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Ed P asked How to control a large motor with an op amp?
I built a voltage comparator circuit that uses 4 aa batteries (6v dc) and want to use it to drive a 12v dc seat motor from a car. I plan to do this with relays but am unsure how to pick the correct relay. I tested the circuit with 5v relays that are good for low amperage motors such as hobby motors and the like but a big motor needs a big relay and I have NO clue how to trigger the big relay with my small amperage circuit. The circuit operation in short: Two potentiometers, and a set of light dependent resistors help an lm1458 dual op amp decide witch one of it's out puts to turn on (I plan on eventually using a comparator instead of an op-amp). The out put pins on the 1458 go through diodes to the bases of two 2n2222 npn transistors. the emitters and bases of these then control the 5v relays (yes I put protection diodes on the coil of the relays) which switch power from a separate 12v battery (at this point using 8 aa batteries) to run a hobby motor in either clock wise or counter clock wise direction. Any one care to help scale this thing up to do use-full work? What am I missing here?
And got the following answer:
You're not missing anything.. but be careful of one thing. If your op amp is driving the base of the transistor through a diode, then whenever the output from the op amp goes high, it is effectively shorted to ground through the diode and the base-emitter junction of the transistor.So, you want to put a resistor in series with the output of the op amp of a value large enough to limit the current flow so that you don't exceed the maximum output current of the op amp. The op amp data sheet should have that. Then again, many op amps have short circuit protected outputs, so you may be OK. With respect to scaling it up, separate two things. You want beefier relay contacts to handle higher currents, but that doesn't mean necessarily that you need a larger drive circuit for the relay coil. It all depends on the how the coil was designed. What you're interested in are the relay specifications called the pull-in voltage and the drop-out voltage. The pull in voltage is the minimum voltage that you need to apply to the relay coil to make it switch states. The drop out voltage is the maximum voltage at which the relay will switch back to its original state. If you know these voltages, and you know the coil resistance (also specified in the data sheet for the coil), you can calculate the current necessary to flow through the coil to make it switch. Then, look at the maximum collector current specification for the transistor you're using. If its larger than the current necessary to drive the coil, you're home free. Otherwise, you'll need to switch to a transistor with a higher collector current. The base current will be roughly 1/50th of the collector current (based on the current gain, hfe, of the transistor). You'll need to see if your op amp can drive that amount of current. If it can, again. you're good to go. Otherwise, you may need to add a second amplifier stage. Or, look into using a Darlington transistor. Its a two stage transistor in a single package, with higher gains. Another alternative is to use a solid state relay (www.digikey.com). These relays have high impedance inputs so they don't require much current to change state. Hope this helps, -Guru
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