Solid State Relays

[vertcnc]

Could someone explain the difference is solid state relays. I am looking to control spindle, coolant and etc. from a campbell breakout board. The relays I currently have are Crydon brand. Not sure which part #. The problem I noticed is the output voltage is varied based on the control voltage. I beleive the control voltage marked on the relay is 3-36vdc. Surely must have the wrong type of relay. Help?

[Al_The_Man]

The different types of SSR's are rated with an input control voltage which can be logic 5v only or 24vdc or 3-36vdc, this is the control voltage used to switch the SSR, the output can be various types and voltages, i.e. the output can cover 110vac to 240vac or DC types can cover 65vdc for example. The output should not vary as long as you have the correct input switching voltage.
Al

[pminmo]

I agree with Al, as long as you have move than 3vdc control voltage the ssr should be on. Now, if your load is very small (i.e. millamps or less) you will probably will see a voltage change, but under the loads of what you want to control, it will be very minor. You do have it inline with the hot side of the ac to the load, correct?

Phil

[Mechanoid]

Regarding the relay: Crydom (note the spelling -may help get correct Web searches ) does not have 3-36 VDC control relays; this is rather an unusual range. A more common spec is 3-32 VDC. There is also 18-36 VDC, which can be easily "converted" into 3-36 VDC by erasing the first 1.5 digits - look at the case under good bright desktop light to see if this is not the case. If you describe how the relay looks, I think I will be able to look up its datasheet, for example, "A black box 25x23x12mm with screw {PC mount | lug quick connect | solder lug | etc} terminals, 2 in the corners of the long side and 2 on another long side but closer to the center. {Single | Double} pole, {Single | Double} throw, normally {Open | closed}, etc - all you know about it or see or directly measure.

RE your measurement: please describe your setup - what voltmeter used? AC or battery powered? was the case grounded? If uncalibrated, why you'd think you would trust it - objectively, i.e. how checked? Where exactly did you connect probes? everything in detail. My impression is that you somehow measure part of or whole control signal together with load voltage.

[ViperTX]

Okay, well solid state relays are not like the mechanical relays. So, the input voltage can vary, but it's the current which actually turns the SSR on. you limit the current to the amount specified by the data sheet...usually 20 ma or less. SSRs are problematic in that each vendor specific model operates differently then another vendor's similar model. They don't turn on and off exactly the same. I would never use them to control anything that required that something be ON or OFF at some particular time for some specific time, unless you have very wide tolerances for those times.

[ViperTX]

Also, the output voltage of the SSR does not vary with the input voltage....the output is either on or off or transitioning from one state to the other...which is hard to measure unless you have an oscilloscope. I won't get into commutation for the AC types...just mentioning in case we have EEs reading this thread

[pminmo]

All the DC controled Crydom's I have ever used have the control volt current limited to 20ma. As long as you present a DC voltage capable of delivering 20ma between the max and min control voltage there isn't a problem. As to turn on and turn off timing, there a heck of a lot better than a mechanical relay. When control electronics are around, the zero crossing ones are the best, because you don't spreay emi at turn on and turn off. Otherwise almost all turn on within one ac cycle (.016 sec) or most turn on within 1/2 cycle (.008 sec).

Phil

p.s. Referring to the SSR's output is a misnomer, it's like saying a light switch has an outout. An ssr's output presents a high resitive state or low resistive state like an open closed switch from a source to a load.

[vertcnc]

Thanks for the input. I will have to get the exact model #(there at a different location presently). When I was playing with these relays I had 12 volts dc across the input. A small computer box fan 120v ac in series with the output terminals. I didn't put a meter across the load to check the voltage.
I could tell the difference in the rpm of the fan as i varied the input voltage.
I suspect after reading posts that the current draw is maybe not enough to test correctly.

[imserv]

Originally Posted by vertcnc I could tell the difference in the rpm of the fan as i varied the input voltage.

This is probably not an SSR as they are either fully on or off.

There are other devices that vary output voltage with a varying input voltage. Some are packaged with the same physical dimensions as an SSR.


Fred Smith - IMService
http://www.cadcamcadcam.com/hobby

[vertcnc]

The part# on the Crydom Relays is D1225.

[Al_The_Man]

That part is zero cross ac switching rated for .04 to 25 amps AC at 24vac to 140vac rms output. The switching voltage is 3-32 vdc. You should not expect to get a variation if the load is over .04amps.
Al

[vertcnc]

Thanks for the reply. I suspect that my initial test using a very nominal load was my problem.