Optocoupler datasheet response time - fixing Vce, Ic, and Rl simultaneously? - Page 1
Forum section: electronics / beginners

author: tinfever
author_rank: Regular Contributor
timestamp: June 30, 2023, 08:32:51 pm
content: In the datasheet for many optocouplers like an LTV-816 or PC817, in the test for response time, they specify the test conditions such as Vce=2V, Ic=2mA, Rl=100 Ohm.How is it possible to fix all three of those? If the Rl 100 Ohm resistor is just pulled up to a 2V source to get the Vce = 2V, won't IC be (2V-0.2V Vce-sat)/100R = 18mA?I guess you could use a current source but they don't show that in their test circuit. You could adjust the input forward current to limit Ic to 2mA, but then the output transistor isn't saturated so the output only swings between 1.8V-2V? This seems like the most likely since they don't actually specify what Rd on the input is, but it also seems like a kind of useless number then?

author: gnuarm
author_rank: Super Contributor
timestamp: June 30, 2023, 09:58:44 pm
content: Quote from: tinfever on June 30, 2023, 08:32:51 pmI'm not sure, but I would expect the current spec to be on the input current at the LED. This is normally specified in several ways, but to characterize the output, this input current must be specified, unless it is simply assumed to meet the input specs. But Ic would be the current on the collector, so, who knows?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 30, 2023, 10:39:57 pm
content: Double-check your assumptions. You use Vce(sat), but is I_c high enough to cause that condition?I_c is simply set low enough that the voltage drop across the resistor is small. 2mA * 100Ω = 200mV so the Vcc might be 2.2V. I_f will be small as well (probably a bit more since CTR is so low at this current, maybe 2-4mA, also depending on grade).And, note that the output amplitude is very small -- 200mV evidently. You need a comparator to turn this into a real logic-level signal; or a cascode to increase the voltage swing. Of course the cascode won't furnish a logic-level output, but it can drive another BJT with reasonable speed, or if a bit more Vce is tolerable, a folded cascode could be used, for logic-level output without a comparator per se.Tim

author: tinfever
author_rank: Regular Contributor
timestamp: July 01, 2023, 02:20:56 am
content: Thank you. I guess I incorrectly assumed V_ce of 2V meant, 2V high, 0V low on that waveform they show. Not 2.2V high 2V low. Seems a bit sneaky to me.If you designed something that operated like that, by adjusting the forward current to limit I_c and keep the output transistor from going in to saturation to get the faster response time, I'm guessing that would be highly dependent on the CTR, and so would vary a ton part to part?

author: MrAl
author_rank: Super Contributor
timestamp: July 01, 2023, 04:29:28 am
content: Quote from: tinfever on June 30, 2023, 08:32:51 pmHello there,It has been a long time since i worked with an opto coupler like the 4N35 for example but it may be the mistake here is in the interpretation of what the numbers 2v and 2ma actually refer too. They do not have to be taken together simultaneously.In other words, 2v when the device is 'off', and 2ma when the device is 'on'. That's still a little strange though but many of these devices are spec'd in a strange way. That would mean Vcc=2v and with 100 Ohms that would also mean that the transistor, when 'on', would look like a 900 Ohm resistor. In turn that might imply that they assume the designer would follow with an amplification stage.As i said it has been a long time since i had to use one but i remember it was better to test some of them rather than depend on the data sheet. The 4N35 is about the same way, except you get a 5th lead where you can place a resistor from the transistor emitter to ground to speed up the total turn off time. The turn off time on these is pretty crappy so if you connect a resistor from base to ground it gets better. The downside is that the CTR suffers because you are effectively shunting some of the photo current to ground, but the upside is the increase in switching frequency.I used this for an RS232 isolation circuit a long time ago and i found that the baud rate would be too low without that resistor to ground. I also remember that there was no data on that to be found so i had to do my own testing over several units.Sometimes i wonder if they are afraid to publish a better test setup because then the public would be able to see how poor some of them are.Try interpreting the 2v and 2ma as above and test a few and see what you get. Then try some better values like 5v and 1ma or similar.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 01, 2023, 04:44:03 am
content: Quote from: tinfever on July 01, 2023, 02:20:56 amSomewhat. The other thing you get speed from is keeping it biased. The delay is basically due to B-E capacitance, and the B-E junction is the photosensitive part. That is, the equivalent circuit is like a photodiode in parallel with a BJT (B-E). When it's dark, B-E discharges towards zero, and when lit after being dark, it has to charge up to Vbe (even fairly low Vbe since we're talking relatively small Ic here), which adds a significant hard delay (i.e. no output change until after it charges up). If you keep it biased rather than completely off, or use a small-signal receiver, it can be a bit better.By "small signal receiver", I mean, the opto's output voltage might be say 100 to 600mV, with an amplitude of 50-200mV. If you have an AC signal, i.e. the logic level is changing frequently, with some maximum high/low pulse rate guaranteed by line coding or nature of the data, then a peak-to-peak detector obtains the logic-high and logic-low levels and simply a divider halfway between, and comparator, recovers logic-level data.But you getmuch morespeed by just using a better device, like 6N136 (or better yet SFH6345), 6N137 or other logic-type optos, or other digital isolators (usually monolithic capacitor- or transformer-coupled). The latter are available to 50Mbps and higher, and don't cost much more than optos do (indeed they may be less when considering the multichannel types).Tim

author: Terry Bites
author_rank: Super Contributor
timestamp: July 03, 2023, 06:50:33 am
content: Its not representative of a practical opto circuit. A very fishy bit of speccing.

author: gnuarm
author_rank: Super Contributor
timestamp: July 03, 2023, 07:09:21 am
content: Quote from: Terry Bites on July 03, 2023, 06:50:33 amTypically, a data sheet is about how to use a part. But for some parts, which are less complex, the data sheet is only telling you how they are tested. You are on your own to figure out how to use it.