A relay question....

[GonzoEngineer]

OK, I have noticed a lot of talented and experienced members here, so here is a question I need advice on.

I have commercial DC Power Supply that supplies 600VDC @ 50 Amps.

It has a control signal input to set the voltage. The input is a 0-10 Vdc signal to program the output voltage. (Input impedance is 10K ohms)

I need to implement a safety circuit to disconnect the control voltage and ground the control input in cases where I have a failure of the control computer.

It has been suggested that I use a relay. Personally, I hate relays, I call them RMD's...(repulsive mechanical devices!)

I was told by the "powers that be" to use a relay.

I was informed by another Engineer that "I need a special type of 'Dry Contact' relay since I am switching such a low current."

Unfortunately, this other Engineer has a habit of blowing smoke up peoples ass's.....so I would really like to learn if what he is saying is something I need be concerned about.

I have been googling all morning, and really can't find anything to support this statement.

Anybody here got any experience here on this?

 

[audioguru]

Most relays switch high currents so they have silver-plated contacts. But silver corrodes easily then won't switch low or no current. With a high current the spark burns away the corrosion.

A dry circuit has low or no current and gold-plated contacts are used for switches and relays. Under the gold is the usual silver contact because if an idiot uses it with a high current then the very thin gold plating vapourizes.

Since the gold is so thin then it costs almost the same as without.

 

[Leftyretro]

Yes, I've seen problems with the switching of low level currents (low milliamps or less) with relay switch contacts. Many relay contacts use a cross-hatch pattern and rely on the spark gap generated to burn clean the contacts with the normal switching action. If the relay spends 99.9% of the time deenergized then the contacts can tend to oxidize over time.

Solutions include using hermetically. Our solution was to use hermetically sealed relays that used either mercury wetted contacts, which may not be as available these days, or contacts with gold plated contacts. This usually solved the problem.

As it's a safety application you might consider using either 2 parallel relays or use a relay with double contacts and parallel the contacts for extra redundancy.

Lefty

PS: the term 'dry contact' at least as used in the industrial control world means that an external circuit will provide the voltage to be switched by the relay contacts. In fact, all electromechanical relays have 'dry contact's, it's what the contacts are wiring to that determines which side of the switching circuit has to provides the voltage source for the contact loop.

 

[GonzoEngineer]

Thanks.....two excellent posts!

 

[k7elp60]

Thanks.....two excellent posts!

You may want to consider the JWD series by TYCO. They are sealed reed relays. I did a search on mouser.com for JWD and got lots of choices.

 

[eblc1388]

You will need to use a SPDT reed relay. The pole is connected to DC supply control input and the two contacts to 0~10V input control signal and return respectively.

Wire up the relay such that its coil is energised while in normal running operation. If the relay coil failed, the DC supply will shutdown to 0V.

 

[GonzoEngineer]

You may want to consider the JWD series by TYCO. They are sealed reed relays. I did a search on mouser.com for JWD and got lots of choices.

Perfect!.....Thanks for the lead!

 

[ericgibbs]

Perfect!.....Thanks for the lead!

hi,
If you ever wanted to get user friendly with those 'RMD's' look at this pdf, very useful as a reference doc.

 

[GonzoEngineer]

Thanks Eric!

 

[armyrule]

can samebody show me how connection of dc motor control using 2 relay (fwd n bwd) and program using EASY68k?