Three Golden Rules for Choosing a Power Supply (No Maths!)

Often the last consideration in a maker project, the DC power supply is the heartbeat of many electrical projects and the correct selection requires some consideration. Power supplies go by a lot of different names, you can call them Wall Warts, AC Adapters, Plugpacks, or AC/DC Converters, whatever takes your fancy.

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Below are my three generalised golden rules for selecting a power supply for any maker electrical project. 

1. Your Power Supply Voltage Output must be the same as your Hardware Demands. If too high it will break your hardware. If too low it will not allow the hardware to work as it should. Usually referred to shorthand as V. 
2. Your Power Supply Current (Amperage) Output must be higher than what is expected. The current pulled out of the power supply is determined by the load. The amperage load (your project) will draw what is required and no more. Usually referred to shorthand as A. 
3. Your Power Supply Total Wattage Output must be higher than what is expected. The world is full of inefficiencies and electronics are no different. Having a higher value of Watts available will prevent brownouts. Brownouts are when hardware does not act as it should because it does not have enough current. Usually referred to shorthand as W. 

If you follow these three rules, your electronics will be happy. Furthermore, after doing this a couple of times selecting the right power supply will become second nature. See some examples of power supplies below.

 
Hopefully, this will help you on your electronic forays! As always if you have any questions, queries, or things to add please let us know your thoughts!

Your chosen hardware and power supply will provide Voltage, Current, and Watt specifications in their datasheets. You can find this information at the bottom of any of our product pages. Often in the Product Description section of our page, you will find this information as well. See an example of this highlighted from our Product Page for the 12V 2Amp DC Power Supply, seen in the image below. To explain further 12V DC means it will Output 12 Volts of Direct Current to a project. The 2A means it will Output a Maximum of 2 Amps of electrical current to a project.

Sometimes, as above manufacturer will only supply two of these three crucial pieces of information (Watts, Voltage, and Current). That is totally okay thanks to Ohm Law! We have a whole guide on Ohms Law so check this linked location for that (and as I promised no Maths that's as far as I'll go in this guide). From our provided information above and Ohms Law, you will be able to figure out that the Max Watt Output of this Powersupply is 24 Watts. There is another way to gain this information too. Looking at your power supply it will have a specification sticker that looks a lot like the image below.

Looking at this the most important information, Output Voltage and Output Current can be found (as well as determining Total Watt). Looking at the above image we can see the text |OUTPUT:12V --- mA |. That means it has an output of 12Volts, the middle symbol means DC Current, and mA means milli-amps. milli-amps equals 1 amp. Thus we have the 2 Amp Output knowledge. Take a glance at the Input section on the label for that Sine Wave (Wavy) Symbol. That symbol means AC (alternating) current.

Now sometimes a Voltage is required from our electrical setup that does not fit a Standard Power Supply. To deal with this we have two great options. Use a Variable Power Supply or Utilise Buck/Boost Converters. Variable Power Supplies have dials that we can rotate to provide the exact voltage we need. Buck/Boost Converters have Potentiometers on them that will allow us to alter their output voltage to exactly what is required. We will still be feeding the electricity from a standard Powersupply into these converters. Buck Converters decrease voltage (at the cost of inefficiencies (heat) but do increase the total MAX current coming out of them). Boost Converters increase the voltage (at the cost inefficiencies (heat) and a lower MAX Current coming out of them). Buck/Boost Converters are also referred to as Step-Down/Step-Up Converters. Linked Here is a great guide to diving into Buck Converters to drive High Power LEDs. See this linked Buck Converter in the image below.

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Now DC Power Supplies are not all made equal. Some Power Supplies are made big, and some are made to fit a very small footprint. These have trade-offs. Often you will hear about Switching Power Supplies and Linear Power Supplies. The big difference between the two (without getting into the weeds) is the Switching Power Supply can be made significantly smaller (tinier footprint) however the electricity coming out of it will be noisier. Now to the human ear Maker level Power Supplies are silent, so what do I mean by Noise? In electronics, noise is an unwanted high-frequency disturbance or interference with the electrical signal. For advanced electronics, like a Raspberry Pi Single Board Computer, a Noisy Power Supply can cause problems. It can cause issues for analog or measurement systems as well. For simple electronics, like DC Motors or Lights, Noise at this level is not a problem. Learn more on this here.

Naturally making sure you have the right connector to easily incorporate the power supply into your system is crucial too. Be it to the wall, which changes depending on your country, or the other end. Make sure the other end will plug into your system (whether it terminates with a Micro-USB, USB-C, or DC Power Line). I often use DC Barrel Jack Adapter (Female) in my projects. 

To be ultra clear with this guide's terminology (however full understanding of the below is not crucial or needed when it comes to picking a power supply, feel free to gloss over):

- Voltage (also known as electric pressure, electric tension, or (electric) potential difference) is the difference in electric potential between two points. One volt is defined as the electric potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those points.

- An electric Current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. One ampere of current represents one coulomb of electrical charge (6.24 x 10^18 charge carriers) moving past a specific point in one second.

- Electricity is measured in units of power called Watts named to honour the legendary James Watt (the inventor of the steam engine way back in ). A Watt is a unit of electrical power equal to one ampere under the pressure of one volt. 

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lambac

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DC power supply and DMM - advice and questions

« on: April 23, , 02:37:34 pm » I'm a university chemistry researcher and my current project involves more electrical work than I am typically used to.

I need to purchase a DC power supply and a DMM, but am a bit lost in terms of options, capabilities and best practices.
Would anyone have any recommendations?

As this project is based around the building and testing of a bench-top system, the bosses suggested that I future-proof my power supply and DMM purchases.

My budget for a DC power supply and DMM are approximately €/$ USD/£, but there is a bit of flexibility here.
Both the DC power supply and DMM need to be able to connect to a computer for data logging.

At the moment I work under 30 V, (limited by the power supply) but in the future I could be running closer to 200-500V, or possibly more, depending on what the budget allows.

In terms of a DMM, something with multiple channels (at least 4) would be ideal.
Something like the Siglent SDM-SC looked promising, but when using the scanner card the voltage across each channel is 110 V max, rather than the V from the front panel input. I also don't need 12 channels.
If a single DMM like this can't be found, I can always use multiple DMMs?

Any suggestions would be greatly appreciated.

jwet

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Re: DC power supply and DMM - advice and questions

« Reply #1 on: April 24, , 02:51:44 pm » I'll try to give some guidance.  These are actually pretty unusual requirements. 95% of power supplies are sub 30v and 99% of DMM's are single channel.  You didn't talk at all about your application or any detailed needs.  Future proofing can be lead to compromises and who know about the future.

Some Questions for you to ponder-
1. On the DMM.  Resolution and accuracy requirements- how many digits basically and basic accuracy?.  Long term accuracy might be important if you doing things like logging- most DMM's in electronics are used for single measurements.  Is it all just DC volts?  Generally pretty easy.  Besides volts, what other parameters do you want to measure?  Environment- lab only or sheltered outdoors, etc.  Type of computer interface- options are IEEE-488- legacy but still very popular and well supported, USB- point to point, Ethernet- growing and RS-232- legacy and shrinking.  USB or ethernet are probably preffered- USB must be isolated.

2. For your DMM needs, you might want to consider a solid single channel benchtop instrument like the Sigilent and a Data Logger for you multichannel needs.  The 100V Sigilent might be a good bet if you get a little inventive.  The HV input spec might be tough to meet but you could make or buy a 10/1 divider for those cases over 100v.  A DC divider is just a pair of good resistors- ac is a bit more complex, but not bad.  Any EE grad student should be able to whip something up for a case of beer.

3. On the power supply- you'll probably want two power supplies.  A grunty 30v unit- perhaps 2A output unit.  Dual output is a common requirement.  If you find something you like, consider buying two. See if you really need a computer I/F, this can add cost.  Also look at accuracy and resolution= its not difficult to get 10 mV of resolution- more gets expensive.  Do you need current limiting, how precises?  Think about how much noise you can tolerate on the outputs.  What are you powering?  Switching supplies are smaller and more powerful for the money but are a bit noisier.  Linear supplies are quiet but get bulky over about 100W (30v 3a, etc).

4. On the HVPS, decide how much current you need.  Only 100 mA at 500v is 50W- this is also a potentially dangerous piece of gear- a few mA at 500v is enough to get a lethal shock.  Go through your needs as above.  There is much less selection.  Consider looking around for something used or from another department, etc.

5. Save some of your budget for test leads and interconnect stuff and possibly software to automate things.  The meter will come with basic leads but consider buying an assortment of pomona banana to banana leads in red and black and maybe green and yellow at 1 meter and 2 meter lengths, these can be a bit expensive ($25/pair).  Get some alligator and other connector assemblies compatible with standard bananas (4 mm) Get a quality power strip with surge and some filtering.  Cabling for your computer interface- hubs, switches, etc.

Good luck.  We can iterate some if you have questions- I'm sure others will chime in now that I've broken the ice.