Also, if it *is* going to be a problem is there a simple fix so I can keep one on off switch?

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What would a 7.4 lipo battery do to the controller?


It doesn't matter what battery you use. The important thing is you should never have power on the "H" bridge unless the control logic has power.

Most robots have 1 battery supplying power for motors and logic so this is not a problem. When you turn the switch on everything gets power at once. If you have separate batteries and switches then the controller needs to get it's 5V supply from a regulator powered by the motor battery.


Oh okay, because my controller fried the other day so i thought that it may be because of the extra 0.2 volts. Stupidly, however, i'd sometimes switch off the logic and forget to remove the power from H-bridge. Also because I was using two different batteries there was no common ground either. I'm just suprised that it did not destroy my controller instantaneously but happened after a few tests with my robot..

My personal opinion is that you should never use two batteries on a robot. Your logic should always derive it's power from the same battery that drives your motors using a regulator or in some cases a DC-DC converter.

Aside from forgetting to use a common ground, if your battery supplying the logic goes flat when the battery supplying the motors is still good then there is a good chance your H bridge control logic will fail and smoke the H bridge.

deleted - moved to reply on comment

Usually the 5V supply is derived from the batteries that drive the motor so they both come on at pretty much the same time and it isn't a problem. One switch is all that's needed.

If your motor power is completely seperate from your 5V supply (they must still share a common ground) then you should power your logic first to ensure that your motors do not turn on unexpectedly.


Thanks -

I have wired a 9v battery to a 2.1 mm plug that powers the arduino.  The 5v output from the arduino is connected to the logic terminals on the rover.  I have a 6x 1.2V NiMH battery pack which is connected to the Rover's motor power terminals.  

To be honest, it never occurred to me to wire them both from motor power (!)

Would it really make a difference if the Arduino has a separate power supply?  Mechanically both circuits are closed/opened by the single switch.  (I tried to decipher single/double pole/throw but couldn't)

Is the only issue that the motors might turn on?  Could I deal with that by expressly set the motors pwm to 0 in setup()?  It's not like I'm going to get smoke?

In re common ground: I assume this is what the gnd pins next to the pwm pins are for? Or is there something else I have to wire?

When you are using multiple batteries then the grounds of each battery should be joined together with the positive terminals being switched. The reason for joining the grounds is that when your controller sends a control signal to a device that is powered from a seperate supply then it needs a return path.

All processor boards, motor controllers etc. are designed for a common ground.

It is unlikely you would get smoke with no power to the logic as the FET's gates are pulled up/down as required to prevent them turning on accidentally. The logic chips would have to malfunction as a result of a static discharge to get smoke.

Some people seem to prefer separate batteries for logic and motors to avoid electrical noise. This is a simple but ultimately expensive and clumbsy way of solving the problem.

You are better off using a single, good quality battery that is powerful enough to drive your motors easily. The use of noise suppression capacitors, twisted motor cables and sheilded sensor wires will take care of the rest.


Admittedly, it looks like when I changed a pin I knocked out the logic power wire out of the terminal.

Now I'm not game to power it up