I'm probably just showing my ignorance, but I was rather intrigued by the mention on last night's 'Great Model Railway Challenge' on Channel 5 of a power supply system using capacitors - presumably fitted to locos - to help them over short interruptions in power supply, i.e. pickup issues.
This sounds to me like an electronic form of flywheel.
Can anyone tell us more about the system, and why aren't we all using it already?
David.
I think it was a layout that used DCC and they did mention "stay alive" capacitor. I'm not sure that the capacitor they showed was actually of the type used in these systems. It was such a brief shot, but it looked more like the sort of thing that might be used to smooth rectified a.c. in a circuit or controller.
Yes, Stay Alive capacitors definitely exist and the Lenz DCC system even has what I think they call "read through" where the DCC signal can penetrate though insulating layers a loco can still respond to commands when contact is lost and it is running on stored power.
https://www.youtube.com/watch?v=2f2hmeNNCOs
There is a cost factor of course, but even then space not only for a DCC decoder but also a big capacitor might be an issue in 009.
In terms of being a flywheel, it acts as one in a way, but the terminology grates rather from an electronic point of view where an inductor rather than a capacitor is thought of as an electronic flywheel. A capacitor is more like an emergency reserve fuel tank.
Thanks for your comments, Graeme, and my sincere apologies for unintentionally offending your electronic sensibilities - I'm afraid I wouldn't recognise a farad if I bumped into one (though I'm guessing a microfarad is probably too small to be seen with the naked eye?)
Your comments all seem to relate to DCC operation, and I'm tempted to ask why (presumably) they don't apply to DC as well, but I won't as the answer would probably only cause further embarrassment...
David.
Hi David,
As you've not asked I won't reply to you but you may be interested to know that almost all* the capacitors used for loco stay alive jobs are big and will only store electricity one way round, positive to one terminal, negative to the other; they really don't like being connected the wrong way round.
So stay alive for DC locos would only work one way, if you reverse the loco you would destroy the capacitor.
DCC is different as the supply on the track is turned into a steady DC voltage in the loco, which drives the DCC controller chip. This steady DC can be stored on a big capacitor and so it works OK for DCC.
Hope this helps
Frank
*almost all as some big and relatively expensive capacitors can store electricity either way but only at low voltages.
Too much information perhaps, but one variety of the sort that works just one way round (electrolytic capacitors) can make a nice loud bang and spray confetti a bit like a party popper if connected the wrong way - not to be recommended. Fortunately they have a + and - or equivalent marking. (The best were ones dipped in wax as found really old B&W TVs.)
A common analogy (or "lie" as Terry Pratchett would put it) for visualizing a capacitor is as a bucket. The bigger the bucket, the more juice (groan!) or electric charge it can hold. A bucket can hold more if it has a larger diameter and taller. Here we can imagine voltage as "tallness" (head of water). With DCC the full track voltage is always available (except when it isn't - when there is a break). Unfortunately with DC the only time the bucket/capacitor is full or nearly so, is when running at top speed, but the time you are most likely to need a reserve to get over a break in the track is when running slowly and the bucket is nearly empty.
Thanks, guys, for assisting my education. What strikes me is that while most electronics - memory, bulbs/LEDs, valves/transistors and so on - seem to have got smaller over the years this doesn't seem to have happened to capacitors, which it seems can only be shrunk so much. Ditto smart switching which might enable them to reverse polarity. Oh well... it was just a thought (born out of ignorance).
Talk of capacitors in old TVs reminded me that my dad worked for at a time for a firm in London (LEMCO) which made them. But he never brought his work home, so I never learned about them (but as he was an accountant that wouldn't have helped much anyway, I guess).
Obviously RC is the answer to all these issues, Frank!
David.
Yes, for capacitors (and inductors) there has been much less miniaturisation - basic laws of physics see to that. It comes down to how to roll up as much area as possible or get as much surface area on a sponge and put it together as close as possible without breaking down and shorting out when you put a voltage across it. Inductors are worse - it is largely how many turns of wire can be squeezed in.
Quote from: clayliner on October 09, 2019, 11:51
Obviously RC is the answer to all these issues, Frank!
David.
Indeed!
Frank
This is almost certainly too much information but it's interesting that in many electronics applications these days, they get round the difficulty of miniaturising the capacitors and inductors by working at higher frequencies. This is because at higher frequencies you need less capacitance or inductance to get the same effect.
For example, power supplies used to run at mains frequency which is 50 hz and use big transformers and capacitors. Now, switch mode power supplies run at much higher frequencies and so the transformers and capacitors needed are much smaller.
Unfortunately, DC locos still run at DC so this won't work for them.
Frank
I can backup Graeme's comment about exploding electrolytic capacitors, having been in the same room when one has been destroyed! (Many years ago when I worked in Cosmic Ray Physics)
All well written and correct. May I add some more practical detail:
1. Yes, there are 2,5 types of capacitors: foil, electrolytic (Elco), and so called 'gold caps' (silver caps etc.). Like for like capacity, their size is decreasing in the list, i.e. foil is biggest. Reason those 3 are 2,5 is that 'gold caps' are technically a sort of halfway-house between capacitor and battery.
2. In terms of using them inside a model loco as an electronic flywheel only gold caps are small enough for scales 00 upwards, but only big continental narrow gauge stock in H0/ 00 would have the room to house them.
3. Elco can be wired in pairs to cope with changing polarity, but that requires twice the space as you have to fit 2!
4. For a reasonable flywheel effect, e.g. 300mm 'freewheeling', you need a capacity in 5-digits millifarad, that's a fist-size or larger Elco. This is a very rough figure as it largely depends on the electric motor, the rolling resistance, i.e. gear train, train length, track radius etc.).
5. Capacitors block DC current, but allow AC to pass through without much of a resistance (if sized appropriately). Trix used that in their 'E.M.S.' system (carrier frequency control), not only to stop the E.M.S. engine from responding to DC, but although to isolate track from DC operation, while allowing the E.M.S. engine to operate in that section.
6. An insulating 'sheet' (paper, dirt etc.) turns any 2 adjoining contact surfaces into a capacitor, e.g. rail and wheel. Mr Lenz used to demonstrate that at fairs by running digital engines over a piece of paper handkerchief. No surprise as digital power supply is AC! RELCO and similar high-frequency 'track-cleaners' utilise the same effect to keep engines running.
7. A capacitor in AC circuit can change the shape of the AC-wave. This is why you have to remove the anti-sparking capacitor (and coil) from the motors when fitting digital control chips to engines. If operating with digital control (or carrier frequency control) do not fit capacitors, unless you have the expertise to make sure you don't mess up the system. This applies to pulse-width-modulation (PWM) controllers and various other electronic controllers too!
8. The 'staying-alive' systems offered for digital control tend to use gold-caps for that purpose. To my knowledge they stop the control chip from shutting down (power off = emergency stop!), but I don't know if they supply the motor; except on the very big units like LGB, Massoth etc., which do supply the motor?
Hope this helps
Waldschrat