Optocoupler Latch Circuit

Few realize how close an optical coupler resembles a relay. It is so close, in fact, that one can be wired as a Run-Stop latch circuit. One advantage over the relay is that it may be turned off via a normally open contact. Isolated outputs with an isolation voltage range of 1000 to 5000V can be provided simply by adding additional optical couplers. Isolated power outputs can be provided via adding similar SSS or SSR devices (solid state switch or relay devices). Non-isolated outputs may be added simply by adding a transistor. All quite handy.

Optocoupler Latch Schematics

Circuit function

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. 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. See fig 1

For those who need additional security, a normally closed Stop contact is placed in series with the LED. 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. This suits it for more hazardous applications where the Stop function must always work. See fig 2.

Fig 1 shows a 2nd optocoupler driving an NPN transistor. The components indicated should be able to source or sink 500mA with a voltage drop of about 1V. I was able to test mine at 100mA. At 100mA, the voltage drop was 0.75V using a D42C8 transistor.

Fig 3 shows how to connect a non-isolated high-side driver.

Fig 4 shows how to connect a non-isolated low-side driver.

Fig 5 shows how to connect a solid state relay that uses high voltage MOSFETS. This circuit is good for both AC and DC.

Fig 6 shows how to connect a high current solid state relay that uses anti-parallel SCRs. The particular isolator selected is of the zero voltage switching variety. By turning on at zero-crossing, it tends to greatly reduce RF noise. A TRIAC version is also available, but I did not research this option.

Note that unlike relays, high power SSRs, seem to have a short product life thus precluding long-term availablility.


Testing indicated one unforeseen problem. 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. This was solved via the addition of a 0.1uf capacitor across the LED.

Recommended optocouplers

Note that the optocouplers must have a CTR (current transfer ratio) greater than 100% or else the circuit cannot latch. The 4N37 has a minimum CTR of 100% and maximum of 200%. It is possible for one not to function if the CTR falls too close to 100%. A better device is the Fairchild CNY17F4 that has a CTR range of 160 to 320%. Both devices cost less than $1.00 and are currently available from DigiKey.

12V Power source

I recommend a wall wart or other transformer isolated type for safety, isolation and convenience.


Optocouple breadboard

Undocumented words and idioms (for our ESL friends)

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. One such test I used to perform involved rapid switching of a sewing machine motor power that tended to drive circuits crazy-if it passed this test, it was considered GOOD-if not, it did not prove anything-LOL

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