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CIRCUIT DOESN'T WORK

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Russlk

New Member
I hope someone will have an idea why this circuit is not working, I am tearing my hair out. I built it on a pc board and it worked, but I wanted to make changes, so I built a new pc board but did not change this circuit. The first circuit below is what is on the pc board. I took the solid state relay off the pc board and put it on a test board (2nd circuit) and it worked OK. I put the relay back on the pc board with the 1K resistor to simulate the test circuit, but it still does not work.
 

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Optikon

New Member
Russlk said:
I hope someone will have an idea why this circuit is not working, I am tearing my hair out. I built it on a pc board and it worked, but I wanted to make changes, so I built a new pc board but did not change this circuit. The first circuit below is what is on the pc board. I took the solid state relay off the pc board and put it on a test board (2nd circuit) and it worked OK. I put the relay back on the pc board with the 1K resistor to simulate the test circuit, but it still does not work.
I understand your 3 circuits and the conditions make sense to me. What you are not clear on is where is your output?
Are you wanting to get a 2.5V waveform AC coupled to your capacitor?
Please clarify what you would like to have.
 

Russlk

New Member
Thanks for the reply. The signal input to pin 4 should appear at pin 3 when the switch is closed; it doesn't. The test circuit shows that the switch is working, the second test indicates that it is not working on the pc board. I have not found an open or short anywhere.
 

panic mode

Well-Known Member
What is the optocoupler part number? It looks like digital out
and maybe it is not suitable for switching low AC signals.
(What is the amplitude of the AC signal? Only 0.2Vpp?).
Also, what is impedance of the AC signal source? Maybe 1K
load is killing it, so try to replace it for bigger value, maybe 10
or 47k. What is impedance of the circuit that optocupler is
supposed to drive?
 

Russlk

New Member
The opto is Omron G3VM-S2. It has worked before, so there is something strange about this PC board that I don't understand. The input signal is from a LMC660 op amp, the load is 160K input resistor to a low pass filter.
 

Russlk

New Member
You may be interested to know that I built a piggy-back board with only the switch on it, tied it into the pc board, and it works. So I still don't know what is wrong with the PC board, continuity checks are ok and all the signals except the output are as expected. Since it is working with the piggy-back board, I will run it that way, glueing it in place.
 

shokjok

Member
The optos are similar to NTE3085. I haven't used these devices, thought the circuit looks simple and should work as described. What is the controller device driving the relay?
 

audioguru

Well-Known Member
Most Helpful Member
The opto IC is designed to switch hundreds of volts, not low level signals.
It has two identical Mosfets in series back-to-back. Each Mosfet has a diode in parallel to pass a reversed voltage.
When the signal is positive at pin 4 then the pin 4 Mosfet conducts and the diode at pin 3 conducts.
When the signal is negative at pin 4 then the diode at pin 4 conducts and the Mosfet at pin 3 conducts.

It conducts only if the voltage from pin 3 to pin 4 is about 0.7V or more.
But your signal amplitude is only 0.1V peak so the diodes do not conduct.
Of course it works when your signal is 5V.
 

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monty_mm

New Member
Hi there,
i'm looking for an optocoupler for my signal (0.2V pp) and then amplify it to input into a PIC. any suggestions? would appreciate your help.

thanks
 

audioguru

Well-Known Member
Most Helpful Member
The LED in an opto-coupler can have its current modulated by your signal.
A photo-transistor or photo-diode opto-coupler can be biased then used to optically receive your signal. Then you can amplify the signal.
 

Ubergeek63

Well-Known Member
The opto IC is designed to switch hundreds of volts, not low level signals.
It has two identical Mosfets in series back-to-back. Each Mosfet has a diode in parallel to pass a reversed voltage.
When the signal is positive at pin 4 then the pin 4 Mosfet conducts and the diode at pin 3 conducts.
When the signal is negative at pin 4 then the diode at pin 4 conducts and the Mosfet at pin 3 conducts.

It conducts only if the voltage from pin 3 to pin 4 is about 0.7V or more.
But your signal amplitude is only 0.1V peak so the diodes do not conduct.
Of course it works when your signal is 5V.
Actually both FETs conduct, you need two back to back to so you have the two substrate diodes back to back so you can block both polarities.
 

audioguru

Well-Known Member
Most Helpful Member
I tried a photo-FET in a Wien-bridge oscillator to control the amplitude. Contrary to what the text says about linearity, the distortion caused by the FET was horrible even when I attenuated the signal it passed to nearly nothing.
 
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