Identical circuits working differently.  Board design issue? - Page 1
Forum section: electronics / beginners

author: Szewczykm
author_rank: Regular Contributor
timestamp: March 15, 2013, 04:36:42 pm
content: I have a circuit that involves a signal, resistors, optocoupler, decoupling caps, and 74hc4050 hex buffer.I have 5 input signals. I'm using a Lite-on LTV-844s for 4 of the signals and an LTV814s for the 5th signal.The only difference between the optocouplers is that one is a 4 channel, the other a single channel. All specs are the same otherwise.I've attached a sketch of the schematic.Like I said, the circuit is the same for all 5 channels. The only difference circuit-wise is that 4 channels are on one optocoupler and 1 channel is on the other smaller optocoupler.If I connect my signal to channels 1-4, I get a nice signal from the output of the 74HC4050.If I connect the exact same signal to channel 5, I get no output.I've checked all of the physical connections. They are all good. I get all the values I expect between points.If I change the resistor going to the input of the optocoupler on channel 5 from 4.7k to 2.2k, I then get output on Channel 5 through the 74hc4050.I'm perplexed as to why, when all other things are equal, I need to drop from 4.7k to 2.2k on the input to get an output.I have a theory. I'll link a picture of the board.You can see, the physical layout on the board is different. There is ground plain on top and bottom of the board. Am I getting capacitance from how I laid the board out? Why would the lower value resistor overcome it?I have so many questions about this. How should I test? I have a Rigol scope. Will putting a scope on there negate the built up capacitance at all if it's small? Did I violate a basic board design rule with having a ground plane on top and on bottom? Did I violate a rule by having that long trace running in parallel to the top ground plane? Am I being dumb thinking that the LTV844 is the same as the LTV814 except for channel count? Am I being dumb thinking that the capacitance would amount to anything that would have an affect on the circuit?Thanks for any input.Mike

author: AlfBaz
author_rank: Super Contributor
timestamp: March 15, 2013, 05:00:57 pm
content: Here's my guessAccording to the data sheet the current transfer ratio can be between 20 and 300%. Or for 1mA on the input you get anywhere between 0.2 and 3mA collector current in your output for a Vce of 5V.The 4 channel opto probably all have very similar CTR's as they are all produced in the same process on the same die, where as the single one will very likely have a completely different CTR

author: nessatse
author_rank: Regular Contributor
timestamp: March 15, 2013, 07:22:14 pm
content: What is your input signal level. I haven't looked at the datasheet but I expect you would need at least around 10ma to light up the led's. With a 4k7 resistor, that would mean something above 47V. You need to lower your input resistors to get decent current in.

author: Szewczykm
author_rank: Regular Contributor
timestamp: March 15, 2013, 07:27:44 pm
content: I'm reading up on that now. It's very interesting.What I'm using as an input signal varies. I'm pulling signals off of a pinball machine.Flashers = 20v DCInsert Light Matrix = 6.3v ACGeneral Illumination = 6.3v ACSwitch Matrix = 5v DCThe matrix is pretty noisy. I found that if I raised the resistance, it helped squelch out some of the noise. The input is ending up on a microcontroller so what I think I'm going to do is bring the "squelch" down to something like a 1k resistor on the input and then use software to ignore the rest of the noise.I thought that I'd need in excess of 10ma too but I've found that 1 or 2 ma is enough to get a signal. But like you said, I think I need to lower them to improve the current at 5v but try to keep the resistor high enough to not get blown out at 20vI was just leaving too small a margin of error using such a high valued resistor. It seems to make sense. I hope this works out in reality.Thanks!Mike