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I was rather curious about why many buck converters and linear regulators were rated up to 42V, or close to it. It's a rather strange voltage to stop at, after all - why not 40V or 45V?

I read that it was due to an idea in the late 90's to early 2000's where every car was going to use a 36V lead acid battery (which would have a peak charging voltage of 42V.) A higher voltage means smaller wires. And cheaper high-end stereo systems as 36V can drive loudspeakers louder and they can have higher impedances. And smaller, cheaper motors. And a few other things.

However, the idea died down because a lot of car manufacturers already had a large inventory of 12V parts and accessories and didn't want to change them. Additional issues were found with making relays compatible with 42V due to arcing and contact wear.

Yet I still see parts being made with 42V ratings. Why? Just a throwback to the old days? Is there another industry that uses 42V?

Thomas O
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  • There's probably a fair amount of inertia - once you go to the effort of creating a line of power semiconductors for a particular voltage range, if customers buy them for assorted other uses because they are available, it makes sense to keep making them, and other customers will keep using them because they are available. – Chris Stratton Jul 21 '11 at 20:51

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For background see Wikipedia comment below (1)

Excellent 2009 Popular Mechanics discussion here Whatever happened to the 42 Volt Car?

Whoops :-) - Ford says 42-volt systems could begin appearing on cars as early as 2003.

Good (but now dated) discussion 42-volt electrical systems are (not quite) here!

NB "I don't know" with certainty, but one of the drivers of the chosen system voltage was Lithium Ion packs - with 10 cells giving you 36 Volts nominal operating Voltage and 42V charging voltage. That will be still valid in new systems of other sorts. The power tool brigade are after ever higher voltages - just as camera users like more megapixels, even though neither is necessarily "better".

If deciding to make a 36V part or a 42V part the difference is a smidegeon more thickness in the IC insulating layers. Easily done. Extremely low extra cost. Why not :-)?

Here a very positively spun report - no obvious date but clearly now a little dated. 42 Volt Market and enabing technologies

Source - Wikipedia

(1) A 42-volt electrical system was intended to allow more powerful electrically driven accessories in an automobile. Electric motors would be used for power steering or other systems, providing more compact installations and eliminating the weight of drive belts or large wires for high-current loads. Although investigated as early as 1988,[1] 42-volt electrical components are now used in only a few automotive applications, since incandescent light bulbs work well at 12 volts and switching of a 42-volt circuit is more difficult.[2][3]

The proposed new standard was triple the voltage of existing "12 volt" systems. Lead-acid batteries produce around 12.6 volts while discharging, while automotive alternators produce 13.5 to 14.5 volts during normal charging operation.[2] 42 volts is an approximation of the output of the new standard's charging system.[3] The higher voltage was selected to provide greater power capacity for wiring and devices on one hand, and to stay under the 50 volt limit used as a guideline for electric shock hazard.

Although many manufacturers were predicting a switch to 36-volt (lithium ion battery) / 42-volt (charging voltage) electrical systems, the planned roll-out has not occurred and the plans appear to have been canceled.[2] The availability of higher-efficiency motors, new wiring techniques and digital controls, and a focus on hybrid vehicle systems that use high-voltage starter/generators has largely eliminated the push for switching the main automotive voltages.[2] Applications that once were thought to require higher voltages, such as electrical power steering, have now been achieved with 12 volt systems.[

Russell McMahon
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I was rather curious about why many buck converters and linear regulators were rated up to 42V, or close to it. It's a rather strange voltage to stop at, after all - why not 40V or 45V?

It's because of something called safety extra-low voltage (SELV) -- certain regulatory standards allow voltages up to 60V under all conditions, and other ones allow up to 50V. (Unfortunately I can't find any references to standards, and even if I did, you wouldn't be able to view them for free. :-( ).

When you consider a 42V nominal automobile battery under worst-case conditions (load suddenly going away during charging), it can be higher than 42, so there's enough margin to stay below 50V and be able to pass those regulatory standards.

Above the ELV/SELV limits, you can operate but you have to use different connectors, make sure there's no exposed parts that are energized at that voltage, etc. -- so there's a quantum jump of expense and difficulty that starts at that point.

Jason S
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Some power tools that require more power than a drill, e.g. electric lawn mower, are 36 V. That's not the reason for the parts, however.

Brian Carlton
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