# Solid State Relay from CMOS pin

I want to operate the LCC110 solid state relay from a 5V CMOS output pin on the PGA2505. I can see that I will likely need 8mA to drive the relay. However, I don't see on the PGA2505 datasheet if it can drive this current from the CMOS pins.

The input voltage drop of the LED in the solid state relay can be up to 1.4V so it is obvious that[will need to put a 450 ohm (5-1.4)/(.008) resistor in series with the relay. The datasheet for each device are provided below. However, I don't know what I am proposing is okay. The relay is used to switch from ground and 48V.

• You need to add a transistor, its digital outputs are much too weak to rely upon. – Asmyldof Apr 14 '16 at 23:39

Figure 1. Extract from the PGA2505 datasheet showing the likely limitation for this application.

The electrical characteristics don't seem to give a maximum source or sink current but does quote (1) output voltage at a measy 200 µA when high and (2) sinking 3.2 mA when low. I wouldn't push the chip beyond that.

You're going to have to buffer the output. You can choose between a digital buffer, a transistor or an FET.

simulate this circuit – Schematic created using CircuitLab

Figure 2. Darlington drive for LED.

Given that the specification lists the output voltage at a 200 µA drive current I suggest using a Darlington pair to step up the drive. R1 limits the base current to less than 100 µA. With a gain of 80 minimum this should be enough to drive the LED at the min 8 mA turn-on but Q2 ensures that we have enough drive as its gain effectively squares the current gain of the combination.

The Darlington pair will have a 1.3 V drop (2 x $V_{BE}$) and the LED 1.4 V leaving 5 - 1.3 - 1.4 = 2.3 V across R2. If we drive at 10 mA (25% over the minimum) we get $R2 = \frac {V}{I} = \frac {2.3}{0.01} = 230 \Omega$. 220 is the next lower E12 value.

• So assuming I use a Q2N3904 or something similar, taking in the digital output into the base and then outputting the current through collector? I don't have a lot of hands on experience with FETs so I wouldn't know which device to select if I went that route. – Caleb Merchant Apr 15 '16 at 0:16
• See Figure 2 onwards. – Transistor Apr 15 '16 at 7:29