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open-collector configuration

Discussion in 'Embedded' started by roopashree, Jun 8, 2006.

1. roopashreeGuest

hi
im using an optocoupler which has an open-collector
transistor at the output. how should i calculate the output at the
collector. is it Vcc-IcRc=Vo or has it to be calculated by some other
method. The emitter is connected directly to the ground

What is an open-collector configuration and where and
when exactly has it to be used?

Thanks
Roopa

roopashree, Jun 8, 2006

2. Ian BellGuest

roopashree wrote:

> hi
> im using an optocoupler which has an open-collector
> transistor at the output. how should i calculate the output at the
> collector. is it Vcc-IcRc=Vo or has it to be calculated by some other
> method. The emitter is connected directly to the ground
>

You could calculate it that way but since you don't know Ic it is a but
hard. The optopcoupler is essentially a switch so its transistor is either
hard on (saturated) or off. The choice of resistor value depends on the
circuit the opto coupler is driving. It needs to be small enough to source
sufficient current when the transistor is off and high enough to ensure
the is not excessive dissipation in the opto coupler when its transistor is
hard on.

Ian

Ian Bell, Jun 8, 2006

3. Paul BurkeGuest

roopashree wrote:
> hi
> im using an optocoupler which has an open-collector
> transistor at the output. how should i calculate the output at the
> collector.

You'll find a parameter in the data sheet called CTR (current transfer
ratio). That basically tells you how the output current relates to the
drive. So calculate your diode current, multiply by the CTR to get the
collector current, and set the resistor such that the output (the
collector voltage) is low enough for your logic (+noise margin) when the
diode is driven.

> What is an open-collector configuration and where and
> when exactly has it to be used?

You weren't listening at school! It's just the bare collector of a
transistor, with the emitter grounded. You use it when you have to (as
in your optoisolator, or say driving an interrupt line), and it gives
the advantage that the outputs of several devices connected together,
with one shared pullup resistor, are ANDed (positive logic) or ORed
(negative logic- this is often used for shared interrupts, arbitration
schemes etc). Usually called "wired-OR".

Paul Burke

Paul Burke, Jun 8, 2006
4. Spehro PefhanyGuest

On 8 Jun 2006 00:05:11 -0700, the renowned "roopashree"
<> wrote:

>hi
> im using an optocoupler which has an open-collector
>transistor at the output. how should i calculate the output at the
>collector. is it Vcc-IcRc=Vo or has it to be calculated by some other
>method. The emitter is connected directly to the ground
>
> What is an open-collector configuration and where and
>when exactly has it to be used?
>
>Thanks
>Roopa

In the linear region, the static current is Ic = If * CTR, so the
output voltage is Vdd - (If * CTR * Rc), but that falls apart when the
Vce gets low. Also CTR is variable from unit-to-unit, with
temperature, and it decreases with time.

Most applications also have to consider speed- the higher the load
resistor, the slower the optocoupler. Most are spec'd with an
unrealistically low collector resistance (eg. 100R) and will be far
slower in a practical circuit.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com

Spehro Pefhany, Jun 8, 2006
5. CBFalconerGuest

Ian Bell wrote:
> roopashree wrote:
>
>> im using an optocoupler which has an open-collector
>> transistor at the output. how should i calculate the output at
>> the collector. is it Vcc-IcRc=Vo or has it to be calculated by
>> some other method. The emitter is connected directly to the ground

>
> You could calculate it that way but since you don't know Ic it is
> a but hard. The optopcoupler is essentially a switch so its
> transistor is either hard on (saturated) or off. The choice of
> resistor value depends on the circuit the opto coupler is driving.
> It needs to be small enough to source sufficient current when the
> transistor is off and high enough to ensure the is not excessive
> dissipation in the opto coupler when its transistor is hard on.

No, an opto-coupler is just another transistor that gets its base
drive from the incident light. It does not have to be saturated,
that depends on the actual light and the collector circuit.
(Saturation is when the collector voltage falls below the base
voltage) So you have to be careful with the collector circuit to
ensure that collector dissipation ratings can never be exceeded.
Collector dissipation will be maximum when one half the collector
supply is dissipated in the collector resistor (and the other half
in the collector junction).

--
"Our enemies are innovative and resourceful, and so are we.
They never stop thinking about new ways to harm our country
and our people, and neither do we." -- G. W. Bush.
"The people can always be brought to the bidding of the
leaders. All you have to do is tell them they are being
attacked and denounce the pacifists for lack of patriotism
and exposing the country to danger. It works the same way
in any country." --Hermann Goering.

CBFalconer, Jun 8, 2006