Friday, March 16, 2012

Linear Regulated Power Supply (Fixed)

Introduction
This is an easy to assemble, all-in-one linear regulated power supply solution.  It utilizes a TO-220 form factor positive fixed linear regulator such as the LM340 or LM78xx series and takes an input voltage of up to 35Vpk from a wall transformer based on the regulator used.  The fuse, switch wirepads, and all of the linear power supply circuitry are integrated on one board – it is meant to be used inside of a HAMMOND project enclosure and all it needs is reduced VAC from a wall transformer of your choice to function.  DO NOT connect this directly to the line AC from your wall outlet, as you will simply fry pretty much everything on the board with the high voltage!

I think this project will make a good project for beginner hobbyists or those who need a simple but good LRPS.  I don't intend to sell it as a kit or fully assembled since this is supposed to be a DIY project after all.
I realize that there are a lot of other LRPS designs out there already but I think this one has its merits or I wouldn’t have bothered putting it up.  In fact I’ll list a few alternatives right here for you to check out and compare for yourself:
AnalogMetric LV30 LRPS
NightFire Kit #1772
AMB σ25 Regulated Power Supply


Theory of Operation
A reduced VAC (ex.16VACrms) comes in from your wall transformer to the board.  CSH shunts away high frequency line noise and transients.  Five fairly large reservoir capacitors come after the full-wave bridge rectifier to provide an excellent unregulated DC signal.  A bleeder resistor (R2) is implemented for safety purposes.
 
CBY bypasses the input to the voltage regulator IC and helps reduce input ripple.  The 1N4002 diode is implemented as a protection diode for the regulator IC.  Electrolytic, tantalum, and ceramic capacitors (C5, C6, C7) are paralleled at the output of the regulator in order to provide excellent transient performance and low impedance over a large frequency range.
 
Input/output power and your on/off switch are hooked up by terminal blocks which accept between 26-16AWG.  (I recommend you use at minimum 24AWG)  I find that these work fairly well and provide an easy means of wiring stuff to the PCB.  A fairly large ground plane is implemented in order to reduce noise and minimize stray inductance among other benefits.

Project Resources
Schematic
Board Layout - Board layout is 1:1 scale so you can print it out if you want to get an idea of the dimensions
Parts List
Note that I've listed parts for a 15V output, but only slight tweaks need to be made if you want different output voltages for other applications.  Many of these parts are incredibly common and you can probably get cheap alternatives for almost everything from a vendor such as Tayda Electronics.  You can do quite a lot with the board and I encourage you to come up with your own customizations and parts – this parts list is more or less here just to get your started.
Part
Qty
Mouser Part ##
Description
AC Adapters 120VAC to 16VAC 1.0A
1
553-WAU16-1000
Wall transformer from TRIAD Magnetics that dumbs the AC line voltage down to 16VAC; excellent build quality but rather bulky and expensive
ACIN/DCOUT/SW - Fixed Terminal Blocks 2P LS5.08mm 90DEG
3
651-1729128
Durable little terminal blocks from Phoenix Contact that are used for hooking up wires
DCOUT - DC Power Connector 2.1mm Pwr Plug Red Tip Blk Handle
1
502-762
Very high quality 2.1mm ID / 5.5mm OD DC power connector from Switchcraft - very expensive but I like it a lot
*DCOUT (Alt) - DC Power Plug 2.1mm x 5.5mm x 9.5mm
1
N/A
Cheapo part from Tayda Electronics; 9.5mm length
Absurdly cheap and gets the job done
Rubber Grommets (optional)
2
534-740, 534-731
Little grommets you fill holes in your enclosure with to pass wires through; choose your own from the Mouser mounting hardware section
F1 Fuseholder for 5x20mm fuses
1
576-65600001009
The fuseholder for a standard 5x20mm fuse - you could short this and not use a fuse at all but I wouldn't recommend this - I've already blown one myself while testing this thing
F1 Fuseholder Cover (optional)
1
576-66000001009
This is a little cover for the fuse holder if you feel like being neat
F1 5x20mm 0.5A Fast-Acting Fuse
1
576-0217.500MXP
This guy will blow immediately if more than 500mA of current passes through it.  The regulator IC can technically handle 1A of current but it is recommended you don't use this power supply for more than about 500mA of continuous current.
SW - any panel mount switch from Mouser or Radio Shack
1
Find your own (needs to be rated for required AC voltage)
Mouser carries a huge selection of switches; Radio Shack has a good selection as well and you can choose one that you think looks cool
Since the board is fairly small you should have quite a lot of space inside your enclosure to work with so switch dimensions shouldn't be an issue for the most part
CSH/CBY/C8 - Multilayer Ceramic Capacitors (MLCC) - Leaded 0.22uF 50volts 10% X7R 5mm LS (Vishay)
3
594-K224K20X7RF53H5
These little ceramic capacitors are used for high frequency bypassing throughout the circuit; you could use a NP0/C0G ceramic here if you want as an upgrade; alternatively film caps will work well here too
B1 - Bridge Rectifier 1 Amp 400 Volt DO-41 (Diodes Inc.)
1
621-DF04M
DO-41/DB form factor bridge rectifier (plenty of cheapo alternatives available)
*B1 (Alt) - DB104G Single Phase 1A 400V Glass Passivated Bridge Rectifiers (Taiwan Semi)
1
N/A
Cheaper alternate part available from thaishopetc
RB - Metal Film Resistors - Through Hole 1/4watt 10Kohms 1%  (Vishay)
1
71-CMF5510K000FKEK
Bleeder resistor that provides a discharge path for the input reservoir capacitors when the circuit is turned off - this part is included for safety purposes
C1-5 - Aluminum Electrolytic Capacitors - Leaded 35V 1000uF 20% 12.5x20 mm
5
140-REA102M1VBK1320P
Input reservoir capacitors; general purpose part from Lelon
12.5mm diameter / 20mm length / 5mm lead spacing
D1 - 1N4002 Diode Rectifiers Vr/100V Io/1A Glass Passivated (Vishay)
1
625-1N4002GP-E3/54
Standard 1N4002 diode - serves as a protection diode for the VREG
VREG - 15V Standard Fixed Linear Regulator (National Semi)
1
926-LM340T-15/NOPB
You can use more or less any TO-220 fixed output voltage regulator in this spot - the Audinst HUD-MX1 needs 15V so I recommend the 15V edition of the LM340 from National Semi; you can request samples from their site or buy them from Mouser along with everything else
TO-220 VREG Heatsink (required)
1
Find your own
Try to find something that doesn't stick out to the sides or front of the VREG too much or you'll end up running into C6 or CBY
I have a bunch from thaishopetc that work quite well and are very cheap
C6 - Aluminum Electrolytic Capacitors - Leaded 25VDC 150uF 6.3x11.2mm LS 5.0mm (Panasonic)
1
667-EEU-FR1E151B
Aluminum electrolytic output capacitor
C7 - Tantalum Capacitors - Solid Leaded 25V 10uF 10% (AVX)
1
581-TAP106K025SCS
Tantalum output capacitor
RLED - Metal Film Resistors - Through Hole 1/4watt 680ohms 2%  (Vishay)
1
71-RL07S-G-680
Resistor to set LED operating current which will depend on your desired output voltage and LED - you can easily calculate this with one of the many LED resistor calculators available online [http://ledz.com/?p=zz.led.resistor.calculator]
For my 15V power supply and the LED in this parts list this yields about 17mA of current through the LED which I find is decently bright.
LED - Standard LED - Through Hole Blue Round LED
1
941-C503BBCSCV0Z0461
LED from Cree that has a 3.2V typical forward voltage and a desired operating current of around 20mA
You can be lazy and just drill a hole through the enclosure for the light to shine through or you can use an actual LED panel mount (Mouser #749-CR-174)
Project Enclosure (HAMMOND)
1
546-1591XXSSBK
The specific project enclosure that this PCB is designed to fit in - measures 4.3L x 3.2W x 1.6H
You only use two #4 screws to mount one side of the PCB down onto the enclosure
If you want to use another enclosure you can always drill mounting holes of your own into the PCB or just use some sort of adhesive on the backside
*Project Enclosure (Alt)(HAMMOND)
1
546-1591XXTSBK
A possible alternate enclosure if the recommended one is out of stock - measures 4.8L x 3.2W x 2.2H
You'll need to redrill two holes to mount the PCB inside this one
PCB Screws for Project Enclosures
1
546-1593ATS50
#4 screws that HAMMOND makes for screwing down PCBs in its enclosures; you could simply go to a local hardware store and buy a couple as well since these are standard screws anyway
Things like heatshrink, wire, and solder I expect you to find by yourself or just have laying around. For solder I usually use Cardas quad or 62/36/2 from RadioShack.  Heatshrink and wire I usually just buy in bulk off of eBay or Tayda Electronics.

Assembly
All parts on this board are through-hole and fairly well spaced so that even those new to soldering can assemble this project with relative ease.  The design still manages to remain reasonably compact with overall board dimensions of 2.5″L x 2.0625″W.  You’ll end up with quite a bit of free space inside the HAMMOND enclosure to work with.  My only words of caution are to basically not heat things up too much to avoid component damage, and to watch for solder bridges between the leads of C6, C7, and the VREG pins as these are probably the only things remotely close enough to be accidentally bridged together.  Also be mindful of component orientation for the electrolytic capacitors and diodes.

The parts in my recommended parts list are for a 15V power supply – if you need a different output voltage you can use your own parts as necessary.  You can pretty much build a power supply for any TO-220 fixed linear regulator that you can find on the market – just make sure you are familiar with the design and know what you are doing before you go around using parts of your own.  I won’t take any responsibility for things getting blown up – I just provide the board and what you put on it is your own problem.

Safety Note:  Be careful when working with live circuits as they can kill you!  Always ask someone more experienced to help you if you are unsure about something.

You start this project off with the bare PCB as pictured below.  I had 3 of them made for about $20 shipped at DorkBotPDX.
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As for soldering components I start with the 4-pin bridge rectifier IC...
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...then I start working my way outwards.
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The gray box capacitors and blue resistors I just happened to have around - they are not the same as the suggested parts on the list.

Next I put in the regulator and some of its surrounding components.
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Note that the regulator is not supposed to go all the way in like many of the other components - you'll see why this is when the heatsink is installed.  Also notice that I was building a 12V power supply.

At this point I clean the board just a little bit using my handy alcohol bottle and brush...
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...then I start to put on some of the chunkier components like the fuse holder and terminals.
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The reservoir capacitors are added in last because they tend to get in the way of everything else.  The picture shown below was taken during my cleaning step after soldering the reservoir caps on.
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You may have noticed that I have still not soldered a few components - this is because I plan on panel mounting these instead.
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Here's an image of how things look inside with the panel mounted components installed.
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I drilled a couple of holes on the side and top near the regulator IC to provide some airflow.

These are some images showing how to get the DC power connector soldered.
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You basically want to first get everything lined up as in the first picture, then solder the wires to the connector, and finally get your heatshrink and connector cover installed.  You should probably heatshrink the negative lead as well - I was just lazy and I find it harder to get the connector cover installed with it.

The regulator IC can technically handle 1A of current but it is recommended you don't use this power supply for more than about 500mA of continuous current.  Things will get quite hot beyond that and performance will suffer.

Testing
Loading to 150mA of current and AC coupling the output to a 101gain op-amp in non-inverting configuration (then dividing by 101) I measured 0.06mVrms of ripple from my benchtop DMM which is pretty much what I'd expect.  Voltage held steady at 12.05V with a 200mA resistive load.

15V Audinst HUD-MX1 Switching PS
-1.6mVrms off of benchtop DMM (Tenma 72-410A)
-1.68mVrms off of DPScope SE (100 averaging)
-5.74mVpp off of DPscope SE
-Switching frequency ~68KHz?

My LRPS
-0.06mVrms off of benchtop DMM
-0.05mVrms off of DPScope SE (smaller than the DPScope SE can accurately measure)
-0.18mVpp off of DPScope SE

Note that these are admittedly fairly basic tests as I lack the high-grade equipment required for anything more detailed than this, but I think that they're still good enough for a basic comparison.

Disclaimer:  I won't take any responsibility for things getting blown up and/or injuries received if you decide to pursue this project.  The project should be considered unverified with only limited testing conducted by myself.

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