Graham North wrote:

>> +9V_Wall -----------+------------ +5V_Out
>>                 7905_Gnd           
>> 0V_Wall -+- 7905_In 7905_Out ---- Gnd_Out (virtual ground)
>>          |                         
>>          +----------------------- -4V_Out

> Yes that's the circuit, it works quite well to be honest I am just
> worried about a power supply greater than 9V being plugged into it. 
> 
> I was going to fit a fuse in the +9V_Wall line, so will a say 5V1 zener
> on its own be ok? Will it work between the -4V_Out and Gnd_Out lines?
> Where will it sink the over voltage/current to?

I'd put the fuse (either the wire type or a PTC type) in the +9V_Wall line
and the zener (or a TVS for robustness) between +9V_Wall and 0V_Wall. You
don't want to protect the output, you want to protect the input.

> Its confusing me as you can tell.

Think modular. Think of your circuit as above as a "power supply module"
with 0V and +9V as inputs and -4V, Gnd and +5V as outputs. Then think how
to protect the power supply input from an overvoltage:


         [ Protection ][     Power supply     ]

 +9V_Wall --Fuse--+---------------+------------ +5V_Out
                 TVS           7905_Gnd           
 0V_Wall ---------+-----+- 7905_In 7905_Out ---- Gnd_Out (virtual ground)
                        |                         
                        +----------------------- -4V_Out

Gerhard

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