Power Supply Dummy Load

by admin on October 20, 2017


Why would you need or want a power supply dummy load? A linear power supply regulates, or should, all the way down to no load. When testing switched mode power supply (SMPS) they require some amount of load before they begin to operate (regulate) properly.   

Power Supply Dummy Load

An easy to build and very effective, inexpensive Power Supply dummy load is an incandescent light bulb. I suggest and have used an automobile tail light bulb for loading standard computer power supplies.  The ubiquitous 1157 can provide a load to both a 12 and 5 volt supply at the same time or if required to a single 24v supply.  The filaments are rated at:

Stop/High: 26.9 watts, 32 candlepower and 2.1 amps at 12.8 volts

Marker/Low: 8.3 watts, 3 candlepower and 0.59 amps at 14 volts

Two_Bulbs1157_singleI took some old clip leads, cut them up and made a pair.  (I had to buy a two-pack of bulbs.)  My 30watt pencil iron got hot enough to solder all the wires, even the common one to the brass bulb base.  I recommend pre-tinning the leads, then soldering to the contact nipples is very fast as they’re mostly solder.

The resistance of the filaments is 0.5Ω and 2Ω.  The high power, or stop, filament (27W) has the lower resistance.  Make sure you use clip leads that can handle the current that will be going through them which can be as high as 2 to 3 amps.  My connections are:

  • Red/Black: High (27W)
  • Green/Black: Low (8W)
  • Red/Green: test 24-30V

The only thing I haven’t done, that I should do, is to put a label or tag on them spelling out the capacities and lead configuration so that I don’t have to figure it out every time I use them.  It’s fairly obvious when you connect them to a 12V power supply, though I don’t remember the specifications.

If you want to build a power supply dummy load with single filament bulbs, the #1156 bulb is the same as the High or Stop filament in the 1157 bulb and the #67 bulb is, for all intents and purposes, the same as the Low or Marker filament on 1157 bulb.

When ready to actually dive into the SMPS you’ll probably need some help, I recommend this book on part testing for your reading and then continued reference. I review the book in this article.

Please let me know how it goes when you’ve built your own Power supply dummy load or send me your questions.

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