High voltage DC question

[electronicsuk]

i will take some pics in the morning as im now on a night shift,

The coil and relay is quite large, not by any means a small one. The coil is around an inch and a quarter tall and 25mm diameter. I possibly could investigate the coil deeper, but id have to be careful as its sitting on a sheet of asbestos board.

I was thinking about replacing the coil and relay contacts with a modern one, hence me trying to work out the coil voltage!

The only lettering is on the 2 green resistors : AW113, 9Kohm, 5% mJ

Why such concern with matching the coil voltage to the old part? Choose a relay with adequately rated contacts and a known coil voltage, then simply calculate the required dropper resistors for 600v supply and replace the whole lot.

 

[JohnW2]

The point here is trying to find a replacement relay with the correct voltage coil from modern equipment, not 1940's technology.

I'm not clear as to what extent you want to keep as much of the original arrangement as possible (in particular, the two 9KΩ resistors).

If you are not concerned about that, you could simply use almost any modern relay with an operating voltage of less than 600V DC and then change the value of resistors to whatever was required (and that is simply a matter of Ohm's Law), with care to make sure that you used ones of adequate power rating. Your greatest problem may be in relation to the rating of the relay contacts - see below.

If you want to stick with the two 9KΩ resistors, then things are much more restrictive. You then have an available operating current less than 33mA. Reasonable size (as opposed to minature PCB mounting) 'general purpose' relays with coils rated for 24V DC generally have resistances in the range 300-500Ω, hence a current of 48-80 mA - i.e. too much. The next (fairly) commonly-available coil voltage is 48V, with resistances of 'reasonable sized' ones probably being in the range 1500Ω-2000Ω and they might be OK for the job. If you could find one with a coil resistance of 1500Ω,with the two 9KΩ resistors, there would be 45.2V across the coil, which would be OK for a coil rated at 48V.

However, whichever of those approaches you took, there is a potential for a problem in relation to the relay contacts. Are they going to be switching a 600V circuit? If so, I think your greatest problem may be in finding a 'general purpose' relay with contacts rated at anything approaching 600V DC. There are plenty of 'general purpose' relays with 48V coils around, but you'd be lucky to find them with contacts rated at much more than about 100V DC - so you might again be back to relays designed for the few areas now using those sort of DC voltages - trams. underground trains etc. and some electronic and industrial applications!

Kind Regards, John.

 

[Stoday]

If the relay is interrupting inductive dc loads, the contacts might well have magnetic arc quenchers. Not your run-of-the-mill stuff.

I would have thought that rewinding the coil would have been straightforward. You can measure the diameter of the wire and weigh it after stripping. Google "diy coil winding" for lots of coil winding jigs.

 

[bhm1712]

Thankfully the relay contacts are only switching 12 volts DC from the battery. The dewirement indicator is basically for when the pole comes off the wire - the 600 volts to the coil is lost which switches the contacts and allows 12 volts from the battery to sound a buzzer in the cab.

I think i have a solution thanks to your help

I will change the resistors to these:

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=searchProducts&searchTerm=485-3123

And then use this relay :

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=searchProducts&searchTerm=140-713

This should mean if my theory is correct that I will have a total resistance of 26790 ohms, so 0.0223 Amp. Across the relay coil i then should end up with 62.2 Volts. The coil is rated upto 66 volts DC. Its coil power is rated to 1.4 amps, and i have calculated that it will dissipate 1.38 watts.

Am i right?

 

[bernardgreen]

I will change the resistors to these:
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=searchProducts&searchTerm=485-3123

The data sheet http://www.vishay.com/docs/50008/rwm.pdf gives a limiting element voltage of 350 volts for this resistor. While this will not be exceeded with steady current flowing it might be exceeded when current varies due to the inductive effect in the relay coil creating transients. Also transients in the 600 volt supply as traction motors cut in and out could be in the order of a thousand volts or more.

Exceeding the LEV on a wire wound resistor can lead to the break down of the insulation between turns. At best this creates a short circuit between adjacent turns and reduces the resisistance by one turns worth or at worst a hot spot is created and the wire goes open circuit.

On the relay you need to check the insulation between contacts and coil and frame is able to withstand the transient voltages on the 600 volt DC supply.

Am I right?

Apart from the concern over LEV yes and the relay's insulation yes you are.

 

[bhm1712]

thanks. I will check this out.

 

[bhm1712]

The relay data sheet says that the dielectric strength between coil and contacts is upto 5000 volts AC, but the rated insulation voltage is upto 400 volts.

Im struggling to find resistors that can withstand the right voltage!

 

[flameport]

Relay, 220V coil, coil to contacts rated 2500v, coil resistance 40k
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=7150922

2x resistors, LEV of 1250V, resistance 33k each (total 66k)
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0159663

With 600v supply, voltage for relay is 226, with 187 across each resistor.

 

[Spark123]

oh poo, missed page 2

 

[bhm1712]

thanks for all your help guys, its priceless! (apart from ban-all-sheds)

Flameport, spot on! now why couldnt i see that as well! I was getting bogged down with sticking close to the original values!

 

[FDJ]

Have a look at what contactors with DC coils are actually available.
I suspect that a 50v coil is going to be the only candidate for replacement.
You might have to change one of the resistors to suit the operating current of the new coil once its resistance is known.

Frank

 

[ban-all-sheds]

ban all sheds - your answer is useless.

Only to someone not prepared to learn.

We can all google ohms law all day long.

I also said "You, or somebody associated with the rebuild of this tram, is going to have to get their heads round it".

It was quite clear from the things you were saying, and the questions you were asking, that you hadn't got your head round it.

IMO understanding what you're doing is essential.

 

[bhm1712]

Hi guys,

thought i would let you know that the indicator is working again!

In the end i used 22k resitors and a 110 volt dc coil relay (100-120 volt range)

Made a nice job of mounting the new (huge) resistors with heat sinks and relay with din rail and base.

Thanks for your help!