Part Selection Guide
Some of the parts below are optional. For most of these parts, you simply leave them out if you don’t want them. If a jumper is required, it will be mentioned.
Resistors
5% carbon film resistors are the default choice. Resistor accuracy isn’t a big deal in this circuit since the output voltage is adjustable. It would also be reasonable to use 1% metal films, but it would be because of their lower drift and noise, not their higher accuracy.
All of the resistors should be 1/4 W types, with one exception.
R4 should be 1/2 W in most situations. When R3 is 120 Ω, the current across R4 is about 10 mA, so R4 must be under 2.5 KΩ if you want to use a 1/4 W unit. When R3 is 100 Ω, the current is 12.5 mA, so R4 should be under 1.6 KΩ if you want to use a 1/4 W resistor. Even when 1/4 W is sufficient, you may want to use a 1/2 W unit to keep heat in this resistor down to improve long-term accuracy.
If you want to use the Vishay Dale RN/CMF series resistors, the RN55s are 1/4 W and the RN60s 1/2 W at the temperatures you should see in this power supply. They’re specified for lower wattage because their temperature range is so high.
R1
This is a bleeder resistor to discharge the line filter caps. Its value is not at all critical, but it should be high.
R2
Along with C3 this forms a snubber to suppress voltage spikes generated by the magnetics when the power is removed. See the C3 section for further details.
R3
This resistor sets the current used by the regulator to operate. There is always 1.25 V across this resistor, so you pick its value to put about 10 mA through this path. While some LM317 type regulators will operate with as little as 5 mA, they are spec’d to require 10 mA in the worst case situation. Better safe than sorry...
R4
This resistor sets the regulator’s output voltage in conjunction with VSET.
The output voltage formula is:
There is a table on the schematic with several useful R3 and R4 values, assuming that you use a 500 Ω VSET trimmer.
If you want to calculate your own values, then assuming R3 is 120 Ω:
This will put your desired voltage right in the middle of the adjustment range.
R4 is normally a 1/2 W resistor, for reasons stated above.
VSET
This pot allows you to adjust the output voltage over a small range.
You could jumper across R4 and just put a 2.5 KΩ pot here to get full adjustment from approximately 1.5 V to 32 V, but these trim pots are not designed to tolerate frequent adjustments. The idea is to have a small adjustment range so you can dial in the exact output voltage you want. Once it’s set where you want it, you can dab a bit of nail polish or paint on the top of the pot to keep the value from changing.
If you don’t need any adjustment, you can jumper across this pot.
R5
This sets the current through the LED. The LED is more than just a power indicator. It also keeps a small, steady draw on the power supply, which can improve performance in some cases. It also helps discharge the filter caps when the power is removed. If you don’t want a power LED, add R5 anyway, and just jumper across the LED.
You should consider setting the current through this path to be 5 to 20 mA, even though that will make the LED rather bright. This will ensure quick discharge of the filter caps.
Capacitors
C1, C2 (Class X and Y film)
Along with L1 these form the AC line filter. C1 must be a Class X capacitor and the C2s must be Class Y capacitors, for safety reasons. These are sometimes described as interference suppressor caps. Check the datasheet to be sure they are designed to be used across an AC power line, and that they have at least a 250 V or higher rating, even if you expect only to be using it on 120 V AC systems.
These capacitors are optional. You can install just C1 and leave the C2s out, perhaps because you want no coupling to AC ground, or because you can find Class X caps but not any Class Y caps.
Largest Part Size: 20 mm × 8 mm for C1, 15 mm × 5 mm for C2.
C3 (metallized film)
When the power is removed, the fields in the line filter choke and the transformer collapse, generating a voltage spike. Along with R2, this capacitor forms a snubber to defeat this spike so it doesn’t damage parts further down the line.
This board position is currently the same size as the 0.1 µF cap used in the AC line filter (C1); you can use the same cap here. It’s a good value for the purpose, but if you want to make the ideal snubber for your particular power supply, the article Calculating Optimum Snubbers by Jim Hagerman explains how to do it.
The voltage tolerance of this cap should be no lower than what you pick for C5.
This part is optional.
Largest Part Size: 20 mm × 8 mm
C4 (film or ceramic)
These caps suppress any noise generated by the bridge diodes as they switch on and off. The exact value to use here is not terribly critical. Values recommended for this purpose range from 100 pF to 0.01 µF, depending on who you ask. My view is that since it’s a high-frequency application, smaller is probably better. Also, a smaller cap is going to avoid passing lots of line-borne noise.
The voltage tolerance of these caps should be high. To be safe, I’d use caps with a voltage tolerance no lower than what you pick for C5.
Schottky diodes don’t generate large amounts of noise, and they already have a pretty high amount of parasitic capacitance across them. These caps really only make sense if you’re using more generic silicon diodes.
Largest Part Size: 2.5 mm × 10 mm
C5 (electrolytic)
These are the main filter capacitors. You can get away with using just one, but using all four reduces the filter bank’s ESR and reduces pre-regulator ripple.
The voltage tolerance should be at least twice the rated voltage output of your transformer. For instance, if you have a +/-15 V transformer, you should use 63 V caps. The reason is, when the transformer is lightly loaded, its voltage will go up by as much as 40%. Also, the peak voltages put out by the rectifier bridge will be about 1.4× the RMS voltage. Taken together, 2× the voltage tolerance is required for safety.
You don’t want too much capacitance here. I’d keep the total of all the C5s to 5000 µF or lower, myself. Often I’ve used just 2200 µF or so with fine results. Overly high capacitance values will require larger AC line fuses, which reduces their usefulness in protecting against problems. There are other bad effects resulting from overly high filter capacitances. Rely on the regulator to reduce ripple; excessive filtering capacitance isn’t going to make a big difference in the performance of the power supply as a whole.
Largest Part Size: 18 mm diameter
C6 (film)
This is a small, fast film cap to lower the ESR of the main filter bank. Its value is not critical, and it can be left out without a big penalty.
It needs to have the same voltage tolerance as C5, or higher.
Largest Part Size: 2.5 mm × 7.5 mm
C7 (tantalum)
This cap bypasses the regulator’s adjustment pin, increasing the regulator’s ripple rejection.
The default value of 10 µF will work fine if you use a tantalum. You could put a 100 µF electrolytic here instead of the default tantalum, but electrolytics' already short lifetimes drop even faster as the temperature rises. Since this cap is very close to the hot regulator, a tantalum will last far longer.
The voltage across this cap is 1.25 V under the regulator’s output voltage. To be safe, choose this cap’s voltage tolerance to be the lowest value you can find that is greater than or equal to the regulator’s output voltage. For instance, if the maximum output voltage is 24 V, a 25 V cap would be fine. If the maximum output voltage were 26 V, however, I wouldn’t gamble on a 25 V cap: you’ve calculated that you have 0.25 V of margin here, but unless you took component tolerances into account, you might not actually have that margin after all. Better to go with a higher rated cap here.
This cap is optional.
Largest Part Size: 5 mm diameter
C8 (electrolytic or tantalum)
This cap does a bit of post-regulation filtering.
For the LM317 it can be as big as you’d like. A 1 µF or so tantalum or a 22 µF of so electrolytic is a good base value. Performance will improve a bit with larger values. You can leave it out if you want.
For LDO regulators (LM1086, LT1085...) a cap is required here and it must have an ESR of 1 to 2 ohms to guarantee the regulator’s stability. Since higher value caps have lower ESRs (all else being equal), stick to 1 µF for tantalums and 22 µF or so for electrolytics.
If the power supply will be connected to a circuit with large capacitors directly across its power pins (like all of the headphone amps on this site!) you must also consider their ESR and the resistance of the wire between the power supply and the powered circuit when using LDO regulators. For instance, you’re likely to have problems using a STEPS with an LM1086 to power a PPA headphone amplifier that has 2000 µF of rail capacitance. You’d want to use an LM317 here instead, because the whole point of using so many caps is that it makes for a very low ESR value.
By the same token, C8 might be redundant if your circuit has its own rail caps, and the wires between the STEPS and that circuit are short and thick.
This cap sees the entire regulated output voltage across it. Choose this cap’s voltage tolerance to be greater than the maximum output voltage. You want a little bit of margin to account for component tolerances, too: if the maximum output voltage is supposed to be 25 V, better use a 35 V cap here, not a 25 V cap.
Largest Part Size: 5 mm diameter
Diodes
D1
These diodes make up the AC rectifier bridge.
You can use any diode in the TO-220 or DO-201 package here. There are a great many types that will work; the parts table and the schematic give just a few ideas for you to consider. I’m partial to TO-220 types myself, simply because they’re more compact and they will tolerate high current peaks (such as inrush current) better.
Be sure to check the reverse voltage ratings as well as the forward ratings. Pick diodes with a voltage rating at least equal to the fully loaded voltage of the transformer. 35 V diodes with a +/-15 V transformer is a good minimum. It won’t hurt performance to use higher voltage diodes, and it may provide a necessary safety margin.
D2, D3
These diodes protect the regulator when the caps on the OUT and ADJ pins discharge. If you add C7 and C8, you need these diodes. If your circuit has power capacitors directly connected to the power input pins, they are effectively parallel to C8, so you still need these diodes. Generic 1N400x types will work. 1N4001s are probably sufficient, but make them 1N4002s or higher to be safe.
D4
This is the power indicator LED. See R5 for details.
Line Filter Choke
L1 is a common-mode choke. Together with C1 and C2 this choke forms an LC filter to reject line-borne high-frequency noise that would otherwise sail right through the power supply. (The toroid, filter cap bank and regulator aren’t much impediment to high frequency noise.) This noise commonly comes from digital electronics on the same line.
The board was designed with Panasonic’s ELF line in mind. You can get the full line from Digi-Key, and a small subset from Farnell. The board will work with any of the 4-pin chokes with 10 mm × 13 mm pin spacing. The full set of supported types — from smallest to largest — are the 16M, 290, 15N, 17N, 200, 450, 650 and 21V.
If you can’t get the Panasonic ELF series, look into the EV20, EV24, EV28, and RN214 series chokes from Schaffner. You can get these from Newark, Rapid Electronics, and Farnell. I haven’t personally tried any of these; I’m just going by what they say in the datasheets.
When picking a choke, you first need to pick the current rating based on how much current you intend to draw from the supply. Since this part is on the primary side of the supply, the current is reduced by the transformer’s winding ratio. For instance, if your supply will have a constant 320 mA load and you have an 8:1 transformer, the primary side current is only 40 mA. The actual current rating should be much higher to provide a safety margin, and to withstand the power supply’s inrush current. 200 mA should be safe enough in this example.
Once you have the current rating, you can then pick the inductance. The higher the value the stronger the filtering effect. It’s really a question of how much you want to spend. A little ELF type 16M choke will work fine, but some may prefer to spring for a bigger choke.
This filter is optional. The Step-by-Step Assembly Guide explains how to bypass it.
Choosing a Regulator
There are many choices for regulators that will work in this design, ranging from a 40 cent off-brand LM317 to a $10 industrial grade LT1084IT. The standard audiophile reaction to these facts is that the LT1084 must be the best choice. The fact is, there are tradeoffs among these choices, so more expensive isn’t always better, even when money is not a concern.
The lowly LM317 has many advantages: it’s cheap, easy to find, docile, and its performance is very close to that of its more expensive family members. If you want to spend a little more on your regulator, get the National Semiconductor version instead of an off-brand clone, or get the high-spec LM317A variant instead of the plain LM317.
You might also consider the higher-current LM338. Even if the LM317 provides enough output current for your application, higher output current implies lower output impedance, which means better regulation. The LM338’s output impedance is quite a bit lower than that of the LM317.
All the other regulators that work in this board are "low drop-out" types, meaning that the voltage drop across the regulator is lower than for the standard LM317. The downside of these LDO regulators is that they are sensitive to the impedance on their output. This means you must carefully take into account the characteristics of the power supply’s output capacitor, the wire between the power supply and the load, and any power rail capacitors in the circuit being powered. For example, using a STEPS that has an LDO regulator to power a PPA that has 2000 µF of rail capacitance (C1) will probably make the power supply misbehave. For more details on this, read the datasheet for the regulators you’re considering. If you don’t want to read datasheets, avoid LDO regulators.
If an LDO regulator will work in your situation and you want the small advantages it will provide, the cheapest option is the LM1086. This gets you the LDO feature plus a tiny performance boost over an LM317. The next step up from that is Linear Technology’s version of the same design, the LT1086. Beyond that, there’s the 3 A and 5 A versions of this family (LM1085, LT1084, etc.); you probably don’t need the higher current, but to provide that higher current they necessarily have lower output impedance, which improves things a bit further still.
Again, I must caution you about spending exorbitant amounts of money on the regulator IC. There comes a point where the extra money you can spend on a better regulator IC would be better spent on a different, inherently better power supply design. This circuit gives the best value with ICs from the low end of the cost range.
Choosing a Transformer
The STEPS board is designed to hold one of the Amveco (a.k.a. Talema) 5 through 25 VA board-mount toroidal transformers. Digi-Key’s part numbers for these are 7002x through 7006x. Alternately, do a search for "amveco pc toroid" and narrow it to the 5 through 25 VA units.
To choose the proper VA rating, the current you will be drawing should be no more than 80% of the transformer’s rated output current. The transformer will start to saturate otherwise, and therefore start to overheat and otherwise perform poorly.
To choose the proper secondary voltage, use my Power Supply Parameter Estimator.
Choosing an Enclosure
The TEPS board is designed to fit the Hammond 1455N12 series case, mainly so it matches the author’s DIY audio equipment. :) You will have to trim the toroid’s heat fins a bit if you use a 25 VA transformer with this case, in order to allow the assembly to slide into the case unimpeded. If you can get away with a 15 VA unit instead, that’s definitely the better course.
The Hammond is a rather expensive case, so if you just want utility, a plastic project box will be quite suitable.
Miscellaneous Hardware
The board has room set aside around the line filter caps for the Qualtek 723W power input module. (See the stuffing guide for a rough outline of this module in place.) This module has a fuse holder built in, and it’s very nearly as cheap as a plain IEC socket. An alternative to this part is the Schurter 6200 series; it’s very similar in design to the Qualtek socket.
If you don’t want to use this type of power inlet module, you have the option of putting a 5×20 mm fuse on the PCB using standard 5 mm fuse clips. See the assembly guide for information on how this affects the wiring.
For output, you might want to get a DC power cord with the barrel connector molded already on it. Kobiconn has several cords of this type, which you can get from Mouser; it’s a lot more convenient than building your own power cable. You should also get a strain relief for that cable.
You’ll notice four holes near the diode bridge plus one near the output pads, labelled TP1 through TP5. These are test points, useful for testing the power supply. I like wire loop type test points, so I can grab onto them with my meter’s grabber leads. Keystone type 5005 through 5009 wire loop test points work well here. You can also use the pins cut from SIP pin strips, or make loops from resistor lead cuttings, or leave the holes empty to form a crude DMM probe "socket".
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