5v 2A dc/dc voltage regulator's voltage out of spec (low) - Murata 78SR-5/2-C

The Murata 78SR-5/2-C is a 5v 2A dc/dc converter (about $14/each at Digi-Key, which is where I got mine).

The data sheet says the output voltage accuracy is +/- 2%, so I would expect the output to be between 4.9 and 5.1v

However, I have tested four units, and under 0.1A load, they output about 4.88v, and at 0.7A they output approx 4.55v

I've tried two input sources and get the same results (plus or minus a few 0.01v) with both. They are a 24v 0.75A switching mode wall wart and a 10v 1A linear regulator. The 78SR is an efficient unit, so the input current is well within those source's capabilities.

Any thoughts on why I'm having this problem? Was I just unlucky in getting four bum units?

If the DC-DC converters have a good circuit, it should keep the output at 5.0V, until the current limiter kicks in. When the output of the DC-DC converter is lowered, it is mostly loss in the wires, and not the converter itself.

Without load, or with a small load, the output must be 5.0V. When you read 4.88V, there is something very wrong.

It is possible that you have 4 bad converters, but that is hard to believe. Do you use a breadboard ? Those can have bad contacts. When you measure the voltage, use a multimeter and measure the voltage on the pins of the converter, not after a breadboard contact and not after a wire.

Thank you thank you thank you!

I took them out of the breadboard and wired them directly: they're right at 5.0v at all loads.

Their short pins could be what did it.

nice ! 8)

FWIW, I measured the resistance between two wires stabbed into the same row of the breadboard. In some cases, the resistance was as much as one ohm! At the currents I was testing, that would cause a significant voltage drop.

I tested another breadboard - one that might be about 15 to 20 years old - and got resistances on the order of 0.1 to 0.2 ohms.

Quality varies wildly.

I wouldn't expect a breadboard to handle more than 100mA per contact...