Hi there,
I need a solid state relay IC. I am using STK500 and need to turn on/off some electric devices that are operating with voltage of 24V.
What do you suggest? Thanks
Regards
Why don’t you use MOSFETs instead of SSRs? They are much cheaper.
Leon
leon_heller:
Why don’t you use MOSFETs instead of SSRs? They are much cheaper.Leon
How? do you have an example?
Assuming its a 24v DC load (MOSFETs are only good for DC applications versus TRIACs and SCRs for AC), its pretty simple. To control from a microcontroller, get a “logic-level N-Channel MOSFET” (the IRLZ44 is one of many possibilities). Make sure the voltage/current ratings are appropriate. If you are driving more than a few hundred milliamps, get something in a TO220 package and understand that current ratings (50A for the IRLZ44) only apply when driven optimally and with a proper heatsink (which you wont be doing with the technique).
Basic connection is Power to the load, load to MOSFET Drain-pin, MOSFET Source-pin to Ground. The microcontroller output is connected to MOSFET Gate-pin.
When the microcontroller output is high, the MOSFET will conduct to ground. The output is low, it won’t. Pretty straightforward.
In theory you should put a 100-200 ohm resistor between the microcontroller output and the Gate because when you switch the MOSFET on, it looks like a capacitor and it can momentarily exceed the draw of you microcontroller output pin driver while switching on. In practice, I dont bother for my regular hobby projects. Likewise, a 10K resistor between the MOSFET gate and source will make it less sensitive to static which can damage MOSFETs. Again, I don’t normally bother for hobby projects unless I expect lots of circuit noise.
Vraz:
In theory you should put a 100-200 ohm resistor between the microcontroller output and the Gate because when you switch the MOSFET on, it looks like a capacitor and it can momentarily exceed the draw of you microcontroller output pin driver while switching on. In practice, I dont bother for my regular hobby projects. Likewise, a 10K resistor between the MOSFET gate and source will make it less sensitive to static which can damage MOSFETs. Again, I don’t normally bother for hobby projects unless I expect lots of circuit noise.
There’s one other reason for the resistor connected from the gate to the source. When the microcontroller first starts up, the gate will be left floating for a short time before the output registers are programmed. During that time, the mosfet can turn on if there’s a leakage charge from the drain to the gate. The resistor prevents that by bleeding off any charge from the gate to ground. In some circumstances, not putting in the resistor can damage the mosfet because it gets into a state where it’s partially turned on and the heat generated from the partial turn-on is too much for the device.