optocoupler trouble
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Posted by ted on May 30, 2008, 3:45 pm
Please log in for more thread options 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. | ||||||||||||||||||||||||||||
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Posted by Tim Wescott on May 30, 2008, 3:53 pm
Please log in for more thread options 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 | ||||||||||||||||||||||||||||
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Posted by ted on May 30, 2008, 4:06 pm
Please log in for more thread options 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. | ||||||||||||||||||||||||||||
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Posted by Tim Wescott on May 30, 2008, 4:27 pm
Please log in for more thread options 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. -- 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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Posted by Eeyore on May 30, 2008, 4:57 pm
Please log in for more thread options
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. With 10mA LED current NOT 400uA. Graham | ||||||||||||||||||||||||||||
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> 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.