I just got the TENMA 72-8700A bench power supply. A link to the manual is here. I have read it, but I am a bit confused by the wording. I cannot figure out under what circumstances the device is in constant voltage or constant current mode, and when I am able to adjust the current or voltage. Can anyone who has some experience with these things help me out? I want to know when I can adjust the output current, and when I can adjust the output voltage.

I have figured this out. The thing I was confused about was the fact that the readout only shows the current flowing, not the set maximum current. I was expecting the readout to show the set max current, not the flowing current

  • \$\begingroup\$ It is a logical OR max on each control, even though it is linear. \$\endgroup\$ Commented Sep 24, 2017 at 14:19
  • 1
    \$\begingroup\$ Ehm, my recollection is of 45 minutes in chat sorting out your confusion. \$\endgroup\$
    – Transistor
    Commented Sep 24, 2017 at 16:09

1 Answer 1


It's fairly easy.

  • When you switch on the output voltage ramps up until it reaches the preset maximum voltage or the preset maximum current - whichever comes first.

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Figure 1. A typical bench power supply with voltage and current limits adjustable from the front panel. This one also has separate voltage and current readouts allowing simultaneous monitoring of both.

Let's take an example:

I have a 12 V DC motor on a worm gearbox which should draw 0.25 A when running unloaded. I suspect it is stalling but can't spin the output shaft because of the worm gear. Procedure:

  • Set the PSU to 12 V and 0.5 A current limit.
  • Connect the motor.
  • Switch on the output.
  • If the motor/gearbox is OK the voltage will rise to 12 V and the current readout will settle down at around 0.25 A.
  • If the motor is stalled the current will be limited by the motor winding resistance - let's say it's 3 Ω. As the voltage rises up we will reach 0.5 A at 6 V. The current limit will come into play and the voltage rise will stop there.

The current limit protects the motor during the test.

Current limiting is also useful in other cases such as testing LEDs without a series resistor or as a constant current source for charging NiCd or NiMnH batteries. For lead-acid battery charging the float voltage can be set to 14 V, for example, and a limit put on the charge current too.

  • \$\begingroup\$ Set the voltage limit to 5 V. Set the current limit to 20 mA. Connect an LED directly across the terminals (with right polarity). Switch on. Then try Graphing the IV curve of an LED. \$\endgroup\$
    – Transistor
    Commented Sep 24, 2017 at 15:00
  • \$\begingroup\$ Add link to user manual for the PSU into your question (so that everyone can see it). \$\endgroup\$
    – Transistor
    Commented Sep 24, 2017 at 15:04
  • \$\begingroup\$ Got it. See page 6 of the manual: "The adjusting of CV is set when the terminal is open circuit. The adjusting of CC is set when the terminal is short circuit." Short out the output terminals and you should see the CV light turn off and the CC light turn on and the current displayed on the readout. It appears that you are doing all your testing with no load connected. \$\endgroup\$
    – Transistor
    Commented Sep 24, 2017 at 15:12
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    \$\begingroup\$ Some more expensive PSUs allow you to set the CV and CC values with the output disconnected (usually by switch). These are more convenient. \$\endgroup\$
    – Transistor
    Commented Sep 24, 2017 at 15:14
  • \$\begingroup\$ Let us continue this discussion in chat. \$\endgroup\$
    – Transistor
    Commented Sep 24, 2017 at 15:16

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