The next-easiest way to get clean DC is to poach if from your power-pack (MRC, etc.). By attaching a simple regulator circuit to your pack’s “Fixed DC Output” terminals, you can have a solid 12 volt source for only a few bucks and about an hour’s work. Parts for the circuit below are available from Radio Shack, Jameco, DigiKey, and plenty of others. Don’t forget the heat sink — many power packs have 20 volts or more on the Fixed DC terminals, so the regulator will get quite toasty.
If you’d rather build a stand-alone supply, the next two circuits will supply you with up to one amp of clean DC. BE CAREFUL with the AC mains voltage and the primary wiring to the transformer — if you’re not very “electrical,” please get help! Again, parts are readily available, and construction is quite straightforward (remember that heat sink).
“PICK-A-VOLTAGE” POWER SUPPLIES
If you need a power supply with an output voltage other than 12 — or if you’d like a supply with an adjustable output voltage (great for the bench) — the “Pick-a-Voltage” supplies shown below could be just the ticket. You can tailor both the output voltage and the available current; the Component Selection Table shows you how to customize to your needs.
If you want adjustable output, Figure 1 shows how to wire the adjustment potentiometer for the 0-24 volt version; for the 0-12 volt model, simply use a 1 Kohm potentiometer (pot) in place of resistor R2 (in either case, don’t forget to jumper the center and rightmost pins). If you go with the adjustment pot, connect the two leads (marked TO CIRCUIT) in place of resistor R2; in other words, remove R2 from the circuit, and connect the pot/resistor combo between the ADJ pin on the regulator and the “minus” (-) rail. It doesn’t matter which end of the pot goes where (as long as it’s wired as shown).
from two identical “wall-warts.” To see the
drawing, just CLICK here.
DOES CAPACITOR SIZE MATTER ?
Yes. The electrolytic filter capacitors shown in these circuits act as “charge reservoirs” (the “filter cap” is the one between the bridge rectifer and the regulator). They add charge during the peaks of the pulsating DC waveform (from the bridge rectifier), and supply charge during the waveform valleys. To a point, the more charge they can hold (ie, their rating in microfarads or “uf”), the more DC current you can draw from the output. Of course, the tranformer’s secondary current rating must be at least equal to the amount of current you expect to draw — capacitors can’t create current (or matter). This point is well worth remembering. In order to determine the amount of current one can draw from a given filter capacitance, use this formula:
One more thing: capacitors are also have a rated “voltage” — this is a very important spec, as it determines what voltage the cap can actually hold. As a rule of thumb, select a cap rated at twice the voltage you expect to see on that cap. For example, in the case just above, out filter cap is on the output of a transformer with a 12.6 volt secondary; so, use a filter cap with at least a 25V rating (higher ratings are fine).
[Note to the electronically sophisticated: Yes, I do understand the difference between RMS and peak voltages…and I know I’ve simplified the math here. But it works, and keeps us focused on the issues at hand.]
CONSTRUCTION HINTS & TIPS
1. Don’t forget the heat sink on the voltage regulator IC; get the best one you can find. Depending on the output voltage and load current, that little regulator can get very hot!
2. The LED power-on indicator is optional, although I find it handy. If you don’t want it, just omit the LED and resistor R3. I happen to like green LEDs for power-on indicators, but some folks prefer red (or even yellow…but hopefully not blue); if you’re of the red persuasion, cut the value of R3 in half (eg, instead of 2.7K, use 1.5K, etc).
3. If the supply is located more than 12″ from the load, you may want to add another electrolytic cap across the “+” and “-” leads close to the load. I’d use a 1000uf/35V unit (or a 1000uf/50V cap, just like the one shown on the input to the regulator).
4. BE CAREFUL wiring up the transformer primary and connecting to the 115VAC house current; this voltage is DANGEROUS! If you’re not comfortable doing this, get help from a qualified friend or electrician.
A special-purpose variable-output supply intended for use with Digitrax’s PR-1 Decoder Programmer can be found under “DCC ACCESSORIES.” The adjustable output voltage may be useful to you in other projects, or as a bench supply. Yet another supply can be found under “DC THROTTLES,” located with the “Single-Knob Throttle” design. This one is a “dual-rail” design; that is, it has matching positive and negative outputs.