Control Relay Grounding Issue

[mrgilpin]

I'm trying to refurbish a Heliostat and utilize a computer to control the three phase motors that came with the device. However, I'm having an issue with my control circiut for the three phase relays I've purchased.

I'm using two Struthers-Dunn 645RT series SS motor reversing relays which can be found here

http://www.struthers-dunn.com/images/645RT%20Flyer.pdf

The wiring diagram on the 2nd page shows both the forward and reverse control inputs.

I can get both of these to trigger independently utilizing my digital IO board attached to my computer. However, if both of these inputs are connected simultaneously the relays do not work.

My digital IO board only has a single ground, and if both the forward and reverse control inputs share a common ground, the relay will no longer trigger.

The only way to get one of the control inputs to work is to physically disconnect the ground of the opposite input.

I've tried to install diodes on the ground lines to prevent current leakage up into the opposite control but this also didn't work.

Is there a way to electronically isolate the grounds on these control inputs?
I purchased these solid state relays for the switching speed, however, Struthers-Dunn says that these relays are difficult to control unless mechanical relays are used for the control inputs.

 

[Fish4Fun]

An odd note about the diagram on page 2 of the referenced document: It appears the LED input on the right-hand side (Reverse) is biased incorrectly by the battery, while the LED on the left (Forward) appears to be biased correctly. Is it possible the diagram is wrong, and the center terminal is actually a common cathode?

In any case, you might consider using 4 outputs from a pair of 4053's to control your LED inputs:

http://www.onsemi.com/pub_link/Collateral/MC74HC4051A-D.PDF

There are different models with different voltage ratings, but this one should work for you. The 405X series of bilateral switches are really very handy.

Fish

 

[mrgilpin]

An odd note about the diagram on page 2 of the referenced document: It appears the LED input on the right-hand side (Reverse) is biased incorrectly by the battery, while the LED on the left (Forward) appears to be biased correctly. Is it possible the diagram is wrong, and the center terminal is actually a common cathode?

In any case, you might consider using 4 outputs from a pair of 4053's to control your LED inputs:

http://www.onsemi.com/pub_link/Collateral/MC74HC4051A-D.PDF

There are different models with different voltage ratings, but this one should work for you. The 405X series of bilateral switches are really very handy.

Fish

Thanks for the help!

I didn't notice that error in the diagram, maybe that has something to do with it. I think one of the issues might be that I'm using a older model of this relay that was shipped to me by Newark. My relays do not have indicator LEDs, however, I assume that they are using similar circuitry.

Putting a voltage across the two control inputs individualy as indicated in the diagram DOES work though.

Whats strange is it only functions if the two grounds are not interconnected. I can pull the positive input from the opposite control (breaking the circuit?) and the system still does not function. It's only when the ground are physically separated that the system works.

I'll have to give the 405x series chips a try, however, I'm hoping to get this working by tomorrow afternoon so I don't have time to source the parts.

I have a whole box of CRYDOM DC60S3 SSRs laying around. I'll try to use these to separate the grounds for the system...

UPDATE! Using the CRYDOMs to switch the grounds works. If the 405x chips operate similarly I'll be able to solder up a interface box between the computer and the relays. That'll look much better than the breadboard monstrosity currently sitting on my desk

 

[Fish4Fun]

mrgilpin,

I do not think those are indicator LEDs, I would assume they are opto-isolators, most likely OPTO-TRIACs. Great news on the CRYDOMs working! 4053's are a bunch cheaper and smaller, but if the CRYDOM's work, stick with them!

Fish