On Wed, Apr 27, 2011 at 8:44 PM, David Duffy (AVD) <
david@audiovisualdevices.com.au> wrote:

> I have a product that would normally be used with a DC input of 12V -
> 36V.  It uses an LM2575 switching regulator to derive the 5V rail (@ few
> 100 mA) for a PIC and other chips.
>
> The lowest of the maximum Vdd ratings on these chips is 6V.  There is a
> situation where some customers may need to power the board from 5V.
>
> I can provide jumpers to bypass the regulator section, but would like to
> protect the 5V chips from accidental connection of a higher voltage
> while still jumpered for 5V.
>
> Looking at transorbs and zeners (with a series fuse or polyswitch) gives
> me the impression that they will not save the logic chips being
> toasted.  Any ideas?
> David...
>


I'm thinking of something involving a MOSFET's gate being driven in paralle=
l
with a diode with a 5V breakdown in reverse. If the input is higher than 5V=
,
the diode shunts and turns the MOSFET off. The MOSFET can be connected to a
switch which selects the input source.
--=20
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