Driving optocouplers

It's a bipolar junction transistor, not a FET. "On resistance" isn't really a concept that applies -- figure that it'll pull down to about

0.2V if you're pumping enough current in.

You don't specify what your "high impedance" is, but it'll probably pull the enable input down low enough.

--
www.wescottdesign.com

On 6 Aug 2015, Tim Wescott wrote

Thanks Tim. Good to know.

How would I determine in future whether or not such a OC is appropriate? What?s the thought process?

Thanks again, Dave

?

Many of the OCs specify if they are TTL or CMOS compatible. That would help as long as you know the ON/OFF voltages for the devices you are trying to control.

Speed is next, are you passing data or just signal levels? Data would require (depending on the rate) a much faster OC than something that is just isolating a signal to a relay, etc. In other words, what is the clock speed and is Rise Time important?

Hope this helps...

John :-#)#

--
(Please post followups or tech inquiries to the USENET newsgroup) 
John's  Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9 
(604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games) 
                      www.flippers.com 
        "Old pinballers never die, they just flip out."

Some claimed here that the old 4N25 is slow, but still it has been used for video (5 MHz) isolation. You have to control the photo-transistor base in an proper bootstrap circuit.

The key thing is the minimum CTR = Current Transfer Ratio. That's the current you get through the transistor for a given LED current.

There are few phototransistors optoisolators worse than a 10-cent

4N25. It's 20% CTR but that's not taking temperature, different currents, aging or good saturation into account.

It's unwise to run them at much more than 10-15mA through the LED.

So let's say we run at 12mA LED current. That would give us 2mA with

10V across the phototransistor. It might drop 30% at high temperature, and maybe allow another 30% for aging, then halve that to have it decently saturated and we're left with about 0.5mA.

So if the pullup voltage is 5V it would work okay if the pullup resistor is 10K or greater. It will be able to switch at a couple kHz with a 10K resistor, less if the resistor is higher, but that's plenty for an enable signal.

A Fairchild FOD817B300 has 130% minimum CTR at 5mA LED current, so it could be run at lower LED current and would switch more. It also doesn't bring out the base connection, probably a good thing.

--sp

--
Best regards,  
Spehro Pefhany 
Amazon link for AoE 3rd Edition:            http://tinyurl.com/ntrpwu8 
Microchip link for 2015 Masters in Phoenix: http://tinyurl.com/l7g2k48

An SSR might be a better choice.

--

John Larkin         Highland Technology, Inc 
lunatic fringe electronics 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

A VFD (Variable Frequency Drive) is usually designed to be an industrial controller. It usually has isolated inputs. That is, the inputs for the enable function is usually passed through an optocoupler inside the VFD. Furthermore, those digital inputs usually accept dry switch contacts.

If the manual recommends otherwise then ignore my response.

That has been my limiated experance with the VFDs. Some of them have adapter boards that use optical isolators.

Ran into that problem when the company I worked for was in a big rebuilding project. The engineer had just used the solid state output of a PLC to turn on a VFD. The VFD wanted a hard contact, so I used an interposing relay as a quick fix as it was the weekend and needed to get it up and running.

It is really difficult to say the resistance of a solid state device is so many ohms. As pointed out is is the ammount of currrent it can sink and how low the voltage will go.

You are correct, AFAIK. The important thing is to know the input circuit of the VFD. Without that, you are forced to do tests. Which is what I would do.

150 mA (no

ms.

er or not

ll

look

I've used them for isolation of digital signals in the past. I did run into a problem where, even though it is a bipolar transistor, in a noisy environment there could be enough noise conducted in by the bonded out base of the transistor to cause the thing to turn on sporadically. For the speed of the signals I was interested in I moved to good old PC817 parts and haven't had any issues since.

Excessive photons flying around the area?

Jamie

max 150 mA (no

0 ohms.

ance

hether or not

s pull

ant look

Not that I know of, I think just good old near-field coupling or capacitive coupling to nearby traces. The output was acting like the opto was always in the linear region, where they needed off or saturated. Cutting the Base pin off corrected the issue, presumably because of a smaller antenna length or exposure to capacitance between it and other traces on the laminate. I didn't spend anymore time on it beyond changing the opto family and relaying the PCB. Mind you, this was 20 some years ago, and I was working for a different company, so all the details are a bit dragged through the filter of time.