Value (spec) of Motor Suppression Capacitors?

[highpeakman]

I am just involved in selling some of my locos which are DCC fitted. Many of them have the decoder hard  wired in and had the original installed suppression circuits removed at the time.

 

It seems that most people feel they would be easier to sell if they are returned to straight DC operation before sale and so I am just about to start removing the decoders. I feel it would then be  a good idea to replace the suppression capacitors but the refitting the original suppression boards is not always possible and, anyway, it is often difficult to know which ones of those I still have came with which loco. 

 

I know that I can buy the Hornby parts such as X8236 which include the cap and a choke but I feel that those are "expensive" (relatively speaking) when I should be able to buy just the caps more cheaply from an electronics part distributor and in more volume than those available from suppliers of Hornby parts (when postage sometimes costs more than a small number of parts available from any particular model supplier). 

 

Does anyone know the specification - values (in pF) of such ceramic caps please? (And the chokes (inductances) if known?)

 

I have searched this forum and the web for this info but couldn't find the information so apologies if this has already been discussed.

 

Thanks for any responses.

[Nigelcliffe]

I had cause to look online recently, and saw an image which indicated a Hornby item is 0.1uF.    Can't be certain that's correct, but it's what I found.  

 

The suppressors don't assist with DC running, they just stop the loco being a spark transmitter to interfere with other radio devices.   405 line TV's are particularly affected, if you live in 1970.    I'd regard their replacement as of very limited value.  

 

 

[Crosland]

What Nigel said. Maybe even 0.1uf. Ceramic disc.

[highpeakman]

2 hours ago, Nigelcliffe said:

I had cause to look online recently, and saw an image which indicated a Hornby item is 0.1uF.    Can't be certain that's correct, but it's what I found.  

 

The suppressors don't assist with DC running, they just stop the loco being a spark transmitter to interfere with other radio devices.   405 line TV's are particularly affected, if you live in 1970.    I'd regard their replacement as of very limited value.  

 

 

Nigel.

Thank you for responding. 

 

Yes, I appreciate what the capacitors do, or at least, are supposed to do even if in practice, for this application at least, they achieve very little. Originally I was not intending to fit them but, as I am selling the locos, I wasn't sure if some people would expect them to be fitted to a DC loco they purchase.

 

I spent my career in high frequency electronics (Radar, Mobile phone, Satellite transmissions and Broadcast transmissions) and have seen some strange interference problems (but rarely) and would totally agree that it is unlikely that locos would cause much of a problem these days. I suppose I imagined that the Law of Sod taking effect to the point of a customer coming back with "Your loco interferes with my tele"!     🤔  (Yes, I remember 405 line stuff! 😁)

 

Really I suppose I thought it is just a case of better safe than sorry. If a simple 0.1uF ceramic will do the job then perhaps I may as well fit it. I'll think on that.

 

One last thought: Theoretically speaking, is it a legal requirement? (Even if unlikely ever to be traced 🙂)

 

Anyway, Thanks for your comments. Helpful.

 

 

 

 

 

[highpeakman]

1 hour ago, Crosland said:

What Nigel said. Maybe even 0.1uf. Ceramic disc.

Thanks for that suggestion. Sounds about right. 

[highpeakman]

17 hours ago, Nigelcliffe said:

I had cause to look online recently, and saw an image which indicated a Hornby item is 0.1uF.    Can't be certain that's correct, but it's what I found.  

 

The suppressors don't assist with DC running, they just stop the loco being a spark transmitter to interfere with other radio devices.   405 line TV's are particularly affected, if you live in 1970.    I'd regard their replacement as of very limited value.  

 

 

 

I looked a bit further and agree with the value you suggest. The ones i had weren't marked but I found a later one marked as 104 which is 1000pF (or 0.1uF). Available on Ebay at less than £3 (inc postage) for 100 pcs. Will probably buy these and fit where convenient. Thanks for the assistance. 

[AndyID]

11 hours ago, highpeakman said:

 

I looked a bit further and agree with the value you suggest. The ones i had weren't marked but I found a later one marked as 104 which is 1000pF (or 0.1uF). Available on Ebay at less than £3 (inc postage) for 100 pcs. Will probably buy these and fit where convenient. Thanks for the assistance. 

 

104 is 0.1uF or 100nF or 100,000pF 😀

 

It's not impossible that a smaller value, perhaps 10nF, would be more effective as it will have a higher self resonating frequency but you might have to buy a really old TV and TV station to find out.

[highpeakman]

14 hours ago, AndyID said:

 

104 is 0.1uF or 100nF or 100,000pF 😀

 

It's not impossible that a smaller value, perhaps 10nF, would be more effective as it will have a higher self resonating frequency but you might have to buy a really old TV and TV station to find out.

You are right, my error. Thanks.

 

Anyway 0.1uF will do the job I want them to do so I am fitting those as being better than having no suppression at all. 

[Crosland]

On 13/10/2022 at 19:56, highpeakman said:

One last thought: Theoretically speaking, is it a legal requirement? (Even if unlikely ever to be traced 🙂)

 

Second hand sales are a bit of a grey area.

 

It is a legal requirement that new equipment does not cause, nor is susceptible to, interference. It is not a legal requirement that you achieve this by using a capacitor. You will also see inductors (chokes) and multiple capacitors in locos.