Help with 12V DC PNP Sensor and SSR Relay circuit

Thread Starter

Fortel

Joined Nov 3, 2020
13
I'm struggling a bit with a 12v DC project.

I have a 12 volt hydraulic pump motor with three wires: two for v+ and one for ground. The motor is reversible depending on which wire receives the V+. It is controlled by a SPDT switch.

The hydraulic pump has a high pressure and a low pressure circuit (think hydraulic motor used to raise / lower a boat's outdrive trim.) Originally, mechanical relay contactors and various micro limit switches were used to control the 50 amp load. I'd like to use solid state hydraulic pressure sensors and solid state relays.

I have a pretty limited (very little) background in electronics. I know the basics, but obviously am missing something, as I can't quite get this circuit to work.

With the hydraulic motor load wires disconnected, the circuit works- low pressure is sensed and that causes the low pressure sensor to close, energizing the appropriate SSR. The SSR's LED illuminates- and it's output circuit is closed, all as expected.

And further testing with the hydraulic motor connected into the SSR's output, if the SSR is activated with power on the inputs, it works- the SSR LED illuminates, and the pump motor operates as expected.

But when the pressure switch is connected to the SSR input, and the hydraulic motor is wired into the SSR's output, the circuit will pulse on and off about twice per second. The motor doesn't run under this condition.

The attached image shows how I've wired this. I'm ready to redo the circuit if someone can show me the light.... Anyone?

Thanks a lot!

Peter
 

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Thread Starter

Fortel

Joined Nov 3, 2020
13
Do you have a part # for these or a link?
Sure do...

These are pressure switches from Automation Direct.

Pressure sensor Part Number: PSD25-OP-5800H

(And the Solid State Relay: Crydom DC100D60)

https://www.automationdirect.com/ad...easurement/pressure_sensors/pressure_switches

Installation instructions
https://cdn.automationdirect.com/static/manuals/prosense/prosense_psd25_installation.pdf

I've updated the attached image to include the relevant data from the sensor manufacturer. I found my original notes, and believe the circuit is wired per the posted diagram - but I came up with the wiring on my own, and apparently, botched it.

Again- if I take 12v+ and connect it to terminal 3 on either SSR, that SSR will energize and operate the pump.

And, if I disconnect the pump from the SSR, the pressure sensors work correctly, energizing the SSR and shutting off when the set pressure is reached, etc. (I have a manual pump to pressurize the circuit for testing.)

But with the pump motor connected to the SSR, when the circuit is powered, the sensor appears to cycle on / off around two times per second- and the pump motor doesn't run. I can't get my head around what's happening...

Thanks for any help!!

Peter
 

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Thread Starter

Fortel

Joined Nov 3, 2020
13
Good catch- thanks for that.

Actually, they are grounded- although the drawing didn't show that very well. I'd used a sensor wiring diagram from some web site. But now I'm not sure exactly how (can't remember) the sensor is grounded.

My intent was that the sensor is wired in a normally closed fashion- so as soon as voltage is applied to the circuit, if the hydraulic pressure is below a set value, the sensor's output (pins 2 and 3) are closed to ground. Terminal 3 on the SSR is always hot. The hydraulic sensor / switch is only making or breaking the ground for terminal 4 on the SSR.

I'll go back to the project today to check the sensor grounding again, and will report back.

Thanks again! Appreciate your eyes and brain!!

Peter
 
Last edited:

sghioto

Joined Dec 31, 2017
2,373
If you switch the positive terminal of the SSR input through the sensor switch as shown in my drawing you won't need to connect 12 volts to this terminal. You can't switch the ground with a PNP output.
 

Thread Starter

Fortel

Joined Nov 3, 2020
13
This is making more sense now, and I had wondered about the ground switching!

Perfect information! I'll re-wire today, and will post back.

Thanks a lot for your help- looking forward to getting this working and behind me!

Peter
 

Thread Starter

Fortel

Joined Nov 3, 2020
13
If you switch the positive terminal of the SSR input through the sensor switch as shown in my drawing you won't need to connect 12 volts to this terminal. You can't switch the ground with a PNP output.
Ugh... no joy.

The circuit was wired per your drawing- I had originally tried it (a couple of months ago) based on my drawing- but that wouldn't trigger the SSRs. So I studied a bit, and corrected it a few weeks ago. When I saw that it still wasn't working, but with different behavior, I found that the circuit worked- the relays would trigger- as long as the hydraulic pump motor was removed from the circuit. I tested by connecting two different dc motors- one to each SSR. They correctly followed the SPDT switch position- One motor would rotate in one direction and, changing the switch position caused the other motor to rotate, in the opposite direction.

But with the pump motor connected to the SSR, the motor never spins, and the SSR never triggers- it's LED never illuminates.

However, with the pump motor connected, if a seperate power source is used (a 9 volt battery!) to apply v+ to the SPDT pole. and a common ground, everything works! The SSR correctly switches it's load, and the pump operates correctly.

I'm guessing the pump motor circuit must have something internal that is causing the problem. I'll look for a circuit diagram for the pump motor. Wondering if it makes sense to somehow try to isolate the power feeding the SPDT switch.

Thoughts?

Thanks everyone!!

PeterIMG_0349.jpg
 

sghioto

Joined Dec 31, 2017
2,373
Thoughts?
From the description it sounds like the 12 volts is dropping below the minimum voltage required for the sensor switch when the motor is activated. That would explain why it pulses off and on. You have verified this by the fact it works using a different supply for the sensors. You can try adding a diode and capacitor to isolate the 12 volts on the SPDT switch as shown below. Depending on the current drain of the sensors this may provide a enough voltage to the sensors while the battery recovers from the initial current spike.
EEE PNP sensor and SSR.png
 
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Thread Starter

Fortel

Joined Nov 3, 2020
13
This sounds plausible- and in fact, I ordered an analog voltmeter to see if I could catch the voltage dropping, rising, dropping...
Just received the analog meter, and will visit the project again tomorrow.

I know the output from the SSR is optically triggered, so there is some isolation from that. But I was wondering if the common power negates some of that isolation and somehow the Sensor / Solid State Relay / Pump motor was not compatible.

In any case, I'll work on this tomorrow, and will try the suggested circuit in front of the SPDT pole.

I'm sure this will all be sorted out soon, thanks to all the help I'm getting here. It's really appreciated!!

Thanks,

Peter
 

Thread Starter

Fortel

Joined Nov 3, 2020
13
Do you have consider about the current limitation for the input side LED of SSR?
I'm not sure I understand the question...

I believe the SSR's input side requires very little current- a few milliamps.

What has me scratching my head is, if I bypass the Sensor / Switch, and apply v+ using an incandescent test light in series from the battery to the v+ input (Terminal 3) of the SSR, the SSR activates, and the Hydraulic Pump Motor runs fine. The test light barely glows- showing very little current is needed to activate the SSR and keep it going.

But the Sensor is a different matter. Maybe I'll measure the current for the whole circuit, minus the pump motor.

I'm using a car battery to test all of this. Only today did I try activating the sensor using the small 9 volt battery- and everything ran fine under those circumstances- sensor activated correctly, SSR activated correctly, and the pump motor ran without fault.

So, it makes sense that it's either a low voltage issue at the sensor (per Sghioto) or at the SSR, or something else that I've yet to identify...

Tomorrow might shed some light.

Thanks everyone!

Peter
 

ScottWang

Joined Aug 23, 2012
7,048
I believe the SSR's input side requires very little current- a few milliamps.
From the links(Installation instructions) in post #5 that it shown the PNP output current less then 25mA, but the output voltage maybe too high for the LED of SSR, so you better in series with a resistor to limits the current under 10 mA from the Pressure Sensor.
 

Thread Starter

Fortel

Joined Nov 3, 2020
13
From the links(Installation instructions) in post #5 that it shown the PNP output current less then 25mA, but the output voltage maybe too high for the LED of SSR, so you better in series with a resistor to limits the current under 10 mA from the Pressure Sensor.
Still not sure I understand... but I want to learn...

As I understand the specs, the PNP Sensor can drive a 500 mA load, and it consumes less than 25 mA.
"Output Maximum Load Current 500 mA - Current Consumption < 25 mA"

The SSR has an input / trigger voltage range of 4 - 32 volts DC.


Control Voltage Range:4 VDC to 32 VDC
Input Current:11 mA to 14 mA
 

sghioto

Joined Dec 31, 2017
2,373
As I understand the specs, the PNP Sensor can drive a 500 mA load, and it consumes less than 25 mA.
"Output Maximum Load Current 500 mA - Current Consumption < 25 mA"

The SSR has an input / trigger voltage range of 4 - 32 volts DC.
That is all correct and the SSR input has a current limiter built in which explains the large input voltage range, no resistor needed.
 

Thread Starter

Fortel

Joined Nov 3, 2020
13
That is all correct and the SSR input has a current limiter built in which explains the large input voltage range, no resistor needed.
Thanks Sghioto!!

Looking forward to getting this all sorted out! It's for a small airplane- the hydraulic pump provides the pressure to raise and lower the landing gear. So, it's pretty important to me that I get this right! In any case, I will have a backup mechanical relay / contactor with a seperate switch on the instrument panel that can be pushed to activate the override circuit.

Will let you know how it goes tomorrow...!

Thanks,

Peter
 
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