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unfortunately there is a voltage drop for the SCR , as I used an opto with a single SCR , when I checked the open anode cathode voltage without any load it was about 5V instead of 12 volts, when I add a 10Ohms load between anode and cathode the voltage on the load was about 1 volt ( instead of 12 volts) I would be grateful if you help me. thank you in advance

enter image description here

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    \$\begingroup\$ Sorry, but your text is a mess with lack of capital letters and punctuation and your diagram is too. There is a built-in schematic tool with a button on the editor toolbar. Please fix if you wish us to take you seriously. \$\endgroup\$
    – Transistor
    Commented May 31, 2016 at 16:22
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    \$\begingroup\$ Is pin4 of the Opto connected to the cathode of the SCR anywhere? \$\endgroup\$
    – user16222
    Commented May 31, 2016 at 16:49
  • \$\begingroup\$ @JonRB yes , I have checked it not connected but still the problem has reminded \$\endgroup\$
    – ash
    Commented May 31, 2016 at 16:54
  • \$\begingroup\$ Do you have a question? \$\endgroup\$
    – Scott Seidman
    Commented May 31, 2016 at 16:56
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    \$\begingroup\$ @ashkansed an SCR requires a gate-cathode current loop to facilitate an anode-cathode conduction. If you gate circuit does not have a cathode connection it will not be controllable as expected (if at all) \$\endgroup\$
    – user16222
    Commented May 31, 2016 at 16:59

1 Answer 1

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There are a few problems with your schematic.

  1. There is no load on the SCR. When it turns on it will short-circuit your 12 V battery.
  2. There is no ground reference between the two circuits so there is no return path for the SCR trigger current.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. SCR trigger by opto-isolator.

  • Add in the red line shown in Figure 1.
  • Move the transistor to the +5 V. Now when the opto-isolator is turned on the SCR gate will be pulled high and it will turn on the SCR.

Trigger current and voltage

enter image description here

Figure 2. Trigger characteristics for the BT151-500R.

enter image description here

Figure 3. Vishay 4N32, 4N33 \$ V_{CE} \$ saturation voltage.

Since the emitter of Q1 may be as low as 4 V when turned on and the SCR gate trigger may be as high as 1.5 V that leaves us with 2.5 V to drive 15 mA into the SCR gate. \$R_{2max} = \frac {V}{I} = \frac {2.5}{1.5m} = 167~\Omega \$. Set R2 to 150 \$ \Omega \$ for reliable operation.

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  • \$\begingroup\$ Unless the opto is PNP, this won't necessarily work. The voltage at the emitter can only be as much as Vb - 0.7ish so you're not going to pull to 5V. \$\endgroup\$
    – Brendan Simpson
    Commented May 31, 2016 at 17:05
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    \$\begingroup\$ I don't know if \$ V_b \$ is relevant in an opto but you did force me to do another 15 minutes work to get the details for worst case. I've dropped R2 from 220 \$ \Omega \$ to 150. \$\endgroup\$
    – Transistor
    Commented May 31, 2016 at 17:52

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