Re: 7824

Al,
That should be OK, the 7812 and 7824 can operate but will not "regulate"
when the voltage drops below Vreg+3 .
As the load is is simply a FAN there will be not large capacitors
storing a charge and you will be OK.
If however the circuit that is fed by these regulators ( some
electronics ) an that circuit has large capacitors it is customary to
connect a diode across the regulator for give a discharge path for such
stored energy.
In practice your "circuitry" will discharge such stored energy and you
can forget about the reverse diode.

Cathode Anode
|-------|<|-----------|
Unreg V+----|--------regulator--|----load

In all my years I have not had a single failure of a 7812 or 7824 that
could be proven to be as a result of this scenario, What people tend to
do is forget these regulator require a decoupling at the input and
output, and you can find 7812 that are oscillating in the high MHz band
because decoupling was left off..

regards
Hans Wedemeyer


Al Schoepp wrote:

> Hope somebody here can help with an electronics question. I need to
> step down 30vdc to 12vdc and 24 vdc for a the stepper driver I'm
> working
> on. I believe a 7812 and a 7824 chip will work for me. However the
> specs say that Vin must exceed Vout by 3 volts. When the system is
> operating that is not a problem. When I turn it off it will take a
> while for the power supply cap to drop to 0 so at some point in time
> Vin
> won't exceed Vout by 3v. Will this damage anything? If so how do I
> prevent this from happening. The stepped down voltage will be used to
> power small cooling fans, if that makes a difference.
>
> Thanks,
> Al
>
> ------------------------------------------------------------------------
> Welcome to CAD_CAM_EDM_DRO@...,an unmoderated list for the
> discussion of shop built systems in the above catagories.
> To Unsubscribe, read archives, change to or from digest.
> Go to: http://www.onelist.com/isregistered.cgi
> Log on, and you will go to Member Center, and you can make changes
> there.
> For the FAQ, go to http://www.ktmarketing.com/faq.html
> bill,
> List Manager
>
> ----------------------------------------------------------------
> [Image]
> Click Here
>