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The SSR is a PhotoMOS AQY211EHAX rated to drive 1 amp. I seem to be shorting the output side of the SSR.

Is a pull down resistor normally required on the output side or am I missing something fundamental?

SSR Photomos driving LED

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  • \$\begingroup\$ Has the LED ever lit up?\ \$\endgroup\$ – Voltage Spike Jul 9 '19 at 19:32
  • \$\begingroup\$ "I seem to be shorting the output side of the SSR" Is that a guess? is there some evidence of this that you have not shared? \$\endgroup\$ – scorpdaddy Jul 9 '19 at 19:53
  • \$\begingroup\$ Current draw on the power supply goes far higher than it should and stops when I disconnect the output. \$\endgroup\$ – Ryan Mills Jul 9 '19 at 19:57
  • \$\begingroup\$ As well as being bereft of an output resistor, you're applying excessive current to the input LED. Absolute maximum is 50mA and you're giving it about 65mA. \$\endgroup\$ – Spehro Pefhany Jul 9 '19 at 21:37
  • \$\begingroup\$ Please post a link to the datasheet for the LED. \$\endgroup\$ – Transistor Jul 9 '19 at 21:41
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You still need a current limiting resistor on the LED. There is nothing limiting this current through the LED and so the output appears to be shorted.

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  • \$\begingroup\$ This is probably me not understanding this correctly. If the source voltage is 3 volts and the LED is 3 volts how would you select the correct resistor on the output? I assume that resistor would go between pin 3 and the LED? Or are we talking a pull down resistor like one between pin 4 and ground? \$\endgroup\$ – Ryan Mills Jul 9 '19 at 19:50
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    \$\begingroup\$ Each LED has a voltage drop that isn't completely constant, but is usually around 2V. You might be able to find the approximate voltage drop in the datasheet. Assuming that it is 2V you need the resistor to drop the other 1V (for a total of 3V). 5mA is plenty of current to light most LEDs so I would pick the resistance to be 1V/10mA = 200R. Putting it between pin 3 and the LED would work, but it could go after the LED too. \$\endgroup\$ – Hunter Jul 9 '19 at 20:11
  • \$\begingroup\$ Thank you, I still don't grasp why the resistors are required but I think that's out side the scope of this question. Adding the current limiting resistor solved the issue. \$\endgroup\$ – Ryan Mills Jul 9 '19 at 22:34

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