Solid state relay.. will this scheme work?

[Cryogen]

Ok.. we have this system in place already. We use the valve to dispense a drop of liquid onto containers travelling upto 300 per minute speed. This is our normal mode. Lets call it Mode A from now. During this mode, the valve needs around 24V to activate (as it is between 25% to 50% duty cycle). So the output of the PLC is tied to the valve directly.. Everything works well.. But soon we realized we need another mode, Mode B, when the valve has to remain open. We need to add this to make the valve dispense liquid all the time.. Of course we do not want to change much of our existing set up. If we use the same PLC output and leave it ON. The valve gets 24V at this 100% duty cycle when it requires only 13V so the valve coil gets burned.

I have lots of spare outputs on the PLC. I am looking at various options to make both modes work. In my previous post, I used a separate output and a resistor to drop down the voltage from 24V to 13V. I got several suggestons and I decided to go with using a separate 12V power supply thikning it would be easy for people to understand, troubleshoot and replace if needed.. (The valve does work with 12V to remain open)

The PLC output now decides the mode the valve needs to operate - Mode A with 24V or Mode B with 12V. Swithcing between Modes can happen often too but not as high as 300 per minute. Now, I decide to use such a scheme below. This scheme will help ensure only one of the two go to the valve and also offers isolation. But I need to look for the right products..

I am very much open to other views too to make this work. As I mentioned, we want the scheme to be reliable, easy to troubleshoot and preferably compact.

Hope this helps.. please let me know your comments/questions.

 

[rsdoran]

This ain't pretty but should do the trick.

The switching relay can be any standard DPDT, the SSR has been mentioned.

 

[keithkyll]

Leave the 24 volt setup as it is. Use a second output to control the 12 volts. Use an additional relay on the 12 volt side if you only have 24 volts from the PLC.
Put a diode in series with the 24 volts if you want to insure isolation on that side (not the 1N4004).

Update: Ron's post should work fine, and is probably the best solution. If you want to do something else, then the 2 SSR's I mentioned previously (from Crydom) will do it. As long as you are using 2 outputs, then 2 standard SSR's should do it. Set the NO/NC logic in the PLC.

 

[rsdoran]

Leave the 24 volt setup as it is. Use a second output to control the 12 volts. Use an additional relay on the 12 volt side if you only have 24 volts from the PLC.
Put a diode in series with the 24 volts if you want to insure isolation on that side (not the 1N4004).

Update: Ron's post should work fine, and is probably the best solution. If you want to do something else, then the 2 SSR's I mentioned previously (from Crydom) will do it. As long as you are using 2 outputs, then 2 standard SSR's should do it. Set the NO/NC logic in the PLC.

Thank's Keith, now to get an update from Cryogen.

FYI: I was saying the pic was not pretty, not the method.

 

[Rich1955]

Did you ever think of a firing card for the SSR. It takes an 0 to 10 volt input to drive a porportional SSR output. In the sence of pulsating DC,It should stop you from burning out the coil to the valve. Might work or not, just a though.

 

[Cryogen]

keithkyll, you are right.. one of those options should work.. I personally like the diode on 24V side and one SSR for 12V.

rsdoran, I don't clearly understand the scheme you have presented. So besides my pulsing 24V PLC output and the PLC output that determines the mode, I need another output to activate a relay? Is it to turn the valve off? I could do that by just not giving the 24V pulsing output.. Am I missing something.

I really appreciate your help.

 

[rsdoran]

Output 1 would determine which supply voltage is used, when not energized 24v is supplied, when energized the 12v is supplied to the SSR. Which voltage supplied would be based on mode.

The output that pulses to trigger the valve will now pulse the SSR and the valve will receive the supply voltage, 12 or 24, through the SSR.

This would require bonding the commons of the 24v and 12v supplies together or if you want to keep the power supplies isolated from each other then use a 4PDT relay to switch both hot and commons.(http://www.zonefirst.com/products/4PDR.pdf)

If I get a chance may try making a drawing.

 

[milldrone]

Check this other thread

In another thread
http://www.plctalk.net/qanda/showthread.php?t=29437

krk posted a pic of a possible solution at post#20. This may not work for you because you were not to keen on using a resistor. It's simplicity speaks for itself.

 

[keithkyll]

There's one more method. The easiest if the PLC will support it. PWM.
5 pulses per second under normal operation. PWM at 50% duty cycle when you want to keep it open.
Aw, never mind. The inductance from that 1 amp coil will probably make PWM impossible.