Hello PIC.ers, >Date: Tue, 26 May 1998 21:58:57 -0500 >From: David VanHorn >Subject: Re: Opto-isolator Question >> I think CP Claire (or something like that) has an opto designed >>for linear operation. As I recall, there might be two LEDs and two >>detectors that are matched so you can get rid of nonlinearities by >>putting stuff in an op-amp feedback loop. >I did this a while back, using a pair of optos, with the LEDs in series. >The idea is that the photocurrent in the "B" transistor is the same as in >the "A" >transistor, since the LEDs have the same current. It is important to feed >the >LEDs with current, rather than voltage. I expected it to be very finicky, >but in >>fact it was easy, stable, and very repeatable. No fancy pricey special >optos. >Four optos and a dual opamp, and I had a reasonable hybrid. .. ditto the above. I've been using this same dual-opto arrangement for many years in analogue circuits. Originally I used the TIL113 optocoupler, but later found that 4N25s were much better matched for characteristics. The TIL113s would only work if you started out with a box of them and swopped them about till the isolated feedback behaved acceptably. 4N25s don't need this, every installation works a coconut first time. I suppose others will be equally good or better, but I've had no need to try further. Best regards, John .. email from John Sanderson at JS Controls, PO Box 1887, Boksburg 1460, Rep. South Africa Manufacturer & purveyor of laboratory force testing apparatus and related products and services. Tel/fax: Johannesburg 893 4154 Cellphone 082 453 4815