On 27.03.12 01:46, gene heskett wrote: > What do we normally assume to be the fwd drop of the usual LED, I assume > IR, opto-isolated input?
The good old 4N25 to 4N37 range is typically 1.18v at 10 mA, according to the datasheet, so a bit less than the 1.6v we're used to from red LEDs. (OK, I haven't measured one of those in 40 years, but that's still what I expect too.) > Setting up stuff for the 2M542's target 10 ma thru its input opto's, I come > up with about 360 ohms for the current limiter R when the 5 volts is 5.27, > and the diode drop is nominally 1.6 volts. 390 ohms is closer, if your opto is similar. (But what's a mA here or there.) > My old meter seems to have taken a hit, and my new one doesn't have the > beans to measure a diode with more than about .85 volts for its band gap, > so the diodes in the 2M542 are measuring either .75 volts if reverse > biased, or open when conventional 'led' bias polarity is applied. To say I > am a bit disappointed is an understatement, it is a $120 meter with all > sorts of bells and whistles! Relying on some in-built measuring set-up is OK if it works. Isn't it nearly as easy to wire up the 390 ohms, power up, then measure the voltage drop in-circuit? (So long as polarity is right ... but we've been there.) By the way, what's a 2M542? Erik -- The right half of the brain controls the left half of the body. This means that only left handed people are in their right mind. ------------------------------------------------------------------------------ This SF email is sponsosred by: Try Windows Azure free for 90 days Click Here http://p.sf.net/sfu/sfd2d-msazure _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
