Building a test circuit

[rjb]

I am in the process of building at home a test circuit on a sheet of MDF to replicate my electrics at home (except blowing G9 bulbs) so I can practise testing for 2330, 16th edition and 2391 etc etc.

Anyway I want to replicate accurate voltage drop so I want to put resistors in series to replicate say 20m of a 2.5mm T+E run for the ring final circuit representing CU to first socket. You get the picture.

Where can I buy such a product. I will be plugging the whole testing board into my own ring final circuit via it's own CU.

I will of course want various resistances which I fill calculate of course using the tables in 7671 being I am such a swot

Can anybody send a link.

 

[Space cat]

If you want ready made resistors your best bet might be RS Components. There's no point posting a link though because RS keep removing them! I had a quick look at Maplin but their power resistor range is very limited.

Have you considered wattage yet? A twenty metre length of 2.5 sq mm wire is about 150 milliohms. Put 20 amps down it and you'll have 60 watts to get rid of - and another 60 watts from the other core!

I used to make my own high power resistors (for testing amplifiers) using wire from old electric fire elements wound into air spaced coils and strung between nails in a piece of wood. I had a pair of 8 ohm 60 watt ones for years. They lit up nicely at full power. Happy days.

If I was doing it now, I'd probably scavenge the wire from a broken fan heater.

 

[breezer]

i beleive the wire is Nichrome, since it has a very high electrical resistance.

great for putting 12v on a short bit and cutting polystyrene

 

[ColJack]

if you have the cable to spare then just make your board a box with 3x2s and just coil the 20M of T+E under it..

can't get more realistic

 

[TicklyT]

Something like this?
150 milliohms would be about 30mm of wire, but I should think that would have a bit of trouble trying to dissipate 60 Watts.

If you want to wind your own non-inductive resistor, then bring termination out as a centre tap of the winding, and both ends commoned together as the other. You then have two parallel resistances wound in opposite directions, so the inductance is self-cancelling.

http://www.maplin.co.uk/search.aspx...esistance Wire&FromMenu=y&worldid=3&doy=21m10

 

[Space cat]

150 milliohms would be about 30mm of wire

By a remarkable coincidence that's very close to what I got with the old fire element wire, namely 7.5 ohms per inch. I wound mine as coils of about 3/8th inch diameter and spaced the turns a couple of wire thicknesses apart. They used to glow dull red at four amps - which is hardly surprizing since the original element was designed to operate at that current.

If you want to shove twenty amps through your resistor you'll have to use several strands in parallel. Five strands won't be enough unless you space them out. Thanks to the three halfs power law you'll need eight or nine of them. To keep inductance down I would lay the wire out in a hairpin shape.

 

[ColJack]

3 halfs power law?

care to explain as i've never heard of that one..

 

[Space cat]

The three halfs power law - as I've always understood it:

Take a simple example. You might expect that 6 sq mm cable would be good for four times as much current as 1.5 sq mm cable. It isn't - even though there's four times as much copper in there to carry the current. What's going on?

The problem is that you only have twice the surface area through which to lose heat and so you can only afford to generate twice the power per unit length of cable. (Four individual 1.5 sq mm cables would of course have four times the surface area of a single one) The resistance is down by a factor of four and you can generate twice as much power so the current can go up by a factor of 2^(3/2).

More generally, if you multiply a cable's diameter by a factor x, it will carry x^(3/2) times as much current for the same surface temperature. That, in a nutshell, is the three halfs power law. A mechanical engineer once told me that it turns up in lots of thermal problems but I never went looking for any others.