Why are there current and voltage limits in a DC bench power supply?

Question

I have a few questions about bench power supplies.

1. Why does a DC PSU have a low output voltage limit, usually in the range of 18V-24V?

2. Why is there a current limit built into the PSU? My understanding is that the PSU should only supply voltage (i.e. energy to the electrons in the circuit wires) and the supplied current should only depend on the resistance of the wire or the internal resistance of supply or component attached, i.e. a resistor or LED?

3. When we short the terminals of the power supply, why does the output voltage on the supply approach zero? If there is no component attached, shouldn't there be zero voltage drop across the terminals, leaving the output voltage the same as if the output was not shorted?

Answer 1

Q1) Why the DC PSU have low voltage usually in the range of 18V-24V?

Because that covers the operating voltages of the majority of electronics circuits for auto, audio, computing and industrial applications.

Q2) Why is there a current limit builtin to the PSU, a/c to my understanding, the PSU should only supply voltage ... and current should depend on the resistance of the wire or the internal resistance of supply or component attached i.e. a resistor or LED?

Correct, in general, but the current limit has many useful applications:

1. Protection when there is a fault in the circuit.
2. LEDs require constant current drive so setting the current limit rather than a voltage limit is much more useful.
3. The PSU can be used to charge batteries with a current and a voltage limit.
4. Small DC motors can be run in constant torque mode when driven by constant current.

(i.e. energy to the electrons in the circuit wires)

Forget about electrons in wires. It will lead you into all sorts of confusion. Just think of voltages, currents, power and energy.

Q3) When we short the terminals of the power supply, why the voltage on the supply approaches ZERO? Because the VOLTAGE DROP is ZERO (since no component is attached) but the voltage on the supply TERMINALS should not change?

There is a component - a very low resistance - maybe 0.01 Ω, for example. If the voltage were to stay at 18 V then a current \$ I = \frac {V}{R} = \frac {18}{0.01} = 1800\ \text A \$ would flow. The power supply can't deliver that so the current limit cuts in and the voltage collapses to the voltage drop across the component at the current limit. In this example, on a 3 A supply, that would be \$ V = IR = 3 \times 0.01 = 0.03 \ \text V \$.

Figure 1. A typical bench PSU.

Depending on the PSU's voltmeter resolution the display may give an all-zeros reading for a short-circuit test.

Answer 2

1) Mainly cost/benefit. There are high voltage bench PSUs, which are specialized. The vast majority of bench PSU uses are in the range of up to 24~30V. (Adding the extra range would make it a lot more expensive, for little use) Also, there are bench PSUs which have double outputs which you can put in series (or parallel), so its super versatile.

2) An ideal Voltage PSU provides only voltage. A practical Voltage PSU might have some internal over current protection (OCP). A bench PSU, as a test instrument, will have current limiting as a feature. Say, to protect your test circuitry, or lets say you want to drive an LED without a current limiting circuit.

3) Mostly because of the above, and that the leads will have a very low resistance.

Answer 3

Why does a DC PSU have a low output voltage limit, usually in the range of 18V-24V?

The limits on most DC bench supplies are usually more like either 5 V or 15-30 V, and that's because these two ranges cover most electronic circuits. The 5 V range covers most digital electronics, being both the old 5 V standard plus the newer 3.3, 2.5, 1.8 and lower standard voltages. The higher range covers most analog electronic circuits.

Power supplies for voltages outside this range are for specialized uses, and thus are rarer and more expensive than commodity bench supplies.

Some power supplies incorporate both of these ranges, and some only one. A common configuration is two outputs with 24 or 30 V limits for powering bipolar analog electronics plus a single high-current 5 V output for powering the digital section of your circuit.

Many bench supplies have floating outputs, so that in a pinch, you can connect the + and - terminals together to get up to 60 V or so. That covers even many uncommon use cases, further lowering the need for higher-voltage power supplies, thus making them even rarer and more expensive than you might initially guess.

Why is there a current limit built into the PSU?

We'll get to part of it with your third sub-question, but as others have said, it's because it is frequently useful to have one.

One use for it I haven't seen mentioned yet is that it's a good backstop for your design efforts: if you've designed a circuit to draw no more than, say, 330 mA but you hear the current-limiting relay click in while working on the circuit, you know something is going wrong.

Another use is that you should design for the power supply you are eventually going to include with the finished design: you don't want to design a circuit that only works properly when powered from a nice bench supply! Low-quality wall warts will have their own current-limiting behaviors, so you want to test for that before you ship the final product. Enabling current limiting is one way to do that before you have selected the shipping power supply model.

You might think, for example, that your circuit runs just fine on 40 mA but needs about 200 mA for the inrush current at power-on. So, test it: set the bench supply to a 200 mA current limit and see how it behaves. Some circuits will do bad things if current-limited during power-up. If yours does this, then you have two choices:

1. Fix the circuit so it copes with a constricted power current during power-up.

2. Raise your chosen limit: clearly you're going to need a bigger power supply for the shipping version of the circuit.

You don't want to have a collection of dozens of wall warts with different current limits just to test this, any more than you want dozens of wall warts with different voltages. The voltage and current limit features are complementary.

When we short the terminals of the power supply, why does the output voltage on the supply approach zero?

What does Ohm's Law tell you?

If resistance goes to nearly zero and voltage stays high, current approaches infinity, which if allowed tends to create an arc welder.

All real circuits have a current limit. The one in your bench supply is trying to save you the bother of scraping molten metal off your work bench after shorting the terminals. Also your hands, face, and eyes.