How to use a bench power supply

[fog37]

Hello,
I understand that a bench power supply (see picture below) is mainly used as a constant voltage source: once the load is connected to the two terminals, we set voltage $V_{supply}$ by turning the voltage knobs on the right and find that voltage across the load. At the same time, the power supply sends out a current such that $I_{supply}= \frac{V_{supply}}{R_{load}}$.
It is possible to set a limit maximum current $I_{max}$ coming out of the power supply so that the electronics don't get damaged. For example, let's consider the load to be a resistor with $R=1 \Omega$ and the max current set to $I_{max} = 2A$. We set the voltage to $V_{supply}=1V$ and find a voltage of $1V$ across the resistor and a current of $1A$ coming out of the supply. However, if we set the voltage $V_{supply}=3V$, the power supply still sends out only a $2A$ current and the voltage across the resistor is only $2V$. Is that correct?

How do you do set the maximum limit current? Should we first short circuit the power supply and then turn the current knob to the desired limit current $I_{max}$? What is the right sequence of actions to do that?

Also, the ports we use are the red and the black output ports. The green port in the middle is Earth ground, a real physical connection to Earth ground. This green port can be connected to either the red or the black port with a wire, correct? Is that how we would use it? why would we do that? Simply so our circuit is safely connected to ground?

Thank you!
Fog37

 

[anorlunda]

It is possible to set a limit maximum current ImaxImaxI_{max} coming out of the power supply so that the electronics don't get damaged.

Some supplies may have that feature, some not. If it doesn't, the simplest way to do that is with a plain old fuse.

 

[davenn]

Hello,
I understand that a bench power supply (see picture below) is mainly used as a constant voltage source: once the load is connected to the two terminals, we set voltage VsupplyVsupplyV_{supply} by turning the voltage knobs on the right and find that voltage across the load

almost ... you set the voltage to what the circuit / load requires

At the same time, the power supply sends out a current such that Isupply=VsupplyRloadIsupply=VsupplyRloadI_{supply}= \frac{V_{supply}}{R_{load}}

yes

It is possible to set a limit maximum current ImaxImaxI_{max} coming out of the power supply so that the electronics don't get damaged. For example, let's consider the load to be a resistor with R=1ΩR=1ΩR=1 \Omega and the max current set to Imax=2AImax=2AI_{max} = 2A. We set the voltage to Vsupply=1VVsupply=1VV_{supply}=1V and find a voltage of 1V1V1V across the resistor and a current of 1A1A1A coming out of the supply.

yes

However, if we set the voltage Vsupply=3VVsupply=3VV_{supply}=3V, the power supply still sends out only a 2A2A2A current and the voltage across the resistor is only 2V2V2V. Is that correct?

no, refer to your previous statement ... 3V / 1 Ohm = 3A ... you tripled the voltage across the resistor
therefore the current is also going to triple
Therefore 3A will come out of the PSY ... IF it is able to supply 3A

How do you do set the maximum limit current?

use you previous calculation ...

Isupply=VsupplyRloadIsupply=VsupplyRloadI_{supply}= \frac{V_{supply}}{R_{load}}.

That determines the current the load needs to work correctly

With that PSU you can, if you wish, limit the current to something below that amount
Dave

 

[davenn]

Should we first short circuit the power supply and then turn the current knob to the desired limit current ImaxImaxI_{max}?

NO when the PSU output is shorted, it should, if a decent PSU like that one, automatically limit the current to zero
That is called short circuit protection

 

[davenn]

Also, the ports we use are the red and the black output ports.

yes

The green port in the middle is Earth ground, a real physical connection to Earth ground.

yes, it should be ... via the Earth wire and pin in the mains power lead

This green port can be connected to either the red or the black port with a wire, correct?

no, not the red ... it could be if needed be connected to the black ( the 0V terminal) IF you want the 0V rail to be at ground potentialread the handbook/instruction sheet for the PSU and see what it says about the green/ground terminal

 

[Borek]

How do you do set the maximum limit current?

I would start by reading the manual.

The PSU I use let's me set up voltage and the maximum current before applying any voltage to the output. It is just a matter of turning the knob until it displays what I need.

 

[russ_watters]

I would start by reading the manual.

Yah - I think I have the same PSU. The manual is succinct and easy to use.

 

[Tom.G]

NO when the PSU output is shorted, it should, if a decent PSU like that one, automatically limit the current to zero
That is called short circuit protection

Limiting the current to Zero when shorted is called Foldback Current Limiting and is frequently a latching operation. i.e a manual reset, like shutting off the supply, is needed to re-enable output. Some supplies have that feature, many do not. Foldback Current Limiting can actually get in the way if high load inrush current is expected or needed, as with a capacitive load, motor startup, incandescent lamp load, etc. On the other hand, some active circuits need the Foldback for protection under fault conditions. For instance if you have a semiconductor circuit, Foldback may protect it agains reverse-polarity connection of power.

How do you do set the maximum limit current? Should we first short circuit the power supply and then turn the current knob to the desired limit current ImaxI_{max}? What is the right sequence of actions to do that?

For a non Foldback supply, yes. Set the output voltage to the desired value, then, as you said, short the leads and set the Current Limit.

For a Foldback supply, you set the Current Limit Knob to its appropriate calibration mark. You can't short the leads because the supply will shut down.

Cheers,
Tom

 

[Rive]

For a non Foldback supply, yes. Set the output voltage to the desired value, then, as you said, short the leads and set the Current Limit.

Well, usually I set the current limit first, at low voltage.

 

[Guineafowl]

I have that exact supply.

With the leads apart, set the voltage required.

Then short the leads and dial in the max current you want to supply. Leave the middle top button out - it halves the available current. I’m not sure what use that is.

The unit will supply the pre-set voltage (CV mode)at whatever current the circuit is drawing, unless your current limit is reached, in which case the voltage will drop to keep the current within limit (CC mode). LEDs indicate CV or CC mode.

Mine came with a bridge to link the -ve (black) jack to the earth/ground (green) jack. I generally use it without the link, so the output is floating. This seems more appropriate if I’m planning to scope things.

I tested the cal with my decent meter and it’s surprisingly accurate throughout the range - just needed a tweak to get the absolute values right.