Solid State Relay - Normally Closed?

Just wired up my SSR-25DD relay with a 24v PS and 2 12x12 12v heated beds in series, and it appears that the relay behavior normally closed?

Did I get a bum relay?

With the power supply plugged in, and the heated bed (via the ramps) turned off, the bed heats up,

There's no incorrect wiring that could have created this behavior is there?

What is important here is how you wired the input of the relay to the RAMPS board. SSRs are normally open.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Test the SSR by disconnecting its control wires from the RAMPS. The bed heater should then be off.

That model of SSR is basically useless for controlling a bed heater because of its high voltage drop. If you don't have a massive heatsink and fan on it and you have already run it for more than a few minutes with the bed heater output turned on, then it will have overheated and failed already.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

For my understanding, I also use this SSR but I am using 220 V AC current and that works perfect.

On the SSR it is stated on the output side that it is only for AC.
So can you really us it for DC if yes is there then the possibility that there is a polarity for the connections ?

amigob, the SSR-25DD is the DC to DC model, there are other ones in the range for AC.

DC42, what's the Vf on this, and what would you recommend as a better choice? Presumably these things just bundle an opto-isolator, a mosfet driver and one or more mosfets into a package?

Quote
JamesK
amigob, the SSR-25DD is the DC to DC model, there are other ones in the range for AC.

DC42, what's the Vf on this, and what would you recommend as a better choice? Presumably these things just bundle an opto-isolator, a mosfet driver and one or more mosfets into a package?

According to [www.fotek.com.hk] the voltage drop is 1.6V max at rated current and 25C. It will increase with temperature.

These devices use a BJT not a mosfet. To use a mosfet in a 4-terminal SSR and get a low voltage drop, you have to feed power from the control side to the output side to supply the gate voltage, instead of just using an opto isolator. There are some very good DC-DC SSRs made by Crydom, but they are expensive. But there is also this one [www.auberins.com] which claims a low on-resistance and is reasonably priced.

An alternative is to use a 5-terminal SSR such as this [reprap.me].

DC-AC SSRs also have voltage drops of around 1V. However, when used with bed heater running at 115V or 230V, the current they pass is much lower than for a 12V or 24V bed heater, so the voltage drop is much less important.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Ah, that makes sense. Many thanks.

For DC control, where we don't really need the opto-isolation, these things seem a bit unnecessary compared to a simple mosfet (maybe with driver) setup. For AC heaters I guess it's a very different story.

And so I learn that the differences are in the details :-)

My first SSR ordered through Aliexpress was bad. It ran away. They replaced it and now works great. Ended up getting one from the states because I didn't want to wait. I got it here.
[www.mpja.com]

I took a chance and picked up another SSR - Different Manufacturer.. Tested it tonight, continuity tests show it activates correctly, I'm tempted to simply get 2 12v 360w power supplies and another SSR and trigger them both from the heated bed voltage from the ramps. It wouldn't be as elegant of a solution, but it would work.

Sounds like I'm having similar issues with a bad SSR. I just ordered the same as you from Amazon. Out of interest are you running this with heat sink and fans? I was only planning on switch 12v 10amps so was hoping not to need to do that.

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DC42 Kossel 330mm x 2meters
My Thingiverse Creations

Yes I run all my SSR's with heat sinks.

I run a 40mm fan attached to the control side so that it only runs when SSR is tuned on. Simple and cheap.

I like it - will keep that in mind!