#Optical isolator circuit how to
#Optical isolator circuit series

The easiest way to test performance is to connect a DVM between input and output. Adjusting R2 so that R1 + R2 ✴.7K causes the output of U2 to come close to the input voltage signal –further adjustment of R2 will trim for the difference in CTR so that the output voltage will exactly match the input voltage. At the same time the output of U3A presents a positive going signal to U2. When both op amp inputs are equal, the output of U1 stops integrating. U3-B turns on and provides a positive going feedback signal. At any rate, the LM358 is very inexpensive.Ī positive signal input causes the output of U1 to shift positive. There is the LM321 single op amp, but it is unpopular and comes only in the SO-23-5 SMD package. I do this because there is no DIP single device that will do the job (that I know of). I generally use the LM358 dual op amp whenever I need a single ground sensing op amp. To maintain a high voltage rating, use two single devices and match the CTR to some degree. Tease, teased –to aggravate –in electronics it can refer to deliberately injecting an electrical disturbance in order to cause circuit malfunction, or prove that it is robust.While DIP optical couplers have up to about 5kV isolation capability, this circuit cannot support above about 100V or so because one of the outputs is fed back to the input amplifier –this puts the voltage isolation between adjacent pins on the device rather than across the device. Undocumented words and idioms (for our ESL friends) I recommend a wall wart or other transformer isolated type for safety, isolation and convenience. Both devices cost less than $1.00 and are currently available from DigiKey. A better device is the Fairchild CNY17F4 that has a CTR range of 160 to 320%. It is possible for one not to function if the CTR falls too close to 100%. The 4N37 has a minimum CTR of 100% and maximum of 200%. Note that the optocouplers must have a CTR (current transfer ratio) greater than 100% or else the circuit cannot latch. This was solved via the addition of a 0.1uf capacitor across the LED. If the power to the circuit is ‘teased,’ the latch can automatically turn itself on due to transistor miller capacitance that causes it to conduct briefly when the collector voltage is rapidly increased-the led senses this and illuminates the phototransistor and the latch is ON. Testing indicated one unforeseen problem. Note that unlike relays, high power SSRs, seem to have a short product life thus precluding long-term availablility. A TRIAC version is also available, but I did not research this option. By turning on at zero-crossing, it tends to greatly reduce RF noise.

The particular isolator selected is of the zero voltage switching variety.

#Optical isolator circuit how to

This circuit is good for both AC and DC.įig 6 shows how to connect a high current solid state relay that uses anti-parallel SCRs. At 100mA, the voltage drop was 0.75V using a D42C8 transistor.įig 3 shows how to connect a non-isolated high-side driver.įig 4 shows how to connect a non-isolated low-side driver.įig 5 shows how to connect a solid state relay that uses high voltage MOSFETS.

The components indicated should be able to source or sink 500mA with a voltage drop of about 1V. See fig 2.įig 1 shows a 2nd optocoupler driving an NPN transistor. This suits it for more hazardous applications where the Stop function must always work. This is considered more secure because if a wire connecting the normally closed contact fell off, the circuit would automatically drop-something that cannot happen in the case of a normally open contact.

#Optical isolator circuit series

See fig 1įor those who need additional security, a normally closed Stop contact is placed in series with the LED. When the Stop contact closes, the current to the LED is shorted and the phototransistor drops out of conduction so that the circuit remains off. When the Run contact closes, it forces current through the optocoupler LED-the phototransistor turns on and completes the circuit even after the contact is released.