I recently bought an aftermarket AC-DC power supply for a laptop which was woefully defective. When connected to a Chromebox, a boot message appeared indicating that the supply could not provide enough power. Although it was enough for a Chromebook to boot, the battery charge was indicated at 22 hours. Both instances were alleviated by using the original HP-branded power supply.

What what I buy that will allow to put a load on these laptop power supplies to both generate a load and also to test the voltage and amps it provides? A readout similar to my bench power supply would be nice such that I can see a digital readout of these values and to see if the voltage drops below spec as the amps increase.

I believe that what I need is a "load tester" but I don't know enough about this kind of equipment to know what it is I'm buying.


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    \$\begingroup\$ It's called "DC Electronic load" \$\endgroup\$ – anrieff Oct 1 '18 at 13:30
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    \$\begingroup\$ @anrieff Doesn't HP use some third pin/power line communication to handshake the charging/lock out aftermarket vendors? \$\endgroup\$ – winny Oct 1 '18 at 13:32
  • \$\begingroup\$ Do the math and buy power resistors of the right value to act as a 25%, 50%, 100% load. \$\endgroup\$ – Sparky256 Oct 2 '18 at 23:22

To simply test a power supply, all you need is a few power resistors and a voltmeter.

Use Ohm's law to compute the resistance that would draw the full rated load from your power supply. For example, if you have a power supply rated for 12 V at 2 A, then ideally (12 V)/(2 A) = 6 Ω would load it to the limit. To leave a little margin, use the next higher value resistor, like 6.2 or even 6.8 Ω.

Connect the resistor to the power supply, and measure the voltage across it with the voltmeter. If the voltage is still what it's supposed to be, then everything is working. If the voltage is too low, then the supply can't handle the current.

Note that these resistors need to be able to dissipate significant power. In the example above, that would be 24 W. A large 10 Ω or 4.7 Ω wire wound resistor with adjustable tap can be useful here.

However, in addition to all that, powering a laptop is more complicated. Laptop supplies aren't just dumb things that produce a fixed voltage up to some maximum current. They usually have a communication channel and exchange data with the laptop before going to full power mode. Some manufacturers deliberately throttle back charging when the power supply isn't one of theirs.

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    \$\begingroup\$ "Some manufacturers deliberately throttle back charging when the power supply isn't one of theirs." Which is all the more reason I should to test the supply to confirm the source of the disparity. I'm not well versed with electronics; I don't understand what a "wire wound resistor with adjustable tap" means but I do understand that my little through-hole resistor certainly can't drive 24W (65W in the case of this supply). \$\endgroup\$ – Zhro Oct 1 '18 at 13:47

An active load may be linear yet inefficient shunt FET or NPN Darlington and base drive control current, or a switched mode PWM with series/shunt RLC to create a certain step load response limited by rise time L/R or average current response but with protection. These are variations of an “Active Load”

test methods

Monitor I thru a low side current shunt with 50mV to 100mV drop to limit Pd then coax to scope.

  • This may be just a wire folded and twisted in half to null inductance calibrated by a known current , R[mOhm]=[mV/amps]

  • Voltage transients are best AC coupled to scope low R load with ext. 50 Ohms at time RC=T=0.35/f(-3dB) high pass breakpoint, where here R=50 so choose f or T and then C.

    • Measure AC ripple this way.
    • Ground leads MUST be very short to scope 10:1 probes when used for ringing reduction.
    • Vdc can be direct coupled and averaged if noisy and/or LPF added.

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