# Why do relay contacts open when connecting a dc motor on the switch contacts?

The circuit simply goes like this:

The base of the transistor is a logic signal which came from an external circuit supplied by the same 6V source as that of the collector. The logic signal can either be 0 volts or 6 volts. The external circuit consists of a PIC and an op amp.

The 6V supply come from a 6V lead acid battery. Also, I used different voltage supply on the transistor and the dc motor, so I used two 6 Volts battery in all (including the external circuit), separately.

Without the 3 Volts dc motor, the relay switched properly when the input is logic 1 (6V), and closed properly when logic 0 (gnd). So, no problem with the relay.

The problem is: With the dc motor attached on the switch contact of the relay, and a logic-1 is applied on the input, the switch on the relay repeatedly opens and close (that is, the dc motor rotates and stops repeatedly).

I tried forcing the value of the input to 6V without the external circuit that produces it. The dc motor rotates perfectly fine in that case.

My first thought was that the battery is drained or absorbed (that is, the supply lowered down to 3 Volts when measured) because of the external circuit that produces the input to the base of the transistor. If I am correct with my assumption, how would I eliminate the absorption of the battery?

I tried replacing fully charged 6V lead acid battery but the problem remains the same. How would I solve this?

If you need more info, you may comment below. Sorry I may use wrong terms because I am not an expert on this field.

• Maybe (I'm arguing) the magnetic field generated by the motor is opening the relay; try to work on that Jan 24, 2012 at 15:49
• I also thought of that. But how would you eliminate the opening of the relay with the dc motor attached? Jan 24, 2012 at 15:52
• You can't separate them in any way? Jan 24, 2012 at 15:56
• For some reason, unfortunately no. I have to use relays no matter what. Jan 24, 2012 at 15:58
• No, I mean, can you put it a little bit farther? Jan 24, 2012 at 16:03

Your battery is not able to supply enough current to run the motor AND the relay AND everything else. This may be caused by an inadequate battery, or it may be caused by inadequately-heavy-gauge wiring between the battery, the motor, and the relay contacts.

With the relay open, everything is fine. When you key the transistor, current flows through the relay coil, the relay pulls in, closing the circuit to the motor. The motor then draws a huge current, which the battery can't supply. The battery voltage sags, current through the relay coil drops, and the relay drops out, cutting current to the motor. Battery voltage pops back up, the transistor is still keyed, current is restored through the relay coil, and the cycle repeats.

Some years ago, I watched a similar phenomenon on a test rig. We were using a very nice regulated power supply to control some solenoid-operated water jets. With all the jets enabled at once, the total load was more than the supply could handle, and it started pulsing like crazy, as the overcurrent protector kicked in and out. The fix was to energize no more than half of the jets simultaneously.

• But the supply on the relay and the dc motor is separate. Also, the dc motor is only 3 volts and I am powering it using 6 volts (if that matters). As stated, I have tried replacing the 2 batteries with new ones but still no improvements. What can you suggest? Using a higher rating battery may not seem to work. Jan 24, 2012 at 16:10

Probably electromagnetic interference from motor to remote equipment.