The math for a regulator using only an R and a zener shows it to be pretty limiting. You start by computing the maximum zener current, which is the zener wattage divided by its voltage. If you know the applied voltage, subtract the zener voltage from it and you now know the worst case voltage drop across the R. From that you can compute the lowest possible value for the R that will not burn it up. Now you can compute the current at which the R, without a zener would make the desired output voltage, and that is your output current limit. Usually, you end up terribly inefficient using just an R and a zener unless the load is very light. Adding the transistor helps things, but now you have thermal issues. The transistor and zener will tend to cancel with temp, but their curves are quite different. It all depends on how accurate the output has to be. Chris -----Original Message----- From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Clayton Mellina Sent: Wednesday, July 30, 2003 3:33 PM To: PICLIST@MITVMA.MIT.EDU Subject: Re: [EE]: zener voltage regulator design -----Original Message----- From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Dinho Sent: Wednesday, July 30, 2003 3:22 PM To: PICLIST@MITVMA.MIT.EDU Subject: Re: [EE]: zener voltage regulator design >I don't know if i'm saying something completly stupid, but don't you >think that the Zener shoud be after the transistor? the transistor is a voltage follower that increases the current output so the zener needs to be before it for the transistor to have a voltage to follow :) >and actually I think >you don't need the transistor at all. if your power supply could give >you more then 100mA, use just a resistor and a Zener. that is what I would have thought but I have read otherwise (in a ST Microelectronics app note) Also, that design requires a very small value resistor that I don't have and don't want to order. >Dinho Clayton Mellina wrote: >I need a design for regulating 9V down to around 3V with a zener (simple - >no bells and whistles). the current capacity must be up to about 100ma. >attached is a schematic of my design (the parts may not be ideal - but I >designed with what I had). My question is: will this design work for what I >need? I wanted to use just a zener and a resistor but I've read that this >design will only supply up to 50ma (this might be ok but better safe that >sorry). thanks for checking - I've never designed with a zener before. > >-Clayton > > >-- >http://www.piclist.com hint: To leave the PICList >mailto:piclist-unsubscribe-request@mitvma.mit.edu > > > > > > ------------------------------------------------------------------------ > -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu