Electrical relay help

[kNucklBustr]

Stepping down voltage from a 48v 7.5A PS to 24v. This to power these relays from a connector terminal.

To determine the power resistors needed do I use the 1W coil rating specs from the datasheet?

[Al_The_Man]

The resistance of the 24vdc coil is 576ohm which would 1watt, so you need a 576ohm 1 watt minimum, series resistor.
2 watt may be more prudent.
Al.

[kNucklBustr]

Just cking, that what I got also.
However, Im confused, having read a similar setup that someone used 5W. Then complained it got too hot and changed to 10W. Im trying to figure out why he used 5W and it got hot and not make the same mistake.

[Al_The_Man]

Well the relay coil obviously dissipates the same wattage, (1w).
I am not sure about the ones below the 1w, it shows 900w, that must be a miss-print?
I see working it back they must be .9w or 900mw
Al.

[wildwestpat]

Read the full type number on the actual relay as there are two types of coil the type L is a 660 ohm coil and the type D has the 560 ohm coil. (See the sixth character in the part number.) There are further differences on contacts and switching current capability. They look like nice relays. Suggest you use the nearest preferred value wire wound vitreous resistor of 560 ohms or 680 ohms respectively for type D / L should be OK. (The data sheet also gives the tolerance on the voltage across the coil indicating the values are not that critical.) The resistor will get hot - too hot to touch. Use long leads to distance the body of the resistor away from heat sensitive surfaces including stray wires. Typically use heat proof sleeving with a wire on each end of 1/2 ich or longer. Use of a larger 5 or 10 watt wire wound resistor will have slightly lower surface temperature but will still get hot. If using a film or carbon resistor increase the wattage as these types have lower tolerance to overheating compared with wire wound types.

A very neat solution would be to use the metal clad wire wound resistors as these have bolt down cases and are occasionally listed on ebay or are available from Digikey and RS at a cost of $3 each however the list price is too high.

A big thanks for the data sheet link as I was looking for relays with this mechanical arrangement.

Hope this helps - Regards - Pat

[kNucklBustr]

Wire clad as in Nickel-Chromium? also can you expand what is "heat proof sleeving with a wire on each end". Im not familar.

Without going into depth, one relay will be for G540 terminal blk #5. providing on/off 13.6A lathe motor, allowing manual and Mach3 control of Estop.
Relay #2 power on/off a 10A accessory vacuum at same time.

Keeping original lathe switches. But may have to replace Estop, its SPST. Need DPST for manual and remote. Cant find one or they are wayy expensive.

[Al_The_Man]

I am curious why go with the 24v coil when 48vdc are available?
Also if two items are switched at the same time, why not go with a double pole relay?
Or if you already have them, put them in series?
If you are using a push-off/twist-release-on E-stop, you can often get auxiliary contact blocks for existing P.B.'s.
Al.

[wildwestpat]

Heat proof sleeving is a glass fibre woven tube and is available either uncoated or with a varnish coating.

http://www.alliedelec.com/images/pro...E/70139244.pdf

RS International stock this but you should be able to pick it up from any electricians store as it is often used to cover the wires inside light fittings. There s also a polyester sleeving but it is not as heat proof. Ceramic beads used to be available but I have not seen these for years.

Regards - Pat