Capacitors. blah blah blah

[peteski]

Chris,  do you have any more info on that diode circuit?

Those are probably 2 Zener diodes hooked up in series (back to back) to clamp the voltage.  Then they are wired in parallel with the motor. Is there also a resistor in the circuit?  If not, then if the throttle is set higher than the Zener voltage, the diodes will basically pass most of the throttle's current though them (heating up rapidly).

John,  you are correct to say that charging the cap t a higher woltage stores more energy in the cap. But if you install a diode voltage-dropping circuit in series with the motor, then you are simply shifting the operating range of the motor.

As an example, if Chris runs his model at 1.5V and it will stop running around 1V then the operating voltage range is 0.5V.  So, if the track power is lost, the cap will run the model until it discharges down to 0.5V. After that, the remainder of the capacitor's voltage will be simply wasted discharging through the motor windings.

If you then install a voltage dropping 4-diode circuit (which "wastes" 1.4V in each direction) then the model will need 2.9V to run at the same speed as before. The motor will now stop running at 2.4V  The operating voltage range is still 0.5V (it is just shifted up by 1.4V).  So the cap charged to 2.9V will only keep the motor running until its voltage drops to 2.4V. Same operating voltage range (of 0.5V) as in the earlier example.  The rest of the voltage will simply be wasted discharging through the motor's winding.

[jdcolombo]

Ah.  Yep - all my suggestion would do is move the operating voltage range around.  Sigh.

DCC is so much easier!

John C.

[peteski]

DCC is so much easier!

In this particular instance, yes, very much so. But DCC also has its demons too. 

[Chris333]

Sorry I was at work, stupid work 

Here is what Searails says about the diodes:
http://www.searails.com/powermaxproducts/electronics.html

Even those small diodes take up a lot of space. You will notice that they do not recommend "pulse" throttles, but up further they state that the motor was tested on a Rokuhan controller. That is the controller I use.

I should note that I tried running using a regular old MRC pack and it just ran horrible. Very jerky.

I understand how caps work with DCC. The voltage is always the same and all that. Even if I would be caught dead near a DCC system I'm betting whatever micro decoder that would fit in the cab would be around $50 or more and then there would be no room for a cap. Plus then around $300 for a throttle to make the thing move. As things are now I can run it off a single D cell battery. Remember this is Nn3. Here is a Whitcomb that is just about the same size as the Plymouth:



When you want the min voltage. Do you mean while running or hand held with leads clipped on? I'm sure it will start at a lower voltage free wheeling, but not really run on the layout that low.

[peteski]

Sorry I was at work, stupid work 

Here is what Searails says about the diodes:
http://www.searails.com/powermaxproducts/electronics.html

Even those small diodes take up a lot of space. You will notice that they do not recommend "pulse" throttles, but up further they state that the motor was tested on a Rokuhan controller. That is the controller I use.

I should note that I tried running using a regular old MRC pack and it just ran horrible. Very jerky.

I understand how caps work with DCC. The voltage is always the same and all that. Even if I would be caught dead near a DCC system I'm betting whatever micro decoder that would fit in the cab would be around $50 or more and then there would be no room for a cap. Plus then around $300 for a throttle to make the thing move. As things are now I can run it off a single D cell battery. Remember this is Nn3. Here is a Whitcomb that is just about the same size as the Plymouth:

When you want the min voltage. Do you mean while running or hand held with leads clipped on? I'm sure it will start at a lower voltage free wheeling, but not really run on the layout that low.

The minimum voltage I'm referring to is when the loco (running with its intended train) stops running.  For example, you have your train running at the speed you like. Then you slowly adjust the throttle to slow down the train. At some point it will stop running while the throttle is till not fully shut off.  At least I assume that is what you will see. That voltage when the engine is no longer running would be that minimum voltage.

I only want to find out what it is to show you what is the range (window) of voltage from when the train is running ad the preferred speed to the point when it stops running.   If that window is rather small then the super-cap will not be able to keep it running for an extended period of time.  For example, if you run your loco at 2V and it will stop running at 0.5V then the operating voltage window is 2-1.5=1.5V

As I mentioned, unlike a battery (which can provide a steady full voltage for most of the discharge cycle), the voltage coming out of a super-cap (or any capacitor) starts dropping as soon as it starts discharging (through the motor).   So, if the window or your loco's operating voltage is very small, then the cap will not be able to sustain the loco running for any appreciable time.

The best solution would be to increase the window of operating voltage. This would have to be done by increasing the gear ratio.  That way you could run the locomotive at the preferred slow speed but at much higher throttle setting. If for example your loco would have a desired speed at 5V and the motor would keep running down to 0.5V then the window of operating voltage would be 4.4V.  The super-cap would be able to supply a useful voltage for a longer time than in the earlier example where the voltage window was 1.5V.

Thanks for the info about those diodes. Now I see what they are doing.  They are using Zener diodes to waste some voltage on them instead of passing the full track voltage to the motor.  It is a modified version of the standard diode voltage dropping circuit. As far as the size of them goes, they are actually rather large (as far as SMD components go).  That is because the wasted voltage will be dissipated as heat, and the diodes have to be large enough to handle that heat.  Using that circuit will not give you any advantage as far as getting longer run out of the super-cap.