FAQs from Rohde & Schwarz

HMP Series Parallel switching (and sensing) - wrong current output


I tested a two channel parallel setup with my HMP series power supply and surprisingly get higher current measured as expected.

Example setup:

  • 2V Output voltage on CH1 and CH2
  • Current limit set to 100mA for each channel
  • A resistor chain of 2R0, 30R0, 3R3 to simulate line resistance

I would expect to get a current readout of 66,7mA, but in fact the power supply displays the follwing values:


What's wrong with my setup?


Your unit is not defect and also the setup is correct, it's just hard to control that low currents in parallel setup. The reason is, that both channels have (internal) influence on the other channel in terms of internal resistance while controlling the loop.

When the real current flow will be measured with a separate instrument, everything is fine - so this is the proof for the internal currents and also for the output voltage to be set correctly:


The only way to avoid that is

  • either to use the parallel wiring only for higher currents where one single channel is not enough for the lacked current
  • or work with current limits to balance the two channels

This topic is also described in the manual: "For power supplies that are connected in parallel, it is possible that compensating currents flow within the power supplies."

"We also recommend, that you use the current limit settings to distribute the load to multiple channels evenly. Distribute the total current proportionately to the current limits of the connected channels. This procedure also distributes the power loss more evenly, thus improving the heat dissipation and protecting the semiconductors from damage."