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Voltage Regulator not regulating?

Discussion in 'General Electronics Discussion' started by Shadow351, Sep 8, 2011.

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  1. Shadow351

    Shadow351

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    Sep 8, 2011
    Hello all, I'm new to building electronic circuits and I'm having an issue with my most recent project.

    I'm am trying to build a portable radio to run off my 18 Volt drill batteries. I came up with a simple circuit with a capacitor and 7812 voltage regulator with 2 heat syncs. I hooked it up as shown in the attached image and had the stereo playing. I wanted to check the actual output voltage of my regulator circuit, so I pulled out my multimeter and found the voltage regulator was putting out 17.4 Volts with the stereo on! I unhooked the battery quickly as I didn't want to risk damaging the stereo and thought I'd seek help before I do anything more.

    Am I doing something wrong or is the voltage regulator malfunctioning? Is there a better way to run a car stereo on an 18 V drill battery? thanks in advance

    btw the stereo I'm using is a Pioneer CD-MP3 unit which has an allowable voltage range of 10.8 V to 15.4 V
     

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  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    It sounds like you may have swapped some pins around on the regulator.

    Check very carefully that the input pin goes toward the 18V batteries and the output pin is toward the device you're powering.

    It is also typical to have a capacitor across the output of the regulator (100 to 1000uF)
     
  3. davelectronic

    davelectronic

    1,087
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    Dec 13, 2010
    7812 regulator

    Hi there. Like Steve said sounds like the input to the regulator is going strait to the radio, first pin is input 18 volts, middle pin is ground, third pin is volts out, you do need a decoupling capacitor on the out put, i use decoupling on in and out, find a diagram, work from that, test with a meter until you get the correct voltage, i do exactly the same if i need quick portable power, it works fine if circuit is put together pins the right way round. Dave.
     
  4. duke37

    duke37

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    Jan 9, 2011
    Stating the obvious, check the output voltage before connecting the amplifier.
     
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    Hi Shadow,

    as Steve said sounds like you put the regulator in back to front
    here's a drawing of the 7812 with the pinouts labelled
    so show it as shown with the markings on the device facing you

    pins left to right are IN -- GND -- OUT

    [​IMG]

    also for a battery supply you dont need 1000uF on the input, a 100uF would be ideal and the same on the output :)

    cheers
    Dave
     

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  6. shrtrnd

    shrtrnd

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    Jan 15, 2010
    I'd put a reistor in there, to drop that 18V value on your 12V regulator.
     
  7. Shadow351

    Shadow351

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    Sep 8, 2011
    I double checked that I hooked it up properly and it is hooked up as shown in a previous post as well as the package it came in.I added a capacitor to the output but it didn't help so I removed the capacitor for now (I used 1000 uF because it is what I had). I am getting 18.16 V unloaded out of the regulator and 17.8 V loaded (i used a small 19V motor out of a printer to load it)

    In the picture the small red wire is + from the battery the alligator clip on the heat sync screw is - from the battery the green wire goes to the multimeter. I have also tried a wire to the middle pin instead of the alligator clip on the heat sync screw but with the same results.

    How would you suggest I hookup a resistor to drop the voltage from 18 to 12V? I don't know the impedance of the stereo and I doubt it is constant. I would think just using a resistor would cause major voltage fluctuations, wouldn't it?

    Any other ideas? thanks for the replies.
     

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  8. jackorocko

    jackorocko

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    Apr 4, 2010
    When you mounted the VR to the heat sink you didn't create a short across the input/output did you? If you are not sure, remove the heat sink and test it for a few seconds. But without a load, it shouldn't be dissipating that much power being there is only about 10mA of ground current
     
  9. Shadow351

    Shadow351

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    Sep 8, 2011
    The Heat sync is not shorting the terminals on the regulator, however that made me think about measuring the resistance between the in and out pins on the regulator. I have a 7805 from another project and the resistance between the V in and V out pins is 65K Ohm while the resistance between the V in and V out of the 7812 is only 5.4 Ohm (not 5.4K Ohm just 5.4) I know they are different regulators but this seems too different. Could this be my problem?
     
  10. jackorocko

    jackorocko

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    Apr 4, 2010
    yeah, it sounds like there is a short. Bad IC, something.
     
  11. duke37

    duke37

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    Jan 9, 2011
    These regulators do not like a voltage being placed across them in the wrong direction and a diode is often placed across the regulator to protect it. If you had connected it up the wrong way round it could be popped off.

    Try the 7505 and get that to work, then change to the 7812 again.
     
  12. davelectronic

    davelectronic

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    Dec 13, 2010
    78xx

    Hi again, the data on the chip is small, ive had what i thought was a 7812, and after a lot of head sratching i was using a 7815, ok so that wont give you 18+ volts, but drill batterys can touch 20 volts, and there is i think a 7824 + 1 amp regulator, probably not but an idea. Dave.
     
  13. davenn

    davenn Moderator

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    Sep 5, 2009
    Think you are thinking about some thing else dave ;)

    the OP is wanting to drop 18V down to 12V using a regulator, not go up in voltage.....


    to shadow, definately sounds like you have killed the 7812, try another one and report back :)

    cheers
    Dave
     
  14. Shadow351

    Shadow351

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    Sep 8, 2011
    I didn't hook it up backwards so that's not what killed it maybe the fact that i was asking more than 1A through it is what killed it? I just expected it to shutdown in that case but no big deal, I picked up another one and put it into the same circuit (didn't change anything) and it worked perfectly so I searched for a way to regulate higher current and found the following schematic on http://powersupplycircuit.net/2n3055.html and built it and was listening to music at a nice safe 11.8V thanks for the help
     

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  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    That circuit will work, but as you've seen, the output voltage is not well regulated.

    There are other options which regulate far better, but in your case I suspect that close enough is good enough.
     
  16. Shadow351

    Shadow351

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    Sep 8, 2011
    luckily the stereo allows quite a bit of error (10.8 to 15.1V) (I was mistaken on the 15.4 V I stated earlier) so I think this circuit will work nicely as long as it can deliver the current and the heat syncs are sufficient.
     
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    heat *sinks*

    You now have no current limiting, so be careful.

    If you haven't placed insulation between the devices and the heatsink(s), make sure the heatsinks don't touch anything or each other.
     
  18. Shadow351

    Shadow351

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    Sep 8, 2011
    The Heat sinks (I thought it was sync, my mistake) are a good 3/4" apart and separate. As far as current limiting I was just going to fuse it at 10A (the current rating of the 3055 Power Transistor) Would that be sufficient? Should I add a 1A fuse before the 7812?
    Thanks in advance.
    -Brad
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The transistor would expire long before you got to 10A without a massive heatsink.

    A 5A fuse would be more appropriate, but you would still be at risk of frying the transistor.

    We can work through the reasons why if you like (it's thermal design).

    What current do you actually require? Design something for that.
     
  20. Shadow351

    Shadow351

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    0
    Sep 8, 2011
    I hooked the circuit up and used the 10A ammeter built into one of my multimeters to check the actual current draw. it appears to be just under 1A continuous with peaks of about 1.5A. the 7812 is staying nice and cool as i'm sure very little current passes through it but the 3055 is getting quite warm so I think I'll pickup a bigger heatsink for it. Would one of these be a good choice or should I go bigger?
    http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_326748_-1
    http://www.ebay.com/itm/5pcs-Transi...ltDomain_0&hash=item19c833fb25#ht_2968wt_1185
     
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