Opto Coupler 20V?

Hi,

I am almost done with my Arduino getting started kit, and the last task is to hack something else using the Arduino.

I have since the beginning had the ideer that I wanted the Arduino to control some slot cars. I have measured the slot car controller, and it is running 20V, and the controller is a variable resister.

My ideer was to exchange the controller for a Optocoupler controlled by the arduino. I suspect that I will need the two wires from the controller coming into the breadboard, og soldered directly to the optocoupler, but I suspect that I will also need a resister so I do not short the circut? Do I need a resister, and if so, how large?

Is it possible to do with a Optocouple like sugested in the book, and then control the speed using Duty cycles?

Will the optocouple be able to handle the 20V coming from the slot car track?

The one I have is a 4N35. I suspect it to be this one: http://www.vishay.com/docs/81181/4n35.pdf

I hope that you will be able to point me in the right direction.

Best Regards Brian.

No, a "regular" opto-coupler cannot provide enough current to directly power a slot car. If the existing controller is an active circuit (using a transistor or MOSFET) you also cannot replace a variable resistor with an opto-coupler.

[u]Here is an example of a MOSFET motor driver circuit[/u]. The example shows 12V, but 20V would also work.

...and then control the speed using Duty cycles?

Yes. You can use [u]PWM[/u] with the MOSFET (or a similar transistor circuit) to rapidly switch the voltage and vary the average current through the motor. This is more efficient (and easier) than controlling the voltage & current linearly (with a rheostat or variable resistor).

Thank you for your reply.

Will this IRF520 MOSFET be able to handle the load? http://www.jameco.com/1/1/465-irf520-transistor-n-channel-power-mosfet-100v-10a-220abfor-more-about-transistors.html

Also I want to understand the specifications on the Optocoupler it sais that the Collector emitter voltage is 70V, but the emitter base breakdown voltage is only 7V. What does that mean. 70V sounds like a lot, but what's with the 7V?

Please bear in mind that I am new to all of this, so you really have to speak down to me :)

The 7V Ube is the typical maximum reverse voltage of a bipolar silicon transistor BE diode, which behaves like a Z-diode. As long as the photo transistor is driven only by light, from the built-in LED, you’ll never have to worry about the BE diode (leave base open, if ever accessible on a pin).

Take care to use a logic-level FET, that fully opens and closes on Vcc (5V, 3.3V) and 0V gate voltage, as provided by a PWM pin.

DrDiettrich: The 7V Ube is the typical maximum reverse voltage of a bipolar silicon transistor BE diode, which behaves like a Z-diode. As long as the photo transistor is driven only by light, from the built-in LED, you'll never have to worry about the BE diode (leave base open, if ever accessible on a pin).

Is that when you use it for a motor, and it produces back current when you switch it off, but is it still running for a while?

If that is the case, and it support 70V, why is it no good for my 20V slot cars?

The transistor connected to the motor must be protected against back current, typically by a diode over the motor.

The optocoupler supports 70V but not enough current to drive a motor. That's why a power transistor is needed in any case, and you can steer it with or without use of an optocoupler.