# Need help understanding MCUs and SSRs

Discussion in 'The Projects Forum' started by Ingeniir, Jul 19, 2011.

1. ### Ingeniir Thread Starter New Member

Jun 29, 2011
19
0
Hi all,

I'm working on a project where I am using an Arduino that can output 5VDC and 40mA to switch 120VAC via an SSR.
Here is the website that has the datasheet for the SSR (S202T01): http://search.digikey.com/scripts/D...ang=en&site=us&keywords=425-2408-5-ND&x=0&y=0

According to the datasheet, the SSR can handle 240VAC and 2A, and the input voltage is listed at 1.2VDC, and the recommended operating current for the ON state is 16-24mA.

I don't fully understand how my MCU, or these solid state relays work, however. Do you need to hit a certain voltage across the input terminals for the relay to activate, or do you need to hit a certain current? Is this correct: if you direct the MCU to output a high voltage of near 5V, the current drawn depends on the resistance of the circuit?

2. ### praondevou AAC Fanatic!

Jul 9, 2011
2,936
489
First a few facts:

- the S202T01 has a LED inside which provides isolation between the control circuit and the power circuit.
- it is controlled by current and the minimum LED trigger current is 6mA
- the LEDs forward voltage drop depends also on the temperature.

"Is this correct: if you direct the MCU to output a high voltage of near 5V, the current drawn depends on the resistance of the circuit?"

Yes, the current drawn depends on the load on that pin.

You should be good to go with the configuration you have if your arduino really can source 40mA. I suggest you work with at least 10mA to be sure you trigger the relay. That means from the output port of your MCU you put a resistor of 390R in series and connect the resistor to the anode of your relay LED (pin 3). Pin 4 returns to ground of your control circuit.

Don't place the triac near your MCU, it's a non-zero cross triac and can generate considerable noise.

Make sure you provide good electrical isolation, putting the power circuit well separated of you control circuit, otherwise you might blow up your control unit and get some unhealthy shocks.

Last edited: Jul 19, 2011
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