DC-DC step down converter & increase the input voltage?

[Tinstaafl]

Background: Bought a cheap mail order DC-DC stepdown converter. It was advertised as a 12-24V to USB converter ie 12-24Vdc to 5Vdc. When it arrived the input voltage was marked as 12V ie no mention of a 12-24V range.

So, what happens if I use it with a 24Vdc supply? Will it release the magic smoke? Will it work but the output voltage be different? Or will the output voltage still be a nominal 5V but maybe a different amperage?

[good spark]

you did say it was cheap, so plug it in, brace yourself and see what happens.





gs

[grounded27]

Buy a meter, the amperage is not a factor (unless your electronics draw more than supplied). If you are worried about the 5vdc out a meter is the only way to be sure.

[ampclamp]

Mate, it's no good to you sitting there.
Just try it.No smoke...good start... check output volts with a good meter , (not your usb device), and if 5vdc , no problems.
Try it on something cheap [ if you have something,] to give it a load for a while to see if it gets too hot and smoky under load before putting the precious Iphone/pod/navigator etc on it.

[MurphyWasRight]

This is almost certainly a switching "buck" regulator that regulates the output by adjusting the on time of a switch feeding an inductor/capacitor comibination.

The output voltage will stay at the set point (5v) over the input range.

If anything the unit should be slightly more effecient (run cooler) at higher input voltages, less switch on time == less loss in switch.

A potential issue is that if the unit is designed for 12-24V (usualy limitied by breakdown voltage of control IC or switch) it may fail when suplied by a nonminal 24V system.
In automobiles (nominal 12V except for my first car a VW bug) the actual "normal" voltage can range from 8V or less (cranking) to 14V or so when charging. Minor faults can result in even higher voltages.

Not clear if the 12-24V was a hard limit or meant that the unit was designed for use on either 12V or 24V systems.

On the plus side it is much more likely to fail in the "no output" condition than with dangerours overvoltage.

Additional protection could be had by using a ~6V zener diode accross the output to clamp the max voltage. If you do that also put a fuse between the convertors output and the zener.

Most usb devices immediatly convert the 5V down to 3.3V using a swtiching converter so a moderate input overvoltage is unlikely to damage anything.

[flying lid]

If it's made in China chuck it away before it sets your house on fire.

Lid

[mike-wsm]

Murphy is right, of course.

And there are additional precautions that can be taken, depending on your level of education and the equipment to hand. If you have a 'proper' power supply start at 12V and crank it slowly up to 28V (the high side of 24V) whilst monitoring the input current. Set the power supply to a reasonable current limit.

If you live on a desert island with no equipment, then use a series resistor on the input side to limit the input current, and monitor the input current with a suitable meter. You need to know ohms law and how to calculate the resistor power rating.

Or ask your local radio ham, it should be within his ability.

[Tinstaafl]

Thank you everyone. I bought it so I could use it in my car and in aircraft I fly - hence wanting an input range of 12 to 24vdc. My issue is that the plug only lists 12vdc on it and that's why I'm concerned about plugging it into a 24vdc system. I don't really want to risk damaging it if a bit of knowledge beforehand indicates that damage is likely. After all, I can leave it permanently in my car and look for an alternative for aircraft. But....if it turns out that 24vdc on the input side of it isn't likely to be an issue then I'll give it a go.

[Gulfstreamaviator]

Please remember that putting a Volt meter on the output will not be that accurate, as until a load is drawn, the indicated voltage might be very high.

Only when a representative load, and thus the products regulation is in service will a true working voltage be see.

If using a multi voltage sector enhanced supply, increasing the selected voltage can actually produce the required output voltage.

glf

[Loose rivets]

I do hope not! Floating voltages are a thing of the past.

Any modern bit of kit should be no more than say, .2v over the designated voltage. One volt would be unacceptable.


Making an assumption that everything's okay because it goes for a time is plain wrong. Cheap kit will put up with over-volting, then simply fail - sometimes to the detriment of vulnerable stuff downstream.

A resistor on the input line will simply cause instability. Scoping the line in might well reveal exotic waveforms. V in, needs to be solid.

Internet search of the specs would be a good idea, and if nothing is found, probably better to not take it into the air. Planes cost lots of money, crowbaring the accessory socket might prove expensive . . . and smokey. :uhoh:

[mike-wsm]

Anything you use in flight should be checked out by a qualified aero engineer. If this bit of kit is, for example, the one pound forty nine pence 'D@ffodil' I'd confine it to ground use.

Airborne electronic kit has to be designed and certified to meet all airborne requirements including storage temperature, operating temperature, pressure, input supply variation (including spikes), fungal, humidity etc specs.

Don't fly anything that is not inspection stamped for flight use.

[Tinstaafl]

Erm... this plugs into the cigaretter lighter socket. The plane has several of them + plus an accessory power socket (still a cigaretter lighter socket though).