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Dramatic voltage drop at voltage regulator output

  1. Apr 14, 2014 #1

    jim380

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    First time posting a problem on here. Be gentle on me :)

    So I ran into this problem when I was testing the power supply block for my senior project.I attached a pic (https://7b6aa69e-a-59896e8d-s-sites...xn8zXySvNTzHjn4QovXQMIMjf3Sfs=&attredirects=0)showing what my design looks like, and specs that need to be met at each output. The problem was that as soon as I hooked up the servo motor to the entire circuit, the voltage at the green node (output of the 6v voltage regulator) immediately got dropped to around 2 volts. Also, the voltage at blue node as well as the one at purple node got dropped, too. Down below is what my testing procedure looks like:

    1. Set up the circuit and everything. (12v battery*1, 5v voltage regulator *1, 6v voltage regulator *1, and a couple push-to-on switches)
    2. Hook up display to the circuit. ----Everything Works fine
    3. Add micro-controller into the circuit ---- Still fine
    3. Hook up servo motor to the circuit -----BOMB! Problem happened

    I have been stuck at this problem for a while. Any help would be greatly appreciated. Thanks in advance.


    Datasheets:
    5v voltage regulator: https://docs.google.com/viewer?a=v&...HV8ZWNlNDR4MjAxMzM4fGd4Ojc2NjQxNzJlOGU1ZDEwZQ
    6v voltage regulator: https://docs.google.com/viewer?a=v&...V8ZWNlNDR4MjAxMzM4fGd4OjcxODQ5YzRiNDliNDRlNDU
    Micro-controller: ATMEGA32U4
    Servo Motor: https://docs.google.com/viewer?a=v&...V8ZWNlNDR4MjAxMzM4fGd4OjQ2ZDA4ZGNjMzM2NzYyYTY
     
  2. jcsd
  3. Apr 14, 2014 #2

    analogdesign

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    It looks like the issue is the 6V regulator is shutting down. I presume that if you hook up the servo motor before the display or ucontroller is fails, correct?

    It could be any number of things but I suspect the 6V regulator is getting too hot since the servo motor is pulling half an amp. Have you installed a heat sink for the 6V regulator? Does the circuit work when the servo motor is idling (not engaged?)
     
  4. Apr 14, 2014 #3

    Baluncore

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    Welcome to PF.
    1. The regulators need a ceramic capacitor from ground to input and from ground to output. See the test and example circuits in the data sheets. Without capacitors the circuits will oscillate.

    2. You need to heat sink your regulators. Do they get hot?

    3. Is the 12V battery healthy?
    Measure the battery voltage when load is connected, if it falls under load it explains your observations.
     
  5. Apr 14, 2014 #4

    jim380

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    first of all, thanks for your quick reply.
    yes. the 6v voltage regulator is thermal protected, so it shuts down when it's getting too hot. the weird thing is, even if I only hook up a 6v voltage regulator and the servo motor with the battery, the same problem still happens. the voltage regulator gets very hot, and it shuts down by itself.
     
  6. Apr 14, 2014 #5
    My first thought is that the servo circuit/connection is drawing too much current. What happens if you connect the servo power up by itself? Have you put a resistive load on the 6V output to verify it will provide the current you need? If you want to check the 6V by itself 6v/400mA = 15ohms, so load it with a 15 ohm resistor. Or can you grab a lab power supply with current limiting and bypass the 6v regulator you are using?

    Try to isolate the problem, but my guess is that the main power exceeds your available current and it is limiting. Also when you connect the servo, it is usual for the motor to seek for a few milliseconds, so you should hear it start to turn briefly even if the power pulls back.
     
  7. Apr 14, 2014 #6
    Have you tested the servo elsewhere? Do you have a different one? Are you sure your connection is not shorting out the voltage? Or maybe you have the + & - of the servo reversed.
     
  8. Apr 14, 2014 #7

    jim380

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    1. I thought caps are not necessary in my case. Also, I think the circuit example shown in the datasheet is just for demonstration purposes. I'll try adding caps anyways.

    2. They indeed got really hot, especially the 6v one, which is connected to servo motor.

    3. good point. I will report back once I get to measure it.
     
  9. Apr 14, 2014 #8

    jim380

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    Exactly what I thought when I first ran into this problem. A couple things I've tried:
    1. Directly power up the servo motor using 4 AA batteries----> Motor works fine.

    2. I believe I've tried putting resistive loads (ranging from 2-10 ohms) on the 6v output. But it's like a couple weeks ago, so I don't recall any specific data at this moment. What I do remember is that the voltage regulator shut down itself at some point so the current measured was almost unnoticeable.

    3. I do have access to a power supply with current limiting setting. And I already tried that. What I did was that I set the current limit at 0.6A first, and then tried monitoring the output voltage at the 6v regulator as I was slowly increasing the current. It turned out that the output voltage got dropped dramatically when the current is around 0.5A. I assume the 6v regulator just shuts down itself when the current exceeds 0.5A.
     
  10. Apr 15, 2014 #9

    jim hardy

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    Baluncore and analogdesign have figured it out.

    In that datasheet you linked take a look at "Table 2 Thermal Data" on page 2.

    How much power is your regulator trying to handle? If it drops from 12V to 6V which is 6 volts, X a half amp, that's 3 watts.

    Table 2 says that at 3 watts, even the hefty TO220 package will heat up by 150 degrees C above room temperature. That puts it above its operating range Top in Table 1 just above.

    It is certainly entitled to. A good heatsink should help a lot.
     
  11. Apr 15, 2014 #10

    Baluncore

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    I suspect you set the power supply voltage to 12V and the current limit on the high side. Then as you gradually reduce the current limit you will reach the point where the current required is equal to the current limit and the power supply voltage will fall very rapidly in order to limit the current to the lower value set. Once the voltage falls below the drop out voltage of the 6V regulator, (about 8V), the regulator will begin to fail and so it's output voltage will fall.

    Your regulator is a current sink, the power supply is a current limited source. In effect the currents are being compared, the voltage on the 12V output will change very rapidly depending on their relative magnitude.
     
  12. Apr 28, 2014 #11

    jim380

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    Sorry for putting you back on this question, but I'am still suffering from the voltage drop issue. I already tried installing a heat sink on the 6V regulator. When it's unloaded, the output voltage is almost exactly 6V. But when I put a load (a 12 ohms resistor, which is supposed to be equivalent to the resistance of the servo motor I use) onto it, the output voltage drops to 1.7V-ish, which also makes the output current extremely small. Here is how I tested it:

    A 12V battery is hooked up to the 6V voltage regulator, and then a load resistor is placed between the output of the regulator and common ground. That's it!

    I wonder what is causing this problem. Please help me out. Any suggestion will be greatly appreciated. Thanks in advance.
     
  13. Apr 28, 2014 #12

    jim380

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    Sorry for putting you back on this question, but I'am still suffering from the voltage drop issue. I already tried installing a heat sink on the 6V regulator. When it's unloaded, the output voltage is almost exactly 6V. But when I put a load (a 12 ohms resistor, which is supposed to be equivalent to the resistance of the servo motor I use) onto it, the output voltage drops to 1.7V-ish, which also makes the output current extremely small. Here is how I tested it:

    A 12V battery is hooked up to the 6V voltage regulator, and then a load resistor is placed between the output of the regulator and common ground. That's it!

    I wonder what is causing this problem. Please help me out. Any suggestion will be greatly appreciated. Thanks in advance.
     
  14. Apr 28, 2014 #13

    jim380

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    Sorry for putting you back on this question, but I'am still suffering from the voltage drop issue. I already tried installing a heat sink on the 6V regulator. When it's unloaded, the output voltage is almost exactly 6V. But when I put a load (a 12 ohms resistor, which is supposed to be equivalent to the resistance of the servo motor I use) onto it, the output voltage drops to 1.7V-ish, which also makes the output current extremely small. Here is how I tested it:

    A 12V battery is hooked up to the 6V voltage regulator, and then a load resistor is placed between the output of the regulator and common ground. That's it!

    I wonder what is causing this problem. Please help me out. Any suggestion will be greatly appreciated. Thanks in advance.
     
  15. Apr 28, 2014 #14

    jim380

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    Sorry for putting you back on this question, but I'am still suffering from the voltage drop issue. I already tried installing a heat sink on the 6V regulator. When it's unloaded, the output voltage is almost exactly 6V. But when I put a load (a 12 ohms resistor, which is supposed to be equivalent to the resistance of the servo motor I use) onto it, the output voltage drops to 1.7V-ish, which also makes the output current extremely small. Here is how I tested it:

    A 12V battery is hooked up to the 6V voltage regulator, and then a load resistor is placed between the output of the regulator and common ground. That's it!

    I wonder what is causing this problem. Please help me out. Any suggestion will be greatly appreciated. Thanks in advance.
     
  16. Apr 28, 2014 #15

    Baluncore

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    What does the battery and regulator input voltage do when you connect the 12 ohm resistor to the output of the regulator?
     
  17. Apr 28, 2014 #16

    jim380

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    They were around 12V, as they should be.
     
  18. Apr 28, 2014 #17

    Baluncore

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    If the battery voltage does not also fall drastically then either the regulator circuit is incorrect or the regulator device is faulty.
     
  19. Apr 28, 2014 #18

    jim380

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    what if the battery voltage (or the input voltage at the regulator) does drop drastically?
     
  20. Apr 28, 2014 #19

    Baluncore

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    If it falls then the battery is dead, probably an open circuit connector or a dry cell.

    Are you are definitely using a 7806 regulator chip?
    Have you checked your pin numbers? Input, ground and output.

    Turn off the power switch on the B+ lead.
    Measure the resistance between the common ground and the load resistor ground.
    Measure the resistance between the common ground and the battery ground terminal.
     
  21. Apr 28, 2014 #20

    jim380

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    Thanks. I'll let you know as soon as I get to check it.
     
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