Floating chassis...

Thread Starter

Externet

Joined Nov 29, 2005
1,459
Hi.
Inspecting an old scanner I have (Bearcat 700A) , noticed its chassis is not connected to battery minus power jack. It is a 12VDC supplied unit meant for mobile use, or with a 120VAC-to-12VDC supply.
Its antenna jack is connected to chassis. There is several small capacitors between chassis and negative power jack. There is thumbscrews meant to hold it to a holder that is supposed to be grounded to the car, joining its chassis to car body.
An internal 7805 regulator puts almost all circuitry to work on 5VDC
Why is it built that way ? What is convenient about having it in such configuration ?
 

ericgibbs

Joined Jan 29, 2010
9,538
hi Ext,
On some mobile units, intended to run on a 12V vehicle battery, the metalwork/chassis is connected to the negative terminal of the battery.

Some older vehicles had their battery positive terminal to the chassis.

I have seen where the user has tried to connect his mobile unit to the battery of a positive chassis vehicle, its OK until he places the negative grounded mobile device on any part the metalwork of the vehicle.

This effectively shorts out the battery.

One way to prevent this is to ground the chassis of the mobile unit via capacitors to the 0V internal supply, this is a DC path blocking method.

It sounds similar to what you are describing

E
 
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