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How is this for a Supply....

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  • How is this for a Supply....

    This is out of a electronics catalogue which has the following points...

    "By adding a PNP power transistor to a positive regulator, the output current can be increased above the normal rating of the regulator itself. The circuit shown (I have re-drawn it) can be expected to to deliver in excess of 4A with the PNP heatsinked."

    So, will this really work using 3x 1 Amp Reg and 3 Power Transistors, which should beable to deliver around 12 Amps?

    System: P133, 64MB Ram, 2x 1.05GB HD, 16x2 LCD, 12 Key Keypad, 300W Inverter, FM Modulator, 45x4 CD.

  • #2
    I beleive you can just use the one regulator, and a whole lot of transistors. i.e. you need one regulator to keep the circuit "regulated"/stable, and then the transistors just sense the regulator output. You could also get a transistor handling more amps.

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    • #3
      And if you supply 12A at 5V with this (assuming a running battery voltage of 14.4V), your power dissipation will be 12*(14.4-5) = 112.8 Watts. You better have one mother of a heatsink...!


      Rob
      Old Systems retired due to new car
      New system at design/prototype stage on BeagleBoard.

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      • #4
        I don't see offhand why that wouldn't work. Although I wouldn't try it for 5V because of what Rob said. But for 12V it should work and you'll be dissipation will be around 29 Watts with the car running. Which isn't to bad.

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        • #5
          Nope, for 12volts it won't work, to run a circuit like that the input voltage has to be higher than that which the regulator would normally use, eg. for a regulatored 12volt output, you would need at least a 15volt input, which, in a car, you can't get, unless you drive a truck or something with a 24volt battery....

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          • #6
            But, this circuit would work for your 5volt power, this is what I am using.....

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            • #7
              Originally posted by THE_SKINNY_MP3_KID:
              <STRONG>But, this circuit would work for your 5volt power, this is what I am using.....
              </STRONG>
              Technically, yes, it will work. But you will need a massive heatsink and fan to keep those transistors cool at 5V.
              Player: Pentium 166MMX, Amptron 598LMR MB w/onboard Sound, Video, LAN, 10.2 Gig Fujitsu Laptop HD, Arise 865 DC-DC Converter, Lexan Case, Custom Software w/Voice Interface, MS Access Based Playlists
              Car: 1986 Mazda RX-7 Turbo (highly modded), 1978 RX-7 Beater (Dead, parting out), 2001 Honda Insight
              "If one more body-kitted, cut-spring-lowered, farty-exhausted Civic revs on me at an intersection, I swear I'm going to get out of my car and cram their ridiculous double-decker aluminium wing firmly up their rump."

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              • #8
                It will work for 12V as well, but it may cut out if you listen much with the car off. Maybe this is just me, but I've had a large stereo in my car for almost 7 years now and I've probably listened to it with the engine off for a total of about an hour. With the car running my alternator puts out almost 14 and a half volts so something like this should work fine for 12V. Although maybe some of you do listen with engines off a lot , then you wouldn't want this.

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                • #9
                  Won't work for 12...
                  Will work for 5...
                  You can try it if you want, but, all that will happen is that your computer will crash because the input voltage is too low for the regulator to regulate properly...
                  It won't generate too much heat with 5volts, if you use a 10amp transistor, then only draw 4-5amps the heatsink will get warm, but not hot....

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                  • #10
                    Actually, I will change what I said, the regulator on its own needs atleast 14.5volts to regulate properly, add a transistor and it would be more like 16volts....
                    IT WON"T WORK!!!!!!!!!!!!!!!!!!

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                    • #11
                      There are some regulators that require 12.5volts minimum input, this is what I am using, add a transistor and the input would have to be 13.5 - 14.5 volts, which is still too high,
                      eg. you have your headlights on, and blast your horn, the voltage drops .5volt, the regulator cant regulate properly and your computer crashes.....

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                      • #12
                        Speaking of Heatsinks and fans.....
                        How would this do????

                        And mount it like this....

                        That is what I have done.....

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                        • #13
                          you sure do ask alot of questions before waiting for answers.

                          the transistor on the 5V rail using the above circuit WILL get hot.... and very hot at that.... even with a P75/P90... if you try that with say a P233+ you will be frying eggs on the transistor/heatsink not powering computers with em!!!

                          your best bet is to look at building a switchmode supply like sproggy's.... www.sproggy.com.... check out the power supply section a few guys there are getting pretty serious about building PSU's...
                          Project - GAME OVER :(

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                          • #14
                            Originally posted by THE_SKINNY_MP3_KID:
                            <STRONG>It won't generate too much heat with 5volts, if you use a 10amp transistor, then only draw 4-5amps the heatsink will get warm, but not hot....
                            </STRONG>
                            It will generate a tonne of heat at 5V. Someone else did the calcs and it was over 100W. That's not a lot of heat? Put your hand on a 100W lightbulb and tell me that's not a lot of heat. Using a 10A transistor will make no difference at all. It will get just as hot.

                            Oh, and that heatsink you pictured looks to be for a PII or PIII. Remember that the power dissipation of these chips is only around 40W (if I remember correctly). So your looking at a heatsink three times as big, or one the same size with three times the airflow.

                            I would highly suggest looking at building a switching supply instead of a linear one.
                            Player: Pentium 166MMX, Amptron 598LMR MB w/onboard Sound, Video, LAN, 10.2 Gig Fujitsu Laptop HD, Arise 865 DC-DC Converter, Lexan Case, Custom Software w/Voice Interface, MS Access Based Playlists
                            Car: 1986 Mazda RX-7 Turbo (highly modded), 1978 RX-7 Beater (Dead, parting out), 2001 Honda Insight
                            "If one more body-kitted, cut-spring-lowered, farty-exhausted Civic revs on me at an intersection, I swear I'm going to get out of my car and cram their ridiculous double-decker aluminium wing firmly up their rump."

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                            • #15
                              Actually, I will change what I said, the regulator on its own needs atleast 14.5volts to regulate properly, add a transistor and it would be more like 16volts....
                              IT WON"T WORK!!!!!!!!!!!!!!!!!!
                              Look at the schematic, the transistor and regulator aren't in series, so you don't need to add the voltages like that. With the transistor like that I'm guessing that Vce would be around 1-1.5V (I'd look it up but I'm lazy ) which with a 14.5V input leaves you 12V at the output. Now unless the 3.3 ohm resistor causes a rather large voltage drop the IC will still have plenty of voltage above 12V to work with because the 3.3 ohm resistor would have to cause around a 1.5V drop to cause the regulator to drop out. I'm not saying that this circuit is a good solution, just that in theory it would work. A switching supply is like what Jeff Mucha made is a far supperior solution.

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