Testing with a DC Electronic Load Beyond its Limits

[enut11]

I needed to test an Agilent E3612A power supply rated at 60v/0.5A/30W. My E-Load is rated at 30v/10A/150W so obviously the problem was with the voltage limit. I could, with difficulty, change the E-Load MOSFETs to higher rated devices but a much easier way is to use an external power resistor to manage the excess voltage.

Simply insert a resistor of appropriate value and power rating in series between the DUT and E-Load.
The external resistor is chosen so the E-Load never sees more than its rated voltage at the test current.

Calculated resistance
Rext = E/I = (60-30)/0.5 = 60 ohms
and power dissipated
Pext = EI = 30x0.5 = 15 watts

So, a resistor of 60 ohms/15 watts can be used, with my E-Load, to test the E3612A to 60v/0.5A.  Such a resistor is shown below as multiple smaller power resistors (50+10) mounted on a fan cooled heat-sink.

Careful setup and methodology is required to avoid excess voltage at the E-Load input terminals.
1) Wire the external resistor between DUT+ and E-Load+ terminals
2) Monitor the voltage drop across the external resistor with a DMM
3) Initially, set the DUT output to the highest voltage limit of the E-Load (30v in this example)
4) Adjust the E-Load to sink the test current (eg, 0.5A)
5) Slowly increase the DUT output voltage to the test limit (eg, 60v)
6) Disconnect the DUT output first to prevent excess voltage at the E-Load

NOTE: The E-Load will always indicate the correct amps but the indicated voltage must be added to the external resistor voltage drop to calculate the correct DUT power.

[enut11]

Picture of the test setup.

DMM indicates voltage across the (50+10) power resistors.
E-Load shows voltage at its input terminals when the current is 0.5A

(33.2v+26.76v)=59.96v

Power supplied by the DUT (Agilent E3612A) = 59.96x0.5 = 29.98 watts, ie in spec

[enut11]

The Agilent E3612A power supply has a high voltage range, up to 120v/0.25A, which I also want to test.
This time we need to drop 90v before the E-Load so extra care needed here to avoid damage to equipment and personnel.

Calculations for the external load:
Rext = E/I = (120-30)/0.25 = 360 ohms
and power dissipated
Pext = EI = 90x0.25 = 22.5 watts

The setup and test procedure is similar to the first post:
1) Wire the external resistor between DUT+ and E-Load+ terminals
2) Monitor the voltage drop across the external resistor with a DMM
3) Initially, set the DUT output to the highest voltage limit of the E-Load (30v in this example)
4) Adjust the E-Load to sink the test current (eg, 0.25A)
5) Slowly increase the DUT output voltage to the test limit (eg, 120v)
6) Disconnect the DUT output first to prevent excess voltage at the E-Load

I don't have a suitable load resistor so ordered a 360 ohm 50 watt unit on eBay and will report results at a later date.