optocoupler trouble

T

ted

Guest
I'm getting strange results from an optocoupler (aka optoisolator, and
this is the kind with transistor output). In brief: nothing I do
seems to make the transistor "conduct", except very feebly.

Here's my setup:

Diode anode: 0V/1.07V, 0.4mA
Diode cathode: ground

Transistor collector: connected to a 330 Ohm resistor which is
connected to 4.92V.
Transistor emitter: ground
Transistor base: open

When the diode anode is at 0V, the transistor collector is, as one
would expect, at 4.92V.
But when I give the diode anode 1.07V, the transistor collector drops
to just 4.88V. I want it to go down to 0V and I don't understand why
that's not happening.
 
ted wrote:
I'm getting strange results from an optocoupler (aka optoisolator, and
this is the kind with transistor output). In brief: nothing I do
seems to make the transistor "conduct", except very feebly.

Here's my setup:

Diode anode: 0V/1.07V, 0.4mA
Diode cathode: ground

Transistor collector: connected to a 330 Ohm resistor which is
connected to 4.92V.
Transistor emitter: ground
Transistor base: open

When the diode anode is at 0V, the transistor collector is, as one
would expect, at 4.92V.
But when I give the diode anode 1.07V, the transistor collector drops
to just 4.88V. I want it to go down to 0V and I don't understand why
that's not happening.
Click to expand...
How much current to the diode? Applying a fixed voltage to a diode will
get you a wide range of currents.

0.4mA in the diode is a recipe for minuscule currents in the transistor.
What does the optocoupler data sheet say? You need about 15mA to pull
the collector down close to zero (you won't get it all the way), that's
a lot for an optocoupler with a plain transistor.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
ted wrote:
Thanks. I'm going to try answering your question about the datasheet,
though I'm not quite sure what I'm looking for. Is it the "DC Current
Transfer Ratio"? For that it says 20%, at I_F=10mA and V_CE=10V.
Click to expand...
Yes. That says that the collector current will be 1/5 of the diode current.

You should check the maximum diode current, too -- that 10mA is
certainly suggestive of what you ought to be able to do. Figure 10mA
in, 2mA out, means you'll need to use at least a 2500 ohm resistor to
pull the collector all the way down.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
ted wrote:

I'm getting strange results from an optocoupler (aka optoisolator, and
this is the kind with transistor output). In brief: nothing I do
seems to make the transistor "conduct", except very feebly.

Here's my setup:

Diode anode: 0V/1.07V, 0.4mA
Click to expand...
You're only putting 400uA through it ?

There's your problem. Try something more like 10-20 mA depending on the
precise device.

Graham
 
ted wrote:

Transistor collector: connected to a 330 Ohm resistor which is
connected to 4.92V.
Click to expand...
330 ohms ? You're hoping for high speed operation are you ?

If you don't need that use 4k7.

Graham
 
ted wrote:

Thanks. I'm going to try answering your question about the datasheet,
though I'm not quite sure what I'm looking for. Is it the "DC Current
Transfer Ratio"? For that it says 20%, at I_F=10mA and V_CE=10V.
Click to expand...
It means if you put 10mA into the LED the transistor will conduct 2mA with
its collector-emitter voltage held at 10V.

It also means (assuming the CTR hasn't wildly dropped off) if you put
400uA into the LED the transistor will conduct 80uA.

Why are you using such a low CTR ?

Graham
 
Tim Wescott wrote:

ted wrote:
Thanks. I'm going to try answering your question about the datasheet,
though I'm not quite sure what I'm looking for. Is it the "DC Current
Transfer Ratio"? For that it says 20%, at I_F=10mA and V_CE=10V.

Yes. That says that the collector current will be 1/5 of the diode current.

You should check the maximum diode current, too -- that 10mA is
certainly suggestive of what you ought to be able to do. Figure 10mA
in, 2mA out, means you'll need to use at least a 2500 ohm resistor to
pull the collector all the way down.
Click to expand...
With 10mA LED current NOT 400uA.

Graham
 
Thanks. I'm going to try answering your question about the datasheet,
though I'm not quite sure what I'm looking for. Is it the "DC Current
Transfer Ratio"? For that it says 20%, at I_F=10mA and V_CE=10V.
 
You should check the maximum diode current, too -- that 10mA is
certainly suggestive of what you ought to be able to do. Figure 10mA
in, 2mA out, means you'll need to use at least a 2500 ohm resistor to
pull the collector all the way down.
Click to expand...
Thanks, and would you please explain why 2500 ohm is the correct
resistance, indeed why the resistance at that point makes any
difference to what the voltage is going to be at the collector pin?
(I was under the impression that when the transistor is "on", it has
no internal resistance...)
 
ted wrote:

You should check the maximum diode current, too -- that 10mA is
certainly suggestive of what you ought to be able to do. Figure 10mA
in, 2mA out, means you'll need to use at least a 2500 ohm resistor to
pull the collector all the way down.

Thanks, and would you please explain why 2500 ohm is the correct
resistance, indeed why the resistance at that point makes any
difference to what the voltage is going to be at the collector pin?
(I was under the impression that when the transistor is "on", it has
no internal resistance...)
Click to expand...
No, it is NOT 'ON' or 'OFF' as in infinite or zero ohms.

A transistor will conduct a CURRENT determined by its input drive (in this
case the LED current).

This is fundamental basics and you clearly need to read up about how
transistors operate and how to use them.

Graham
 
Wim Lewis wrote:

" In between "on" and "off", the transistor is in
what's called the "active" region, and it acts like a controllable
current-limiter.
Click to expand...
Like a 'transfer resistor' in fact from which the word trans_istor is derived.

Transistors were not originally designed as switches but as linear devices.

Graham
 
ted wrote:
You should check the maximum diode current, too -- that 10mA is
certainly suggestive of what you ought to be able to do. Figure 10mA
in, 2mA out, means you'll need to use at least a 2500 ohm resistor to
pull the collector all the way down.

Thanks, and would you please explain why 2500 ohm is the correct
resistance, indeed why the resistance at that point makes any
difference to what the voltage is going to be at the collector pin?
(I was under the impression that when the transistor is "on", it has
no internal resistance...)
Click to expand...
Because with a 0.2 current transfer ratio and 10mA to the diode, the
transistor will only sink 2mA. If you want to drop 5V, you need at
least 2500 ohms of resistance to do it.

As pointed out elsewhere, transistors are never really "on" in every
sense. They are only fully on in the sense that they are voltage
limited, which depends on the circuit they're in. In this state any
more drive to the transistor will not result in any more voltage drop at
it's output -- but in your case that means you can't ask the transistor
to sink more than 2mA.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
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