Interference suppression for Hornby Dublo.

[Dubloduke]

I have several 3 rail Hornby Dublo locos that have the suppressors removed. This is not a problem with modern TVs and radios. However, I enjoy listening to radio on long wave. The  obliteration of the radio signal from these locos is almost total.

       

Please remember that I`m `electrics` illiterate, but having looked at suppressor fitted locos it looks to me that there is a thing called a choke in the power feed and another thing that looks like a capacitor going from the live brush wire back to the chassis. I reckon I can make these suppressors if I know exactly what is required.

 

Visiting different internet suppliers to purchase these things I am amazed at just how many different sizes and power types there are.  Please can someone help with the correct size / power rating of these things.

 

Dublo locos are 12 volt 3 pole motors that draw between 0.20 amps and 0.60 amps.

 

Richard.

[Crosland]

I would start with just a 100nF (nanoFarad) 25V ceramic disc capacitor across the motor brushes. Buy some 1nf at the same time, to compare.

 

Maplin will have them, but  you will pay through the nose. RS are now offering free next day delivery for on-line orders, no matter how small the order. Worth a try

 

No harm in fitting similar across the track, even multiple ones around the layout. Standard Hornby power clips include a suppression capacitor.

 

The capacitor acts like a short circuit to the high frequency noise (long wave radio waves!) generated by the arcing between the motor brushes and the commutator (completing the circuit between one brush and the other), but an open circuit to the DC supply (so it has to go through the motor). The chokes, if fitted, act like an open circuit to high frequency (blocking it from escaping from the loco) but short circuit to the DC supply (allowing it through).

 

I know this is the non-DCC forum, but do not do this on a DCC layout

 

Andrew

[Dubloduke]

I would start with just a 100nF (nanoFarad) 25V ceramic disc capacitor across the motor brushes. Buy some 1nf at the same time, to compare.

 

Maplin will have them, but  you will pay through the nose. RS are now offering free next day delivery for on-line orders, no matter how small the order. Worth a try

 

No harm in fitting similar across the track, even multiple ones around the layout. Standard Hornby power clips include a suppression capacitor.

 

The capacitor acts like a short circuit to the high frequency noise (long wave radio waves!) generated by the arcing between the motor brushes and the commutator (completing the circuit between one brush and the other), but an open circuit to the DC supply (so it has to go through the motor). The chokes, if fitted, act like an open circuit to high frequency (blocking it from escaping from the loco) but short circuit to the DC supply (allowing it through).

 

I know this is the non-DCC forum, but do not do this on a DCC layout

 

Andrew

Thank you Andrew,

 

                              What is the difference between a 100nF and a 1nf capacitor, and what do I look for when I try them? 

Someone has suggested I try a 470pf capacitor, boy am I confused.

 

With regards the choke. Are there different sizes and which one do I need?

 

Richard.

[Gordon H]

What is the difference between a 100nF and a 1nf capacitor, and what do I look for when I try them? 

Someone has suggested I try a 470pf capacitor, boy am I confused.

 

100nF (or 100000pF) is one hundred times more capacitance than 1nF (or 1000pF). Usually a 100nF will have the numerals '104' written on it, and a 1nF will say '102'.

In each case the first two figures are the two most significant digits of the value, i.e. '10', and the third figure is the number of zeroes to follow (i.e. power of ten), always expressed in picofarads (pF). So, '104' means 10 x 10^4 pF and '102' means 10 x 10^2 pF.

The other proposed value of 470pF would be a '471'.

Your confusion is perfectly understandable, because it is very difficult to know what value will work best for this kind of thing as there are so many variables. Typically, lower capacitor values attenuate higher frequencies better, but have less effect on lower frequencies. The suggestion of 100nF is a reasonable starting point, especially if you are trying to guard against AM HF radio interference. It goes without saying that the capacitor needs to be a non-polarised type as the voltage on the motor can be of either polarity.

For the choke (or inductor), essentially the higher the value the better, so long as the inductor is adequately rated for the DC current that the motor requires without saturating, and fits in the space available.

[Dubloduke]

Andrew, many thanks for your patience.

 

This evening I have been digging through old Hornby Dublo stuff and have come up with this.

 

     "The Hornby Dublo capacitor wired across the motors was originally 500pF, then when preferred values came in, it was 470pF. However, it seems that Wrenn used 1nF ones on later production."

 

and this :-

 

        "The RS 1 amp suppressor chokes were 4.7uH"

 

I have found a source for 470pF capacitors and RS 1 amp chokes with a vaue of 4.7uH. Is there any corrolation between 470pf and 4.7uH?   Also is 1 amp sufficient for the motor, which draws a max. of 0.65 amps?

 

I suppose I now have to decide on capacitor values - 470pF or  100nF or  1nF.

 

Richard.

[Crosland]

As I said, in the first instance, I wouldn't bother with the chokes.

 

You are doing a subjective test of the effect in your particular environment so tune the radio to your favourite station, run a loco, try a capacitor across the motor brushes and see what the effect is. Try another and see if it's better or worse

 

Andrew

[Dubloduke]

Thank you Andrew, I will do that and see how it goes.

 

Richard.