Using a BC546 NPN Transistor to trigger an SSR from a current sensor. Sensor to give 1.5-2.7 V out to drive the transistor. SSR requires >= 3 V and uses up to 10 mA.

I want the transistor to fully trigger the SSR (V>3, I=10mA) when a current flows through the sensor. With R2 = 470 Ohms and best sensor settings (D0) I can get:

Sensor off: V(CE) = 4.57, D0 = 0.29, V(BE) = 0.29

Sensor on: V(CE) = 2.4, D0 = 1.4, V(BE) = 0.62

Sensor on not connected to transistor has D0 = 2.7V

I'm getting 2.5 V(CE) across my transistor, which should have less than 1 V in saturation according to my reading of the datasheet.

I've attempted R2 = between 400-4000 with the SSR only partially operating.

Here's my schematic:


R1 is a possibility for a constant current circuit, which I don't fully understand (currently shorted/not used).

  • 4
    \$\begingroup\$ Maybe you should attach a schematics of how do you connect things. Just in case... \$\endgroup\$ Commented May 18, 2018 at 5:22
  • \$\begingroup\$ Are you sure about those voltages? Are you sure the sensor is capable of providing such current? \$\endgroup\$ Commented May 18, 2018 at 5:22
  • \$\begingroup\$ 2.5V Vce and 5V over the SSR adds up to 7.5V. The SSR is specified as 12V 7.5mA. Which by the way is not "compatible with CMOS IC or TTL" \$\endgroup\$
    – Oldfart
    Commented May 18, 2018 at 5:27
  • \$\begingroup\$ Power is via USB PSU: 5V - SSR - (Collector - Emmiter) - 0V. Just tried a 47 Ohm base resistor, still no fully powering SSR. 5V powers the SSR nicely. Sensor will output in the range of 2.5V, depending on settings and sense current \$\endgroup\$
    – phaseform
    Commented May 18, 2018 at 5:33
  • 1
    \$\begingroup\$ My question was is the sensor capable of providing current you require. Did you measure that voltage under load? \$\endgroup\$ Commented May 18, 2018 at 5:48

1 Answer 1


You are using sensor WCS1800 which has maximum current output 2mA. You are exceeding it.

Lower the base current to much lower value, by increasing base resistor.

  • \$\begingroup\$ Is there an equation I could use for optimal values? The sensor is supposedly TTL..? \$\endgroup\$
    – phaseform
    Commented May 18, 2018 at 6:02
  • \$\begingroup\$ To the OP: What exactly are you trying to do? I think you should update the original question by editing it and adding an explanation of what the circuit is supposed to do according to you, and what you are using it for. I am afraid it may not work the way you want it to unless what you want is, well, kind of strange. \$\endgroup\$
    – user57037
    Commented May 18, 2018 at 6:17
  • \$\begingroup\$ done - added details \$\endgroup\$
    – phaseform
    Commented May 18, 2018 at 6:25

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