hi, i want to design a mobo type bd to control something. the interface to out world, using opto coupler PC-817. its very cheap indeed, that i want to use it in many apps. the 1st thing come to my mind is, how come i use these isolation method, but using one power supply? as you know, i dont want to bother with two/more ps, due to the high cost.
Posted on 2003-09-23 06:59:17 by dion
well, the only occasion you'll use opto-coupler is when you have different power sources :)
it's useless to put an optocoupler in your schematic, but anyway you can use it in exacty the same way you used it in that keyboard
Posted on 2003-09-23 21:06:20 by Ultrano
You can simply use the same supply in design. The opto-coupler couples the 'information' across two branch circuits, yet electrically are issolated. If you have a surge you destroy the opto-coupler and/or place elevated voltages on the rail(s).

If your affraid of elevated voltages, then you can also place zeror diodes to clamp and shunt surges to ground.

:NaN:
Posted on 2003-09-24 17:00:41 by NaN
uhm.. so who is right here?
Posted on 2003-09-25 05:47:03 by dion
He :)
he said it in a more clear way.
Posted on 2003-09-25 12:04:02 by Ultrano
uhm... thanks guys :)
Posted on 2003-09-26 07:48:39 by dion
If you use the optocoupler as an interface between your circuit and another one to provide isolation, then in your circuit you have your P/S and the device you connect to has its own power supply, which you can use. This way you have two power supplies available, but you only provide one, the other one is "free".

There are situations where you need level shifting in your circuit, even though it only uses one power supply. The optocoupler comes in handy. It does not provide "isolation" (the stages share the power and ground), but it does transmit the info between two circuits which are at different potentials.
For example, you can drive the opto's LED from a uC (with respect to GND or Vcc), but its transistor is connected with the collector to Vcc and the emitor drives the load, or the emitor is grounded and the collector drives the load. The transistor is isolated, in fact you can do anything you want with it, including shorting out a part of the circuit where none of the connections is neither GND nor Vcc.
Just observe the polarity, C to "+", E to "-".

The only thing you need to pay attention to is their relatively large variations and dependance on temperature. Make sure you have enough LED drive current for the lowest gain device at the extremes of the temperature (generally at the lowest temp) and that the highest leakage (at highest temp)will not affect your circuit. Check the datasheet carefully for min/ max parameters and curves with their variation.
Posted on 2003-09-29 11:51:26 by VVV

If you use the optocoupler as an interface between your circuit and another one to provide isolation, then in your circuit you have your P/S and the device you connect to has its own power supply, which you can use. This way you have two power supplies available, but you only provide one, the other one is "free".


could you explain more what it mean with "free", VVV?

thanks
Posted on 2003-09-30 19:48:02 by dion
What I mean is that the unit you connect to has its own power supply which can be used on one side of the optocoupler. Since it exists, you do not have to provide it, it is "free".

For example, you build a circuit to connect to the OBDII on your car. The side of the optocoupler that is connected to the OBDII is powered from the car's battery. This is the "free" source. The other side of the optocoupler, which is connected to your circuit is powered from your circuit's battery. This is the source you have to provide.

Although this may seem like an unnecessary complication, it is good practice to have isolation between your circuit and one built by someone else (especially an expensive one) and only transmit DATA over the isolation barrier.
If something goes wrong in your circuit, the only thing seen by the other circuit will be bad data (that does not make sense), but the levels would still be correct, you would not cause any other problems.
Posted on 2003-10-01 11:30:45 by VVV

If something goes wrong in your circuit, the only thing seen by the other circuit will be bad data (that does not make sense), but the levels would still be correct, you would not cause any other problems.


i am thinking about related issue. the major problem with opto-coupler is, it is hard to inspect quickly if it is burned/bad or good. i just wonder if someone would make an opto-coupler with visual indicator (i.e. led) in both side, so, if something goes wrong, i can quickly noticed it by look on it :)
Posted on 2003-10-02 05:48:33 by dion
hello all, still curious on optos isolation.
i want to design a mobo type bd, to be used for redemption machines.
i was talking 'bout it because i want a perfect isolation to many "heavy" devices, such as motors, solenoids, etc. even in logic level, i ever come across a bad bd with 2 stage ( so each stage contain a chip), so, this make me think and care about isolation. as you guys note before, i insist to use opto-coupler, since i dont know if theres another component can be used here. then its became complicated, because you all said it cant be, because i just have one power supply. and i dont plan to use more. anyone could help me out?

thanks
Posted on 2003-11-27 02:22:53 by dion
I used to design vending machines and bill changers. I don't know what a "redemption" machine is. The problem is that it's been a long time so I don't remember everything.

We used heavy duty opto couplers for AC motors. IIRC, the inputs were simple logic while the outputs could handle the motors. These were cube shaped devices, maybe 2 inches by 2 inches and cost $5 or less. It was easy to tell if they were working or not by just measuring the voltage on the led side to see if it was on, then checking the AC side.

I don't recall if we also used DC motors but I'm sure we did.

One of the biggest problems was protecting the logic board from spikes when the solenoids and motors clicked on and off. In particular we used big caps on the digital ground lines to the chassis ground, IIRC.
Posted on 2003-11-28 10:08:29 by drhowarddrfine