In car power supply

[Terry Sare]

     I sent this directly to Mike but will add to list.
     
     
     Current in == Current out
     
     12V --> 5V out means you dump 7V at 800ma across the regulator. This 
     is about 5.6Watts. Efficiency is about 20% on a linear. If you cannot 
     handle that much power loss, you need switching types. Digikey sell 
     power trends for about 18.00 that run about 90% eff. (I went to a 
     power trend seminar the other day and got samples).
     
     Digikey:
     Page 153 of Catalog 974B Power Trend Integrated Switching Regulators
     
     Yes, this is a switching regulator about the size of a 7805T with heat 
     sink. Self Contained, runs about 500Khz or higher. If you run any 
     analog DA or AD, a low pass filter would take care of the noise. They 
     have a web page at:
     http://www.powertrends.com/isr/index.html
     
     For your application, you might want a 1.5 or 2 amp version for a 
     safety margin. About $18.00. a 1 amp version is $13.13 if you don't 
     have any surge problems. Dissipation is around 1W with this part. 
     
     Yes, I figured your real power dissipation was somewhat higher. The 
     13.8 volts is nominal, you also have surges from 60-100V and reverse 
     dumps. You need to protect any part with a diode (1N5406) in series 
     with the input and a TVS to ground (SA20ADI->Diodes Inc) sa20adict-nd 
     digikey.
     
     Terry
     PT5101A/78st105hc 
     in ---- Diode -------------------- reg ----- out
     |      |     |         |
     TVS   .01uF  10uf     100uF
     |      |     |         |
     ground------------------------
     Note: this is off the top of my head -- actual values may vary. Get 
     app notes from power trend for real cap. values.
     
     ______________________________ Reply Separator 
     _________________________________ Subject: Re: 7805
     Author:  DRAINVILLE at paprican.ca at dell_unix Date:    10/28/97 4:48 PM
     
     
     Thanks for the reply.
     You mention 'power trend' in Digikey.
     I saw in Digikey DC-DC converters, is it the same. If not is it a 
     switching type device?
     I'm dissipating 11W.
     When the car is running the voltage is at 13.8V * 0.8AW Mike
     


______________________________ Reply Separator _________________________________
Subject: Re: In car power supply
Author:  owner-diy_efi-outgoing at coulomb.eng.ohio-state.edu at dell_unix
Date:    10/28/97 4:46 PM


> Hi everyone. This is my first post to this list. I'm building a
> dynamometer and performance analyser for my car, using a Atmel 8952. 
> I'm now in the power supply section and faced to the heat dissipation 
> problem. I'm not using a DC-DC converter because of the price.
> I tried to use a 7809 and 7805 together and also a 7805 with a bypass 
> transistor but both setups generate a lot of heat. (7 to 8 watts).
> My circuit draws about 800ma including LCD and backlight.
     
Try this:
take about 7 7805's and parallel them.  IT would seem at first that they 
would tend to fight over each other having their outputs tied together, 
but they don't and tend to stay very well behaved.  IF your worried 
problems use a .1 ohm resistor from the output to the "tie point", to keep 
them all balanced.  The load will be split among all the 7805's, and the 
heat should be a bit easier to handle. (heat sink them if necessary)
     
This is documented in Popular Electronics and I have tried it myself and 
it DOES work, even though one might think that the outputs would try to 
feed into each other.
     
Dave