[sci.electronics] WAIT! - Re: 12 to 9 volts dc

asd@cbnewsj.att.com (Adam S. Denton) (02/27/91)

DON'T use the posted circuit -- you will cause Discman Frying.

In article <erk.1356@americ.UUCP> 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

tomb@hplsla.HP.COM (Tom Bruhns) (02/28/91)

dunne@motcid.UUCP (Jim E. Dunne) writes:

>  Can you provide part numbers and makers?  When I was investigating 
>  a power supply for my 9V electric guitar effects, I could only find
>  12, 8, and 5 Volt regulators in the TI book.  I know there are 
>  "adjustable" ones, but I operate on the KISS principle when it comes
>  to circuits!  I'm trying to get 9V from a really dirty 12V wall-type 
>-- 
>  
>  Jim Dunne                    Motorola Inc.             uunet!motcid!dunne

(hee-hee-hee --  you're with Motorola and can't find semiconductors?? ;-)

Try, for example, Panasonic AN7805, -06, -07, -08, -09, -10, -12, -15,
-18, -20, and -24, all from Digi-Key, on Pg. 46 of their Jan-Feb 91
catalog.  Similar series for negative voltages, all at 1 amp max,
and down to 4 volts in a 100 mA series.  But personally, I prefer the
adjustable ones because it keeps my personal parts stock lower, at
very little additional hassle.  And for efficiency, try the new
switching regulator series from National or a similar one from Linear
Technology.  Another favorite of mine is a LM2941CT:  it's adjustable,
and designed so if you _reverse_ the input polarity, it gracefully
stops destructive electrons from visiting your circuit.  Also handles
high input voltages fairly well.  Also has low drop-out voltage (low
input-output differential required for operation).

elliott@optilink.UUCP (Paul Elliott x225) (03/01/91)

In article <1991Feb26.235009.1389@cbnewsj.att.com>, asd@cbnewsj.att.com (Adam S. Denton) writes:
> DON'T use the posted circuit -- you will cause Discman Frying.

I knew someone would follow up on this -- good treatment!

> 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.

I understand that it is *not* good practice to boost the output of
the 78XX-series regulators by putting diodes (Zener or otherwise)
in the ground lead.  Something to do with the short-circuit protection
(I haven't analysed it myself).

I prefer the LM317-series of regulators.  These let you set the
output Voltage with two resistors and are more stable than the 78XX
regulators.  If you *must* use the 78XX regulators in boosted-
output Voltage applications, here's what I do:

You can use a series resistor from the ground pin to circuit-ground, 
or better a resistor from output to the ground pin, and another from 
the ground pin to ground:

           7805
        ------------
in o----|in     out|-----+-----o out
        |   gnd    |     |
        -----+------     R1
             |           |
             +-----------+
               Iq ->     |
                         R2
                         |
                       -----
                        ---
                         -

The regulator quiescent current (Iq) flows from the gnd pin, through R2
to ground, and is typically 4.75 mA in the 7805. 

The Voltage across R1 will be 5V (for a 5V regulator), and the current 
through it will be 5V/R1.

Set the value of R1 so the current through it is about 5 * Iq, to provide
for some swamping of Iq variations.  This gives a R1 value of:
R = E / I, R = 5V / 23.75mA, R1 = 210 Ohms 
(approx, use 220 for I(R1) = 22.7mA

Choose R2 so that the current through it (Iq + I(R1)) gives the Voltage
drop required to boost the regulator output to the desired value:
Vout = 9V (in this example)
Vreg = 5V (for the 7805)
Vboost = 4V
I(R2) = Iq + I(R1), I(R2) = 4.75mA + 22.7mA, I(R2) = 27.5mA (approx)

R2 = Vboost / I(R2), R2 = 4V / 27.5mA, R2 = 145 Ohm (approx)

If you use a 120 Ohm resistor for R2, you get:
Vout = Vreg + (R1 * I(R2)), Vout = 5V + 3.3V, Vout = 8.3V

Power dissipation in R1 is about 0.114W, in R2 is about 0.133W

All in all, the LM317 is a better bet, though.

-- 
      Paul M. Elliott      Optilink Corporation     (707) 795-9444
            {uunet, pyramid, pixar, tekbspa}!optilink!elliott
 "If I had known it was harmless I would have killed it myself." - P.K. Dick

asd@cbnewsj.att.com (Adam S. Denton) (03/01/91)

In article <5825@optilink.UUCP> elliott@optilink.UUCP (Paul Elliott x225) writes:
>I understand that it is *not* good practice to boost the output of
>the 78XX-series regulators by putting diodes (Zener or otherwise)
>in the ground lead.  Something to do with the short-circuit protection
>(I haven't analysed it myself).

Sounds reasonable; I have never done this myself.  The data books all
use resistors anyway.  Serves me right for brainstorming...!!

>I prefer the LM317-series of regulators.  These let you set the

So do I!  Why I forgot to mention it, I don't know.  The only concern
is that the 317 (like the 78's) requires a minimum differential voltage (the
value eludes me at the moment) which isn't too small (maybe 2-3V).  On
the other hand, the transistor ckt can technically operate with a very
small differential (~0.4V) but that can throw the various part values
way out of whack.  The LM2931 series are good for low dropouts but
can't hack the current (i.e. 100mA is their limit).  Those new
National mini-switchers look pretty nice...gotta try 'em soon.

[good explanation on using resistors with 78XX deleted]

>All in all, the LM317 is a better bet, though.

Yup.  Although I have a vaccuum-tube circuit that's better :-) :-)

>      Paul M. Elliott      Optilink Corporation     (707) 795-9444

Adam Denton
asd@mtqua.att.com

gsteckel@vergil.East.Sun.COM (Geoff Steckel - Sun BOS Hardware CONTRACTOR) (03/01/91)

The whole thread of `how to run X from a automobile battery should be in
a FAQ file.

The LM317 is a good chip for this sort of linear stepdown application.

BUT:
  the automotive alternator/battery/voltage regulator combination can produce
  violent transients, AND
  most power semiconductor devices fail shorted under these conditions

Do you really want to send the 140 V load dump or the 1500 V spike through
your homebuilt regulator into your $200+ portable stereo?  I didn't think so.
(:-)  The monolithic voltage regulator can only do so much.

You want at least a series fuse, a zener/MOV overvoltage protector, and some
RFI/EMI protection - the little microprocessor in a Discman gets confused easily.
Reverse voltage protection is nice, too.  Only about $2 for parts, too.

Could someone who has them accessable (= not under 1000Kg junk in attic) post
references to application notes, etc., describing the automotive electrical
environment?  I will as soon as I find mine, but that might not be soon...

	geoff steckel (gwes@wjh12.harvard.EDU)
			(...!husc6!wjh12!omnivore!gws)
Disclaimer: I am not affiliated with Sun Microsystems, despite the From: line.
This posting is entirely the author's responsibility.

depolo@eniac.seas.upenn.edu (Jeff DePolo) (03/01/91)

In article <4582@eastapps.East.Sun.COM> gsteckel@east.sun.com (Geoff Steckel - Sun BOS Hardware CONTRACTOR) writes:
>The LM317 is a good chip for this sort of linear stepdown application.
>BUT:
>  the automotive alternator/battery/voltage regulator combination can produce
>  violent transients, AND
>  most power semiconductor devices fail shorted under these conditions
>
>Do you really want to send the 140 V load dump or the 1500 V spike through
>your homebuilt regulator into your $200+ portable stereo?  I didn't think so.
>(:-)  The monolithic voltage regulator can only do so much.
>
>You want at least a series fuse, a zener/MOV overvoltage protector, and some
>RFI/EMI protection - the little microprocessor in a Discman gets confused easily.
>Reverse voltage protection is nice, too.  Only about $2 for parts, too.

I think this is getting a bit silly.  First of all, the adapter that
Sony sells doesn't have that much stuff in it - probably a simple
regulator, maybe a filter cap for minor de-spiking, reverse polarity
protection (diode), and a fuse.  That's it.  There is no RFI/EMI protection,
no MOV, _maybe_ a zener for OV.  How much do you think they're going to give
you in a $20 adapter (maybe $5 worth of parts)?

The easiest way I can think of is also the simplest (KISS).  Take a
78H09 regulator, mount it to something (a very small project box),
put a back-biased diode across the 12v input, and a fuse in-line with
the output.  Total cost: $4.00 + cost of cigarette lighter plug.

EMI/RFI - forget it.  An MOV is your only hope of protecting the unit
from high-voltage spikes, but again, I've never seen the need.  Look
at all of the car electronics that don't use them - is everyone else
wrong?  I don't think so.  It's overkill, IMHO.

--
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 Jeff DePolo  N3HBZ             Twisted Pair: (215) 386-7199                  
 depolo@eniac.seas.upenn.edu    RF: 146.685- 442.70+ 144.455s (Philadelphia)  
 University of Pennsylvania     Carrier Pigeon: 420 S. 42nd St. Phila PA 19104