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Ten things to consider when buying DC Power Supplies

DC Power Supplies are typically big, heavy, and expensive to ship.  The last thing you want to do is buy a supply that doesn't work for your application.  With just a few simple steps and a little knowledge, you can make sure to get the right supply the first time every time.  

1.  Voltage and Current output:           See, I told you this was going to be simple!  Of course you need to know what your output requirement is - right?  But the sad truth is that you probably won't be able to buy exactly what you need.  Say you need 42 volts at 16 Amps.  You won't find exactly that value.  Generally the power supply manufacturers break their supplies into wattage classes.  Some simple math tells you that 42V x 16A is 672W.  So you would probably look for something in at least a 750W unit.  But you are still unlikely to find exactly what you need off the shelf.  Say you found a supply that puts out 50V / 20A.  That would be an almost perfect solution in a 1 kW supply. 

You really don't want to be running your power supply at its maximum power output for extended periods of time anyway.  That's like driving a car with the gas pedal floored.  It works for a while - until it doesn't.

2.  Form Factor and outputs:     These are generally only considerations in lower wattage power supplies.  Above around 1 kW you are pretty much stuck with a rack width (19") supply with a single output on the rear panel.  At 1 kW and below, you can still get the same rack form factor but you have a few more choices.  In these lower wattages, you can also get a smaller portable / benchtop style with front or rear outputs, or multiple outputs may be useful for prototyping and general lab use. 

Today, power supplies are becoming more and more compact.  Just a few years ago, manufacturers were starting to put up to 1 kW in a 1U package (1.75" tall).  Today, we commonly see 1500W in this same package size and are starting to see supplies over 2 kW in 1U.

3.  Programmability:        Most DC supplies are fairly simple "brute force" instruments that you control manually with the front panel knobs and/or keypad.  If that's all you need, please proceed to #4.  Do not pass go.. Do not collect $200 (but you might save a few dollars).  If you're still reading, that means you probably are using your power supply in an automated system.   Or maybe you're just a tech geek that likes reading about power supplies.

Historically power supply automation / programming has been primarily via GPIB (IEE488 bus), and sometimes RS-232.  Newer supplies are now commonly controlled over LAN (ethernet), or USB, however GPIB and RS-232 are still widely used. 

If you're replacing an existing supply or duplicating a test setup you already use, you may need to buy the same exact model power supply since anything different would require rewriting the code.  In that instance, we may be able to work with you to repair your existing supply, or move its programming interface into a replacement unit if necessary.  Occasionally we can even find a newer product that emulates the programming language of your older obsolete supply.

4.  Input Voltage:              Common North American input voltages are 120V, 240V, 208 3-Phase, 480 3-Phase.  International AC supply voltages can vary by country.   Aside from some smaller power supplies that can work on both 120 and 240V, most are factory set to work at a specific line voltage and can't be converted.  So it's critical that you get a supply that works with your available voltage source.

Generally larger wattage supplies work with higher input voltages.  The largest ones require three phase power.  In practice, we typically see 120V supplies up to about 1500W, 240V supplies up to about 3 kW, and above that we typically see three phase power being used. 

5.  New or Reconditioned:         Comparing between new and reconditioned, you have three basic areas to consider

  • Price                          Reconditioned equipment will normally save somewhere between 20-90% off list price.
  • Availability              If you need a very unusual power supply or a large quantity, you may only have new product as an option.  Lead times, however can be up to several weeks for new product while               reconditioned may be available for immediate shipment.
  • Quality                      When you buy a new product, you can have a reasonable expectation that it will be complete and work properly out of the box.  On the used market, quality varies widely between vendors.         If you want to get the product correct and complete the first time, you will want to deal with a vendor like AccuSource who knows power supplies thoroughly. 

99% of the time, those five things are all we need to talk about.  But I said ten things - so here's five more considerations for the discerning tech geek........

6.  Switching vs Linear:   Back in the 1970's, if you bought a DC Power supply, it was almost certainly a linear supply. It was pretty simple.  A big transformer, rectifier, regulator circuit, and that's about it.  Lots of them are still in operation today.  They were big and heavy and expensive.  Since then, the market has moved toward switching or switched-mode power supplies.  They are much smaller, lighter, more efficient, and less expensive to manufacture.  I only know of one manufacturer that is still manufacturing linear DC Power Supplies today.  Switching works for 99.9% of applications. 

Linear does still have an advantage or two though.  Aside from the simplicity, they can have a cleaner output than switched mode supplies.  The switching circuit creates high frequency noise.  Both emitted and at the output terminals.  This noise is generally filtered out to an acceptable level, but could be a consideration in some situations.

7.  Constant Current Vs Constant Voltage:              Typically, power supplies are operated in constant Voltage mode (CV).  What this means is you set a voltage output on the front panel.  The output immediately goes up to that voltage.  You also may set a current limit, but the output voltage stays constant, and the current varies depending on the load.  Typically modern supplies operate in CV mode until the current limit is reached and at that time they switch over to constant current mode.  For instance if you set the output to 10V / 10A.  When your load is drawing 9.99 Amps or less, the voltage stays at 10.  Once your current reaches 10A, if the load keeps increasing, the voltage output will decrease.  That is constant current mode.  Most power supplies automatically cross over as described above, but if you plan to use your supply in CC mode, you will want to verify this.

8.  Accuracy, Ripple, Noise and Regulation:                       If you have a demanding requirement, you may need to review these specifications.  Some supplies are considered general purpose, and some are considered lab grade.  You will of course pay extra for lab grade supplies.  Most applications, you don't have to worry too much about these specs as long as you buy a decent brand of power supply from a reputable dealer that knows how to thoroughly test it.  (Are you getting the hint??)

9.  Safety Covers:              Most power supplies have both an input and an output safety cover supplied by the original manufacturer when purchased new.  Sadly, when power supplies are de-installed from the original owner, few people take the care to re-install the safety covers.  Only a few secondary market dealers go to the time and expense of purchasing replacement covers, much less fabricating covers when OEM replacements are unavailable.  When buying on the second market, make sure you know what you're getting.  "Refurbished" doesn't mean the same thing to everyone.

10.  Shipping / Freight:   The larger the power supply, the more difficult it is to ship it safely.  UPS and Fedex will ship boxes up to 150 lbs, but we typically ship anything over approximately 100 lbs via pallet freight.  The handling is generally much gentler and the cost is fairly comparable. 

That's it.  Now you are a power supply buying guru.  But just in case, we are always here to help.  Call 770.538.0061 or email