I'm the inventor/licensor for a commercial electronic device that operates at 12VDC using a wall power adapter. I plan to change the operating voltage from 12VDC to 5VDC so it can operate via a standard 5VDC USB 1.0 or 2.0 port. The 12 volt device will look exactly like the 5 volt device and will use the same input power plug. I need to use the same plug since changing it would require a new injection mold costing thousands of dollars. Is there a cheap way in the circuit board to limit the voltage so if someone plugs in the old version 12VDC adapter into the new version 5VDC device the electronics won't fry?

 

How much current does the device take at 5VDC?

 

Can you post a schematic of you circuit?

 

use voltage regulator, preferably switcher, in case you need to reduce voltage from 12V to 5V.
and if supply is less than 5.5V, just bypass it. how much space you have available?

 

LM2596 accepts unregulated voltage up to "37V" or "57V" for the "high voltage version" and outputs the needed voltage of "5V". The max current is "2A-3A", if this is enough for your device.

There are different versions of "LM2576" offering a stable "5V" output or an adjustable "output voltage".

EDIT:
Unfortunately there is a voltage drop which will lower the input voltage, so I suggest using "3.3V" to power your device and output from your device to the "USB" if there is such an option. When connected to a "5V" or "12V" power supply, the end voltage will be "3.3V" which is a stable output by "LM2576-3.3". The USB will work with "3.3V".

 

How much current does the device take at 5VDC?

The device takes about 400mA at 5VDC.

 

Can you post a schematic of you circuit?

I can't. The PCB is proprietary.

 

As suggested, can you change the operating voltage to 3.3VDC?

 

Any linear voltage-reducer would have to dissipate (12-5)*0.4 = 2.8W, in addition to the 2W your device needs. So the device would have to handle a lot of extra heat. A switch-mode regulator, as mentioned in post #4, would avoid that.

 

Here is a 5V 1A SMPS that will accept input voltage from 4V-36V.

LMZM23601V5SILR

 

The one I gave is exactly a switch regulator.