SSR question

Is there a difference between the operation of a DC SSR and an AC SSR? I've burned out the one SSR I have that is DC and don't really want to drop another 25$ if one of the AC SSR's will work, but I also don't want to risk them if it will destroy the equipment

AC SSRs will work only with AC.

Right... AC SSRs are usually made with a [u]TRIAC[/u]. Once a TRIAC turns-on it doesn't shut off until current flow stops. With AC, it turns-off at the next zero crossing (if there is no voltage on the gate), but with DC it will latch-on.

DC SSRs are usually made with a MOSFET and they only pass current in one direction. You might fry it with AC.

There are some "small" SSRs that work with AC or DC.

Can you share what you are trying to control?

In general if you can find the datasheet for the SSR in question it will specify what it is capable of.

John

trying to control a 110W water pump. the ssr is an FOTEK SSR-25DD. the pump is on a 15A fuse. the whole thing worked for about 5 cycles and then failed closed. no current passes through, no indicator lights for the ssr receiving power from the power supply

trying to control a 110W water pump

OK, it is a 110 Watt pump.

Do you think it could just possibly be important to state whether that is an AC or DC pump, and whether you are powering it with AC or DC, and what the AC or DC voltage might be?

well, if i'm asking about dc ssr's, and the pump worked for 5 cycles on the dc switch, seeing as how if it were an AC switch, it wouldn't work at all, i'd have to say that it's probably a DC pump

it's a 12v dc pump, drawing 9amps during use, has a 20amp fuse. the relay has a fairly hefty heat sync on it.

i'd have to say that it's probably a DC pump

If that indicates the state of your knowledge, please choose another project. Otherwise, you might kill yourself.

no... I was being sarcastic as that's how you came across.. ac pumps won't work on a dc circuit. the magnets won't engage and there for, the spice will not flow... i'm not an idiot.. but thank you for assuming that i am one..

Please read and follow the directions in the "How to use this forum" post.

Here is another tip: read and follow instructions, and you won’t seem like an idiot.

Some points to consider:

  1. You attempted to use a 24-380 VAC, 25 Amp SSR on a 12V DC circuit.

  2. The startup current of a DC motor is 5 to 10 times higher than the running current. Given that the running current is about 9 Amperes, the startup current will be in the vicinity of 50 to 100 Amperes. Not something a 25 Amp SSR of any sort can handle.

Does that help you understand why things aren’t working well?

If this is the relay you have (had) then it should work on a DC circuit FOTEK SSR-25DD

You stated your pump is: 110 Watt, 12V, ~ 9 amp DC pump.

However I could not find that exact device on the Fotek website. The closest I could find is: Foltek DC Device

Thoughts:

On the eBay offering the following was stated:

  • When using an inductive load, it is usually necessary to have a voltage control device with a specific clamping voltage, such as a bi-directional Zener diode or a varistor (MOV), at the output. For varistors we recommend 1.6-1.9 times the rated voltage.

1) The SSR spec sheet I found does not specify any type of motor capability, unusual. Perhaps this particular relay is only capable of resistive loads.

2) If the output is damaged due to some kind of over loading that should not damage the input, which you said the there were no indicator lights.

3) Does the pump work properly if you make a connection right to the supply?

4) I would suggest the input might have been reversed but you said it worked for a while before failing. With that said, how were you powering the input?

If all checks out I would suspect an inferior SSR. Or an SSR with no motor capability.

sorry for not getting back to this thread... the forum language settings have certain words set to permban... now that this has finally been resolved... I can get back to this a little bit before I move to Colorado.

JohnRob, in answer to your questions: 1- this is what I have found to be the case. I'm now looking for a relay that is capable of handling a motor 2- correct 3- the pump works properly when hooked directly to a 12v battery. 4- the input is being powered by a mosfet shield fed by an arduino mega.

I'm currently trying to figure out how to put a capacitor in line with the pump to handle the inrush current, but I'm not that familiar with circuit design as of yet and could use some assistance with this.

midiean: sorry for not getting back to this thread... the forum language settings have certain words set to permban... now that this has finally been resolved... I can get back to this a little bit before I move to Colorado.

JohnRob, in answer to your questions: 1- this is what I have found to be the case. I'm now looking for a relay that is capable of handling a motor 2- correct 3- the pump works properly when hooked directly to a 12v battery. 4- the input is being powered by a mosfet shield fed by an arduino mega.

I'm currently trying to figure out how to put a capacitor in line with the pump to handle the inrush current, but I'm not that familiar with circuit design as of yet and could use some assistance with this.

Inrush current for an AC motor is usually one or two AC cycles because of the inductance. On a DC motor the inrush current is only reduced by the rotation of the motor producing back EMF to reduce the current.

The question for you is "how much time does it take for your motor/pump to come up to speed? That determines the inrush current curve you will have to support with a capacitor.

Paul

Do you have a free wheeling diode (at least 10 Amp rated) in parallel with the pump (cathode toward +) to suppress inductive kickback?

Midien,

I agree with JohnRob, Paul_KD7HB and Outsider. I would say you SSR needs to be capable of handling the stall current of the motor, plus some overhead for safety, maybe 25% to 50% in addition to the stall current.

Your idea of using a capacitor to handle the inrush current doesn't work because either you put it before the SSR, in which case the SSR still has to carry the inrush current, or you put it between the SSR and the motor, in which case the capacitor will be discharged at the point the SSR turns on so won't supply any of the current to the motor, in fact it will need to charge so will add to the current the SSR has to supply.