Large Scale Central

Nonpolarised capacitors help required

Electronics Power and Sound
#1

Having a break from largescale for a year or two, I have taken an interest in British ‘OO’ scale. Commonly found on mass produced locomotives are three-pole ringfield motors. To smooth out the impulses and make for a better, smoother running motor, it was suggested to take two capacitors (4700uF, 15 volts dc) and solder the two negative leads together and then solder the free positive leads to the motor terminals. This parallelling of the capacitors apparently gives a non-polarised capacitor with resultant 2350 uF rating. The purpose is to smooth out the 120 degree pulses in the motor (three pole armature).

The author then suggests placing a ‘greencap’ across the motor terminals to further smooth the motor pulses. What is a greencap? Also, will the capacitors have an impact later if I change over to DCC control or would I need to disable analogue control on the decoder?

#2

http://www.talkingelectronics.com/pay/BEC/Page18.html

#3

Mike,
thankyou, I read the link. Seems a difference in opinion as regards ‘uF’ value when parallelling capacitors. My source states the rating is halved and the link provided states the rating is doubled when connecting in parallel. Series connection (negative leg to positive leg) gives a halving of the rating value of one capacitor. Connecting the negative legs together is obviously parallel connection, so now actual value is up in the air.

  Seems like a greencap is just a small non-polarised capacitor.
#4

I don’t know anything about capacitors and one of my mates laughs at my ability to destroy the best protected circuits, actually I am going to start hijacking your thread Tim. I am having a break from 24th scale myself and getting back to some H0 scale modelling, I hadn’t done any for a long time and I am really enjoying myself, I will get back there one day but I have a lot of unfinished H0 to complete first. Gives one a fresh look at the hobby.
But my 2 bobs worth Tim is to work hard at tossing those crappy old ringfield motors away if you can, there are a lot of nice smooth running mechanisms out there now, you may even be able to buy replacement chassis for some of the older English stuff that have a far better running worm and gear drive.

Wayne

#5

Wayne,
being a masochist, I actually like ringfields. I believe that it is lack of power pickups that is their downfall. I am suprised just how smoothly one can make them run, both Lima and Hornby with just a little TLC. Similarly, I was feeling a little peeved with constant largescale kitbashing and needed a break with something more ready to run. The British railway scheme has rekindled a childhood interest from fifty plus years ago. I even have old Tri-ang motor bogies working beautifully (smooth as silk).

#6

I remember small caps being rectangular and green, long time ago… I think they were mylar or mica or whatever… nonpolarized, and in the small mfg range like .01, .001

Noise is normally filtered with caps in the .1 to .01 mfd range.

Greg

#7

I’m not sure what to make of Tim’s wiring configuration except to state that for electrolytic capacitors it’s neither serial or parallel. A series connection requires + to - just like putting batteries in series. The resultant total capacitance is expressed as the reciprocal of the sum of each capacitor’s reciprocal. The shortcut calculation for two capacitors in series is the product of the values divided by the sum of the values. (Also, the total working voltage rating of a series capacitor assembly is the sum of all the working voltages.)

To “add” the capacitor values requires a parallel connection. In Tim’s case this would require that both - leads be connected in common and both + leads be connected in common. These combined leads would then be connected to the DC motor terminals taking into account the maximum voltage with margin for any back EMF coming out of the motor.

Greg is right…in general, for any given volume and working voltage non-polarized capacitors will have much less capacity than an equivalent polarized (i.e., electrolytic) capacitor.

Frank

#8

Wiring a pair of electrolytic (Polarized) caps IN SERIES back to back is an old engineering trick to make a larger valued Non-Polarized cap. Since the caps are wired in series, the value is 1/2 of the specified capacitance value, but the voltage rating is equivalent to the specification for one cap. So a pair of polarized 4700uF, 15 volts (DC) caps wired in series with the negative terminals tied together would be rated as a non-polarized 2350uF, 15 volts (AC) cap.

Why do this? Try to find a non-polarized 2350uF, 15 volt capacitor needed for this application. If you find one it will be orders of magnitude larger than the locomotive. Check out some AristoCraft or LGB motors where pairs of caps have been added to snub the voltage spikes and reduce the RFI emissions.

Also the specified voltage should be at least twice the max working voltage, so I would recommend using at least 25 Volt caps for an OO locomotive.

The addition of these caps can cause problems for some DCC decoders, especially for those with BEMF speed control.

Ken

#9

It’s called a bi-polar capacitor. There is a manufacturer mentioned frequently on these forums who uses now a noise suppressor board on the back of the motors. Early ones used two I think 22uf electrolytics, in a bi-polar array. First one of these I got, gee, ran forward just great, would not back up unless full power was applied, and then it just crept. Took it apart, removed the board, connected directly to the motor…ran fine in both directions.
Looked at the board. CLEARLY marked for “+” and “-” leads of the caps…one was installed backwards.
Doesn’t matter if you use electrolytics or ceramic or mylar…as long as the array is the desired voltage and capacitance.

#10

Amen to the above. I recall making non-polarized caps that way in 1974 at IBM under the watchful eye of the senior electronic technicians. Classic old school technique that works fine if size or what you have on hand require such a solution.

Then for “fun” we would hook up the tantalum capacitors backwards and watch them explode in a puff of orange smoke.

That was probably bad.

#11

Or take a HUGE electrolytic, bend the wires, stick them in a wall outlet for a couple of seconds, then lay it on a workbench.
And wait for someone to walk along and pick it up.

#12

Tantalums probably bad??? Bad is getting one or more 2200 uf aluminum electrolytics installed backwards in a 500 VDC linear supply. Big boom! Lots of hot electrolyte! Holes in suspended ceiling! Use to be standard occurrence in old school US manufacturing facility, but current state of art in one Asian facility.

WARNING - The same explosions can happen if the caps are installed incorrectly on motors. The smaller caps will yield a smaller but still dangerous pop.

Simple definition of a Defibrillator: a HUGE electrolytic capacitor discharged across a chest. Not good for healthy humans. For pranks try SMALL electrolytics - say 10 to 47 uf. You will still be able to smell a little burnt flesh from your former friend or acquittance.

Ken

#13

Since the wires of the cap were about 1/2" apart (the distance of an outlet), getting across it, arm to arm through chest, was not something that happened.
Instructors went ballistic…almost as bad as when we’d bring .45ACP’s into metal shop and stick one in the forge.

#14

I will touch nothing on the TOC workbench in the future!!

Paul

#15

Carp (sp.) Here I’ve been charging these things up just to catch you.

#16

Reminds me of an old story about Ford ‘trembler’ coils and metal gentlemen’s urinals. Apparently, the charged coils were placed awaiting someone to ‘complete’ the circuit.

#17

The family story goes, my Dad and his brother took a Model T Ford coil (wood box, vibrator type…looking at one right now) before a family gathering, set it up with a battery and knife switch in one room, stuck the wires through the wall, connected to bare thin copper wire wrapped on the toilet seat…one leg on one side, one leg on the other…and when someone sat down, they completed the output circuit, and you could hear them scream for blocks.
Found one of these coils years ago in an antique store and bought it, just to have something to show folks when I related the story.

#18

Glad it’s the coil you show people.

#19

Funny…most folks nowadays are ignorant of how a Model T worked…the manually shifted automatic…the way you depressed and lifted pedals.
There were 4 coils, in wood boxes, in a tray/shelf on the scuttle (firewall), and the vibrators caused them to throw continuous spark. There was no distributor, no points, nothing. When the air/fuel mixture got compressed enough, it fired.
Thermo-siphon colling system, with no water pump.
Fuel tank under the seat, gravity fed…long uphill runs, had to swing into a driveway and back the rest of the way up when it started to starve out.
Early ones had brakes on the back axle only.
But, they sold a zillion of the buggers.

#20

Radiator leaks were repaired with oatmeal.

Greg