hi, just looking up some specs on the 7805 regulator , one site says a max of 35vdc input and this says 25vdc max input
what is correct ?
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 ?
thanks,'
mark k
National Semiconductor's datasheet says 35 volts - but I suppose that other makers can do their own thing (but it would be nice if they changed the part number if they don't follow the originator's specs.)
A series zener diode of appropriate current and power rating?
-- Peter Bennett, VE7CEI peterbb4 (at) interchange.ubc.ca GPS and NMEA info: http://vancouver-webpages.com/peter Vancouver Power Squadron: http://vancouver.powersquadron.ca
I'm pretty sure that your best bet is to redesign your 5V regulator using a LM317T adjustable regulator. It tolerates 40V in and is rated at 1.5A out so no problem getting 1A out, unlike plain 780x series regulators, of which, I have NEVER achived the full 1A out, but always had them start limiting early.
Good god, why are you feeding it such a high voltage to begin with? The regulator will dissipate 6 times the power of the load! That ridiculous! Reduce the value of the filter cap so you get a lot of ripple. This will reduce average voltage. Put in a power resistor to dissipate some power and drop the voltage. A 7805 will work with 8V at the input.
Depends on what you mean by "easily". You can put a string of 12 1N4001 diodes in series. They'll drop about .6 volts each at lower current, and as the current approaches 1 amp the drop will increase to roughly .9 volts per diode. You'll be dissipating ~ 10 watts in the diodes at full (1A) current, so keep that in mind in the construction.
Ed
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).
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
I've had good luck with the LM2595 "simple switcher" - it only needs four other parts (input cap, coil, diode, output cap) and can handle up to 40v input.
It's the middle chunk of this board:
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.