Adding a resistor to a locomotive question

Don't add a resistor, but rather pairs of 3A diodes connected in a reverse fashion for each pair.  Resistors will not give you a set voltage drop, but the voltage will vary with load.  If you add around 5 pairs of diodes, you'll drop the motor voltage around 2.5-3.0 volts and you'll have that much more voltage on the track for comparable speeds.

Here's a page on dropping voltage with diodes that illustrates the connection of the diodes.  Dropping voltage with diodes

Here is also a string put in an engine

www.jcstudiosinc.com/BlogShowT...=488&categoryId=

Dale H

Thanks Dale, I figured you had that there someplace.  

I would make one comment.  I think 6A diodes are massive overkill for anything that I've ever seen.  Since each diode only conducts on a half cycle, you're adding capacity for 12 amps to the motor!  If your motor is drawing anything close to that, you have a MAJOR problem, and diodes won't fix it!  3A diodes are considerably smaller and should be more than sufficient.

Dale, if the diodes are in series with the AC voltage, there's no way you're only using one side of the string.  Since the bridge rectifies on both halves of the AC alternation, both sets of diodes will be conducting equally as well.  Take another look at how a bridge rectifier functions.

John

I do  not know if we are on the same page here. Above is the diode string in question. It goes in series to the motor(s),in series with DC current (pulsed DC 120 cycles) from the rectifier in the reverse board.  Only 4 of the 8 diodes will conduct current,which 4 depend on train direction. 

If the diodes were place in series in an AC circuit all 8 wold conduct and share the load.

Dale H

Dale I don't think guys are on the same page. You are putting your diodes in the dc current stream (after the recitifier) and John is putting his in the AC current stream (between the pickups and the rectifier). Both ways work and both of you are correct in your thinking as far as I can see.

Ron

You hit the nail on the head Ron, when I saw his last post before yours, I realized why we were at odds.

Actually, he's more correct than I am in this case, since I believe the motor lead is where it has to go. 

Of course, if that motor is drawing more than 3 amps, I'd be very concerned anyway, but might as well be safe if the 6A diodes fit.

Although I haven't tried it I don't see why your way wouldn't work just as well. You would still be sending ac to the Bridge or e-unit just less voltage. 

Ron

That's true, I guess there's no downside, and you get to use smaller diodes.   I forgot the tender has it's own pickups for the sound...

If the diodes are in series with the MOTOR ONLY, this will work. However, if you put the diodes in series with the pickup rollers and the rest of the circuitry, you do not solve the original problem of having enough voltage for the Tru Sounds without going too fast.

In this application, to accomplish what the OP wants to do, the diodes MUST go in series with the MOTOR LEAD and no where else.

Isn't the sound in the tender with it's own pickups?

Ahah I forgot the original post. The K-Line Tru-Sounds get there speed in steps from the  motor voltage out so Rich is correct.

Ron

An excellent observation, and I yield the point.   Obviously, the diodes in the motor lead are the solution to this particular issue, Dale is 100% correct.

I've used the diodes in the pickups for conventional running sets with the whistle in the tender, and it's worked out well.  I've never tinkered with the K-Line Tru-Sounds, I didn't realize it was tied to the motor directly.

Actually the Tru Sounds is a separate system they seems to have 3 steps and as you increase the voltage it jumps to the next step. It was crude by today's sound systems. If your voltage is in between 2 steps it will jump back and forth from slow to fast. The diodes will have to be attached to the motor to slow it down a little because when you get to the voltage for fast chuffing sound the locomotive can fly off the track. I run mine on 054 curves minimum and it still goes around the curves to fast for comfort. Eventually I will upgrade to a Dallee sound unit but for now until I can I'll have to live with the Tru Sounds.

If the system had no connection to the locomotive, you could put the diodes in the locomotive pickups as well.  Like I said, I really don't know how this board or particular locomotive is wired, so I really don't know if that's practical.

The only pickups are on the engine so the diodes have to be connected directly to the motor. So would I need 4 pair of 3 or 6 amp? 

Well, the size is a judgment call, as well as what will fit in the space you have.  I'd probably use 3A diodes, but as Dale points out, they will be carrying the whole motor load.  If you run large consists or have steep grades, the 6A diodes may be a better choice.

I'll try the 6 amp first and if it don't work I'll try the 3 amp. I need to run a couple errands this evening so I'll swing by RS and pick them up. I want to thank everyone for your help and I will keep you posted on the results. 

Gene

Gene

3 amp or 6 amp will function the same. The only question is fit and load on the diodes. If 6 amp ones fit I would go with them, Not much cost difference, especially at Radio Shack.

Dale H

SUCESS!!!! 

It works great now with the 6 amp diodes. The Crusader boiler has plenty of room for the diodes. I'll return the 3 amp tomorrow.

Again I want to thank you all for the help.

Gene

John,  I still don't understand your logic on diodes on the AC side.  Yes, full rectification uses both sides, but current flows through one set and returns to common via another.  If the load is a 3amp load those diodes will be at there rated levels.  It is a series path, not parallel.

Many inexpensive or early units used 4 amp rectification.  Those units don't hold up.  Later models use 6 and 8 amp rectification.  I agree a dc motor should not draw that high, but twin motor units do draw closer to the 3 amps.  G

GGG, look at the animation I linked.  On the positive peaks of the AC one pair of diodes conducts, on the negative side, the other pair conducts.  The average current flowing through any one of the diodes is only half of the current consumed in the circuit.  You make the point for me, all the diodes are in use at some point in a complete cycle, but only for half of it.

The diode chain we were discussion has the same characteristics, on one side of the AC waveform, half the diodes are conducting, on the other side, the other half are conducting.  Each diode only sees 1/2 the average AC current for the circuit.

Now, I can see the argument for a single component bridge rectifier rated at 3A may not handle 4-5 amps, since the rating is for the part.  OTOH, the 3A diodes are rated for 3A continuous (with proper cooling), and if they're seeing only half that if there is 3A AC current flowing in the circuit.

I can't think of how to state it any clearer than that.

If I add one more pair of 6 amp diodes to the string will it slow the locomotive a little more? It could stand to slow just slightly.

John, the typical maximum rating for a diode is

"Average Rectified Forward Current (single phase, resistive load, 60 Hz at some specific temperature for ambient or case", not a DC current.

The RMS heating due to the halfwave current should be higher than an equivalent sustained DC current.

In other words, a 3 amp diode should be able to handle 3 amps DC better than 3 amps halfwave rectified (or a 6 amp fullwave bridge circuit.)

Yes, each pair of diodes will slow things down more.

That being the case, the average current those 3A diodes see with a 3A AC load would be 1 1/2 amps, right?  That's what I said I believe.  You would have to have a 6A load on the pickup connection to exceed the ratings of the 3A diodes.  Not sure where I said anything else.

I was commenting on your statement "3A diodes are rated for 3A continuous (with proper cooling)" to clarify that it is continuous halfwave, not DC.

For antiparallel strings of diodes in the roller circuit, I agree that the diodes can be rated at half the continuous AC current - 1 1/2 amps for 3 amps total current.  If the antiparallel string is after a fullwave rectifier so that it can feed a DC motor, each string of diodes must be rated for the full current - 3 amps in your example - since only one string is active for each voltage/current polarity and motor direction.

I think this is what you have been saying.

(If there is a large capacitor after the fullwave bridge, the RMS input current increases, requiring a higher rating on the roller diodes.)

OK Dale, I think we're on the same page.

However, it's not continuous half wave, it's average current.  "continuous halfwave" would imply 1/2 the current, no?  The other half of the wave would not have the diode conducting.

In any case, I think we're in agreement, just not stating the case clear enough.

Flogging a dead horse for the sake of science:

A halfwave rectified signal has

An average value (DC equivalent) of 2/PI/2 or .32 of the peak sinewave value

An RMS (heating equivalaent) value of .707/2 or .35 of the peak sinewave value

Then we get into what type of meter you are using - true RMS responding or average responding with "RMS calibrations".

It is the RMS value of the current that really determines the amount of heating in the diode.

For our work here, I would ignore these differences and just add a 50% safety margin for reliability.

I apologize to those Forum members who have their eyes rolled back by now.

I had a very good boss who defined proper Engineering as

Knowing with certainty in advance that the design WILL work.

Boy, am I glad we got that straight. For a minute there I thought we were going to get into advanced particle physics and maybe even string theory. In any case its nice to know that many of the contributors have the knowledge background to back up their suggestions.

Al

I guess I should use my true RMS bench meter for my next measurements.

Now, on to the particle physics that Al wanted...

If particle physics is how to determine the size of the particles in a coal load I'm all for it! 

Ron

We'll get right on that Ron.

I did it using 5 pair of 6 amp diodes and the speed if perfect using a Z4k at 15 volts.