# Phototransistor question.

• posted

Is there a mathematical large-signal model for the behavior of a phototransistor optocoupler with the base terminal brought out and used simultaneously with the coupled LED, in any way comparable to the current-controlled or Ebers-Moll model of an ordinary BJT, for the purposes of circuit analysis?

• posted

Yes. Most SPICE models simulate the photocurrent as a current source (corresponding to the B-E photodiode), a big R || C (to simulate storage charge) and an average transistor (with fairly high beta ~800, which is actually typical).

4N25 models are all over the place, check out a few for details.

Tim

```--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms```
• posted

The generic Spice model simply uses a current source connected between collector and base of the transistor, and proportional to input LED current.

If you're trying for a more robust model, I'd suggest looking for a photo-diode model that includes voltage across that junction as a second variable.

But I don't know if such a model exists.

Phil Hobbs is a good source of photo-diode information ;-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
• posted

Hi Jim, thanks for the reply. I'm sorry I wasn't more clear - I guess what I had in mind was if there were any equivalent models for doing "pencil and paper" circuit anaylsis, rather than SPICE. I have been playing around with some weird phototransistor circuits in LTSpice, and while they simulate fine I can't seem to figure out an easy way to analyze their behavior on paper since both the base connection and the LED input are being used.

• posted

Probably just treat it as a current-gain device with a fixed Vbe. Then write the transient equations. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
• posted

Hmm, so from a current-gain perspective we have a photogenerated base current, say CTR/beta at the LED operating point, and via the external base connection one can either enhance or reduce that current +/- ib*beta, so Ic = beta(CTR/beta +/- ib) = CTR +/- beta*ib. That simple?

• posted

Should be adequate for most situations. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
• posted

It'll do for any situation where a phototransistor is appropriate! (How's that for damning with faint praise?)

Cheers

Phil Hobbs

(Phototransistors stink on ice.)

```--
Dr Philip C D Hobbs
Principal Consultant```
• posted

Well! They do pretty well for on/off mundane applications ;-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
• posted

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Phototransistors are slow, and they require external components for those "on/off mundane applications."

• posted

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Yes. Tell us all about what is required. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
• posted

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• posted

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This from the guy who despises beta-dependent circuits on principle? Phototransistors are pretty cheap, and they do respond to light, if slowly. They're just the ticket for figuring out whether your fridge light really goes off when you shut the door, and other high-tech jobs like that. For any actual measurement, a 5-cent photodiode and a quarter of a 6-cent LM324 will blow the doors off a phototransistor.

Cheers

Phil Hobbs

```--
Dr Philip C D Hobbs
Principal Consultant```
• posted

used

Bitrex was trying to evaluate something like a 4N25 opto-coupler (read back up the post) where the base comes out on a pin.

You're deviating the discussion off the path :-)

I don't know if I've ever seen a _discrete_ photo-transistor. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
• posted

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There are lots of them, for a few cents each. Octopart lets you search keywords as well as part numbers, so typing in "phototransistor" and searching on price pulls up a whole bunch.

Cheers

Phil Hobbs

```--
Dr Philip C D Hobbs
Principal Consultant```
• posted

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After I posted I remembered seeing some TO-18 devices with a lens; and also remember decapitating a TO-18 to let light in. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
• posted

On a sunny day (Fri, 22 Jun 2012 09:22:27 -0700) it happened Jim Thompson wrote in :

You could also scrape the black paint of a Philips OC13 or OC71. They sold the same version without paint as phot transistor I heard. Ge transistors :-)

• posted

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Reminds me that some telephone circuit I worked on years ago used a tubular optocoupler, with a neon bulb on one end and a CdS light- dependent resistor in a TO-5 can in the other end. (It must have been a ring detector.) I took it apart to make a darkness-operated lamp. The LDR looked like one of those here:

• posted

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I have rolled my own opto-isolator with an LED, a tight fitting tube (ABS), and a photo-diode, where I need multi-KV of isolation. PCB was even slotted to avoid creep. IIRC, medical application. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
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Only if you get it right, and be "discreet" ;-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |