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Optocoupler question: Why is my rise time so high?

Hello,

I have some (seemingly) simple optocouplers (datasheet:

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model HCPL-2631. They're acting funny, and I'm hoping that someone here can point out a basic mistake that I'm making in hooking them up.

These are open-collector optocouplers. The emitter inputs consist of the anode and cathode of an LED. The detector outputs are your standard (Power, ground, collector) trio.

Right now I have the chip on a breadboard. The LED cathode is connected to a 330 ohm resistor, which is also connected to ground. The LED anode is connected to a frequency generator that outputs a square wave at 5v. The open-collector output is pulled high via a

4.7K resistor, to 5v. Power and ground are connected to 5v and ground, respectively. There is only one ground in the circuit (i.e., the 330ohm resistor and the detector ground are on the same node).

The problem is the rise time to go from 0v to 5v; it's about 4 microseconds. The data sheet states that the rise time for these optocouplers should be in the tens of nanoseconds. The fall time from

5v to 0v meets this spec. I can't figure out why the rise time is so awful.

I thought it might be stray capacitance, but if that's the case, why is the fall time so low?

Any help appreciated!

Reply to
porkbyproduct
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I think this is almost totally a case of stray capacitance. The total capacitance on the collector pin (the transistor's internal collector capacitance, the pin to pin capacitance, the breadboard capacitance of that node, the scope probe capacitance [you are using a divide by 10 probe, I hope], etc.) are being charged in the positive direction by a 5 volt supply through 4.7k, so with about 1 milliamp peak current. When the transistor turns on, it is speced to dump at least 50 milliamps to ground, a very unsymmetrical situation. To prove that this is what is happening, reduce the pull up resistance to 2.2k and see if the rise time falls by half.

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Regards,

John Popelish
Reply to
John Popelish

Is this the

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type breadboard ??

With so few pins, you could solder wires to the opto pins and check if its the breadboard causing your problems, which I suspect it is.

donald

Reply to
donald

Yes, it's a similar style breadboard.

I actually put this breadboard circuit together after I integrated the optocouplers into my circuit, using solder and a protoboard. I observed the problem first on a soldered circuit, and then again in the breadboard circuit.

I have a big pile of 4.7K resistors lying around, and I thought, "Well, if they're good enough for I2C pullups..."

Reply to
porkbyproduct

So, parallel 2 or 3 of them.

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Cats, coffee, chocolate...vices to live by
Reply to
Ecnerwal

This was the problem. I dropped the pullup capacitor to 1K or so, and the rise time dropped accordingly.

Thanks!

Reply to
porkbyproduct

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