Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!lll-winken!elroy.jpl.nasa.gov!swrinde!zaphod.mps.ohio-state.edu!pacific.mps.ohio-state.edu!linac!att!cbnewsj!asd From: asd@cbnewsj.att.com (Adam S. Denton) Newsgroups: sci.electronics Subject: WAIT! - Re: 12 to 9 volts dc (kinda long) Message-ID: <1991Feb26.235009.1389@cbnewsj.att.com> Date: 26 Feb 91 23:50:09 GMT References: <1991Feb10.220719.17078@cbnewsl.att.com> Organization: X Lines: 89 DON'T use the posted circuit -- you will cause Discman Frying. In article erk@americ.UUCP (Erick Parsons) writes: >>In article <1991Feb10.220719.17078@cbnewsl.att.com> wally@cbnewsl.att.com (Brian S Walden) writes: >>I want to take my old Sony Dicsman D-5 in the car with me, >>the problem is that it requires 9 volts and 1/2 amp. >> >>I'm not a e.e., but from the course in college (a few years, now) >>I remember something like this (12 to 9 volts) is just a resistor, >>a zener diode, and an NPN transistor. > >Good Memory Brian > >>I figure someone should have done this already with real parts. >>So if you'd like to share a schematic with me, I'd be very >>grateful. > >Here goes: >---------- The posted circuit will do nicely to fry the Discman. The series PNP will saturate, essentially shorting the collector to the emitter, giving about 13.6 V (13.8-.2) to the Discman: > \ / (Relative to the Base that is) > _\/__/_ This is a PNP POWER X-sistor. > | > |B > __|___ > /\ Zener Diode 9.7 Volts 1/4 Watt > /__\ 1/2 Watt to be safe > | > | > / > \ ___Current limiting Resistor (To save the Zener > / From overcurrent and all it's nasty side fx) > \ 285 Ohms, 1/4 watt again 1/2 watt to be safe. > | > __________|_________ Ground for + 9VDC > | > ___|___ > _____ > ___ Chassis Ground > > >Theory of Operation: The B-E junction of the PNP establishes 13.8-.7=13.1V on the base, NOT 9.7V as intended!! Use the following, which is very common: IN (13.8V) -+---- c e ----- OUT=9.0V | \___/ R | b |------+ | ----- /_\ Zener | GND The transistor is NPN, 2N2222/3904/4401/etc. variety. R is typically 100-1000 ohms. The diode is 9.1V Zener (for 8.4V out) or 10.0V Zener (for 9.3V out). Example: Zener = 9.1V, output current = 0.5A, Beta=50 (2n2222), -> base current = 0.5/50 = 10mA. Want minimum zener current to be 20%, or 2mA, for reliable operation at minimum input voltage of 12V, so total current thru R is 1.2*10=12mA. At 12V in, base is at 9.1, so R must be smaller than (12V-9.1V)/12mA= 241 ohms. Use 220 or 180 for reliability. At 1/2 amp and 16V input, the transistor will dissipate (16V-8.4V)*0.5A = 3.8 watts. Oops! Can't use 2n2222 etc. -- try a heat-sunk 2n3055 (overkill!). This has beta of 30, so you'll need to re-calculate the value of R (I get 145 ohms; use 120). R's wattage: P=V^2/R = (16-9.1)^2/120 =0.396 W; use 1/2 or 1 watt. R's current: (16-9.1)/120 = 57.5mA. Zener wattage is thus P=VI=(9.1)(.0575) = 0.523 -- use a 1 watt unit. Now you're all set! Enhancement: Bypass the Zener to ground with a large electrolytic (100uF) to improve ripple rejection. If you do this, DO NOT SHORT the output, as the cap will discharge through the b-e junction, causing meltage of the transistor. Another solution is to use a 7809 if you can find one, or a 7805 with a 3V Zener (or 3-4 series-connected diodes) in series with the ground lead. If you go the latter route you will need capacitors at the input, output, and ground terminals of the 7805 to GROUND in order to ensure stability. Adam Denton asd@mtqua.att.com