Solid State Relay Design For 24VAC

Thread Starter

DJ_AA

Joined Aug 6, 2021
22
Hi All

I am new to the forum.

I am working on a project, where i need to control 24VAC solenoids using my microcontroller. Previously i have used standard bulkier relays, but i now need some thing much smaller in design, therefore aim to use the AQH2213AX SSR.

Can anyone advice, if the design below is correct or do i need to have any additional components for any type of filtering?

My power source will from a 240-24VAC transformer.


ssr.jpg


Thank You
 

MaxHeadRoom

Joined Jul 18, 2013
23,805
It would appear that you could drive the SSR without the need for Q4.
If you think you need it, a 2N7000 would be a better choice.
 

Papabravo

Joined Feb 24, 2006
17,030
What state will transistor Q4 be in if GPIO_A is uninitialized, or high impedance, and unable to control the base of Q4. Do you want the relay to be default "ON", or defaut "OFF"?
I agree with Max. An external pullup resistor will default the relay to "OFF", and you can pull it low to turn it on. If you are determined to use a transistor then you can use a common gate/base arrangement to do it.
 
Last edited:

Thread Starter

DJ_AA

Joined Aug 6, 2021
22
What state will transistor Q4 be in if GPIO_A is uninitialized, or high impedance, and unable to control the base of Q4. Do you want the relay to be default "ON", or defaut "OFF"?
I agree with Max. An external pullup resistor will default the relay to "OFF", and you can pull it low to turn it on. If you are determined to use a transistor then you can use a common gate/base arrangement to do it.
I will be adding a pulldown resistor. I must have missed it out
 

Thread Starter

DJ_AA

Joined Aug 6, 2021
22
It would appear that you could drive the SSR without the need for Q4.
If you think you need it, a 2N7000 would be a better choice.
Q4 will be needed, as this design is likely to be powered with 5V and my Microcontroller from another board is working with 3.3V.

The reason for using a MMBT3904LT1G is because i have multiple reels of them which need to be used.
 

Papabravo

Joined Feb 24, 2006
17,030
Q4 will be needed, as this design is likely to be powered with 5V and my Microcontroller from another board is working with 3.3V.

The reason for using a MMBT3904LT1G is because i have multiple reels of them which need to be used.
Where will you be adding the pulldown? What will be the value with respect to the base resistor going to Q4?
 
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