According to the Application notes for the LM338, This formula will determine the output voltageVout = 1.25x(1 + R2/120) + .00005xR2On Page 8 they have a diagram for a 1.2V->25V variable output using a 5k Trim POT for R2.Whenever I do the math though I get:Vout = 1.25 x (1 + 5000/120) + .00005x5000 = 53.58VoltsThis is way over the 25V output.(pdf link)http://www.ti.com/litv/SNVS771AThe reason I'm jumping through these hoops is because I like to go from Lenz->Digitrax Zephyr (I like the throttle/cab). Problem is the Zephyr doesn't allow you to control the track voltage which sits about 12.75V. (A bit low for HO DCC) According to the manual, the track voltage = input power source - .5 Volts. So I thought about getting a cheap 19V laptop computer power supply and using the voltage divider circuit. But the math doesn't add up.
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!
Not a big deal. Set the pot to half way. Adjust the pot for the voltage you want. Remove the pot. Measure pot resistance. Insert the nearest value from your resistor selection.
I have made a bunch of these regulators.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
A simple on line calculator from a Google search. I know, some hate to hear, Google search. Store the link everyone.
http://www.reuk.co.uk/LM317-Voltage-Calculator.htm
Thanks Rich,
Much appreciated. I thought the application notes looked a "lil off" when they put a 5k pot in there. (I guess 2.5k pots are harder to come by and why they used the 5k in the example circuit)
Ok, that is why your post sounded so confusing.
Many examples use a 5 k pot BUT, if you look carefully, you can see the supply is variable voltage type. They assume you will select the proper fixed resistor.
I have picked up some 2.5 k sometime ago. There are many on line surplus electronics dealers around.
That diagram has been pretty much a standard since I started using regulators back in the late 1970's.
Once in a while, some diagrams might show only fixed value resistors.
You don't mention it but I hope you have a multimeter if you work at what I call the component level.
An even simpler answer would be to get a laptop power supply closer to the range you require. You can get 16 volt varieties on ebay for less than ten bucks. Your output would be right around 15.5 volts which should be just about right.
Mark.
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I'm not so sure that "input voltage - .5V" is accurate - the PS315 power supply that came witht he Zephyr is a 15V power supply. And do not exceed the maximum input voltage on the Zephyr in pursuit of higher output voltage. You can still use your Lenz booster, or one of the inexpensive Tam Valley ones, to get more voltage on the track. Plus the Tam Valley booster is 3 amps vs the 2.5 of the Zephyr. The Zephyr output can run stationary decoders while the Tam Valley booster powers the rails.
And to drop the power supply voltage, no need for complex circuits. Just wire a few diodes in series. Each diode will drop about .6 volts, so 4 of them will drop 2.4 volts. Just make sure to get diodes with current ratings at least equal to the load, 3-4 amp diodes will be sufficient for powering the Zephyr. Probably cheaper than some complex regulator circuit as well.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Mark R. An even simpler answer would be to get a laptop power supply closer to the range you require. You can get 16 volt varieties on ebay for less than ten bucks. Your output would be right around 15.5 volts which should be just about right. Mark.
Just check the specs in the listings. I have two laptop supplies that are rated for 19 volts DC and measure 19 volts DC. Both good for 3.5 amps.
One regulator. two caps and two resistors are not complex. The beauty is you have a regulated constant voltage source. If you try to draw too much current or the regulator gets too hot, the regulator protects itself by shutting sown until you reduce the load.
I'm pretty sure Zephyr has a regulated output. Their other boosters do. If that's the case, increasing input voltage won't increase output voltage. More heat to dissipate will be the result. Reducing input voltage will reduce output but diode pairs are simpler.
Randy's suggestion to use the Zephyr to control another booster that has adjustable output would be the way to go. It sounds like you already have the equipment needed.
Martin Myers
Per the manual
Maximum input: 3 Amps, 16.8 Volts DC, 42 VA continuous. DO NOT exceed 16.5V DC input. Digitrax recommends the use of a 12V DC battery or a dedicated PS314 power supply or equivalent for use with the DCS51. PS314 output is rated at 13.8V DC and allows up to 3Amps to enable correct operation of the unit. A 2mm power connector is needed to connect a 12V DC battery or SELV UL Class 2 rated DC power supply that meets the stated requirements. Output (to track): DCC digital waveform of approximately 0.5V less than input DC voltage, which meets recommendations for most HO and N scale locomotives with DCC decoders installed.
Maximum input:
3 Amps, 16.8 Volts DC, 42 VA continuous.
DO NOT exceed 16.5V DC input.
Digitrax recommends the use of a 12V DC battery or a dedicated
PS314 power supply or equivalent for use with the DCS51.
PS314 output is rated at 13.8V DC and allows up to 3Amps to
enable correct operation of the unit.
A 2mm power connector is needed to connect a 12V DC battery
or SELV UL Class 2 rated DC power supply that meets the stated
requirements.
Output (to track):
DCC digital waveform of approximately 0.5V
less than input DC voltage, which meets recommendations for
most HO and N scale locomotives with DCC decoders installed.
19V 3.5 Amp DC power supply $16
500 Ohm Pot (10 turn 5% accuracy) $16
14.25 V is standard HO DCC
So that's 14.75 VDC I need at the terminals.
15V power supplies are fairly common as well. No extra components needed.
rrinker 15V power supplies are fairly common as well. No extra components needed. --Randy
Sounds very pratical Randy. 14.5 VDCC would be great voltage to work with. However it becomes a real pain when I have to hook it up to other boosters running 14.25 VDCC. As there is a 3 Volt drop across the regulator circuit @ 3 Amps, I have to grab the next highest common voltage >=18 Volts, which is 19 VDC.
You see now why my track power comes from a DB150 and mY Zephyr just sits there and acts as the command station.
But the volateg athere shouldn't be an issue - you shouldn;t use one moster supply to power all boosters, they should have their own.
Or I'm missing something. The old Zephyr definitely does not follow the .5 volt less than input, since the default power supply is 15V and the track output is around 12.5V. If the Xtra is .5 less than input, they you'd be getting 14.5 to the track which sounds liek what you want. A half volt or so less difference isn't much of a jump and will generally be unnoticeable, it's when there's 2+ volts difference that the locos noticeably jump.
While the fancy regulator appeals to the tinkerer in me, it offends the keep it simple engineer side of me.
When I had only the Zephyr, I never felt things ran too slowly - most locos run faster than they should anyway, particularly on our model layouts, where we think we are being generous with 32" radius curves but those are actually quite sharp. But then, my interests lie mainly in way freighs and switching, where very slow speeds are the order of the day. Reliable slow speed runnign is my goal, and if my freights can get up to 45-50mph that's plenty fast enough.