Electronic – Relay Voltage Rating Question

Using just 2 transistors will work, but you'll waste a significant amount of current from the power supply.

The reason you can't get enough V across the relay coil is that its resistance is much lower than the 47 ohm resistors -- they limit the current too much.

If the relay really requires 3 V, and is 16 ohm, you'd need to make the 47 ohm something like 1.6 ohm or lower in order to allow 3 V across the resistor. Now, the 'ON' transistor will need to sink both the relay coil current and the supply across the 1.6 ohm -- 2 A !

That would require a base current of ~ 100 mA which an Arduino won't be able to supply...

Using 5 V makes things easier, but not very practical yet -- you can use about 10 ohm, so the transistor currents become about 500 mA instead of 2 A

Best solution is to use an H-bridge -- replace each 47R with a PNP transistors shown here.

Your circuit (as you show) will also need clamp diodes -- else the back emf from the relay coil will cause large + spikes and damage the NPNs the 1st time you turn off the NPNs.

You can use MOSFETs instead of the NPNs and PNPs -- for Q1 & Q2, then you don't need R1, R3. For Q3, Q4, you don't need R2 & R4.

This circuit should work for 3 or 5 V.

^{simulate this circuit – Schematic created using CircuitLab}

1. The max input current for this SSR is 12 mA, so you have to use a current amplifier. It can be for example any general purpose NPN BJT connected with a common collector. I suggest using 10 V power supply for this BJT and using 0 V / 7 V DCA output for switching. Do not go to +10 V! This may damage your DAC.

Alternatively, you can use a general purpose N channel MOSFET with "logic level" switching. Then you have to drive it by 0 V / 10 V from ADC.

2. No, SSR switch operates "smoother" for its load then mechanical switch. The latest one causes several on/off transients (ringing) while SSR switches on only once. So SSR can not damage a fan.

3. No, you can not force stop the fan by this circuit. You need a special motor and a special switching circuit with "break" resistor (or more complicated means to return energy) to stop rotation quickly.