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I don't understand why most modern lab PSUs in constant voltage mode don't explicitly display the maximum authorized current.

You have to set the current with output ON starting from 0 A, passing by a few mA where the device state is blinking/unstable because it is missing energy, up to the maximum driven current, and blindly go a bit higher in case operation requires slightly more current at some point - while I'd like to set voltage and current with output OFF, and then power the circuit.

I really don't get why most lab PSUs don't display the maximum current that you set. It looks the bare minimum to me, just like the set voltage. I've been working with a dozen models, and only one had a "review" button letting you know the voltage and maximum current.

Am I misusing lab PSUs? Or why isn't there a maximum current indicator on all lab PSUs?

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    \$\begingroup\$ It would probably help if you talked about a specific model, but a possible answer is that you're buying cheap stuff, and it only has two displays. You'd always want to see the voltage, so the choice for designers is show the current at the moment, or show the limit. The current at the moment is way more important. You can check the limit any time you want just by shorting the output \$\endgroup\$
    – BeB00
    Nov 19, 2017 at 23:15
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    \$\begingroup\$ The bench supplies I've used have a button you hold then twist a knob to set the current limit. \$\endgroup\$
    – Ron Beyer
    Nov 19, 2017 at 23:51
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    \$\begingroup\$ Can you do this? turn the voltage down, short the output , set the current limit then set the voltage. \$\endgroup\$ Nov 20, 2017 at 0:09
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    \$\begingroup\$ Have you considered the fact that older bench supplies use analog meters and potentiometers? \$\endgroup\$ Nov 20, 2017 at 2:29
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    \$\begingroup\$ Basic UI: every datum added distracts from all other data; it can cause a heart attack to see 2.50A in the corner of your eye while working on something sensitive. \$\endgroup\$
    – dandavis
    Nov 20, 2017 at 3:15

4 Answers 4

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Most bench power supplies, even most of the ones with the fancy digital meters on the front, are actually very simple internally.

They are basically adjustable regulators, linear or SMPS, with a pot that adjust the output voltage, and another pot that sets the maximum current it will put out, plus a couple of standard meters that measure the voltage and current at the output.

schematic

simulate this circuit – Schematic created using CircuitLab

That is, when you adjust the voltage to 12V on the display, you are actually adjusting the output, and reading it on the meter, not setting the target voltage. Similarly, with the current limit, you are simply changing a resistor in some current limiter circuit somewhere in there.

The point is, the meter has no idea what you have set. In fact, the meter often does not even have a one to one relationship between knob position and output values. They will hold the output at whatever values you set, but the values themselves are entirely operator controlled.

With better power supplies, you are actually telling some micro in there what voltages and currents you want to establish and it, with the aid of DACs and ADCs, controls the voltages and currents for you. Having that control gives those supplies the ability to show you what you have set. It is also why better power supplies cost more.

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I don't understand why most modern lab PSUs in constant voltage mode don't explicitly display the maximum authorized current.

I've never seen one that displays both actual current and current limit setpoint on separate displays but the better or more useful ones do have an online / offline display mode that works as follows:

enter image description here

Figure 1. A TTi EX-R series bench power supply. Note the DC output on/off switch circled. This usually indicates that the voltage and current limits can be set offline.

  • Offline: The displays show the maximum voltage and current that the PSU will supply before limiting. The Farnell range used to blink the decimal points on the current display to indicate that offline setup is selected.
  • Online: The displays show actual voltage and current. These are accompanied by CV (constant voltage), typically green, and CC (constant current), typically red, LEDs which light to indicate the mode the PSU is operating in.

You have to set the current with output ON starting from 0A, passing by a few mA where the device state is blinking/unstable because missing energy, up to the maximum driven current, and blindly go a bit higher in case operation requires slightly more current at some point.

"Missing energy" isn't the right term. "Setpoint limit exceeded" might be better, if a bit wordy.

I'd like to set voltage and current with output OFF, and then power the circuit.

You can still do this in a fashion by setting the voltage with load disconnected and setting the current with a short circuit on the terminals. It may be possible to hack your PSU or make an extension box with suitable switches to do this giving you the required features on a low budget..

I really don't get why most lab PSU don't display the max current that you set. It looks the bare minimum to me, just like the set voltage.

The reason is cost. The ones you like require closed loop control between the setpoint and the output with calibration, etc. The simpler ones can have a separate current and voltage regulator circuits with a voltmeter across the output and an ammeter in series. The regulators don't have to be accurate and may not be particularly stable either and the limits may drift with temperature.

There is plenty of choice. You set the user requirements and eliminate any models that don't meet it.

I've been working with a dozen of models, only one had a "review" button letting you know the voltage and max current.

I've never seen the review button on a PSU.

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  • \$\begingroup\$ Lol, I have one of those supplies sat on my desk beside me. Decent PSU. \$\endgroup\$ Nov 20, 2017 at 0:24
  • \$\begingroup\$ On the other hand we have a £40k Agilent PSU in the lab at work that displays both set point and current values at all times. But then that one does have an LCD display. I guess you get what you pay for. \$\endgroup\$ Nov 20, 2017 at 0:26
  • \$\begingroup\$ Sort of decent ( I got a couple as well). The connection to the 4mm are not rally designed for 20A for any real time \$\endgroup\$
    – user16222
    Nov 20, 2017 at 7:05
  • \$\begingroup\$ These 4 mm banana plugs from Mouser are rated at 30 A. If contact and wire resistance is an issue the TTi PSU in Figure 1 has remote sense to eliminate the voltage drop between the PSU and load. \$\endgroup\$
    – Transistor
    Nov 20, 2017 at 8:22
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Most likely, because it's more difficult. A simple ammeter is an off-the-shelf component.

Working out what the current would be if you shorted the output terminals is another matter. That's especially true if it's a basic bench power supply with analog controls.

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This depends on the model. Some don't even show current limit at all. Some have a button. Some show it when the output is off.

Some show it always, like the Rigol DP832:

rigol dp832 screenshot

It's a feature that makes the supply more expensive. Your wallet determines if you want this feature or not.

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