Stay Alive on DC?

[retroaudio]

Hi,

 

Stay alive is a term seen many times in relation to DCC and the module seems to be an add on unit to a dcc decoder. It is also used as a cure for intermittent sound module operation. In practical terms, I see it's most important use being to avoid momentary stalling on dead frogs and dirt spots on the track. I might seem to have rambled on a bit there but what I am looking for is a simple capacitor/diode circuit that could achieve this on DC. Has anyone got a practical circuit that would work? I would imagine it being a capacitor with a diode in the negative line to protect it when the loco (and voltage) is reversed. Would this mean one capacitor for each direction?

 

I'm keen to give this a try in some form so I would welcome ideas.

 

 

Cheers,

 

Richard.

[spamcan61]

A big bipolar capacitor should do the job. Built in inertia, could make stopping at the buffer stops interesting!

[Black Sheep]

was just thinking that you'd have to get used to stopping in the right place!

 

but would be useful, especially in N gauge where short locos can stall easily.

[spamcan61]

was just thinking that you'd have to get used to stopping in the right place!

 

but would be useful, especially in N gauge where short locos can stall easily.

 

Unfortunately, having had a quick look at Farnell & Digikey it's tricky to find a bipolar capacitor small enough for OO nevermind N :-/

[Skinnylinny]

Sorry to rain on your parade, but I believe I've seen this idea before... Unfortunately, you'd need a fairly huge capacitor (both in capacitance terms and in physical terms). Capacitors are great for smoothing out bumps and changes in DC voltages, provided you're not trying to draw a large amount of current (such as with, say, a motor), which will discharge the capacitor in a matter of milliseconds... Even with polarised capacitors. As an example, look at the capacitors used in a CDU for point motors. Yes, they're for 16V rather than 12V, but they're always fully charged at 16V, and still discharge through a solenoid (which is a coil of wire, so basically the same as a motor) in what, a tenth of a second. Imagine from that how long a capacitor on a loco running at 6V (medium speed) would last over dirty track or an insulfrog point. And that's a fairly hefty capacitor or capacitors.

[retroaudio]

Braking on the cheap! Brilliant!

 

But the biggest bipolar I can see is 470uF? Is this big enough to do the job?

[retroaudio]

Hi skinnylinny,

 

I agree the capacitor would have to be large, but I was thinking of a loco with a pemanently coupled rake and using the 1st brake coach to put them in. What size do you think would be satisfactory?

[noiseboy72]

The capacitor on a DCC system is more for running the decoder electronics than the motor. Add in the fact that DCC will be using some sort of pulsed motor control and you can see that DCC makes stay alive easier and lower power than conventional DC.

 

You can put a normal capacitor plus a protection diode, but this will of course only work in one direction. This will give you a wider selection of caps though.

 

For a DC layout, I would look at adding more pickups, possibly a flywheel and plenty of wheel and track cleaning. I don't think any form of stay alive will be the solution that it often is in DCC.

[retroaudio]

Thanks Noiseboy,

 

I understand the use on DCC better now. I like the idea of a pulsed controller but not sure which commercial units are of this type.( I have circuit diagrams of them though). I have ordered a gaugemaster electronic track cleaner to try out as When I last had a model railway (many moons ago! just converting my loft for the next!) I remember the Relco units working well.

 

Realism with slow running is important to me so I am exploring all options. (Not DCC at present due to costs)

[Suzie]

Beyond just putting in a crude bipolar capacitor, you could add a DCC decoder and then you can use a unipolar capacitor. If you make your own decoder it need not cost much more than a few pounds and it will probably improve running over not having a decoder for various reasons - and you will be ready for when you do go DCC.

 

If you want to make a bigger bipolar capacitor you can put two unipolar capacitors in inverse series with reverse protection diodes across them.

[AMJ]

Many years ago I saw and had a go at a mechanical stay alive system at York expo. This consisted of the motor driving through a clutch to a big fly wheel you needed to reverse power to stop quickly.

[Dukedog]

Using the formula t=RxC where t= time in seconds, R= resistance in ohms and C= capacitance in Farrads.

 

If the motor windings have a DC resistance of say 2 ohms (That's optimistic) and the Capacitor is 0.5F (That's huge!) the time taken to discharge the capacitor will be 1 second!

 

Now, IF you can get a capacitor that size is it worth the hassle for a 1 second "stay alive"?

 

My personal opinion is ...Don't even think about it! Just clean the track and use live frog points! Simple!

 

Cheers!

Frank

[Crosland]

Using the formula t=RxC where t= time in seconds, R= resistance in ohms and C= capacitance in Farrads.

 

If the motor windings have a DC resistance of say 2 ohms (That's optimistic) and the Capacitor is 0.5F (That's huge!) the time taken to discharge the capacitor will be 1 second!

 

R x C is the time constant of the exponential voltage decay. The time constant is the time take for the voltage to decay to approx. 37% of the initial voltage. During the next RC period the voltage decays to 37% of the new starting voltage.

 

The charge decays in a similar way since Q = C x V.

 

Andrew

[retroaudio]

Hi Dukedog,

 

I could see the maths working against this and you have proved it! I only considered this as I am exploring every option before building my railway. I will be using electrofrog points and clean rails as slow running is very important to me. As an electrician myself I shall be lokking at the electrics very serioulsy!

 

Hi Suzie,

 

From what you suggest, I can fit a DCC decoder and still run the loco on DC? You mention building my own decoder, this is somthing I would be able to do given a suitable circuit diagram as one of the things stopping me from DCC at the moment is cost. Any more info on this would be most appreciated. Times are hard!!!

 

Thanks Guys,

 

Richard

[Dukedog]

R x C is the time constant of the exponential voltage decay. The time constant is the time take for the voltage to decay to approx. 37% of the initial voltage. During the next RC period the voltage decays to 37% of the new starting voltage.

 

The charge decays in a similar way since Q = C x V.

 

Andrew

 

Yes OK all understood by ME, but please don't try and blind the OP with science.

 

I tried to keep things as simple as possible so that any one reading my reply need not have a HNC in electronics to understand it!

 

Suffice to say using either of our explanations you would need a B***** big capacitor to get any sort of "electronic flywheel effect" And yes I know it can be done in other ways but not in an easy way for the average modeller with little knowledge of electronics.

I could have gone further by explaining about back EMF and it's effect on permanent magnet DC motor control but to be honest I don't think the OP would appreciate that much confusing information.

 

 

As far as I'm concerned simple was all the OP needs to know.

 

This happens far too often on RMWeb, some one gives a simple explanation to a problem then some one else comes along and adds more to the confusion by adding more and more technical information which really doesn't help the OP one bit. just adds to the confusion.

 

Quite often I am certain that some "experts" post just to "score points" over a previous poster and P*** on their chips!

 

 

Cheers!

Frank

[SHMD]

Dukedog is right. It's too impractical and, without, electronics, incompatible with many DC controllers.

 

Dukedog's values, (very sensible example values), show an RC time constant of 1second for an inline resistance of 2 ohms and a storage capacity of 0.5Farads!

However, that 1 second is not at the voltage stored.

after, roughly, 0.7s the voltage across the capacitor has dropped to half of its initial voltage when the power was lost! (the voltage at 1sec (RC) the voltage across the cpacitor will have dropped to about %30 of its initial starting value.

So for the Half Speed (6Volts**) 0.7sec after power loss the voltage would be 3 volts.

The voltage across the capacitor at 1sec will be 2volts.

 

Not much use! Especially if you know that the 0.5F capacitor is about the size of a half-used toilet Roll!

 

Suzies suggestion, of putting 2 capacitors back-to-back, will work in making bi-polar capacitor but the problem is the Total capacitance is 1/Ct = 1/C1 + 1/C2. In other words you need 2 toilet rolls and you end up with half a toilet roll!

 

ALso, if your DC controller is a PWM type then you start messing around with "Power" supplied to the Loco. Capacitors are rubbish at storing energy but they can do it for very short legths of time.

PWM controllers apply FULL VOLTAGE for a percentage of the time - and then repeat it. Varying the length of time ON changes the power - speed - of the Loco (Duty cycle). Usually, you apply voltage on and then off vey quickly proportionally and then repeat (Period), if you don't want to hear your Loco humming, then you make the frequency at which you switch ON and OFF (Frequency!) as high as possible, say above 10 to 15kHz.

Trouble is that's a period of only ~0.7uSeconds. Just the short amount of time that capacitors are good at storing energy!

 

Thus, if you fit a "large" capacitor to your Loco, then you will find that, very quickly, your train will be running at full speed even with the controller set near minimum! - and that's if the frequent surge currents don't cause damage to the controller!

 

In short, follow Dukedogs suggestion and don't try this.

(Without a lot of well thought out electronics inbetween the controller and the Loco's motor - say a (DCC) "Decoder"!)

 

Kev.

(I'm not even going to start on spark erosion problems... Doh!)

 

(PS. I worote this about 15:00 today.)

[retroaudio]

Hi Kev,

 

I agree. I read other discussions on this subject and was not sure how practical it was, hence my thread here to clarify. And it has!

 

Hi Dukedog,

 

Your reply cleared my question up nicely. I am OK with electronics but not quantum physics! I much prefer the blunt and to the point method as opposed to going round the houses so thanks for the advice. I will always get my soldering iron out if it will save me a few bob on buying ready built.

 

Cheers, Richard

[Crosland]

Yes OK all understood by ME, but please don't try and blind the OP with science.

 

I tried to keep things as simple as possible so that any one reading my reply need not have a HNC in electronics to understand it!

 

Suffice to say using either of our explanations you would need a B***** big capacitor to get any sort of "electronic flywheel effect" And yes I know it can be done in other ways but not in an easy way for the average modeller with little knowledge of electronics.

I could have gone further by explaining about back EMF and it's effect on permanent magnet DC motor control but to be honest I don't think the OP would appreciate that much confusing information.

 

 

As far as I'm concerned simple was all the OP needs to know.

 

This happens far too often on RMWeb, some one gives a simple explanation to a problem then some one else comes along and adds more to the confusion by adding more and more technical information which really doesn't help the OP one bit. just adds to the confusion.

 

Quite often I am certain that some "experts" post just to "score points" over a previous poster and P*** on their chips!

 

 

Cheers!

Frank

 

Trying to simplify things does not justify posting stuff that is just plain wrong and I'm not going to contradict information that is correct, am I? Nothing in your post indicated that it was a "simplification" rather than lack of knowledge or understanding on your part.

 

This last section is not aimed at anyone in particular, but what happens far too often on RMWeb, and Usenet and many other fora, mesage boards,.. , is people coming up with half-truths and ill-informed posts, whether through ignorance or a misguided attempt to dumb things down. DCC especially seems to attract more than its fair share of "urban myths", probably because it is more complex.

 

Wld yu crrct chld wh mssd t vry vwl n wht thy wrt t mkit smplr? It's stil mostly intelligible but it doesn't help them learn correct spelling or grammar.

 

A major purpose of RMWeb should be to inform and educate, not to dumb down. If it goes over some peoples head then they should ask for clarification.

 

Andrew

[Baby Deltic]

What you need is a very small fusion reactor within the loco, with a small steam turbine and generator. You will also need a voltage sensing relay to monitor the track voltage. When the voltage falls, the relay drops out and switches the motor power source from the track to the generator, then back to the track once power is restored.

[kevinlms]

Trying to simplify things does not justify posting stuff that is just plain wrong and I'm not going to contradict information that is correct, am I? Nothing in your post indicated that it was a "simplification" rather than lack of knowledge or understanding on your part.

 

This last section is not aimed at anyone in particular, but what happens far too often on RMWeb, and Usenet and many other fora, mesage boards,.. , is people coming up with half-truths and ill-informed posts, whether through ignorance or a misguided attempt to dumb things down. DCC especially seems to attract more than its fair share of "urban myths", probably because it is more complex.

 

Wld yu crrct chld wh mssd t vry vwl n wht thy wrt t mkit smplr? It's stil mostly intelligible but it doesn't help them learn correct spelling or grammar.

 

A major purpose of RMWeb should be to inform and educate, not to dumb down. If it goes over some peoples head then they should ask for clarification.

 

Andrew

 

Increasingly the questions asked here are about replicating the advantages of DCC, without actually using DCC. While some of the items requested MAY be possible, its unlikely to be flexible as the DCC version. Why is that? Answer, because dozens of different manufacturers are working to one set of standards, with a huge multinational customer base and so there is real development happening and what was not possible in DCC a few years ago, is now standard practice. Typically, Back EMF (the ability to keep a locomotive at a constant speed) is such a feature.

 

DC control still has its place for basic running of trains (and many are quite satisfied with that, or advanced features such as interlocking), but really you are looking at customised equipment & wiring to achieve this or a good friend that understands & prefers it. But if your after the advanced features, then DCC is the best option, in my opinion.

 

Kevin Martin

[Dukedog]

After reading several model railway forums and looking at the various DCC forum entries it's struck me that something is not quite right some where.

 

Why do most DCC forum entries start off with the words...

"HELP!" or "I have a problem" ???

 

It seems to me that this DCC malarkey is not a simple as people try and make out.

 

I have always been made to believe that DCC is just 2 wires!

Having seen the underside of some DCC layouts at exhibitions and on the internet I think that is a bit of an understatement.

Most I have seen seem to resemble the wiring of a Boeing 747 or the starship enterprise!

 

Then there are the posts telling us that we MUST modify all our Peco live frog points as there is the danger of shorting out the track.

 

The number of posts I have read where the words "Blue Smoke" have appeared is also amazing!

 

Don't get me wrong, I'm not against DCC or progress, Having worked in industry as electrical technician and worked on Data bus, PLC, and SCADA then DCC is not that complex to be honest.

I just wonder why people seem to have so many problems?

 

Cheers!

Frank

[SHMD]

Hmmm...

 

　

I've been think about this thread and Suzies suggestion of using a DCC Decoder.

　

 

I can see this as a possible solution.

So much so, that I might enter it in the 2012 "Inovation Challenge" - If Suzie and RetroAudio don't mind, that is, and I find a time machine so that I can actually 'make time' to work on the project!

　

 

Making your 'own' Decoder is really a none starter.

It would cost too much, never work, be bigger than your fist, and would look like a birds nest at the end of it - and that's the 00 version!

　

 

BUT... How about modifying a commercially available Decoder?

　

 

Advantages:-

 

Cost - Much cheaper than you can even buy the bits for seperately.

Proven design - lots already running around.

Small - There already small enough to fit into a Loco (minus the energy storage)!

　

 

Disadvantages:-

 

Cost - Expensive.

Risk - you might end up 'wasting' a Decoder or two and all your time!

Time - Developing the software.

No drawings - you would have to "reverse engineer" the Donor Decoder.

Needs to work with different types of DC controller.

　

 

Prerequisites:-

 

The Decoder needs to be big enough for me to see it!

I would need a 'Donor' Decoder to be based of a Pic-Micro that I could use.

The "relevent bits" would need to be accessable.

　

 

Feel free to add more...

　

 

Features:-

 

DC operation from 3.5 - 4.0volts to, well alot, say in the 12 - 18volt range. (Every controller is different! And some would still be incompatible, but 'Relco' type units would definately be a no no.)

Selectable "Stay alive" options - say:- Selectable power output when a power break is encountered (variable limp), Initial Boost (or jump) into the dead section, Brain power priority (sod the motor), economy mode (use what you can according to whats left in the storage capacitors), err any more suggestions? Say a battery option (much better than capacitors but adds another level of control problems/choices).

　

 

This would, primarily, be a software project wit0h some experimenting and tweeking once it was working but reverse engineering the Decoder would also be time consuming.

　

 

Just a thought...

　

 

Kev.

(Hmmm... but not a simple diode and capacitor solution, as initially required, plus the DCC decoders were a cost issue to begin with...)

　

Never lose sight of the original goal!

[puck]

Not being a DCC convert (yet), as I understand it this 'stay alive' is purely to keep the chip powered if the loco hits a dead spot, e.g. point insulation. This being the case then the loco will come to a grinding halt whether its DC or DCC. Only the onboard flywheel of newer locos will allow it to pass across a dead spot.

Andy

[noiseboy72]

Puck,

Your understanding is not quite right. Due to the lower power consumption that comes with Pulse Width speed control, a super cap and voltage multiplier connected to a DCC chip will also power the motor - for quite a few seconds in the case of Lenz et al.

 

The issue is that this does not work on a pure DC system, or in a PWM system where you are trying to put the capacitor after the PWM controller.

 

Have a look on youtube for some of the Lenz stay alive videos. There is one of a train travelling 8-10 inches without power, but still controlled due to enough signal reaching it through one wheel!

[Crosland]

 

I have always been made to believe that DCC is just 2 wires!

 

Think of it as one "circuit", rather than two wires, for the whole layout.

 

It's much simpler than cab control that requires one circuit back to the control panel for each and every switched section plus the circuits from each switch to each cab.

 

Andrew