Marine Rugged Mount Power Suppply for Garmin 650t

[normanj]

I'm exploring using my Garmin 650t as a data source for a chart plotter based on a Raspberry pi. The Li battery in the Garmin is a nominal 3.7V so I assume the GPS needs something like this to function. However the Marine Rugged Mount power input requires a nominal 12V so it appears that there is a some voltage stabilisation used to drop this to the required 3.7V.

I've been trying to use the mount in conjunction with a 5V supply but this seems not to do anything, which I find a little surprising.

Is my observation that trying to power the GPS with 5V via the rugged mount cable is not an option? I suppose I could used the 5V via the USB socket, which is how I normally charge the battery, but this would introduce yet another cable into the mix, and would expose the Garmin internals to a salty atmosphere. I don't understand why Garmin would use a voltage converter that cannot handle less than 12V, even though it is comfortably more than the device needs.

 

[Nuvi-Nebie]

I don't understand why Garmin would use a voltage converter that cannot handle less than 12V, even though it is comfortably more than the device needs.

12V - 24V is used because the mount is designed to be fitted into a vehicle that runs off 12V - 24V voltage it's as simple as that, there is no supply in the vehicle less than 12 V that could be used for for permanent wiring

 

[normanj]

Thanks. I understand why the mount or, more accurately, the Garmin contacts to the mount can accept these voltages. It enables them to be powered by lead-acid batteries etc. My point was that it should not be necessary for the power source to be that high, given that internally it is being reduced to around 3.7V to drive the unit. Most voltage regulators will happily stabilise a broad range of inputs provided it is at least a little over the required final value.

 

[Nuvi-Nebie]

If a simple voltage regulator was used then it would be required to dissipate a lot of heat, let us say the unit requires 5 Volts at 1 Amp to run the unit and charge the 3.7 V battery, as some vehicles are 24 V the voltage regulator would be dissipating 19 Watts to supply the unit with 5 Watts, a better way is to use a switched mode power supply aka a DC to DC converter, this can convert an input of 24 V at 250 mA to an output of 5V at 1A or an input of 12V at 500 mA to an output of 5 V at 1A, however the range of input voltages has to be designed in, so it would only work from say 30 V down to 10 V

Voltage regulator = 24 V at 1 A input for 5 V at 1A output = 19 Watts of heat
Switched mode = 24 V at 250 mA input for 5 V at 1 A output = 1 Watt of heat