enter image description here

As per the image, an operation for manual & Auto logic where manual can overwritten auto command and vice versa. The red circled area is an SCR(Silicon Controlled Rectifier). I'm looking for a SCR which can conduct at ZERO Gate Voltage(Vgt=0) i.e when the Mechanical switch is turned 'ON' the SCR turns 'ON' and it starts conducting 230V AC and when Gate Voltage is provided to the SCR,it turn 'OFF' the SCR and hence turns 'OFF' the load lamp.

I can't use a 'RELAY' or a 'TRANSISTOR' as the current flow in the circuit should be unidirectional.Also in case of 'RELAY' space constraint is an issue.

But either way if I can't find any SCR which conducts at 'Zero' Gate Voltage(Vgt) won't mind using transistor.

Any suggestion for an SCR which can conduct at 'ZERO' Gate voltage as mentioned in above explanation shown in below circuit.


As soon as the mechanical switch is closed micro controller gets the signal through the opto-coupler that the 'Load Lamp' is operated from then mechanical switch also the SCR above the 'Load Lamp' starts conducting for 'Zero' Gate Voltage now if one wants to switch off 'Load Lamp' from remote location it can be done through micro controller which triggers Gate Voltage to SCR. Similarly if one wants to turn 'ON or OFF' the 'Load Lamp' from the remote Location it can be done by triggering SCR located on left. Opto-coupler connected after the 'Load Lamp' is for feedback from the 'Load Lamp' to the micro controller.

Hence for the logic mentioned above one need an SCR connected between the mechanical switch and 'Load lamp' which can be operated at 'Zero' gate voltage

An Opto Coupler which can operate for 230V AC directly.

Part No of both Opto Coupler which can operate at 230V AC and SCR which can conduct at 'Zero' Gate Voltage?

  • \$\begingroup\$ Why don't you explain what functionality you want to expand the question to different and potentially more pleasing solutions. \$\endgroup\$
    – Andy aka
    Feb 12, 2016 at 8:54
  • \$\begingroup\$ Just have one SCR controlling the load. Feed your mechanical switch into the micro-controller and sort out the two-way logic in there. \$\endgroup\$
    – Transistor
    Feb 12, 2016 at 10:47
  • 1
    \$\begingroup\$ SCR "operated at 'Zero' gate voltage" sounds as though you are looking for a "normally closed" SCR. These don't exist, as far as I know. They all need a trigger to turn them on. \$\endgroup\$
    – Transistor
    Feb 12, 2016 at 10:50
  • \$\begingroup\$ @transistor I'm using mechanical switch as a backup,the area I live in has patchy internet connectivity hence 'Load Lamp' can't be controlled sometimes and as I want to keep Mechanical and Auto logic different hence if can't connect the controller directly to the mechanical switch \$\endgroup\$
    – ankur
    Feb 12, 2016 at 11:22
  • \$\begingroup\$ (1) This looks like the same question as your previous one which received three answers, none of which were accepted by you. (2) Are you aware that SCRs will only give you current on positive cycles only? (3) You can't keep mechanical logic separate. (4) Your micro will maintain state even if internet connectivity is lost. Therefore one of the solutions in the original question will satisfy your requirements. (5) Where did you get the term 'zero gate voltage'? \$\endgroup\$
    – Transistor
    Feb 12, 2016 at 13:30

2 Answers 2


You could consider using a MOSFET-output SSR with normally-closed output (also called "Form B"). They conduct AC (or DC) and can be controlled from a microcontroller directly with a single resistor, and also include isolation.

It's not clear to me what exactly your load is, but perhaps something like this which can switch up to 0.5A at 220VAC. Lower current capacity devices in smaller packages are available from various manufacturers.

enter image description here


You can do that, but not with a plain SCR. You need to use a GTO (Gate Turn-Off Device). They are expensive, and still require a positive trigger on the gate to turn them on, but they can also be turned off with a negative gate trigger.

They work, but do have issues, and the triggering of them can be touchy. Most actual GTO devices steal some of the forward current to assist in the turn-off function, some require you to supply the full turn-off current, which is usually about 1/4 of the actual forward current.

GTO technology still exists, but it is uncommon. It was one of the earlier methods of phase power control for AC Variable Frequency Drives.

  • \$\begingroup\$ @R Drast is there any other option that is cheaper?? \$\endgroup\$
    – ankur
    Feb 12, 2016 at 12:13
  • \$\begingroup\$ I would suggest just use a diode and a transistor honestly. Using an SCR, you are already dropping half of the AC wave. It depends on what you are doing. Saying "Manual/Auto" where either can be overwritten is basically saying "Three Way Switch Set". Most Hand-Off-Auto controls require selecting hand or auto to begin with. \$\endgroup\$
    – R Drast
    Feb 12, 2016 at 12:51

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