Resistor size to "step down" DC output?

[Anonymous]

I have two older MRC Tech II model 1500 DC controllers that I want to use to power my DC accessories, like Tortoise switch machines and the motor drive for my Atlas turntable. However, the Tech II puts out 20v fixed DC, and I need to drop that down to 12v (the turntable motor 'races' with the full output). Anyone know what size resistor I need to put in series with the output to drop the voltage from 20v to 12v?

I did a search on resistor calculators in Google, but the vast majority I found are for figuring out the value of a resistor from the color bands, or for figuring out the size resistor needed for an LED, neither of which particularly help me. [banghead] I did find one that calculates voltage divider circuits, but that's not quite what I'm doing either.

So if you could suggest a resistor I should use, or point me in the direction of a calculator that'll figure out the resistor needed for a particular voltage drop, I'd appreciate it.

Oh, and Happy Holidays, one and all!

[rrinker]

Use the same calculator as for the LED< but where it asks the LED voltage, put 12v.

You want 12v, the power source is 20v, so you need to drop 8v. You also need to know how much current the motor uses. One Tortoise is about 12ma. 20 of them would be about 240ma, or .24amp. So your resistor will drop 8 volts into a .24amp load. V=IR, or R = V/I. So 33 ohms. BUT you also need a power value for that resistor. Current through the resistor is going to be V/R, or 20v/33 ohms, or .6 amps. Time the voltage, 20v, is 12 watts. Yes, TWELVE watts.

Bottom line, don't use a resistor for this. Use a bunch of diodes. Forward biased IN4001 type diodes drop .6 volt each. String 12 or 13 of them together to drop the 20v to 12v. 1amp diodes will run a LOT of Tortoises.

Or even BETTER, use a 12v voltage regulator. I would use a filter capacitor on the input to smooth the DC, plus you need a coupel of small capacitors for the regulator itself, and if you mount it on a heat sink you can get a solid 1.5 amps from a 7812 12v regulator, assuming the source is up to supplying that much power. This is the best choice of the 3 so far.

Option 4, get an old computer power supply, and take a look at this web site. http://web2.murraystate.edu/andy.batts/ps/powersupply.htm A used but workign PC power supply can be nearly free if you look around, a switch and come binding posts and a 10 watt power resistor from Radio shack will run you a couple of bucks, and you will have a handy 12v power supply that can supply enough current to run every Tortoise, turntable, and structure light for the entire neighborhood. A fuse on the output line would be a good idea, too. Sending 10-15 amps at 12v through parts of your layout into a short is not a good thing, plus the power supply will be pretty much instantly destroyed if the outputs are shorted.

--Randy

[Anonymous]

Hmm, that's a lot more work than I want to get into to. I have a filtered and regulated 12v power supply from my ham radio days laying around -- guess it's time to dig it out of its box and put it to use. [:)]

Thanks, Randy!

[Leon Silverman]

I have a question for jsalemi:
Are you using that power pack for actually running trains. or just to power your accessories. If you are using it for your accessories only, then run it off the variable DC.
The rated Volt-Amps of a power pack are degraded by what ever accessory load you put on it. In this case. (.24amps x 12 volts) equals 2.88 Volt-Amps. You never said what the current draw on the Atlas turntable motor is, but that will also detract from the current available to run trains on.
To rrinker, you power requirements are wrong. The voltage drop is only eight volts, not 20. This gives a power dissipation of only 1.9 watts. I wouldsize the power capacity at double this power draw, or about 4 watts, in order prolong the life of the resistors. I do not know what power ratings in resistors are available, but the required dissipation could easily be achieved by connecting the resistors in parallel (e.g. (4) -1 watt or (8)-1/2 watt resistors) to run the Tortoise.

The formulas are:

P = IV = I(SQUARED)R = V(SQUARED)/R.

[Anonymous]

No, I'm just using the power packs for DC power; I've converted my RR to DCC. Actually, I'm using one of the power packs as a 'jump throttle' for my Digitrax Zephyr, but I guess I can use the other one as a DC power only supply, and set the variable output so it only puts out 12v.

Boy, that's a Homer "D'oh!" if I've ever encountered one. [B)]

Don't know what the draw of the Atlas turntable motor is -- their documentation doesn't say.

[rrinker]

Oops. Hey, it's been nearly 20 years since I had to actually know this [:D]

If you plan to run the Tortoises directly, you might want to set that variable output to slightly under 12v. If you power the Tortoise through a simple DPDT toggle, running them on a bit less than 12v makes them even quieter. Dunno what the Atlas turntable requires though. On 8-10v it might be a bit too slow.

--Randy

[Anonymous]

Seems to work fine on 12v output (one of the advantages of telecommuting is I got to hook up the other MRC power supply this morning [:)]). I'll be running most of the Tortoises through DPDTs (about 6 right now), though I picked up a NCE Switch-It to play with and may run one or two additional Tortoises through that instead. Haven't decided yet if I'm going to go with DCC control or stick to switches -- figure I can sell the Switch-It on eBay if I decide not to use it...

[rrinker]

One of the disadvantages of telecommuting is that I have to resist the tempation to go down and work on the layout RIGHT NOW. [:D]
I ORIGINALLY was planning to use Switch-Its for turnout control, but I've been leaning toawrds a CMRI implementation for that aspect. Under my original plan, what I was goign to do is this: Since my layout would need more than the 2.5 amps of the Zephyr to run anyway, I was going to expand with a Chief or Empire set, likely SEB first, keeping the Zephyr as the command station for the seperate programming track and readback, since I wouldn't likely be running more than 10 locos at a time, and then later adding the Chief when I needed to handle more than 10. Since the Zephyr track voltage is not adjustable and not an exact match for the DB150/DCS100 N or HO scale settings, I wasn't going to power any track sections with it - rather, this would become the dedicate supply to the Switch-Its and thus the Tortoises.
Lacking this, it's a good idea to power the accessory decoders from a seperate power district through a circuit breaker of some sort, because if they are powered from the local bus and you have a short because of a mis-thrown turnout...oops, no power to change it, and as soon as you manually move the Tortoise to clear it, the power will come back and push it right back to the 'bad' position!

--Randy

[Anonymous]

Good tip on putting the accessory decoders in their own power district. Thanks!

My layout room is right next to my home office, so it's easy to walk 10 feet and do some little job while the computer is churning away at something. Amazing how much you get done on the layout when you break the work up into 5-10 minute chunks a couple of times a day. [:D]