Re: 7805 regulator input voltage confusion ??

mark krawczuk wrote:
hi, just looking up some specs on the 7805 regulator , one site says a max of 35vdc input and this says 25vdc max input (http://store.americanmicrosemiconductor.com/7805.html) ,,,,,, i`m pretty sure the max is 35vdc ?

what is correct ?

Whatever the manufacturer of the particular part you're using says. So if you need 35V, and some manufacturer's parts say 25V, specify the manufacturer (or the pertinent parameter).

also i have nearly finished building a fixed and adj power supply all in one , and after the filter cap i have a reading of 35.9vdc , now i`m relying on the max being 35 vdc for the reg , so how can i EASILY knock the voltage down to about 25 volts or so ?

Check your peak voltage -- that 35.9V is an average reading, the peak could be higher, and may go considerably higher if the power line goes over-voltage. Given that reading I'd design for _at least_ 40V on occasion if not 45, and I'd probably figure on it dropping down to 30V or 25V on a bad day -- all subject to the specific way you built your circuit.

A zener, or diode string, as already suggested. Just put some load ahead of the regulator to make sure you pull enough current to get you down to your required voltage.

Or preregulate with a zener to the base of an NPN pass transistor.

Or choose a different transformer for your 5V supply.

(I'd suggest a switcher, but the level of questions you're asking suggests that it'd be too big of a bite for you at the moment, _and_ there aren't many spoon-fed circuits out there to go from 40V down to 5V).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
.

Loading