I am interested in using the GeoChron on 24V in a street sweeper. The documentation lists the input voltage at 7-12V. However I see that there is a mc34063smd in the device that is rated to 40V. Does anyone see any issue replacing the C17 100uF Cap with one rated at 50V and running the device on 24V?
Darryl VK2TDS
You’ll need to replace the 14V MOV (“R1”) on the input, since this is a nominal 14V part.
I suggest you replace it with a much larger TVS diode, eg SMB package. The existing tiny protection device is really not suitable for an automotive environment anyway (spikes & surges could easily have enough energy to vaporise it).
It is actually really tricky to choose a device for a 28V (nominal) input that will clamp the voltage below the maximum 40V the chip is rated to. Look at the datasheets for some TVS / MOV protection diodes & you’ll see what I mean. You need the clamping voltage to be less than 40V under “real world” spikes / surges, but you don’t want it to start conducting at normal battery voltages.
I suggest you put a surge-rated resistor and polyfuse in series with the power supply input, to make it easier for the (replacement) TVS / MOV to clamp the voltage below 40V.