# noob trying to use a SSR with low voltage

Discussion in 'General Electronics Discussion' started by rbrooks, May 17, 2012.

1. ### rbrooks

34
0
May 17, 2012
Hi.

As the title says, I have a solid state relay that I'm controlling with a 24 prox switch.

It's working fine, but the voltage I have to control is low: 5vdc reference voltage.

It's having a switching action, but only from 5v on, to 3v off.

Can you use, for lack of a better term, a pull down resistor to get a more positive TTL type reading? 5v on, but 0v off?

If so, how would one calculate the necessary resistor value? At the moment, all I have is a 249 ohm resistor and it's not doing it.

Thanks in advance for any help.

2. ### GreenGiant

842
7
Feb 9, 2012
Having a more complete picture of the circuit would help a lot

Im assuming that by "24 prox switch" you mean a 24V proximity switch, so you are using a relay that is controlled by 24 VDC and controls 5VDC

I have some suggestions but like I said I need to see your circuitry

for now try putting the circuit as follows...

5VDC supply --> resistor (249 is fine to try for now) --> relay --> ground
then attach the reading element (im assuming microcontroller of some sort) to the point in between the resistor and relay

3. ### rbrooks

34
0
May 17, 2012
Here's what I have and am conceiving.

With no resistor, the VOM reads 5v when the relay is activated, but due to it being a SSR, the relaty allows voltage to pass even when the relay is off.

What I'm hoping is to put a resistor in the loop, so that when the relay is in the off condition, the current will just go across the resistor back to - rather than across the relay.

I guess I would need to know the resistance value of the open relay, right?

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4. ### rbrooks

34
0
May 17, 2012
I tried your scheme with the 249 and got 2.5v open, 5v closed. Closer to what I need, but there's a bit of float. I was thinking a pull down would give me 5v and 0v.

5. ### GreenGiant

842
7
Feb 9, 2012
I see what you are trying to do

The voltmeter is hooked up incorrectly

And the resistor where you have it wont really do anything

I have attached a modified wiring that would indicate on the voltmeter when it is tripped

what is your end goal with the other side of the SSR?

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6. ### GreenGiant

842
7
Feb 9, 2012
Try a larger resistor, there is math to figure out the size that you need but its more complicated than is worth getting into with this sort of project

Try something around a 1K or 5K range

7. ### rbrooks

34
0
May 17, 2012
I am sending a signal to a National Instruments DAQ system. It operates on 5v max. Basically my application will look for this 5v signal and change a condition. When this switches off, it would change the condition again.

I suppose I can make it work with 5v and 2.5v, or any two distinct numbers, I was just hoping for more of a positive transfer, IE, the steady 5v that I get and a steady, quick-switched 1v or something. More likely to avoid bounce or false readings if it's two distinct numbers.

If I'm making any sense.

8. ### rbrooks

34
0
May 17, 2012
I connected it the way you have described, and it definitely gives me 5v and 0v, just on the inverse of what I was thinking. When the relay is on, I get 0v. When it's off, I get 5v.

I can work with that. 249 ok for the resistor or should I use a different one?

9. ### GreenGiant

842
7
Feb 9, 2012
249 should be fine

10. ### rbrooks

34
0
May 17, 2012
Wonderful. I have piles of them around here.

Thanks much for the assist. I've used pull up and pull down before with reed sensors and collectors but nothing like this.

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