what capacitor for decoder

[andel1]

hi all

one of my locos is stoping at every point so the decoder ive fitted has the two wires coming out from the back of it for attatching a capacitor but i need to know which one and which wire is positve blue or black

regards

andrewp

[Chrissaf]

It would help if you specified the decoder brand so that the original manufacturer 'stay alive' options could be researched on the Internet**. With regard to capacitor type. Typically these will be Electrolytic can type capacitors with a 16 volt rating. Personally I would go with 25 volt rating as a minimum to provide a safety margin. With regard to the capacitor value in uF (micro-farads). The biggest value you can fit in the space available.

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Note** without knowing the decoder brand, it may be that the manufacturer has used different colours for the 'stay alive' feature compared to the normal decoder function wires. Before connecting a capacitor to the blue and black wires, I would put a voltage multi-meter across the wires and check polarity of the measured voltage (about 15 volts DC give or take a bit). One would expect the 'blue' wire to be positive and the black wire to be negative, but this can not be assumed.

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Super capacitors could be used, but these need special consideration with regard to voltages and 'in-rush' current protection. This previous post has more information on using Super Capacitors.

https://www.hornby.com/uk-en/forum/post/view/topic_id/13404/?p=2

[andel1]

Sorry chris its a lais 4 function decoder 

[RDS]

According to the Instruction Leaflet for that Decoder, (as Chris correctly says) the Blue wire is +ve and the Black is -ve.

http://laisdcc.com/LaisDcc_Decoders_Manual.pdf

[andel1]

thanks ive found all that out but dosent the capacitor have to be ac not dc

[Chrissaf]

Absolutely not AC.

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The capacitor is Electrolytic and will be destroyed if put across an AC voltage. In a 'stay alive' scenario, the capacitor is placed AFTER the bridge rectifier that converts AC to DC (pulsed). The function of the capacitor is to be charged up by the DC output of the bridge rectifier, it then holds that charge to be used when needed. When track voltage is lost, the capacitor discharges into the decoder to make up for the loss of track current. As an analogy think of the capacitor as being a small capacity rechargeable battery. Other types of capacitor are designed for use with AC. Electrolytics are not one of them.

[Chrissaf]

This is a typical Electrolytic capacitor that could be used for a decoder 'stay alive' function. Notice the 'short lead' nearest the camera. Notice how this is marked 'negative' on the capacitor body (the band of long oblong boxes on the light colour background). Notice the 'long lead' furthest away from the camera. As is common with any electronic component that is polarity sensitive. A 'long lead' denotes the 'positive' voltage connection. Thus the decoder 'blue wire' would connect to the long positive lead and the decoder 'black' wire would connect to the short negative lead.

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[andel1]

so were dose the bridge rectifier go

[Howbi]

Andel1............the bridge rectifier is already part of the decoder..........you only have to solder the 2 wires to the capacitor as Chris has already described.......HB

[Chrissaf]

Andel1, as HB says, the bridge rectifier is included in all DCC decoders.

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Original image taken from Brian Lambert's (Flashbang) web site and modified to represent the additional blue and black 'stay alive' wires of the Lais 4 function decoder. This schematic represents a typical 'stay alive' equipped decoder circuit. Maybe answering your question in the form of a drawing will help make the answer as clear as crystal.

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[andel1]

Thanks guys that's fantastic  now my princess Victoria will run like the others 

[81F]

 Funnily ednough I was going to ask a similar question for the same type of Chip!

 

However, could someone tell me how large the 2200uF capacitor is as I wish to fit it to a cornby open cabbed pannier (or if it's a bit tight I'll exchange chassis with a Junty, J83 or my own 1701 saddle tank which has more room inside.

[37lover]

81F,

I'm sure Chrissaf wil be along shortly; in the meantime I can advise that a 16V 4700 uf cap that I used for lighting in my Pullman is 12mm dia x 25mm long. Connection wires, that can be bent, protrude up to 30mm.

[Chrissaf]

Thank you 37L, just logged on and seen the posted question from 81F.

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81F, as I said in my first reply, fit the largest uF value that will fit in the space available. However in general, 16 volt Electrolytics will be smaller than 25 volt ones. There will also be some dimensional size differences between different capacitor manufacturers. Super Capacitors will be even smaller but need to follow the construction guidelines I gave in the link in my first reply. Three 5.5 volt Super Capacitors mounted in series to obtain a 16.5 volt rating could be made in a cylindrical size package about 12mm diameter by about 16mm long.

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This Super capacitor for example is 12mm x 4.5mm. Three of these in series would give a capacitor of 16.5 volt rating with a combined in series value of 33,000uF. Remember to include the resistor / diode in-rush current protection described in the linked post if contemplating a Super Capacitor implementation.

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/media/tinymce_upload/aa823ef1776483c777e70b9e78275911.jpg

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This standard 2,200 uF 16 volt Electrolytic capacitor by comparison is only 6.7% of the (three 0.1F combined in series) Super Capacitor value and physically larger at 12.5mm x 25mm. Excluding terminating leads. This example size could be larger or smaller depending upon who made it.

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/media/tinymce_upload/cf3cdce4029ac22c734c602a8aae637c.jpg

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PS - The images and stated sizes were obtained by a quick rough and ready search of what was selling on ebay. The physical sizes were 'as stated' in the ebay product descriptions.

[Nick_]

I think a stay-alive is going to need a lot more capacitance than 2,200 micro-farads to be of any use. I'm guessing at least 10 times as much, if not more? 

i have found the stay alives from DCC concepts do not contain enough energy to keep a loco moving, whilst the TSC keep alive will allow an 0-6-0 tank engine to keep moving for 10+ secs without track power!

[Chrissaf]

In principal I would agree with you. The 2,200uF value was just shown to indicate the style of an Electrolytic capacitor and demonstrate the size difference between that and much higher capacity Super Capacitors. However that said, I paste below an extract of a post made by Daz1966 in a previous post on the subject.

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"I haven't purchased the components to make a super capacitor yet however I had some 3,300uf 25V caps in stock so I tried one of those on an erratic running class 56, its now by far my best running loco, will creep around my 18' x 8' layout that contains dead frogs and crossovers without the slightest hesitation."

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This shows that how effective smaller value capacitor values are, is somewhat dependent upon the track environment they are operating in and the current consumption of the loco in question under different loads.

[81F]

 Many thanks for the help.