# Decoupling issue with DC Motor?

I have the following circuit :

That's basically everything, excepted a button on DC1 to turn on/off the motor, but shouldn't impact.

The problem is, I was prototyping with the DC Motor on the LM7805 and the other parts powered by the +5V of the arduino I was using to program the ATTiny45. I assume the DC Motor resistance at "full speed" is $50\Omega$. (Around 0.1A at 5V).

Now I've moved everything on the LM7805 and when I turn on the motors the ATTiny shutdown after few seconds. The time he stays up depend on the value of RV1 who manage the speed of the motor, controlled by the attiny with the BC337 transistor controlled with PWM.

So, I started to use bigger caps but I can't stabalize the circuit. I've tried with 4700uF on C1/C2/C3 and the improvement is not soo great, a bit better but still shutdown pretty quickly. They are placed as close as possible to "their pins".

So, what can I do to leave the motor at that place ? Put it on a separated LM7805 ? Add a capacitor somewhere ? (I really need to put it on a voltage regulator)

• Can you use a separate voltage regulator for the motor? Jan 13, 2014 at 8:33
• @alexan_e : As mentioned in the question I was considering that, but I would prefer to do it in another way (if it's possible) Jan 13, 2014 at 8:37
• As a general rule, if at all possible you should attempt to run the motor without a (linear) voltage regulator: As the motor current rises, so does the heat generated at the linear regulator. Next, the motor will draw maximum current not at full speed, but when stalled i.e. not able to move. At full speed it will draw the minimum current. Also, there doesn't seem to be a decoupling cap across the motor itself - Add one 10 uF capacitor between the motor terminals, and consider adding two more, one from each motor terminal to the ground rail - make sure caps are soldered close to the terminals. Jan 13, 2014 at 8:57
• @AnindoGhosh : Damn, caps on the motor terminals... I forgot them and that solve the issue... (The most annoying is that I've done that in my previous project...) Time to rest a bit. Thank you, you can post it as answer if you want :) Jan 13, 2014 at 9:04
• @EmmanuelIstace Either drive them at a lower duty cycle using PWM, use a current-limited drive rather than voltage-limited, use a motor rated for the available raw voltage, or (least preferable) use a switching regulator instead of a linear one. Jan 13, 2014 at 9:46

• The suggestion included a third cap which is not swown in the schematic, 10 uF capacitor between the motor terminals, across the motor itself Jan 13, 2014 at 9:27