Value of a smoothing capacitor

Can you read the value of the cap on the old board.  If your engine will see up to 20volts AC then a minimum 35volt rated capacitor would be needed.  The larger cap may cause your engine to coast a bit. Dont know what kind of engine you are running.  You can use a bleed resistor if its a problem.

If you are just powering the motors directly, about 470uF should do the job nicely, but larger wont hurt, aside from the coasting already mentioned.  Keep in mind you won't have any direction control if you connect the motors directly to the Rectifier/Cap.  If you are going to have any sort of electronics running off the DC power you should also add a capacitor to catch any little spikes, something like .01uF.  As mentioned already, use AT LEAST 35V caps, and I'd recommend 50V.  I've seen over 40VDC come from a "16" VAC transformer through a bridge rectifier and cap, when the whistle is pressed.  

Steve, John

 

thanks for the information.  The .01uf capacitor used for spikes, how does it get wired in.

 

Thanks

RAD400

Wire it across the DC out off the rectifier in parallel with the larger electrolytic cap.  It catches higher frequency ac spikes that the larger cap has trouble responding to.

Lionel sometimes I think put a small NP cap for signal interference on their AC motors for  TMCC. For coasting on DC motors you would need a large NP cap. You can make an NP cap out of 2 polarized ones. For instance take 2, 1000 uf, 35 volt polarized capacitors. Wire like terminals together such as the 2 + terminals. The remaining 2 terminals can be hooked across the motor(s). The result will be an NP capacitor of half the capacitance and twice the voltage. So in this case you would have a 500 uf 70 volt NP capacitor. K-Line used to put this across the motors of some of their engines. It does smooth things out a bit. 

 

On super street cars I run them on DC off the transformer using a bridge rectifier and 4700uf capacitor. They run a little better that way. I also put caps on each block of a block system,so that when the vehicle stops,there is a bit of coasting. This is much less effective in an engine but still has a positive effect. 

 

Dale H

 

 

No need for an NP cap with a plain bridge rectifier as the polarity will always be the same at the output.  A single electrolytic will do the trick.  I'd go for perhaps a 1000uf at 35 volts and see how you like the performance.

 

 

Thanks everyone there responses.  i will try it out this weekend and post a reply.

 

Thanks,

 

rad400

I was talking about on the motor of an engined ,powered by a reverse unit where polarity would change with direction.

 

Dale H

Would  unfiltered DC work just as well at lower speeds? Pulse power?? didn't we have something similar in our HO days?

Apples and oranges Dale.  Since the original poster said this...

 

I am replacing a burned out circuit board in an engine with a bridge rectifier to be used with two DC can motors.

 

I was answering that question.

I run a 10,000uF 50V cap across the bridge rectifier on the circuit board of an older Williams Trainmaster. In parallel with the cap I run a TVS diode and a 10K resistor for bleeding the cap. It might coast a bit but I don't really notice it. The motors draw down the cap quite fast. If you actually run the numbers, assuming the motors provide a 10 ohm load and the cap is charged to 20VDC, the cap discharges in 0.5 seconds. I do notice a huge difference in pulling power. I also noticed that the engine has trouble reversing from neutral due to the cap staying charged. To get around this I wired the light bulbs in the Trainmaster in parallel with the capacitor. I need to hold the reverse lever on my ZW for about a second to make the Trainmaster reverse. Running the light bulbs off the capacitor caused them to glow brighter. To get around this, I added a resistor in series with the bulb.

Since you're using a reverse unit, I'm assuming you are also using a non-polarized cap?

 

Its polarized. I'm not sure you can buy a 10,000uF non-polarized cap. The cap is connected across the + and - outputs on the bridge rectifier. The output of the bridge rectifier is fed into a transistor H-Bridge to control the motors.

 

I've also upgraded the bridge rectifier (higher current, lower forward voltage drop) and transistors (lower on state resistance) in this engine. This engine had some issues with a burned circuit board when I bought it and it didn't run well.

maybe I am missing something. The reverse unit works on DC can motors by reversing DC polarity. Hooked as you describe,when the engine reverses, in one direction the capacitor has reverse polarity input voltage also. If you don't put a diode in series the capacitor could be damaged or explode. 

 

Dale H

Dale, I'm wondering the same thing.

 

If the circuit board was for controlling the motor, and that will be replaced by a bridge rectifier, to power to DC motors, then the engine will run in one direction only.

 

If that is your intent, so be it.

 

I don't see why one HAS to have a capacitor if one is just running simple conventional. Nice, but not necessary.

 

You can't just model the motor as a resistor either, it has back emf.

 

Electrolytic caps dry out. It make take two hours (if you exceed the temperature rating by a large amount) or it might take two decades (long after you are dead), but it will happen.

 

quote:

The cap is connected across the + and - outputs on the bridge rectifier.

 

He has the capacitor connected to the bridge rectifier. How is that going to change polarity? What am I missing?

In rereading what he said, you're probably right.  He's connecting the capacitor directly on the reverse board.  Don't know the implications of that, but if it works, it works.

 

Your not missing anything.

 

The only issue I ran into was trying to reverse the engine while in neutral. The only load on the capacitor during this time was the circuit boards electronics and the 10K resistor I added. To get around this I simply wired the two light bulbs across the capacitor.