# Simple Benchtop Power Supplies

#### Wendy

Joined Mar 24, 2008
21,848
Back in the 80's if you wanted to build a power supply you had to get a transformer, diodes, and a large capacitor. It also meant people new to electronics and electricity had to build a project using line voltage, which can be hazardous if you make a mistake.

Wall Warts

During the 90's wall wart's started becoming common. If you don't know what a wall wart is, think of a power supply box that plugs into outlet directly. They can be awkward, but more importantly, they are extremely safe to use. Nowdays you can buy pretty much what you need in the form of voltage and current. Some of them can have some pretty sophisticated circuitry to handle much higher currents than the older days.

A word to the wise though, they come in all flavors, and I do mean all. You can buy a simple AC wall wart, basically nothing but a simple transformer in the wall.

Most are simple DC power supplies with no regulation. You can usually find the specs on the case. Pay special attention to polarity, as there are no standards concerning any of these devices. As a general rule of thumb they also do not use a ground, though there are exceptions to this. It is highly unusual for ground to be connected to one of the power supply leads, though again, you can find a few exceptions.

I'm going to assume a simple DC model, or a switching unit that is highly regulated. The simple units will have non-replaceable fuses inside, if you blow them you are expected to throw them away. The unregulated models do not have stable DC voltages either, a 12VDC 1A can have as much as 18VDC out with no load, and it can be worse. You will need to check this before you use it.

To make a modern bench supply with one is simplicity itself. There are families of 3 terminal regulator chips out there, very inexpensive, very effective, and very old. You can buy many of them from Radio Shack, they are that common.

The best range in wall warts to go for is a 1A unit, 15-40VDC. Prices vary hugely, I paid under $6 for a 24VDC 1.3A switching unit. Digikey had them for under$20. It really does pay to shop for these units, and local stores can have some unexpected bargains.

Enclosures (Boxes)

This is more important than you might think, though not critical. A metal case is always to be preferred, since it is unlikely you will melt it. Most simple power supplies generate heat, sometimes lots of it. If you do decide to use a plastic box be sure to keep the heat generating components away from the plastic.

Be sure to get a box big enough to hold everything. Using a wall wart dramatically reduces the size a box needs to be, but there will be some circuitry inside, knobs on the outside, and possibly more. A simple sketch of the layout before you start buying parts and cutting holes will save you a lot of heartache.

It is worth buying a jack that matching the plug on your wall wart. This is because you don't want the wires to wear out from when you store the unit, unless you are one of the few who will have a permanent bench setup. Most of us don't.

Regulators

This is the heart of the power supply. Like I said earlier, the job is made a lot easier with modern regulators. I highly recommend the LM317. It can be used in the following schematic.

This setup will support up to 1.5A, with proper heat sinking. The LM317 can adjust between 1.25V to 37V, do not exceed 40 V on the input.

Heat Sinking

Whatever you use for the regulator, unless you are using very small currents, it will get hot. Sometimes extremely hot. Hot enough to melt plastic, this was what I was warning against earlier. Place the heat sink where it can have air flowing over it. If you are planing on using maximum currents you might even consider adding a small fan.

I would consider a heat sink an absolute necessity, but don't over think this one too much. Just plan on having one (it can be as simple as a thick chunk of metal) and allow fresh air to reach it.

Fuses

Even though the wall wart likely has a fuse, odds are it is not replaceable. This means it is a really good idea to include a simple fuse, possibly with a fuse receptacle on the outside of the box, though it can be as simple as an inline fuse inside the box. Either way, I highly recommend using one.

Voltmeter

This is optional, but I recommend installing a simple panel meter. The one I built has a 15V with a 15KΩ impedance, I added another 15KΩ resistor and switch to create a X2 scale. Radio Shack still sells the one I used.

They are pretty common from electronic outlets, you can easily beat the price.

Other Features

You'll note I haven't mentioned any other features. This because I wanted to keep this project simple. My unit has basic banana jacks, a meter, and a power indicator. It started off as an AC powered unit using a transformer (it was built in the 80's), I'm adding a wall wart to boost its current output. I'm also keeping the transformer as a secondary internal power supply, the wall wart jack I bought has an internal switch.

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#### spinnaker

Joined Oct 29, 2009
7,815
Nice article Bill.

I am thinking of building a bench supply myself. I have seen the wallwart used in other power supply projects found on the internet.

I was not sure if this was a good way to go or not.

1. Is it possible to use a wallwart to have both positive and negative voltages? From my rusty memory of power supplies you need a center tap for that. Not sure how you would do it here.
2. I was considering adding some fixed voltage outputs. It would be nice to have different colors for the various outputs but everywhere I look, all I can find is red and black. Anyone know where I can find other colors?

#### Wendy

Joined Mar 24, 2008
21,848
Yes, just use a LM337 for the negative side, and have a second wall wart. Many laptop supplies also have plus/minus. They exist.

If that doesn't appeal I have another article that would work.

Creating a Virtual Power Supply Ground

Tanners has blue, yellow, orange. They're out there. If you can't find them online PM me.

#### BMorse

Joined Sep 26, 2009
2,675
I also like to build my own Bench top supplies, except for I use ATX power supplies, since the outputs are already regulated and you have multiple voltage outputs to choose from (+3.3,+5,+7,+12, -5, -12, etc.) I am currently working on finishing my bench supply, still in progress but already working...... Just finished the enclosure a couple of weeks ago and now I am working on adding a cutoff switch for each voltage output, that way I can cut the power to one output without having to turn off the whole PSU (+ fuses on each output)..... I also added Indicator LEDS that let you know when the output is on and I used a bicolor LED to show when the PSU is in standby (RED) and when powered on it uses the output of the PSU to signal all voltages are OK to light the Green LED.... It is still in the process of being finished.... here are a couple of pics....

although I need to get some binding post of different colors.... the Black is of course Ground, Yellow = +12 Volts, Red = +5, Light Green (I need an Orange for this one) = 3.3, the dark green is going to be switchable between -5 and -12 volts.... also, I will be adding a current meter on it, in case I need to measure current draw on the circuit under load....

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#### Wendy

Joined Mar 24, 2008
21,848
Test equipment is one of those items it is not usually practical to DIY. Power supplies are a major exception to this.

I've added this article to my blog already. Truth, I thought of being even more verbose (gasp, can you believe it!), but kept it simple. When people are ready for more advanced power supplies they usually have the basics down.

I'm impressed by the shear number of switchers out there. Computers offer one source, but not the only one. I've got another thread sitting on the back burner where I'm thinking of building a ± 24V 6A power supply. I'm still chewing on the regulator circuitry.

Again, I have a local source of things like binding posts. I'm willing to be a middle man since Tanner's doesn't do internet.

Also,

http://www.bgmicro.com/index.asp?PageAction=VIEWCATS&Category=272

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#### beenthere

Joined Apr 20, 2004
15,808
Check those wall warts, though. I just put a drop-out relay on my big stereo amp. I went through the jumk box, and found one supposed to output 11 VDC @ 600 ma. It actually had 14.0 VDC across the T92 coil, just a bit high.

So I grabbed another rated for 10 volts @ 400 ma. It gives 11.65 VDC across the coil, so I went with that one.

Wall wart specs are more like suggestions.

#### Wendy

Joined Mar 24, 2008
21,848
Yep, guess I should have mentioned that. An unregulated 12VDC 1A wall wart can output as much as 18VDC with no load. Many times the specs assume a load, which is reasonable from their point of view since these units are usually sold with an appliance.

#### PackratKing

Joined Jul 13, 2008
843
OK. I have a million and one wall wartz, and concur that they have their uses, tho' I found you have to build a custom unit to do what you need it to do.

I wouldn't trade my 100% home-brewed, 0 - 12v., optional 0 - 24v., infinitely variable ac/dc 8 amp, paralelled LM317 regulated -- next to no ripple -- for a dumptruck full of warts.

Secondly, what is it with you guys and the aversion to building things offa the household mains ????? Ya gotta power that transformer from somewhere ! Sounds as though some of my other power schemes for troubleshooting, would give y'all the beejeebers.

I am safety conscious, tho' sometimes ya gotta take the bull by the horns, and make him work for you, and have a rather large inventory of packratted components to burn ........

I have no qualms about configuring a .1 ohm - 200 watt ceramic power resistor, with an open slider as a "pot" to reduce line voltage to whatever ac value needed. Ya just gotz to be darn sure you hooked everything up right with solid connections prior to throwing the switch.

The above mentioned bench supply is based on an old transformer. I have the intent to upgrade its feed to [one of the ] honkin' [ 5 - 600 watt, 2 - 1 Kw ] UPS unit isolation transformers I snagged in a fortuitous dumpster-dive.

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#### bluebrakes

Joined Oct 17, 2009
245
i did make my own power supply many moons ago, using an old transformer and rectifier from a car battery charger. unrectified it was pushing 15v at 4 amps.

a few heatsinked voltage regulators for 5v, 9v 12v and unregulated 15v.

Since then i had been stalking ebay for a 2nd hand TTi PL330QMD dual power supply.
I managed to buy two for £100. Worth every penny! Ability to current limit, etc.

Isolated mode can provide two variable current/voltage outputs. 0-32 volts @ 0-3 amps

parrallel mode one output at 0-32 volts @ 0-6 amps

series mode one output at 0-64 volts @ 0-3 amps

#### Wendy

Joined Mar 24, 2008
21,848
Nothing against, but wall warts are safe, and they work as well as transformers. You can argue this as you want, but it is a simple truth. Why? Because they the same thing, just moved into a different box. The switchers are even better, smaller, more power in a small space.

The big thing is safety. If you are qualified to work with AC that is good, but a lot of people aren't, but they still want to experiment. This just lets them do it safely, and be creative. You'll note these weren't plans, just a set of guidelines and suggestions. And it is as custom and designed for the job as anything using older methods.

#### studiot

Joined Nov 9, 2007
5,002
Over the years I have had many sorts of bench power supply.

Here is one I made in the late 1970s or early 80s.

I managed to acquire a crate of former British Airways slave clocks. These were a robust case of useful size with a rectangular front cutout showing 4 nixie tubes. Inside was a very nice toroidal transformers and an LM309 on the back.

I blocked off the front cutout with a fibreglass/resin panel from a sewage works I was instrumentaing at the time and put sockets for +12; -12; +5 & 0.

Adding internally another transformer, rectifiers, and two more regulators I had a very simple but useful psu at 1 amp per outlet.

Remembering the other recent thread about cases these are smashing for small projects.

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#### studiot

Joined Nov 9, 2007
5,002
Two more views plus one of some more recent power supplies.

Note the orange Variac on top of a twin 6amp supply and a twin 2amp supply to the right, both fully curreent or voltage controlled.

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#### gtojer

Joined Nov 18, 2009
1
I am verry new to this. This is my first post. I would like to do what you mentioned as far as doubling the scale of a DC panel voltmeter. You said your voltmeter has 15K ohm impedance. The Radio Shack spec sheet lists the internal resistance as 85 ohms +/- 10%. My meter is 10 V full scale, measures ~9.9K ohms across the terminals. How do I determine the impedance in order to find an appropriate resistor to double the scale?

#### Wendy

Joined Mar 24, 2008
21,848
When you got the voltmeter, was there a 15KΩ (or 10KΩ) resistor with it? Mine had one. You use the resistor in series, and add a switch in series with another 15KΩ on the switch. The switch either shorts the 15KΩ or not, with effectively doubles the range. Given these are 5% parts I was surprised how accurate it was (and is).

It has been well over 20 years since I built mine, specs can change, memory can drift.

You already did what you had to to find the resistance of the meter, a ohm check across the terminals. From what you said, your meter is 10KΩ, you probably need another 10KΩ resistor for a X2 scale. It is a simple series arrangement. For a X3 scale you would use a 20KΩ. I prefer a fixed resistor, 5% is fine. If it is an odd value or you think you need more accuracy then you can use a small 10 turn pot.

The AAC book has a chapter on these kind of meters. Depending on where you put the resistor, it is a voltmeter or a current meter, the internals are the same.

Welcome to All About Circuits!

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#### spinnaker

Joined Oct 29, 2009
7,815
BTW I found that mouser carries a number of different colors for the output plugs.

My question is which type? The flush type have a nice clean looking mount. But the post type give you the option to easily connect regular wire w/o a banana jack.

What is the best way to go?

And why are those banana jacks with wires so blasted expensive. Unless I am looking at the wrong thing, I am seeing $6-$7 per wire!

#### Wendy

Joined Mar 24, 2008
21,848
The externals will allow a plain wire to be connected, which is good. I used flush style on mine, but since it is low amperage this is OK too. It just means you have to have a set of leads for the power supply.

If you are designing from scratch I would go with the posts (not the flush) banana jacks. Not having something that can be knocked off is nice too, so you have to look at your likely use. If the current is over 2A there would be no question, posts can handle heavier wire.

#### studiot

Joined Nov 9, 2007
5,002
And why are those banana jacks with wires so blasted expensive
You will find that proper jack/probe connecting wire is expensive because it is specially flexible for the job. Probe wire usually also has thicker insulation than 'ordinary' wire.

#### jgessling

Joined Jul 31, 2009
76
As a relative beginner I think wall warts are great. I have a bench supply for prototyping but for a finished project, having an appropriate wall wart simplifies things, and is safer. Maybe later I will build power supplies, but for now this fine. I second the notion about checking what you got, output voltages can vary a lot from what's on the case. As for sourcing, my local flea market generally has many available for $1 or$2 each for people like me willing to sort through the piles. One more organized vendor has them in separate boxes by voltage.

One question has been bugging me though. Is there any difference between a supply designed for something that includes a battery, like a cell phone or laptop vs something like computer speakers that doesn't have any battery, like computer speakers? Is the circuitry that controls charging in the wart or in the device? This will help select an appropriate model.

#### BMorse

Joined Sep 26, 2009
2,675
One question has been bugging me though. Is there any difference between a supply designed for something that includes a battery, like a cell phone or laptop vs something like computer speakers that doesn't have any battery, like computer speakers? Is the circuitry that controls charging in the wart or in the device? This will help select an appropriate model.
The wall wart just supplies the power and most of the charging circuit is in the device it self...... but some wall warts will put out DC (rectifiers built in??) and some put out AC that has to be rectified for DC use...... When I power any of my projects off of wall warts, I usually still run the power coming in through a bridge rectifier, this also helps if you end up using another wall wart that has the positive and negative swapped at the connector (most power connectors will have the outside of the plug as -, and the inside as +, but I have seen them the other way around.) If you do use a bridge rectifier, you have to have a wall wart that can still put out enough voltage for your project + whatever the diodes drop....

#### spinnaker

Joined Oct 29, 2009
7,815
Bill another follow up question on this post. If you already had the filter caps, would you add some larger caps right on the output of the wall wart? Or just go as is?