# Unexpected voltage drop across resistive load.

#### Mark Flint

Joined Jun 11, 2017
145
My bench DC supply is set at 12.0v with no limit (equals 5 amps) on the current setting. I place a 21w incandescent auto lamp on the supply and it reads 1.82 amps at 12.0v.

I then measure the voltage drop directly across the lamp and see 11.70v. There is nothing 'downstream' in series with the lamp except for the negative lead of the power supply

Can I presume that there is a resistance in this negative lead which is causing the 0.3v discrepancy - or can there be another explanation?

#### Papabravo

Joined Feb 24, 2006
21,313
The power supply has an internal source resistance of:

$$\cfrac{0.3\text{ V}}{1.82\text{ A}}\;\approx\;160\text{ mΩ}$$

Ohm's law man -- Ohm's law!
ETA: Ideal voltage sources do not exist!

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#### Jon Chandler

Joined Jun 12, 2008
1,098
Resistance of BOTH leads and the resistance across connectors accounts for the voltage drop.

Ohm's Law:

V = i × r ==> r = V / I

r = 0.3 / 1.82 = 0.165 ohms.

#### Papabravo

Joined Feb 24, 2006
21,313
Resistance of BOTH leads and the resistance across connectors accounts for the voltage drop.

Ohm's Law:

V = i × r ==> r = V / I

r = 0.3 / 1.82 = 0.165 ohms.
Are you saying that the power supply has no source resistance? The resistance of the leads would not be zero and maybe contribute to the externally observable voltage drop from the power supply to the load.

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#### dl324

Joined Mar 30, 2015
17,137
Can I presume that there is a resistance in this negative lead which is causing the 0.3v discrepancy - or can there be another explanation?
Wires have resistance. Use a beefier test lead, or parallel a couple, and measure again.

Any decent power supply would sample the output voltage and minimize the affects of its internal resistance.

#### Papabravo

Joined Feb 24, 2006
21,313
Wires have resistance. Use a beefier test lead, or parallel a couple, and measure again.

Any decent power supply would sample the output voltage and minimize the affects of its internal resistance.
Since we do not know the make or model, we can not speculate on the "decency" of the unit.

#### ElectricSpidey

Joined Dec 2, 2017
2,885
The OP has already stated the output voltage of the supply with the load attached.

#### Papabravo

Joined Feb 24, 2006
21,313
The OP has already stated the output voltage of the supply with the load attached.
Yes, but we don't know if the supply is sensing the voltage across the load and compensating for any internal resistance.

#### Jon Chandler

Joined Jun 12, 2008
1,098
Are you saying that the power supply has no source resistance? The resistance of the leads would not be zero and maybe contribute to the externally observable voltage drop from the power supply to the load.
The voltage was measured at the power supply terminals and across the load. That was the question asked, and everything known about the power supply.

The internal resistance of the power supply is out of the question as the voltage was measured at the output terminals. The resistance of the leads and connectors is the only thing in the equation.

The TS was on the right path, but I thought it important to point out the resistance of BOTH the supply lead and return lead enter into the calculation, based on the phrasing of the original question.

#### Papabravo

Joined Feb 24, 2006
21,313
The voltage was measured at the power supply terminals and across the load. That was the question asked, and everything known about the power supply.

The internal resistance of the power supply is out of the question as the voltage was measured at the output terminals. The resistance of the leads and connectors is the only thing in the equation.

The TS was on the right path, but I thought it important to point out the resistance of BOTH the supply lead and return lead enter into the calculation, based on the phrasing of the original question.
Believe what you want. I don't care.

#### MisterBill2

Joined Jan 23, 2018
19,453
The power supply has an internal source resistance of:

$$\cfrac{0.3\text{ V}}{1.82\text{ A}}\;\approx\;160\text{ mΩ}$$

Ohm's law man -- Ohm's law!
ETA: Ideal voltage sources do not exist!
Certainly IDEAL VOLTAGE SOURCES EXIST!!! But not in our physical realm. They are found in text books and in simulators, and in places where they are "close enough to ideal" so that it does not matter.
But average inexpensive power supplies are not perfect, nor are real world connections. And some varieties of clip leads are so thin that they seriously affect anything they are used with.

#### boostbuck

Joined Oct 5, 2017
570
Since we do not know the make or model, we can not speculate on the "decency" of the unit.
It seems you are?

#### Jon Chandler

Joined Jun 12, 2008
1,098
Believe what you want. I don't care.
[Edit – Sorry, my high-quality schematic didn't get included in the original post. It clearly shows what's happening here.]

I believe exactly what was provided. 12 volts, measured at the power supply. 11.7 volts measured at the load. 1.82 amps measured flowing through the circuit. Therefore 0.3 volts is being dropped between the supply output and the load. Period.

The internal resistance of the power supply is N/A because the voltage was measured at its output terminals. Please illustrate exactly where the power supply internal resistance enters into this.

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#### MisterBill2

Joined Jan 23, 2018
19,453
Real wires have a bit of resistance, as do real connections. Thus all current carrying circuits do have some voltage drop, which in many cases is ignored because it does not cause any problems.
SOME clip leads have a much greater resistance and often cause problems.

#### Sensacell

Joined Jun 19, 2012
3,504
Real wires have a bit of resistance, as do real connections. Thus all current carrying circuits do have some voltage drop, which in many cases is ignored because it does not cause any problems.
SOME clip leads have a much greater resistance and often cause problems.
I have found modern clip leads to be almost garbage, poorly crimped wires, ridiculously small wire gauges etc.
This often shows up as unexpected voltage drops in my test setups.
Maybe Pomona makes some good ones, \$50 each?

#### Jon Chandler

Joined Jun 12, 2008
1,098
Watch out for banana plugs where the spring section rotates freely. They are crap.

#### MisterBill2

Joined Jan 23, 2018
19,453
The one benefit of those very poor clip leads is that they also double as fuses if there happens to be a shorted connection. The clips and covers work well enough when I replace the wires with pieces of "super flex" from damaged cable assemblies..

#### Mark Flint

Joined Jun 11, 2017
145
I always make my own clip leads from substantial multi-strand that is rated for ~30A. But the leads on the bench supply are original. Replacing them is on my TODO list now.

#### MrChips

Joined Oct 2, 2009
31,087
I would not expect to find leads to be supplied with the bench supply.
Yes, try replacing the leads with heavier gauge wires and better connectors.

#### Jon Chandler

Joined Jun 12, 2008
1,098
I added the schematic to post #13 that I meant to include. Sorry I forgot it.

It shows the situation described and why power supply internal resistance doesn't matter in this case.