# Need advice on choosing a diode

#### Michael Woods

Joined Apr 25, 2017
6
I have a 12v device on an automobile that draws about 1.5 Watts. I’m alittle concerned that the ground wire may be back feeding because the device is attached to the OBD port which has another source to ground. I want to put a Diode on the ground wire but I know very little about choosing the right Diode. What should I look for to handle the wattage rating of the device and also to handle the 12v?

#### SamR

Joined Mar 19, 2019
4,911
W=IV .: I=W/V so I = 1.5W/12V = 125mA
So that is what the circuit requires as a minimum, realistically use a fuse or diode based on the wire or components ampacity. If the wiring/fuse/components of the circuit are rated for 10A then the diode should be able to handle that as a minimum. You want the fuse to break the circuit, not the diode/wire/components to burn out first.

#### LowQCab

Joined Nov 6, 2012
3,580
What do you mean by ""the Ground back-feeding"" ?
The OBD-Port will usually have redundant Grounds.

I would say your problem is elsewhere,
and a Diode is just going to lower the available Voltage by ~0.7-Volts.

Worse-yet,
by putting it in the Ground-Wire,
now you are messing around with the Voltages of the Can-Bus Communication-Lines,
not recommended.
.
.
.

#### Papabravo

Joined Feb 24, 2006
20,602
If you can't measure more than 0.6V potential difference between the grounds then the diode will be as useless as a screen door on a submarine. Why do people keep inventing non-existent problems? We should ostracize the next person that uses the term "back-feeding"!!

#### Michael Woods

Joined Apr 25, 2017
6
Hi. My electrical knowledge is next to 0. Here is what I am fearing is happening in more detail.

There is a device that I will name ‘Primary Device’, that has a power and a electrical ground wire. The ground wire that this device is connected to also has other devices that ground onto this wire and eventually the negative terminal of the car battery.

This same device is ALSO connected to the OBD port as it gets measurements and readings from it but NOT power and ground, however the OBD port lso has a ground.

What I am fearing is the possibility that other devices connected to the electrical ground wire / negative battery terminal, may be feeding through the primary device and into the ground port of the OBD which means current is going through my primary device to reach ground as it sees a path to it through the OBD port instead of through the wire and to the negative battery terminal.

I was thinking by making my electrical ground on the Primary device a one way, that I would be able to prevent current from feeding into my Primary device to reach ground via OBD.

#### LowQCab

Joined Nov 6, 2012
3,580
That's not the way to fix this situation.
A Diode in the Ground-Wire will do nothing but cause more weird problems.

A Schematic,
for every device used in this scheme,
and a detailed explanation of what you are trying to accomplish is required.

There may be 10 different ways of getting the same end-product,
and half of them won't work as expected.
.
.
.

#### djsfantasi

Joined Apr 11, 2010
9,131
That’s not the way it works. As a matter of fact, the grounds of all the devices MUST be connected. If not, the signal wire to the OBD can not be read because it has no common reference point.