Voltage regulation

Discussion in 'FAQs' started by smkettner, Dec 9, 2005.

  1. smkettner

    smkettner New Member

    I have a simple over and under layout around my Christmas tree and I wish the train could run at a more constant speed as it travels around and over the loop. The engine is an Athern Diesel C44-9W with all 12 wheels driven. The power pack is a Tech4 Rail Power 250. When I connect a digital volt meter to the rails I noticed the voltage would drop just below 7 as the train climbs and then on decent the voltage rises to 10.5 before it settles to about 9 on the flat section. All this without changing the throttle. My thought is to have a better regulated voltage so that the speed would be more constant. I think as it climbs the engine draws more amps and the power supply allows the voltage to drop causing a greater drop in speed. Are there any recommendations on a true voltage regulated power supply? Or am I doing something wrong with what I have? I am not really interested in a DCC system as this is a rather basic single track loop.
  2. kadidle

    kadidle Member

    Welcome to The Gauge. From what your describing your pulling too many amps/too much grade. Once you exceed the current capacity of the pack, the voltage will drop, causing you to loose speed. I don't know what the current (or VA) of the pack is, but I would expect it's overloaded. Also, if your grade is simply too steep, you might try reducing it a bit.
  3. ezdays

    ezdays Out AZ way

    The Tech 4/250 is rated at 25 watts. You don't say what scale you're running, but that should be enough to run one train up a grade in anything up to HO. You also don't say how many cars you are pulling or how steep is the grade.

    MRC doesn't spec how much of that power goes to the track, but kadidle is right, if you are dropping voltage as the train goes up a grade, you don't have enough power and so as the amps increase, the volts will drop based onwhat the power pack can deliver.

    If you are running other accessories, you might disconnect them and see if that makes a difference. You could try running it at a slower speed, lower the grade or disconnect some cars to see if that makes any difference.

    One other possibility for the slowdown is dirty tracks, or you may try adding an extra voltage feed to the tracks in that area. Measure the voltage at the power pack output first, and if it drops there, than you are stressing the power pack and cleaning the track or adding the extra feeds won't help in that situation. If the power pack has full voltage even when the tracks don't, then you have a track problem.

    Let us know how you do.
  4. railwaybob

    railwaybob Member

    What you are experiencing is normal for DC operation. As the loco experiences more friction on the rails as it travels around, it requires more power to overcome the friction. Hence the voltage drops and the amperage increases. This is part of Ohm's law which says that voltage equals the amperage times the resistance (V = I*R). The equation must always remain balanced. So, as the resistance increases, the amperage decreases and the voltage remains constant. Or, to put it another way, as the resitance increases, the voltage increases and the amperage remains the same. In most cases, it's a combination of the two where the amperage increases, the voltage increases.

    Since you have no control over voltage and amperage, (unless you want to buy a different type of power pack - and that can get expensive) you have to take a look at the resistance. While you have no control over the electrical resistance, you can control some of the physical resistance.

    You mention that you are using an Athearn AC4400. This is a 6-axle loco. What is the radius of your circle/curves around the tree? If they are less than, say 36", there will be physical resistance which will result in electrical resistance as these long trucks try to negotiate tight curves. Can you increase the radius of the curves?

    You mention that you have an over-and-under layout. How steep are these grades? What is the maximum height of the over-and-under? What length of track does it get to the top of the grade? I assume that as the loco climbs, it also goes around a curve. These are all elements of physical resistance (and hence electrical resistance) as the loco tries to climb the grade. Is there any way you can lengthen the track as it goes up the grade? Can you reduce the height of the over-and-under?

    Have you lubricated the bearings, the drive train and the motor? Follow the manufacturer's instructions in lubricating.

    How new is your loco? If it is new, it has a high physical resistance in the motor, the drive trains, the bearings, the wheels, and hence a high electrical resistance. Run it around the track with no load for a couple of hours. Then change it end for end and run it around the track with no load in the other direction for another couple of hours. This will break in the loco. And don't forget to lubricate before breaking it in.

    Finally, given that it is an Athearn, these locos draw a larger current than, say, a higher priced Atlas or Kato locomotive. And, as it is a 6-axle compared to a 4-axle, it will also draw more current.

    As long as you've reduced the physical resistance to the extent possible, you've lubricated the loco, you've run it in, and the voltage and amperage is within the parameters of your power pack, sit back and enjoy your train!

    Or, better yet, go down to your local hobby shop, pick up a couple of switches, and some track, and convert that over-and-under circle of track into a layout with some sidings.

    Bob M.
  5. kadidle

    kadidle Member

    Something that just popped into my head, is the power decrease happening well away from the connection to the Tech4? If so, run a second set of wires from the Tech4 to the midpoint of the track. As it sounds like a temporary track, you may also have some loose/dirty connections at the joiners.

    As Bob pointed out, you really can't violate Ohm's law... :D
  6. jim currie

    jim currie Active Member

    does the tech 4 use a
  7. jim currie

    jim currie Active Member

    does the tech 4 use a pot to control speed if so the more amps the loco draws the more power is wasted in the pot.
  8. ezdays

    ezdays Out AZ way


    The Tech 4 uses digital technology to control the voltage. It actually uses pulse width modulation to change the effective voltage. I've got a Tech 4/250 and two Tech 4/350's. You can program the maximum output with the control knob set to full. At that setting, you get a true DC out. As you lower the knob, the DC turns to a series of pulses, the lower the voltage setting, the further these pulses are. At half setting, the output has a 50% duty cycle to give you effectivly half the voltage setting.

    You can also program momentum and braking for slow starts and stops. I've put a scope on the output and watched the pulses get wider and wider, then narrower and norrower using these features.
  9. smkettner

    smkettner New Member

    Thank you everyone for the insight. It is an HO gauge. Eight cars pulled. 22 to 33 inch radius in a continuous double loop. It is snap together track with integral road bed (ugg) and I did check voltage that is goes around continuous without drop. The grade is 4inches over 70inches or a scary 5.7%. I think the locomotive is in good shape and the wheels and tracks are clean. It will run smooth at slow speeds if I manipulate the throttle continuously as it goes up, down and flat. The Tech4 is rated total output of 17VA with three outputs listed as 18VDC, 16VAC, 21VDC. I have no other accessories connected. I have used the programming and it works fine for what it is but I cannot get what I want.

    I would be OK spending some good money if there was a controller that truly would control voltage and allow the current to increase. Or have some type of regulated speed control to maintain same speed up or down. What is a top brand that I could look at or am I kinda dreaming. I understand Ohms law and the pulse modulation control of my power pack. I guess I was looking for something better, way better than what I have.

  10. smkettner

    smkettner New Member

    This is effectively what I think is happening so I am looking for a fully electronic regulated supply that will do its job irregardless of the load.

    I would prefer something designed for model trains but I am thinking of buying a general regulated power supply for electronic hobbies. I see some for about $100 shipped. I may need to go to my local electronics store.
  11. jim currie

    jim currie Active Member

    sounds like the TAT 4 throttle i built in the 80's

    Smkettner was looking at a ad on back of a MR that had a MRC that will compensate for up hill and down hill.
  12. Ray Marinaccio

    Ray Marinaccio Active Member

    I have the same situation on my layout.
    With some locos I have to throttle up the hills and reduce the throttle going down. I consider this normal operation.
    The locos that I put gear reductions in (such as Ernst regear kits) will run a relatively constant speed up and down the hills. I am assuming this is due to the lower current draw from the motor as it doesn't work as hard with the lower gear ratio.
    I would think that a power supply with higher amps would get your train up the hill with less reduction in speed But I'm not sure that it would run down the hill at the same speed.

    I just had a thought. If you want to let the train run in one direction constantly, you could divide your layout into 3 electrical blocks with an insulated track joiner on one rail.
    One would be the accent, one decent and one on the level. Hook the power supply to the accent block. Wire 4 diodes in series across the accent and level blocks so current will flow from the accent block to the level block. This should give you an approximate 2 volt drop in voltage between blocks. Do the same between the level block and the decent block and 8 diodes in series between the accent and decent blocks giving you an approximate 4 volt drop between the accent and decent blocks. These diodes must be wired so the current will flow from the accent block to the decent block.
    Theoretical this should regulate the voltage in each block automatically.
    It could also be wired to run in both directions with additional diodes wired in the opposite polarity across the insulated gaps and adding another power tap to the block we were calling the decent block. The power leads to the insulated rail would then need diodes installed to give full voltage to the accent block for each direction.
  13. ezdays

    ezdays Out AZ way

    Jim, A few years ago I had downloaded a throttle circuit that I was going to build. It even included a walk-around. Then I was at Roy's Trains one day and they had the Tech4/350 on sale. I figured that even though I had most of the parts already, I couldn't build one that looked that good and I could use the time for other things so I bought one. That's when I put the scope on it and found out how it worked. I went back and bought a second one a few days later. As I remember, the sales price was cheaper than I could find on-line.
  14. ezdays

    ezdays Out AZ way


    You are right, that is a scary grade, and I'd expect it to slow down going up and speed up going down. You are also right in that the 250 has only 17 VA output, I thought that is was 25, but I checked mine and it says "17 VA total output." Now that means it includes the accessory outputs as well, so it's not clear what kind of power is available for the rails. My 350's are only 30 VA, again, total output. Some day when I'm in the mood, I'll check that out and see just how much rail power is available and how much is reserved for accessories.

    But, what you're saying is that your train is running more prototypical in that it slows down going up, and speeds up going down. Ray has a good solution using diodes to give you voltage drops, just be sure if you try it that you use diodes that are rated for the current. The 1N4000 series are rated for an amp, the 1N5000's are good for three amps. Another similar solution is to use more than one power pack, each set to a different voltage, but then you're getting into using more than one cab to control one train.

    I would think that if you had a power supply that would be large enough to give you contstant voltage at these loads, the train would still slow down going up and speed up going down due to gravity, wheel friction and load weight.
  15. railwaybob

    railwaybob Member

    Once upon a time, they used to make an SCR throttle. This operated the same way as the speed control on your car. The loco would run at a constant speed, regardless of the hills or grades.

    The 22" - 33" radius, along with the 4" grade is definitely going to suck up a lot of juice with a 6-axle loco. One solution would be to get an Atlas or Kato 4-axle loco, rather than a new power pack. You might be able to get a used one at a reasonable price.

    If you're feeding the track from only one place, you might want to run some additional track feeds. Remember that voltage drops as it travels along nickel silver (or any other) track. By the time it gets out to the farthest end, you might have a considerable voltage drop. Copper wire of about 16 AWG can carry a lot more power with less voltage drop than nickel silver track.

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