How to use a solid state relay

Other Hardware Motors, Mechanics, Power and CNC
1

Hi everyone,
sorry for the newbish question - I've been struggling with this for a while, read plenty of posts, done lots of searches, etc... today I had a more knowledgeable and experienced friend come round and help, and he was stumped as well, so definitely time to turn to the forums. I want to get this 100% right before I plug it in to the wall.

I am attempting to build a thermostat/heater using the following equipment:
Freetronics Twenty Ten (Arduino clone)
DS18b20 temperature sensors
220-240VAC Aquarium heater
and a SY4080 SSR (http://www.jaycar.com.au/productView.asp?ID=SY4080, datasheet http://www.jaycar.com.au/products_uploaded/SY-4080.pdf)

If you don't want to read the entire datasheet, here's the important specs:
Voltage Range: 3- 32V DC
Control Current: 15mA Max
Input Resistance: 2.2kohm min.
Output Circuit - Max load current: 3A
Nominal Line Voltage: 240V AC
Load Voltage Range: 24- 280V AC-

As far as I understand, the specs of this relay are perfect for use with an Arduino. I've read several forum posts that say it's not necessary to use a resistor or any other circuitry with SSRs, so I just wired it straight up to the Arduino like an LED, with a 2-second 'blink' program on the Arduino, and tried to measure a connection across the load side of the relay using a multimeter. Nothing happens (I tried both orientations of wires to the poles on the relay). There is a voltage of 2.6V running across the switching circuit when it blinks, however, and the LED I left in series is blinking, but dimly.

I'm assuming that the voltage on that circuit has dropped from 5 to 2.6V because of the resistance of the relay (2.2K) ?

Then I thought maybe the AC side needs a current running across it in order to switch... so I wired up a small circuit with a 6V battery and some LEDs, and also tried making one using the 5v output from the Arduino (I am under the impression that DC power can be used here even though it is for an AC load, correct me if I'm wrong). No change. But now I have noticed that the "Load Voltage Range" is 24-280V - does that mean the relay won't work with less than 24V on the load circuit? Is that my problem?

Otherwise, can anyone suggest any other problems with this setup or any ideas on what might be going wrong?
Is this actually a good relay to use for this purpose? If not, any suggestions for others that are relatively cheap and preferably already available in Australia?

thanks for your help, and feel free to speak to me like a two year old - I'm still getting the hang of basic concepts in electronics, so may need some explaining.

2

Hi,

SSRs aren't quite as straightforward to test as electromechanical relays:

  1. You won't measure anything across the load side of the SSR with a multimeter because SSRs do the switching with triacs. To test it, connect the load side to mains power in series with a mains lamp instead. You can test the relay using a battery connected to the control inputs (instead of the Arduino) first if you like.

  2. Don't put a LED in series with the control side of the SSR, the LED may drop enough volts to prevent 3v getting to the SSR (that's why you only measure 2.6v across the SSR inputs). Connect the control inputs of the SSR directly to an Arduino pin and ground instead, ensuring correct polarity (i.e. + to Arduino pin). If you want to have a led showing when the relay is energised, connect a LED and series resistor combination in parallel with the SSR control inputs. Some SSRs have this built-in.

  3. SSRs designed for switching AC won't work on DC because they wait for the zero crossing before turning on. If you want to test it using a lower voltage than mains, you will need 24v or more AC. Also, SSRs pass a few milliamps to the load even when off (for yours, it is 5mA max).

3

Thanks for your response.
For future readers reference, I got some proper screw terminals and wired it up as safely as I could, removed the LED from the circuit, and plugged it in. It worked with no problems!

Now I have to figure out how to put all the pieces together to complete my system :slight_smile:

4

Also, SSRs pass a few milliamps to the load even when off (for yours, it is 5mA max).

That means when the SSR is OFF the wiring is not safe to touch, it will still byte you if you do, it is only powered down.

5

Thanks for that, important info.

6

Hi, I need turn the 3-phase motor with arduino.
I´m using a SSR Relé de estado sólido,Relé para automóviles,Controlador de temperatura
5-32v
I need reverse the motor about 200 times per minute.
Is it possible?

thanks

7

I need reverse the motor about 200 times per minute.
Is it possible?

That SSR will be able to turn on and off the motor, but not reverse the direction of rotation. To reverse direction of rotation of a 3 phase motors requires switching any two of the three phase wires to the motor, that would require additional switching of the motor leads. This would require a special switching circuit and your 200 times a minute spec may make it almost impossible to find. You do know that most 3 phase motors have a 'inrush current' that is several multipules of it's running current and as such are usually limited to how many restarts per unit time they can be subjected to before over heating. I would recommend you check out the motor's datasheet for compliance of what you want to do with it.

Lefty