Optocouplers

[FunkMasterBob]

Hi.

Now that that's out of the way, I need to hook up a dc motor to a sequencer. Its a simple situation- I just need to turn the thing on and off. No reverse directions or anything.

In the simulation I just use a logic gate connected to the led inputs of an optocoupler and it seems to be fine, but I'm afraid of the current in real life destroying the optocoupler. On the other side of the optocoupler I attach a 12V power supply to the collector (upper portion) and then the motor to the emitter (bottom) then to a resistor and then to ground and the negative side of the power supply.

I purchased two optocouplers, but neither seems to work. They may jsut be old and faulty, but now I'm unsure of myself. Does this circuit sound like it should work? The next step is buying brand new optocouplers from an internet source, and I don't wanna pay shipping only to find out my whole idea was dumb to begin with.

 

[FunkMasterBob]

oh, and this is for a project. That's all.

 

[KMoffett]

bob,
Optocouplers' output transistors cannot drive a motor. They are very low current devices. They could be used to drive a higher current bipolar junction transistor or MOSFET, that would drive the motor. What's the motor's current rating...or the resistance of the motor winding (assuming it is a 12VDC motor)?

Ken

 

[FunkMasterBob]

Unfortunately, I have no idea. I was givent his motor from a supply, with no information on it. I've run it at range from 5 V to 12V with no problems. I don't know the current rating either. When I attach it directly to the power supply. the supply says it's outputting around 200 mA. However, when I use a multimeter to attempt to read this current, it says 7 microAmps. Seems ridiculously too low.

Would you mind expounding on how I would determine the appropriate transistor or MOSFET to use?

 

[KMoffett]

The attached schematic should work...I think. Not sure if you have access to Radio Shack, but all the parts should be there. The diode is added to protect the Q1 form the motor spikes.

Ken

 

[FunkMasterBob]

Wow, thanks. Any transistor would work there, or is the 1N4001 specific to this sort of issue? Also, if i just needed an electronic switch to activate a buzzer, an optocoupler is good for that, right? Also, waht is the purpose of the contained resistors? I really would like to udnerstand the circuit, not just implement it.

 

[KMoffett]

Any transistor would work there, or is the 1N4001 specific to this sort of issue?

By "transistor" do you mean "diode" 1N4001...D1? Almost any diode would work...but I would suggest one in the with a voltage rating =/> 50V and a current rating of =/> 500mA.

Also, if i just needed an electronic switch to activate a buzzer, an optocoupler is good for that, right?

Again it would dependent on how much current the buzzer requires. By the way, what opto-coupler are you using?

Also, waht is the purpose of the contained resistors? I really would like to udnerstand the circuit, not just implement it.

R2 pulls the base of the PNP transistor high (to +12V) to assure thar the transistor is "off" and not susceptible to noise when the opto's transistor is off. R3 limits base current to an acceptable value when the opto's transistor is "on" essentially shorting its collector to ground. Google for: transistor tutorial.

Ken

 

[FunkMasterBob]

Well, by transistor I meant transistor, but by 1N4001 I meant MPS2907.

I don't know how much current the buzzer requires. Once again, no information and the multimeter I think has a lying problem. It's got a self-contained battery, so I have to jsut connect the two portals or whatever the word is. Man, I can't remember anything today. I was planning to use an optocoupler to do this (which worked in the simulation of course, but my confidence in that grows dimmer every moment.)

The optocoupler I have now is a MOC3023. But it doesn't seem to be working even with ttl logic voltages... so I'm open to ideas.

Ok, I think that transistor information is coming back a little. I used to understand mroe, but it was a rough understanding from the beginning.

 

[KMoffett]

.....Stop right there!...an MOC3023 is a "triac" output coupler. It "will not work" on a DC circuit. It will only work on AC circuits. You need a different type of photocoupler.

Ken

 

[FunkMasterBob]

So it's not the same as one that uses a transistor?

 

[KMoffett]

No! Do you have any other couplers?

ken

 

[FunkMasterBob]

Not with me, no. I'm going to have to purchase some others. Why doesn't the triac work?

 

[KMoffett]

A triac can be turned on with its terminals at either polarity, but won't turn off until its supply current is removed. With an AC supply this happens every half cycle...8.3 mSec. If you apply DC, once your LED in the coupler turns it on, it stays on.

Ken

 

[FunkMasterBob]

Well, it wasn't even doing that for me earlier. But I can't use it anyway with that information, so I'll have to pruchase another/

 

[FunkMasterBob]

Ok, so I've tested the circuit, and I think I understand how it works, but I have one issue. The motor does not stop when I "turn off" the transistor. A small voltage is leaking through. It's not enough to start the motor, but it's enough to keep it going. Also, the transistor I'm using (a BC640) gets incredibly hot. Is that going to be a problem?

 

[KMoffett]

Can you post a schematic of your circuit with the component values you are using?

Ken

 

[FunkMasterBob]

I am using the schematic you posted. Same values. 10 k from emitter to optocoupler, and 4.7 k from base to optocoupler.

I have played around with it some, and only succeeded in ruining two transistors and a few optocouplers. So I came back to the original design, so far it being the only working one, and have the same overheating problem, as well as one I did not notice before- the motor runs without the optocoupler being activated. And I can't seem to figure out why. Any ideas? Thanks for your help thus far. It has been incredibly valuable.

 

[ericgibbs]

I am using the schematic you posted. Same values. 10 k from emitter to optocoupler, and 4.7 k from base to optocoupler.

I have played around with it some, and only succeeded in ruining two transistors and a few optocouplers. So I came back to the original design, so far it being the only working one, and have the same overheating problem, as well as one I did not notice before- the motor runs without the optocoupler being activated. And I can't seem to figure out why. Any ideas? Thanks for your help thus far. It has been incredibly valuable.

hi,
If your motor draws 200mA, the base current to the transistor should be about 20mA.
With a 4.7K, you are driving the base with only about 2mA, the transistor will not saturate and will run hot.

What type of opto and transistor are you actually using.?

 

[FunkMasterBob]

The optocoupler is is an LTV827, and the transistor is a BC640. The opto is transistor based, (not triac) and the transistor is a PNP. I'm not sure what information you want specifically.

 

[ericgibbs]

The optocoupler is is an LTV827, and the transistor is a BC640. The opto is transistor based, (not triac) and the transistor is a PNP. I'm not sure what information you want specifically.

Hi,
I'll get the datasheets and look it over, get back to you.