So how do you get rid of the 12W dissipation at the zener without a
heatsink?

Brian kraut wrote:
>
> I don't know what the maximum voltage input on a 7805 is, but I have used them at
> 28V.  The big problem with using them with higher voltages is that they dissipate
> a lot of power.  At 30V input is is 30V-5V+25V times 1A=25W of heat dissipated.
> I manufacture some devices that run on 28V and don't have room for a heat sink on
> the regulator so I use a 12V zeener in series with the input voltage to knock it
> down some.
>
> There are some switching regulators that work up to about 50V and don't generate
> a lot of heat, but they are a whole lot more expensive than a 7805.