Step-by-Step Assembly Guide

WARNING: Before I get to the information on how to assemble this, I want to make an important point: I am not selling STEPS power supplies. I sell a circuit board that can be used to make a power supply. The distinction is that you made the power supply, so you must take responsibility for your own safety when building, testing, and employing it. If it fails in any way, it’s not my problem. Power supplies can kill. Be respectful of that power.

1. Add the short stuff

Add all the resistors.

Then add VSET, D2, D3, the C4s, C6, C7 and C8. The C7 and C8 capacitors are polarized, so be sure you put them in the board the right way!

You could add the power LED and test points now, if you want.

2. Add the TO-220 parts

Add the bridge diodes, being careful to get the orientation right.

Next, add the regulator. The best way to do this is to put a thin layer of thermal grease on the regulator, then bolt the regulator to the heat sink, and insert the whole assembly into the board and solder it down.

3. Add the AC input wires

The wire pads are big enough for 18 gauge wire.

The three wires to the right side of the picture are for AC line (Lx), AC neutral (N) and earth ground (G). There are two pads for AC line. The LA pad is the one to use if you will be using the on-board fuse holder. If you will be using an IEC inlet module with its own fuse holder, use the LB pad instead. Under no circumstances should you avoid using a fuse!

If your AC inlet module uses FASTON type lugs, crimp the mating lugs onto the each wire for attaching to the AC inlet module. Keep the wires short. If you’re putting the power supply into a metal case, you should crimp two wires into the ground lug: one wire goes to the board, and the other goes to the case for safety reasons. If the case is aluminum, sand the inner surface around the bolt hole to remove any surface oxide and anodization to ensure good contact with the case ground wire.

The A through D pads configure the transformer’s primary side for your country’s wall voltage:

100-120 V power: Put a short jumper from the A pad to the B pad on the board. Jumper the C pad to the D pad. (This is the configuration shown in the picture.)

220-240 V power: Put a short jumper from the B pad to the D pad on the board.

If you won’t be using the line filter, jumper across the filter choke. You need two wires, each one going between the two pads joined by the curly wire symbol on the board.

4. Add the big stuff

Add C3.

Add the C5s, paying attention to polarity. The big copper rectangle is the ground plane, so all the negative leads go towards it.

Add the transformer. The side of the transformer marked 115 V goes towards the long edge of the board.

5. Add the line filter

If you want the line filter, add L1, C1 and the C2s.

6. Test the power supply

Plug the IEC cord into the power supply first, and then plug the other end into the AC outlet. This keeps you away from the power supply when it first turns on, in case something is hooked up improperly. It also keeps your fingers away from the bare wires at the rear of the AC input jack when power is applied. Power supplies can self-destruct violently, so you may want to avert your face. An exploding capacitor is...exciting.

If nothing exploded or caught fire, measure AC volts between test points 1 and 2. Be careful not to short them together. This is the AC output voltage of your transformer. With no load on the power supply, it will be higher than the rated output voltage of your transformer. With the Amveco 70053 (±15 V), I get about 37 V here. Your transformer’s datasheet may have this value in it, so you can check that you’re getting the correct value.

Next, measure DC volts between test points 3 and 5. This is the unregulated DC voltage at the filter capacitors. You should get a bit under 1.4× the AC voltage you measured previously. (You’d get exactly 1.414× the AC voltage with an ideal bridge rectifier. Real rectifiers have a small voltage drop across them.)

Next we can check that the unregulated ripple voltage is reasonably low. If you have a suitable digital multimeter, you can just measure AC volts across test points 3 and 5. You should only read some tens of millivolts, the exact value depending on how much filter capacitance you used. If your reading is over a volt or you have an analog meter instead of a digital one, you need to put a large capacitor inline with one of the meter’s leads to strip off the DC voltage. How large a cap you must use depends on the meter. For an analog meter (low input impedance), it may have to be several microfarads, necessitating a very good (expensive, large) film cap or an electrolytic. If you use an electrolytic, be sure to get the polarity right. For a DMM, most any large film cap should work, since the input impedance will be much higher than for an analog meter.

Next, measure DC volts between test points 4 and 5. This is the output voltage of the regulator. You can adjust the voltage with VSET now, if you wish.

If you have a measurement preamplifier (such as my LNMP), you can also measure AC volts between points 4 and 5 to see how much ripple the regulator let through. This should be such a small value that without the preamp, all you’d be measuring here is your meter’s noise floor, unless you have a very high end meter. Even a Fluke 189 isn’t sensitive enough to give the true residual ripple value for a properly functioning STEPS without an LNMP! It can be useful to do this test without a measurement preamplifier anyway, just to be sure that output ripple is all but zero. If you don’t get a near-zero reading and aren’t absolutely sure your meter strips DC on the AC settings, put a cap inline with the meter as recommended for the unregulated-side ripple test above. (Incidentally, the LNMP does strip DC, so you can use it with even cheap meters.)

7. Do the casework

For the strain relief I recommended in the parts list, you need a 7/16" hole. Run the DC output cable through the hole in the panel before soldering it to the board since the barrel connector might be too big to fit through the strain relief hole.

For the recommended AC input jack, you need a rectangular hole about 27 mm × 33 mm. If you use the Hammond case, put the hole 5 mm from the top of the panel and 8 mm from the left edge.

If you’re using a metal case, run a wire from the AC ground to a bolt on the case, as shown in the picture above. If your case is aluminum, the anodization or oxidation will need to be removed around the bolt hole so the ground wire makes a solid electrical connection.

If you have the older v1.0 version of the board, it is a smidge too wide to fit in the Hammond 1455N12 case. (This was fixed in v1.1.) You can take a Dremel with a cutoff wheel and take about a millimeter of material off the edge towards the regulator circuit. Sand paper will also work.

If you will be mounting the board into the bottom of your case, the mounting holes are at the corners of a 3.65" × 4.425" rectangle.

You’re done!