Gerhard,

That makes sense, thanks.

G

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
Of Gerhard Fiedler
Sent: 30 May 2007 15:21
To: piclist@mit.edu
Subject: Re: [EE] Strange Linear Supply Protection

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

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
######################################################################
Attention: 
This e-mail message is privileged and confidential. If you are not the 
intended recipient please delete the message and notify the sender. 
Any views or opinions presented are solely those of the author.

This email was scanned and cleared by NetIQ MailMarshal.
######################################################################

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist