I have a 110 VAC water pump that is currently controlled by a 24VDC relay (Aromat JA1a-TM-DC24V). This mechanical relay is intermittently failing. I would like to replace it with a solid relay, such as the Sharp S108T02, which I have used with in arduino projects.
How can I make this work?
The mechanical relay...
has a nominal power consumption of 1.2W.
can switch up to 15Amps AC
The pump is rated at 1.8 Amps
The Sharp relay
is rated for switching up to 8 Amps AC, and 400 V drm.
I'm not too good at reading data sheets. Since the power consumption of the mechanical relay is higher than the solid state relay, I don't think I need to worry about the draw on the DC power supply. Also, since the mechanical relay is in an existing DC circuit, I am assuming that a back EMF diode must already be in the circuit. But I don't think the solid state relay needs one anyway, so nothing to do here.
The solid state current and voltage ratings for the switched circuit seem to be capable of handling the pump circuit. If I understand correctly the S108T02 requires about 8 mA to turn it on, and it has a maximum current rating of 50 mA. I think the relay is essentially a diode in the circuit. For 50 mA in a 24V circuit, I think I need a current limiting resistor, and it needs to be about 500 ohms. But such a resistor in this circuit will dissipate about 1.2 watts. I think that is a huge resistor.
If I use a 2k resistor instead, then the circuit will draw 12 mA, and dissipate 288 mW (a little over 1/4 watt). If I use a 1/2 watt resistor I would have more than a 20% safety margin.
Is my thinking correct? And is there a better way? or a better idea?
You should be able to connect that SSR to an arduino pin with a series 330 ohm 1/4 w resistor. This will put about 15 mA through the LED inside. The 8 amp rating of that SSR should handle your 1.8 A pump without any problems. I would connect pin 4 on the SSR to common and pin 3 thru the 330 ohm to your pin on Arduino. Then just drive the pin HIGH to turn on the SSR. A WORD OF CAUTION ON CONNECTING TO PINS 1 & 2 OF THE SSR - USE A MINIMUM OF 16 AWG WIRE AND KEEP IT CLEAR OF ANY LOGIC LEVEL WIRING !!! Hope this helps.
Tom
"You should be able to connect that SSR to an arduino pin with a series 330 ohm 1/4 w resistor. This will put about 15 mA through the LED inside."
You forgot to account for the 1.3V drop of the LED. 5V-1.3=3.70V R=E/I or 3.7V/.02 = 185 Ohms. If you read the data sheet, it recommends from 16 to 24mA Forward Current. The LED also degrades over time, so you really want to hit it a little harder than 330 ohms. 180 ohms will give you just over 20mA.
The S108T02 might not work properly with the pump as a load due to its inductive component (you may need to use a non zero cross type instead). Check the Design Guide in the link provided. Also I recommend using an MOV.
You could use TRIAC or sloid state relay. If you have knowledge in hardware and AC line then you can choose TRIAC. else you just use SSR relay.
SSR(Sloid state relay) details
Input 2 pins. you can connect directly with your arduino. Like an LED connection with series resistor.
Output 2,3 etc. If you have TWO pins the you just connect series of motor. that is it.
There are different types of SSRs. Some popular types for AC are zero-cross and random or instantaneous turn on. The random or instantaneous turn on type is what you would need to control a load that's inductive. You would also need a snubber or MOV ... some SSRS would have this built-in. Look at spending $40+ for a proper SSR to control the pump motor.
I'm thinking your existing relay contacts would very easily weld when trying to break the pump motor load because you're probably not using any contact arc suppression. Adding a 150VAC rated MOV like this would be a good solution for less than $1.