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Regulated 5v power supply

  1. Mar 1, 2006 #1
    Hi, this might not be electrical engineering, but I found no other forum to question about this.

    I'm trying to get a 5V regulated power supply from a 12V, 300mA transformer. I can't seem to get it to work. I'm using a 5V regulator, two 470 microfarad capacitors and electronic wire in a solderless breadboard. From the 12V transformer I measured 12.30 volts once it was plugged in. Probably it became more because I snipped off the connector and some copper wire. I plugged the negative lead of the transformer into the bottom strip of the breadboard, and the positive lead into the F4 socket. I'm trying to get the regulated 5V power on the top strip of the breadboard. This is my current configuration:

    • J4,5,6 = 5V regulator chip (input, ground, output)
    • I6 to top strip = wire
    • F1 to E3 = wire
    • A3 to bottom strip = wire
    • A7 to bottom strip = wire
    • E5 to F5 = wire
    • E7 to F6 = 470uF capacitor
    • G1 to G4 = 470uF capacitor
    • F4 = positive 12V lead from transformer
    • Bottom strip = negative 12V lead from transformer

    When I put test leads into the bottom strip and any socket on row 4 I get 12.30V, but when I put test leads into the bottom strip and the top strip I get 10V which climbs steadily by hundredths. I'm not sure what that means, but I want a 5V regulated voltage on the top strip. Can you please help me on how to do this?
     
  2. jcsd
  3. Mar 1, 2006 #2

    berkeman

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    What do you mean by the positive and negative leads of a transformer? A transformer by itself is an AC device. Does your transformer assembly include a full-wave bridge rectifier to give you pulsed DC?

    Can you post a sketch of how you are hooking everything up?
     
  4. Mar 1, 2006 #3

    Cliff_J

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    Put a 10k ohm resistor on the regulator from output to ground and measure the voltage across the resistor and see if it is indeed 5V.
     
  5. Mar 1, 2006 #4
    Ya, the transformer takes the 110/120 VAC and regulates it down to 12VDC. I'll try to get a sketch. I'm trying to learn all the electronic stuff. Does negative and ground mean the same thing on a solderless breadboard?

    [​IMG]
     
    Last edited: Mar 1, 2006
  6. Mar 1, 2006 #5

    berkeman

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    What do you mean by 12Vdc out of the transformer? Does it say that? Does it contain an output diode bridge and smoothing capacitor? When you measure the DC and AC output components from the transformer, what do you measure for each?

    As to the negative rail on the breadboard, the answer depends. If you have a single rail power supply, then yes, negative = ground. If you have split supplies, then you will have + something and - something, typically centered around the ground rail. Like, it's common to use +/-12Vdc for opamp circuits, with a center reference rail at ground.
     
  7. Mar 1, 2006 #6
    This is exactly what it says on the transformer:
    PLUG IN CLASS 2 TRANSFORMER
    MODEL: HHD12-300
    INPUT: AC120V 60Hz
    OUTPUT: DC12V 300mA
    POLARITY: - C +
    DATE: 0506
    MADE IN CHINA

    Measured V DC: 12.29
    Measured V AC: 35, 13, 5, infinity (alternating)
     
  8. Mar 1, 2006 #7

    berkeman

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    Okay. The AC 35, 13, 5 numbers are probably in millivolts, so it looks like the wall transformer unit has a pretty steady DC output at no load.

    So now all you need to do is connect your 5V regulator chip to the 12V input in order to make the 5V output. Are you using an LM340 (or LM7805 equivalent)? Which IC package is it in? Are you sure you are using the correct pinout for it? Where are you putting the 470uF cap?
     
  9. Mar 1, 2006 #8

    chroot

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    He's using the word "transformer" improperly, using it to refer instead to an AC/DC adapter.

    The real question, of course, is which specific 5V regulator are you using? There hundreds (if not thousands) of 5V regulators on the market.

    - Warren
     
  10. Mar 1, 2006 #9
    The 5V reg. chip is L7805CP, maybe i'm not using the correct "pinout"? Where would I find that? With the chip facing me I have the 12V input on the left, ground(neg) in the middle and 5V output on the right. I have one capacitor on the same 5-socket row as the 12V with the other end of the cap connected with ground, and the other cap connected to the same 5-socket row as the 5V with the other end of the cap connected with ground. The negative terminal on the caps are connected to ground.
     
  11. Mar 1, 2006 #10

    berkeman

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    That's the correct pinout for the TO-220 package of the LM7805. But the pinout for the TO-92 version is opposite of that. You can find the datasheet for these regulators online many places. Even searching digikey.com can often get you to the part datasheet. The rest of your connections sound correct at this point. I agree with the earlier post about adding a little load to the 5V output. The 7805 regulators generally won't need it, but many voltage regulators have a minimum load current to keep their output in regulation.
     
  12. Mar 1, 2006 #11

    chroot

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    I can't really follow your "connection list," either. From the looks of it, there's no path from, for example, J5, the regulator's ground, to the negative terminal of the AC/DC adapter.

    Yes, "ground" usually refers to the negative terminal of an arbitrary power supply. All other voltages in the circuit are thus measured with respect to the negative terminal of the power supply.

    You'll need to connect the supply across the input (input and ground terminals) of the regulator, in parallel with one capacitor. The output (output and ground terminals) of the regulator should also have a capacitor placed across them. The output of the regulator is measured at its output pin.

    - Warren
     
  13. Mar 1, 2006 #12
    Yes, there is a connection from the reg. to ground, I forgot to include that wire in my sketch. I knew I ordered 10 330-ohm resistors with 2 prongs, instead I got 10 100K-ohm resistors with 10 prongs, are they the same thing?
     
    Last edited: Mar 1, 2006
  14. Mar 1, 2006 #13

    berkeman

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    By a 2-prong resistor, I assume you mean a traditional axial package? And by a 10-prong resistor, I assume you mean a SIP resistor array? Those arrays can either be 5 separate 2-connection resistors, or 9 resistors with one common connection. Look up the part number to figure out which resistor array configuration your SIPs are.

    Looking at your sketch earlier, and adding a ground connection on the voltage regulator seems like it should work. At least if the LM7805 is in the TO-220 package. Does it still not work? If not, maybe clean up the sketch a bit, and add annotations for the voltage that you read at each point in the circuit with respect to the - (ground) line. That will help us to debug the problem.
     
  15. Mar 1, 2006 #14

    berkeman

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    BTW, one "gotcha" that messes up every EE student at least once is the break in continuity in the outside bus bar nets in many prototype plug boards like the one you sketched. Typically, the continuity is only carried through half of the bus bar length down the edges of the proto plug board, and you have to bridge the break if you want your bussing to continue for the full length of the proto board. Maybe beep out the continuity of your proto plug board to understand what sets of holes are actually connected to each other....
     
  16. Mar 1, 2006 #15

    chroot

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    I have no idea what you're talking about with the resistors. Do you mean you have a "resistor pack," which has 5 resistors in one package?

    - Warren
     
  17. Mar 1, 2006 #16
    a sip resistor package on a busbar breadboard? :confused: :rolleyes:
     
  18. Mar 1, 2006 #17

    berkeman

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    Well he called it a "solderless breadboard", which makes me think of the small 0.1" array plug boards. And SIP resistors will fit fine on those proto boards. But come to think of it, a TO-220 package LM7805 will not plug into that kind of small plugboard, at least not without damaging the proto board for future use. We need pictures!
     
  19. Mar 1, 2006 #18
    for some reason i was thinking of a smd resistor array :tongue2:, to-220 will fit, to-92 needs the leads bent.
     
  20. Mar 1, 2006 #19

    berkeman

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    Yeah, but the TO-220 leads are pretty broad, so if you stick them in the wrong way, they bend the spring contacts out enough that the contacts won't be tight for normal-size leads from then on. It might be possible to jam in the TO-220 leads the other way, so that the flat part of the leads lines up with the slot in the spring fingers, but that might also glob up the plastic of the proto board (I'm not sure). Worst case, the OP can solder short wire leads on the TO-220 leads to help him plug the voltage regulator into the plug board.
     
  21. Mar 1, 2006 #20
    Well I got it to work, it reads 5V now, but I think I burned out an LED in the process :). My meter read 10m-ohm resistance for the LED, and 0 for the resistance between the LED and resistor. The part number for the resistor is 30867 at jameco, but I get 100K-ohm SIP arrays instead of those single 330-ohm ones, and I cannot find the pinout. My temp. probe read 40degC for both the regulator and the LED, is that normal?

    Also, how hard is it to make an ALU with TTL chips?

    EDIT: I measured the resistance for one of the 5 resistors on the SIP and it measured 100.5K-ohm. This disappoints me because that is not what I ordered, and they put the part number of the 330-ohm on the bag of the SIPs

    The breadboard I have is P/N 20722 at jameco, and the 7805 fits fine.
     
    Last edited: Mar 2, 2006
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