Using GPIO To Control Solid State Relay - For Both AC And DC Output Loads - Protection For Both?

Thread Starter

Mahonroy

Joined Oct 21, 2014
339
Hey guys,
I am wanting to wire up a solid state relay (SSR) so that it can be used for turning on/off both AC or DC loads (e.g. 24VDC inductive load, or 24VAC inductive load - relay coils and contactors for example come in both DC or AC versions, 12V, 24V, etc.). I would provide the 2 SSR switch wires, and you could then use it along with the corresponding power supply (AC or DC).

Generally you would put a flyback diode across the relay output to protect it from the inductive load generating a spike when turning off. This is only for DC though, so this would not work for the AC load. What would I do here to satisfy both conditions? Does an SSR even need a flyback protection diode?

Here is the part number of the SSR I was using by the way:
VO14642AABTR
http://www.vishay.com/docs/81646/vo14642a.pdf
 

crutschow

Joined Mar 14, 2008
27,408
Yes, you need to protect from inductive spikes as that SSR has a maximum output voltage rating of 60V.
You can put two back-to-back 45V, 1W zener diodes (or 45V transorb or TVS diodes) across the SSR output to protect if from both AC and DC load spikes.
Check the transient voltage of the protection device you select at the maximum load current you expect.
That will allow operation with up to 28VAC or 40VDC.
 

Thread Starter

Mahonroy

Joined Oct 21, 2014
339
Yes, you need to protect from inductive spikes as that SSR has a maximum output voltage rating of 60V.
You can put two back-to-back 45V, 1W zener diodes (or 45V transorb or TVS diodes) across the SSR output to protect if from both AC and DC load spikes.
Check the transient voltage of the protection device you select at the maximum load current you expect.
That will allow operation with up to 28VAC or 40VDC.
Thanks a lot for this information. I have never used a TVS diode before so I looked into them. So lets say I need to have the possibility open that a 48VDC max (might be used). In this case, I could use this then?
SMAJ48A-TR
https://www.digikey.com/product-detail/en/stmicroelectronics/SMAJ48A-TR/497-14743-1-ND/4868912

I'm assuming I would want the "bidirectional" version? However, it seems bidirectional are front-to-front and not back-to-back, is this correct? And that part number does not indicate if its unidirectional or bidirectional... hmm I'm a bit confused.

EDIT:
This looks to be the bi-directional version:
SMAJ48CA
 

crutschow

Joined Mar 14, 2008
27,408
Front-to-front works the same as back-to-back since they are in series and the electrons don't care which diode comes first in line. ;)

I don't know if the one you selected will work since I don't know the maximum load current.
You will need to look at the data sheet to be sure the protection diode you use does not exceed 60V at the maximum load current you anticipate, since that's the peak transient current the diode will see.
 
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