# How is this scheme for multiple solid state relays controlled by a microcontroller?

1. ## How is this scheme for multiple solid state relays controlled by a microcontroller?

I need to control multiple AC mains power terminals which are going to power DC power supplies. The aim of the control is just to switch OFF or ON the power for the DC power supplies when needed(not a nonstop ON OFF). So basically the idea is cut their electricity by a microcontroller board such as Arduino board's digital outputs. Total number can be around six but for simplicity I drew a diagram for three:

In the above diagram D1, D2, D3 are microcontroller 0/5V digital outputs. Q1, Q2 and Q3 are transistors to not to load the digital pinouts. SSR1, SSR2 and SSR3 are solid state relays which control the line terminal for the AC power entry to galvanically isolated power supply units PSU1, PSU2, and PSU3 individually.

So for the moment, I ended up with this SSR for the purpose. Another one is this.

But before I have couple of hesitations and questions:

1.) As in the diagram the SSRs in my case will turn ON or OFF the DC power supplies. So I don't have any motor or inductive type of load it seems. But these SSRs has mainly two switching types: Zero-cross and Random. Would a random SSR work fine for this application? If I use zero-cross type would it have any bad effect for this type of scenario?

2.) Is my way of using the transistors(not to load the digital pins) correct? There is not input impedance of the SSR input given in its datasheet. So I wonder would the transistor need any resistor across(100k in my diagram) or in parallel?

(In another form someone wrote that EMI can be a problem through signal path and high impedance)

I'm really in need to make this work properly.

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2. ## Re: How is this scheme for multiple solid state relays controlled by a microcontrolle

You need resistors in series with the base of the BJT's or you could use fets (which draw 0 gate current) and skip the resistors.

On a quick look the SSR's appear to be voltage controlled with a current draw of 7-16mA. Your circuit is ok in that respect.

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3. ## Re: How is this scheme for multiple solid state relays controlled by a microcontrolle

Originally Posted by asdf44
You need resistors in series with the base of the BJT's or you could use fets (which draw 0 gate current) and skip the resistors.

On a quick look the SSR's appear to be voltage controlled with a current draw of 7-16mA. Your circuit is ok in that respect.
If I use BJTs(with 12V Vcc) can 1k base resistor be sufficient ? You mentioned FET but they pass current through their capacitance as well. What FET would you recommend?

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4. ## Re: How is this scheme for multiple solid state relays controlled by a microcontrolle

Hi,

Calculate 5V/1k. 5mA into the bases. Multiply 5mA by 10 (collector I = ~10*base I). The base resistor value should reflect the desired collector/output current. That 1/10 is well established rule of thumb, best check BC547 datasheet to see roughly expected hFe, both in electrical specifications and in any graphs provided, may need to do some guesstimating by extrapolation.

5. ## Re: How is this scheme for multiple solid state relays controlled by a microcontrolle

Originally Posted by doncarlosalbatros
...... You mentioned FET but they pass current through their capacitance as well. What FET would you recommend?
Their capacitance can pass a small current but only for AC signals, which is not a problem here.

A 2N7000 (or similar) is a common, small N-MOSFET that should work for you.

I would use a zero-crossing SSR as it should generate a smaller turn-on surge current from the power supply input.

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6. ## Re: How is this scheme for multiple solid state relays controlled by a microcontrolle

Hi,

I would use a zero-crossing SSR as it should generate a smaller turn-on surge current from the power supply input.
Really?
I'd say it depends on the load behaviour.
For usual low frequency transformer based power supplies .... zero cross switching is worst case for inrush current.
Better switch them at 90°/ the max. voltage, else you should expect high currents caused by transformer core saturation.

Klaus

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7. ## Re: How is this scheme for multiple solid state relays controlled by a microcontrolle

Originally Posted by KlausST
Hi,

Really?
I'd say it depends on the load behaviour.
For usual low frequency transformer based power supplies .... zero cross switching is worst case for inrush current.
Better switch them at 90°/ the max. voltage, else you should expect high currents caused by transformer core saturation.

Klaus
In my case all the power supplies are SMPS. Did you mean linear power supplies?

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Originally Posted by crutschow
Their capacitance can pass a small current but only for AC signals, which is not a problem here.

A 2N7000 (or similar) is a common, small N-MOSFET that should work for you.

I would use a zero-crossing SSR as it should generate a smaller turn-on surge current from the power supply input.
Thanks for the answer. If I use 2N7000, do I need resistor between gate to GND or/and a series resistor between microcontroller pin and the gate?

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Here is my final version? What do you think?:

Btw this is not a nonstop turn ON/OFF application at high frequency. This is just for DC static operation. Normally the MOSFET will be ON so the relay. And in case it will be OFF. Does Miller capacitance matter?

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