[BTS]

I want to set up a pair of relays on a machine. I am using 12v DC for the coils and I am switching 60v DC. The relay is rated for 15 amps.
The problem I have is the contacts arc when the coil de-energizes and the contacts open. I have read some articles on the net about "snubber" circuits, however I have only found them for low amperage circuits.
I am having trouble determining the values for the resistor and capacitor to use in the snubber circuit.

Thanks

[Al_The_Man]

You could also look at P&B relays that have magnetic arc blow-out, there is a magnet placed between double pole contacts.
Also are you using DC rated relay, the switching current is lower for DC than AC?
What is the actual DC current you are switching?
Al.

[mike_Kilroy]

r&c best for ac load: u will 100% fix ur problem by putting a single diode across the contacts. 12 cents cost

1n4007 good for this size. or just get one at radio shack rated say 4+ amps, 400+ volts

[Al_The_Man]

r&c best for ac load: u will 100% fix ur problem by putting a single diode across the contacts. 12 cents cost

1n4007 good for this size. or just get one at radio shack rated say 4+ amps, 400+ volts

Mike, A diode across the contacts is going to prolong conduction, which is exactly what arc across air does once it ionizes, reverse biased it will be open circuit, forward it will conduct?
A diode across the Load may work to quench the arc.
Al.

[mike_Kilroy]

ya al - too much happy hour here tonite! tnx for correction - that is what i meant - put it across the 60vdc load - then when contact opens, the diode provides the reverse current path for the huge reverse voltage field generated in the inductive load, hence squelching the arc so it does not even happen.

[Al_The_Man]

The problem is that if the relay used is not DC rated and not supplied with arc quench of some kind, when the contacts begin to open and an arc starts to form, it continues as the relay opens, so that conduction carries on just as if the contact were closed, the load voltage does not reverse in this case so the load reverse diode does not come into play, if prolonged of course, the contacts melt and the relay is destroyed, this is the principle behind a plasma torch, and it can be seen that at 200vdc or 300vdc with enough energy behind it the torch arc does a pretty good job on steel plate etc.
We designed a generator field regulator for mobile cranes in scrap yards etc that switch 100A magnets, there can be no mistake made here on the type of contactor!
Al.

[mike_Kilroy]

true of course, but I suspect his issue is similar to one we had last year where we had 90vdc source to power brake actuators. Before sticking diodes across each brake coil we lost 4 relays in 2 days. Put diodes in and machine has been been running for near a year now.

[Al_The_Man]

It also depends on the nature of the load as to how much energy is stored and if a possible drop out delay is acceptable which is a down side to the diode usage.
Small loads it is not so noticeable.
Al.

[Curley]

BTS, Tyco has several application notes on relays.

Application Notes - TE: P&B, Potter & Brumfield, Agastat, Axicom, CII, Hartman, Kilovac, OEG, Products Unlimited, Schrack

Here are two notes from Potter & Brumfield:

http://relays.te.com/appnotes/app_pdfs/13c3236.pdf

http://relays.te.com/appnotes/app_pdfs/13c3203.pdf

These are good sources of information. Bless Google search!

[Al_The_Man]

I use P&B which are now Tyco, PRD-1DH0-110, they have other coil voltages, these have arc blow out fitted.
If it is a 15a 60vdc load that is almost 1kw DC to switch, I would hesitate using a regular general purpose relay in this applications.
Al.

[Curley]

That's the right stuff.

[BTS]

Thanks for the replies!
I am switching a nichrome wire heater that should be pulling 8-9 amps. I think I am using the wrong type of relay. I was using this:

Digi-Key - Z785-ND (Manufacturer - LY1-DC12)

It should work fine for the 40 watt 12v heating element on one system of the machine, but the 60v element is my problem child. The element resistance is right at 15 ohms so it should be about 240 watts.

[Al_The_Man]

Well at least you have a resistive load, not inductive.
But where do you get the 15ohms from? Heating elements are usually rated in watts for the rated voltage, Due to resistance changing between cold and at powered temperature.
If it drew 15amps at 60v when powered then it would be 900w?
The relay in question has a capacity of 15amps at 24VDC so you could be very close or over the limit if 900w.
For that application I would tend to use the PRD type mentioned earlier using the H or J contact code, as you do not have an inductive load, you MAY get away without the arc-blow out feature.
http://relays.te.com/datasheets/prd_ds.pdf
Al.

[BTS]

Maybe this info will help.
The heating element is two 6 foot pieces of 26 ga nichrome wire, wired in parallel and fed with 60v dc. These are then attached to a 1/4" thick aluminum plate that is layered with kapton tape . It measured 15 ohms at the feed point.
I have also looked at some solid state relays such as Digi-Key - CC1038-ND (Manufacturer - D1D20)

[Al_The_Man]

Yes I was going to suggest SSR, I assume this is a portable application where it cannot be run off AC? Otherwise that may be preferable,
If the 15ohms is the resistance cold, it will be quite a bit higher res. when hot.
Al.

[BTS]

It is a floor mounted machine. Using the 60vdc from the machines power supply. Didn't want to use line voltage ac on the bed where people will have to move parts around.

[Al_The_Man]

Is it switching or linear supply? If linear you could maybe come off the AC side?
Al.

[BTS]

It is linear. I am coming off the system bus bar because it has a breaker.