Building an improved adjustable Bi-Polar Power Supply! (Updated: 4th Feb 2021)

Following on from my previous power supply builds in an effort to get good clean audio from a decent clean power supply, the parameters have not changed that much. I decided to design this power supply as the previous dual-rail / bi-polar design did not meet my requirements for the way I wanted to build my next project.

This time the focus was on practicality. Now that I am getting good with KiCad design, I decided it was time to have a go at designing another power supply. I was happy with the single rail design I did previously as it was quite small and served it’s purpose. My objective with this design was to make it to a specified size and make it overall smaller in the process so it would not only be more appealing to me but to others aswell.

Design Parameters

This design is based around the same type of input transformer supporting dual primaries for 110v/220v operation and features the same CRCRC filtering stage as before with the only difference being this time I opted for a rectifier diode bridge IC instead of using 4 individual diodes as its cheaper and less components to solder plus our aim is to save space. I set a specified board size to work with this time (160mm x 100mm) as I wanted it to slide into the rails of a Hammond 1455T2201 enclosure as that is what I will be using for my upcoming project.

You may also notice I have used different regulators in this design and opted for the LM2941T (Positive) and LM2991T (Negative) regulators. I did this because these are more modern LDO (Low Dropout) regulators and have good ripple rejection and are not as noisy overall compared to the old monolithic 78xx and 79xx counterparts. They also have more features so they come in a 5 pin package instead of the usual 3 pin most are used to. More on the features later.

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So what’s changed?

As I mentioned previously, I have made a few design changes to this power supply. Here is a list of the improvements and changes I have made in this design:

• More modern LM29xx LDO regulators used which have lower ripple noise.
• Regulator ‘Enable’ function so you can power the regulators ‘ON/OFF’ while leaving the rest of the board powered up.
• Adjustable output voltage via trimpots for both rails increases flexibility of the power supply plus you get the added benefit of being able to match both outputs exactly.
• An onboard fuse has been added for those that want more flexibility over type of IEC inlet used.
• Mains input voltage selection jumpers now smaller and much clearer with silkscreen showing how to configure.
• Added terminal connectors to make it easier to connect up without having to solder wires.
• LED indicators showing power to the rails protected with current limiting resistors.
• An output LED indicator with external connector option to show when the regulators are powered on which is protected by a CCS IC limited to 20mA.
• Larger output smoothing capacitors (1000uf) to further reduce ripple noise.

Building and Testing

Building the board is fairly straightforward. I like to solder the smaller components first and work my way through finishing with the larger components. There are a few things to bare in mind when building this power supply and there will be some choices you can make depending on your preference. If you have one of my other power supply boards you may notice I have also increased the size of the solder pads on the resistors and regulator pads in particular.

You may have noticed the large ‘dots’ which are marked next to certain components. These indicate the positive side to help make stuffing the components and building a little bit easier. The only exception to this rule is the D2 zener diode. This ‘dot’ simply marks the orientation of the component and the smaller ‘dots’ are reference positions of the footprints to indicate pin 1.

Measurements

After building a prototype for testing I decided to take some measurements on the oscilloscope. If you look at the image below you will see the ripple is practically non-existent. I managed to get it lower than the other power supplies I built previously which is thanks to the use of modern regulators and an increased smoothing output capacitor. Peak to Peak voltage measures at around 1.6mV with the average sitting at around 655uV and the RMS at 724uV which in all fairness is probably noise generated by the scope otherwise it would read much lower. This is indeed one quiet power supply.

Important Build Notes

Before stuffing the D5 LED and the U3 CL220 IC its important to consider if you are going to use an external LED mounted on your enclosure. If using an external LED connected to J10 then you should leave D5 vacant. The CL220 is a CCS (Constant Current Source) LED driver IC which outputs a maximum 20mA of current.

The reason this was used is because it limits the output current to remain at 20mA so when you adjust the output voltage via the trimpots it keeps the D5 or J10 LED at the same brightness regardless of what the output voltage is set to.

Just a word of warning the CL220 does run hot to the touch but nothing to be concerned about.

I measured the ambient temperature with a laser handheld thermometer and it peaked at around 55°C which is fine without a heatsink and won’t cause any issues as it’s well within operating limits. Just make sure it’s not touching any of the output capacitors.

If the temperature of the CL220 bothers you, then you can leave U3 CL220 and D5 LED vacant and connect your own LED on a lead with a series resistor to the positive or negative output if you intend not to change the voltage once it has been set. Doing this will also disable the output of J10 where you would normally connect the external LED.

Alternatively you can connect a resistor to the middle pad and square pad (pins 2 and 3) where the CL220 would usually go and leave D5 vacant to allow you to use the J10 connector for your own external LED. I use a 2.2k for 18v to 20v output like in the image above but it is always best to calculate the correct value that you need.

If you do decide to use the CL220 then ensure it is installed the correct way round. The silkscreen on the board is marked to show which way the tab should face but you can also refer to the image below for clarity.

If you have a REV B board you can connect your resistor to R10 which is located directly underneath U3 CL220. If you do this then you must leave U3 CL220 vacant.

If you have opted to use an external LED connected to J10 with the CL220 installed its important to note that your chosen LED should not exceed 20mA otherwise it could end up dim or simply not lighting up at all.

Also note the small dot marked on the J10 connector, it was actually part of the design footprint of the connector components but it also conveniently marks which side the positive of your LED connects to. This is marked more clearly in the REV B boards.

An oversight on my part was forgetting I had implemented an onboard fuse which meant I should have labelled the terminals for J1 to ensure the fuse is connected to the Live side and not the Neutral. If you refer to the image below I have labelled the correct orientation of connector J1 for the AC input. I have also labelled the Earth too but you can connect that to any of the four mount holes. This has since been rectified in the REV B boards.

All being well if you have built the board correctly you will have a nice quiet power supply like the one I built below. The output voltage can be adjusted easily by turning the trimpots (RV1 and RV2) clockwise to increase the voltage and anti-clockwise to decrease the voltage. Although the operating limits of the regulators are (+/-) 5V to (+/-) 20V, you can actually set the trimpots beyond these limits. This has not been fully tested and you do this at your own risk.

One last thing to mention is the connector J6 which provides the ‘ON/OFF’ or enable function of the regulators. If you wish to use this function then simply connect a latching switch to this connector to ‘short’ the two terminals together or simply omit the connector and solder a bridge or attach a loop to the connector like above if you plan to just power the whole board from an external switch.

For reference, the side of the connector with the dot provides the ‘ON/OFF’ or ‘Enable’ function on this pin and the other pin is connected to ground. shorting the two together simply ties the ‘ON/OFF’ or ‘Enable’ to ground which switches the regulators ON. This function of the regulators is protected by a current limiting resistor and a 3.3v zener diode.

There is also an optional MOD you can perform which allows you to connect an LED to show when the regulators are in the OFF state. (Standby LED). Refer to the image above to see how to connect it. Make sure the LED is a low power LED ideally between 1.5v to 3.0v. A red LED would probably be a suitable choice to signify the ‘OFF’ state. When you switch the regulators ‘ON’, the LED will turn off.

Note: As J6 is primarily used to turn the regulators ‘ON/OFF’, connecting the LED alone across J6 pins does not turn the regulators ‘ON’. You will still need to hard short the two pins in this case.

Bill of Materials

The following components can be ordered from DigiKey:

732-11373-ND x1 F1 Wurth (FUSEHOLDER BLOCKS – PCB – CLIP C)
F4607-ND x1 Fuse Littelfuse Inc. (500mA Slow Blow Ceramic 5mm x 20mm)
399-5464-ND x1 C1 Kemet R46KN322000M1M 0.22uf/250V Snubber Capacitor (X2 Rated Polypropylene)
DF04M-ND x1 D1 ON Semi. (BRIDGE RECT 1P 400V 1.5A 4DIP)
493-1883-ND x6 C2,3,4,5,6,7 Nichicon (UPW Series CAP ALUM 3300UF 20% 35V RADIAL)
PPC5.1W-2CT-ND x4 R1,2,3,4 Vishay (RES 5.1 OHM 2W 5% AXIAL)
102-6161-ND x5 J1,6,7,9,10 CUI Devices (2 POS. TERMINAL BLOCK, SCREW TYPE, 5.00)
102-6162-ND x1 J8 CUI Devices (3 POS. TERMINAL BLOCK, SCREW TYPE, 5.00)
LM2941T/LF03-ND x1 U1 ON Semi. (IC REG LINEAR POS ADJ 1A TO220-5)
LM2991T/LF03-ND x1 U2 ON Semi. (IC REG LINEAR NEG ADJ 1A TO220-5)
HS352-ND x2 HS1,2 Aavid (HEATSINK TO-220 W/PINS 1.5″TALL)
3296Y-203LF-ND x2 RV1,2 Bourns Inc. (TRIMMER 20K OHM 0.5W GW TOP ADJ)
1N5226BVSCT-ND x1 D2 Vishay (Zener Diode 3.3V 500mW ±5% DO-35)
BC4554CT-ND x1 R5 Vishay (RES 27K OHM 1% 0.4W AXIAL)
BC3925CT-ND x2 R8,9 Vishay (RES 1.2K OHM 1% 0.4W AXIAL)
CL220N5-G-ND x1 U3 Microchip Tech. (IC LED DRIVER 220V .02A 3TO-220) (OPTIONAL)
516-1310-ND x2 D3,4 Broadcom (Green 569nm LED 3mm 1.9V Radial)
516-1308-ND x1 D5 Broadcom (Red 626nm LED 3mm 1.7V Radial) (OPTIONAL)
BC4549CT-ND x2 R6,7 Vishay (RES 2.2K OHM 1% 0.4W AXIAL)
493-1831-ND x2 C8,9 Nichicon (UPW Series CAP ALUM 1000UF 20% 25V RADIAL)
1295-1074-ND x1 TF1 Talema (70064K – 25VA – 18V / 694mA)

Alternate Transformer:

TE2260-ND x1 TF1 Amgis (L01-6364 – 25VA – 18V / 694mA)

Board Availability

Drop an email to contact@audiosy.net if you are interested in obtaining one or more boards for your use.

Disclaimer:
The boards are provided as is without warranty of any kind. They are intended solely for personal use and are supplied as a bare board only which is classed as an unfinished product. While every effort has been made to ensure the board has been designed with industry standards in mind, it may not depict an exact representation of these standards and as such there may be minor inconsistencies or oversights in the design process. While these boards have been extensively tested, there can be no guarantees whatsoever that the board is completely free of defects and as such caution and due diligence should be taken at all times when assembling and using the board.