Large Scale Central

Diode help

Electronics Power and Sound
#1

Okay all you electronic gurus take a look at the schematic on page 2 (page 35 of the actual pamphlet) of this link. http://www.cvpusa.com/doc_center/r11a_G2_manual_color.pdf
Is the diode only there to prevent the gel cell battery from back feeding the rectifier? If there is no battery then a diode would not be needed correct?

Also, I found a capacitor with the same numbers but it doesn’t look the same, it resembles a battery with the wire coming out either end of it.

Does a capacitor have a +/- side to it?

Thanks for the help. these are uncharted waters for me

Terry

#2

The diode provides a couple of roles, though I am not sure which is critical here.

  1. It drops the voltage to the gel cel by .7 volts. This may be needed to prevent overcharging or boiling off the gel cel.
  2. It keeps voltage from flowing to the capacitor. This may help the capacitor perform its voltage stabilization/filtering function
  3. It may keep voltage from flowing back to the track, though I believe the bridge rectifier (which is four diodes) already does that, so really, it’s just 1 and 2.

Diodes are cheap. Follow the instructions.

Some capacitors have a + side, and you do not want to get that wrong. If the component you have says 2200uF/50V then you will be fine to substitute it. Assuming you get the ± right. Many capacitors are cylinders, like a stubby AAA or AA cell, with wires coming out the end. One end will be marked with +++ or —.

#3

So this circuit is for charging the gel cell and only from their 15 volt power supply…

the full wave bridge gives 2 diode drops itself, and the third is the one you are asking about… so you have 2.1 volts drop from 15, so your charging voltage is 12.9.

Normally the safe charging voltage is somewhere under 13.1 volt on a gel cell, so this is using a regulated voltage and it will normally charge a gel cell battery and the charge will effectively self-terminate when the battery voltage reaches 12.9 volts, normally a safe “float voltage”, i.e. it can take this voltage forever without damage or overcharging.

The capacitor is pretty weird on the output of a full wave bridge that is running from DC, what possible ripple could you have?

If you were concerned about noise, you would use a disc capacitor in the .01 to .1 mfd range, not a larger electrolytic.

Greg

#4

Yeah, that capacitor does not make a lot of sense to me, you are right that a smaller one would be typical for decoupling (noise reduction). I am willing to believe their engineers got it right, but I’m not sure of the explanation.

As to the source of noise and ripple, I am sure that the electric contact between rails, wheels, and pickups are noisy as heck, every time some grit, dust, or corrosion is encountered.

#5

actually this circuit is for running on track power with the ability to charge a gel cel to run on as well. I don’t want to charge a battery just run an airwire card on track power. The point is moot though as the A/W card I have is a first generation, not a G2 and it won’t handle the track voltage where i want to run it at. This was just going to be a “quickie fix” until I could get a batteryto put in the loco.

Terry

#6

Just a fine point Michael… noise yes, but no ripple possible as long as it’s not coming from the power supply… no matter how much noise… way different frequencies.

Greg

#7

Ahh, I misunderstood the original post. Read too quickly. I’d say then that the capacitor is there to guard against short interruptions in power as suggested above. I’d leave the diode in and see if it works fine. If the track voltage is too high where you want to run, you can throw in more diodes. It wastes electricity, but what’s the harm?

To greg: Yes your point is good, I was being loose on the use of the term ripple. Assuming, of course, that your power supply has no ripple. I imagine it does not matter much either way.

#8

More points:

The diode is NECESSARY to get the right charging voltage to the gel cel… all batteries will experience a large swing in charging current with very little difference in charging voltage (I can explain if necessary)

The capacitor is UNNECESSARY, (unless you have a high ripple power supply) as that the battery the prime source of power and is thousands of times the capacity of the small capacitor.

But as Michael points out, if you have no battery, than it could provide maybe a millisecond of power to the loco (useless)… but of course if no battery, then you can eliminate the single diode as Terry originally asked :wink: because no charging is needed.

Greg