2004 Track Circuit Calculations

[Peter]

Hi,

Thanks for the reply.

As per the bullet no.1 mentioned in the question, I have taken the value as 0.7v. Please let me know if am wrong while considering the values.

Here is the question, for your reference please.

A d.c. track circuit is provided with an additional resistance at the relay end, connected in series with the relay coil. The characteristics of the track circuit are as follows.
Feed transformer rectifier output: 5.0 Volt
Relay coil resistance: 9.0 Ohm
Relay pickup current: 40.0 mA
Ballast resistance: 2.5 Ohm km
Length of track circuit 600 m
1. Calculate the value of the additional relay end resistance that will ensure a minimum rail to rail voltage of 0.7V when the track circuit is clear of trains.
2. Calculate the value of the feed resistance that will ensure reliable operation of this track circuit.
3. Calculate the drop shunt when the feed and relay end resistances have the values you have calculated.
4. Explain briefly the advantage of the additional relay end resistance.

Thanks in advance.

Regards,
Jenni.

That helps a lot. Sorry, I don't seem to have a copy of 2004 paper. What you put down numerically is very good. You have used the correct numbers and appropriate formulae and get a sensible set of values.

One thing, note that 2. asks for the value of the feed reisitor that will give reliable operation. The value that you worked out was to give you the critical voltage on the rails that will just pick the relay when the TC goes clear. This would not really count as reliable so you should have either maded a qualitative statement like "for reliable operation, need ot make sure there is a bit more current, therefore pick a value x% lower than this" or build a margin into the calcs by saying that you want critical rail volts + (say) 20% for reliable operation = 0.84v.

Being the cruel tormentor that I am, I'll not answer your question directly on the purpose of the relay end resistance, but suggest that you run through the calculations for operating the TC without the extra resistor fitted (eg calculate rail volts and hence feed R then DS with just the relay (hint, the rail volts then equal relay volts)) and then try to explain what difference putting the relay resistor in there does and why this might be beneficial.

Peter