Sure. The question is then "What's the chance a transistor will have
20x its expected gain?"
In general use I'd believe a 4:1 spread, when selected for optoisolators, I'd expect the spread to be a little tighter, and surely not even 10:1. What say you, o silicon sculpting sage?
Cheers, James Arthur
P.S. Inside info: actual CTR, measured in circuit, = 100%. ja
You are correct that it's VERY unlikely to have a problem. But, in my world, I NEVER take chances.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
Well I'm tempted to draw a fuse in there, but I guess I have no choice but to bow to the sage. By popular demand, just in case 20:1 isn't enough margin:
If my push-pull opto thing is done right, it will never smoke. And if you're really paranoid, change the LED driver circuit. It ain't rocket science.
I've used total-pole optos in linear mode too, as the output stage of a 400 volt p-p swing opamp; I posted that a while back.
Of course you do. It's just a matter of how many sigmas you're willing to pay for.
We often take chances, particularly in the performance-critical heart of a product. If a phemt is rated at 9 volts max, but tests ok to 25, we'll use it at 12. And often we test parts for behaviors that just aren't specified... we often know stuff about parts that the makers don't.
Extreme performance sometimes requires taking calculated risks. But there's no point in taking significant chances on the routine parts of a product.
I actually sat at a bench and measured some opto's CTR, I think one of the 4N series; anyway, I made a graph from about 0-20 mA, and was astonished when my graph turned out to be a straight line within a few percent. At a previous job, when they needed a linear transfer function, they used either TWO optoisolators, one for reference to linearize it, or they used a V-F converter, opto, and F-V converter.
I had intended to use it in a feedback loop for a design we were working on, but that project died on the vine - we couldn't afford it. I almost bankrupted the guy. =:-O
There are optos where one LED lights two detectors (e.g. Infineon's IL388). By closing a loop on one detector (on the isolated side) you assure linearity. The 2nd, matched detector provides the signal on the signal side.
Pretty good for phone couplers, but not much in the way of absolute accuracy.
Yep, so the one-LED-lighting-two-detectors strategy works well to fix that (aging, linearity, and LED drift). Matching the detectors, then, is the weak link. Works well for audio though--you often see the series I mentioned replacing transformers in modems.
I guess you could calibrate everything out--if you had a multiplexer and a reference on the isolated side--by cycling between signal, zero, and Vref, but with that much hardware you might as well digitize and be done with it.
At first glance I thought that might work, but I think the negative feedback will add Miller thru the opto NPN and keep it slow.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
I don't have the MOCD217, but SPICE simulation on the MOC223 shows tr/tf as about equal at 550ns. It easily handles 100KHz and he can back that input drive off to 0.5mA or so, 2mA is way too much, and there is very little danger of exceeding 100% CTR at that drive level. Yep, so NFB does make the transitions slower.
Inspired by Fred's use of simulation, I plugged an MOC207 into LTspice and found it surprisingly faithful in modelling my original Fig. 2 circuit.
So inspired, I explored some more.
Attractive as John's totem-pole idea was, I couldn't adjust things to jerk the opto output transistors 'off.' Maybe I missed something, but once they're saturated, they're stubborn.
Fred's idea of reducing LED current is a good one; I'd originally set the LED current a bit high for CTR and aging allowance, but I was too conservative. Lower LED current is better.
Here's a faster two-transistor version that minimizes the voltage swing across the opto's output transistor, and thus manages to pass
Wonder what might be done with those devices where the opto-transistor base is brought out?
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
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