Solid State Relay

I have a DRA1-MPDCD3-B solid state relay. I am using a 5VDC power supply that is running through a dry contact on a motor driver across the input of the relay. The 5V to the input is switched on an off through software. The input circuit seems to be working great.

I am sending a 5VDC signal across the output. The 5V output is a separate source from a relay board on a piece of machinery. When 5V is connected to common, this acts as a momentary switch for a heat cycle. When the circuit is open, the heat cycle ends. The problem is that I am not seeing the output switch between open and closed.

This relay in normally closed. I am not the familiar with solid state relays. Should I see continuity across the output when no power is resent on the input? On my motor drive relay, my contact acts as a closed circuit regardless of power to to the drive so I was expecting the same from this relay.

I appreciate any help to get this working.

Hello Jason,

This is my understanding of your circuit. Note that the left hand side purple power supply, dry contact, and ground are independent from the green on the right.

Ideas to explore:

  1. You didn’t mention the load for the right-hand side of the SSR. Curious what you are using.

  2. The -B in the part number indicates that the relay is normally closed. It’s possible you want the DRA1-MP240D3.

  3. Other?

Having a heat cycle controlled by a Normally Closed (NC) relay is problematic. This could be trouble as turning off the motor drive will result in an always on condition.

Please let know if this answered your question.

Best Wishes,



Either the 20mA minimum load current for operation, or the 1.5V maximum voltage drop on the output at 3A, could be the source of the problem.


Sounds like the driven load is likely of the small-signal/control-level sort. If an NC relay function is really what’s needed, I’d suggest that an appropriate electro-mechanical relay might be a better choice.

Reason being is the “solid state” part of “solid state relay.” One doesn’t see a lot of NC SSRs, because most semiconductor switches require a bias voltage in order to turn “on” and if the objective is to make a device that’s “on” when the control signal isn’t… Well, one can expect that compromises would need to be made.

Depending on how the output structure of the SSR in question was implemented, I’d not be at all surprised if it appeared open to a continuity check or a dry-contact signal, but would happily switch a more substantial load, e.g. above the 20 mA minimum Paul pointed out.

Thanks for your feedback. These are all control level signals. Here is a sketch of the circuits.

The relay on my drive output is normally closed and the machine relay needs to close to start a heat cycle. This relay was chosen to allow that functionality. My motor drive does not require power to allow the 5v to pass through to the SSR. If the 5V power supply looses power, the machine fault in a safe was and will not start.

When I replace the relay output with a push button, everything works as expected.

Testing today, it appears that the relay is functioning on the output. But when I connect the relay, the machine turns on only when cycle start circuit opens. There is a major safety concern if I change my software to accommodate its current functionality.

When the SSR output is open, I see 0.7V which seems high. When the SSR is closed, I see 4.7V across the output.

I was thinking about trying a pull down resistor for when the SSR output is open.

I am open to suggestions.


Your description is still a bit unclear to me. In your diagram above, are you passing 5V through the coil of a relay in the box you label “Motor Drive Relay”?

I’m not following what you mean by this statement:

In this statement, to which relay are you referring?

Again, in the following statement, to which relay are you referring?

In the following statement, the function seems backward. If a load is connected to the SSR output and the input of the SSR is OFF, the output should be ON and the voltage between pins 1 and 2 should be 1.5V or less. If the input to the SSR is ON, the output should be OFF
and the voltage between pins 1 and 2 should be very close to the supply voltage on the load side.

The SSR mounted on the DIN rail socket is the MPDCD3-B. Here’s the datasheet for that part. It includes an internal schematic of the SSR, which shows that its output structure is a BJT type transistor.

According to the datasheet, the load current can be no more than 3A and no less than 20mA. If the load current is less than 20mA, it may not operate as you would expect. When the transistor is conducting, the voltage drop from pin 1 to 2 can be as high as 1.5V (probably somewhat less for you, especially if your load current is on the lower end of the specified range, but it will be well above zero volts). Since it is a “normally closed” SSR, the output will be in the ON state when there is no current flowing on the input side (from pin 3 through to pin 4). When the output is in the OFF state (when current IS flowing from pin 3 through to pin 4), the output transistor will be OFF, and it will not allow more than 1mA to pass through from pin 1 to pin 2.

Rather than trying to make this work in your circuit right away, I would first test out the SSR on its own to verify that it works and so you know what to expect when connected to your load.

A continuity tester will not work to test the output of this SSR since the output is a BJT type transistor and not a dry contact. To test it, you could place a small load between pin one and a power source, such as a 5V supply. The load could be as simple as a resistor with a value of about 100 Ohms and rated for 1/2 Watt or more (or two 200 Ohm 1/4W resistors in parallel). You could then test whether it is working or not by measuring the voltage drop across the resistor. When the output is ON, you should measure at least 3.5V across the resistor, if using a 5V supply, and less than 0.1V across the resistor when the output is OFF.

I think I an starting to figure out how to use this forum.

David, Thanks for the detail about how to test this relay. Its appear to be working as you described.

I have motor driver with a normally closed dry output relay contact that I turn on and off through software. Turning this relay on and off controls the input of my solid state normally closed SSR. Even is the drive loses power, my SSR input does not.

Replacing the output of the SSR with a manual and mechanical normally open momentary switch. Pressing the button starts the heat and releasing stops the heat.

Yes, your statements are correct. The machine heat cycle function is backwards when the relay is installed.

I agree that the problem is likely the load current. I considered adding a resistor in parallel to the output of the SSR to boost the current but I don’t have enough information about the machine to choose the correct resistor value.I do not want to fry my machine relay board by producing on over-current situation. I think the best path forward is to switch to a mechanical relay.


Jasen.baskette, I’m glad your experience with the TechForum is getting better. Please keep us posted on your post, of how things are going with your relay project, and if you get it working like you want it to.

I think your conclusion to switch is probably correct.

You might want to double-check your wiring and connections to make sure everything is set up correctly. If it’s still not working as expected, it could be a problem with the relay itself. In that case, you might consider testing it with a different power supply or checking the datasheet for your specific relay model to see if there are any special considerations.

1 Like