L7812CV 12v Voltage Regulator Dropping Down To 3V

Discussion in 'General Electronics Chat' started by gatekeeperamps, Mar 18, 2013.

  1. gatekeeperamps

    Thread Starter New Member

    Sep 17, 2012
    7
    0
    Hello,

    I try not to bother anyone with any issues I have or unless I am completely against the wall with an issue.

    I have built a Plug In The Wall Linear Ham Amplifier with a Non-Regulated 200amp Supply which is besides the point but I am using 6x12v LED 120m Fans on the top for Cooling.

    I have an exact replica of this Amplifier already and the builder used 3 X 7812 Voltage Regulators using 1 Regulator Per 2 Fans(Having The Regulators just on slide on heat sinks that kind of look like Money Clips in a sense)

    HERE IS A SUMMARY OF THE INFORMATION AND PROBLEM:

    Setup:
    USING 3 X L7812CV 12v Voltage Regulators
    Input Voltage is 19v To Each Regulator
    There are 2 X 12v LED Fans on each Voltage Regulator.
    Regulators are on a decent size heat sink(Way Bigger Then the one in my other project)
    I have a 103(10,000pf) 3KV Ceramic Disc Cap on each V. Reg.[One Leg on output Leg and other Leg of Cap on Ground Leg of each V. Reg.]

    Problem:
    Voltage Regulators are dropping to around 3v which isn't enough to even spin the fans. Only to light up LEDs on fans very DIM

    Tested just 1 Fan on 1 Voltage Regulator and Voltage Drops to around 9v(Which is very very Weird to.)

    2xFans only pull .760a on the 19v supply with supplying dropping to 17.9v
    2xFans only pull .545a on a 12.52v Supply(Which I guess is what it would be pulling through the Voltage Regulator Right?

    I have a total exact replica of this same project that are using KA7812 1amp Voltage Regulators and it works PERFECT! Only other difference is they are using 104(100,00pf) Ceramic Caps instead of 103 like me.

    I'm using L7812CV 1.5amp V. Regulators and look at my problem!!!!!!


    ...............................................................................................
    VERY DETAILED INFORMATION IS BELLOW IF NEEDED!!!
    ```````````````````````````````````````````````````````````
    The reason for using these are:

    1. These are 12v Fans and the Internal Power Supply Floats at 18-19v.
    2. When the Amplifier is in use, the Voltage Drops to around 15v which if wasn't using a Voltage Regulator the Fans would Slow Down. Slow Down, then Speed up...Slow Down then Speed up after unkeying the amplifier....you get it.... which is unprofessional. You want the fans to stay at a constant speed.

    H E R E I S T H E P R O B L E M:
    After Wiring Everything, Putting a 103(10,000pf) Ceramic Cap On Output Pin on each Regulator and other Leg to Ground Pin(To Ground):

    All 6 Fans's LEDs CAME ON VERY DIM AND THE FANS DIDN'T SPIN!!! :(

    So I checked the Output of the Voltage Regulars and each Output was sitting at around 3v. (Which I've checked these fans and they need about 4v to even spin)

    So I did some troubleshooting, make sure everything had a very very good ground.

    So then I ask my self...is there a way these fans are pulling to much current.....no way....my other project has 2 Fans on each Voltage Regulator.

    So I checked, just 1 Fan on 1 Voltage Regulator and the Voltage Dropped to about 9v or so......THEN I WAS REALLY REALLY CONFUSED!!!!!! WHAT IS GOING ON HERE!!!!

    So Then I did a Amp Reading on the Fan. Here is what I got, which I now is weird but this is what I got.

    .............................................................................................
    First Reading was using 3X Big Marine Batteries In Parallel(Should be charged decently):
    .............................................................................................

    Batteries Standing Voltage: 12.52v

    1 X Fan = .285ma (Voltage From Batteries Dropped to 11.82v)
    2 X Fans = .545ma (Voltage From Batteries Dropped to 10.97v)
    [The Voltage Drop is Weird but maybe the batteries are weak, I have had them in Parallel for about 2 months and have used them for testing and such]

    Then I took 2 of the same Fans that I didn't care about and tested there amp draw from the Power Supply the fans will be running off of which the Internal Power Supply of this Amp Floats at exactly 19.2v. Here is the data:

    Power Supply Standing Voltage: 19.2v

    1 X Fan = .384ma (Voltage From Power Supply Dropped to 18.30v)
    2 X Fan = .760ma (Voltage From Power Supply Dropped to 17.90v)

    I understand more Volts Less Amps but I'm guessing that the 3 Batteries are just needing charged because once these fans got more Voltage from the Power Supply they pulled more Amps then on the batteries.

    Note: I only hooked fans up for a few seconds so they wouldn't be damaged from the Higher Voltage.

    So, I did that test to see what they would be pulling. And it is no where near the Max of Each Voltage Regulator?

    So why is 1 Voltage Regulator Dropping to around 9v with only 1 Fan On it, and why is 1 Voltage Regulator Dropping to around 3v with 2 fans on it.

    .............................................................................................
    NEXT THING I CHECKED WAS THIS, WHICH I HAVE A ? ABOUT ALSO
    .............................................................................................

    Then I was curious of what kind of Voltage Regulators were being used in the Amplifier that works perfect.(I have had the Fans on the other amp running for hours and never had an heat issue or anything)

    I also checked the fans on the other Amplifier and there amps pull were .400ma per fan, so there fans are pulling even more current then the ones I'm using.

    SO CHECK THIS OUT:

    I LOOKED AT THE VOLTAGE REGULATORS BEING USED ON THAT AMPLIFIER FOR THE FANS AND THEY ARE THIS:

    KA7812 12v Voltage Regulator and only have a capacity of 1A

    I am using L7812CV Voltage Regulators and are 1.5A

    I checked both DATASHEETS and couldn't see a difference really.

    The only Difference I saw in the other Amplifier was that they were using 104(100,000pf) Ceramic Disc Cap on the Output of each Regulator and I am using 103's but I've read I can use either or.

    So my back is up against the wall, what am I doing wrong here??????????

    I am sorry for writing so much but I just wanted to make sure I have all the information so that there are no questions.

    SOMEONE PLEASE PLEASE PLEASE PLEASE HELP! IT WILL BE THE BIGGEST BLESSING, THIS IS FOR A FRIEND OF MINE THAT HAS BEEN WAITING FOR ABOUT 6 MONTHS ON THIS.

    THANKS SO MUCH!
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Here are possibilities that come to mind.

    The LM7812 datasheet calls for a .22 μF ceramic cap on the input and a .1 μF cap on the output pin. There is no need for these capacitors to be rated in the kv range; 50 VDC is fine. It doesn't seem likely this is your problem, but it's a possiblity.

    What is the output voltage on the LM7812 with no load, that is without the fan? It should be at 12VDC plus or minus .2V.

    Does this problem occur during transmitting (RF problem) or all the time.

    Are you sure you have the 7812 wired correctly? The input is on the left as you look at the front of the IC, the ground is in the center, and the output in on the right.

    Can you try running one of the fans on an external power supply, like a 12V wall wart and see how much current each fan draws?

    It's possible your 7812's are defective. Have you tried changing one out? I have bought some ICs that were defective and some that were mismarked.
     
  3. Meixner

    Member

    Sep 26, 2011
    116
    21
    What do you have on the input of the voltage regulators when you connect the fans.
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    Not exactly related to your problem, but more "Volts less Amps" would only be true for a constant power at the load. A fan will consume more current at higher voltages. An SMPS as load would consume less current at higher voltages.

    Since nominal current consumption of the fan seems to be 300mA at 12V I would first try to connect an equivalent resistive load instead of the fan. That's 40Ω/3.6W. Take any combination of resistors to achieve the desired current/power rating.

    My idea is maybe the initial inrush current of the fan could be a problem? That also means you cannot replace the test resistor with a light bulb (because they also have higher initial current)
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Where did you purchase those L7812CV regulators? If it was from someone at a hamfest, online auction, or other non-factory-authorized outlets, you may very well have counterfeit parts. Counterfeit parts have become a very real problem nowadays.

    However, you first need to use caps on both the inputs and outputs of the regulators as Tracecom mentioned. You can use ceramic or poly metal film caps. If you want to use aluminum electrolytic caps instead, then use 10uF to 220uF or so for both caps, keeping the length of wiring from the caps to the regulator terminals as short as possible.

    I have seen 3-terminal regulators oscillate in the MHz range; when they oscillate, the output voltage goes way down. Small capacitance combined with parasitic inductance of the wiring creates a resonant circuit and resulting regulator instability; adding large caps on the inputs and outputs swamps the oscillations; it slows the rise and fall times enough that the regulator can keep up with the changes.
     
  6. PackratKing

    Well-Known Member

    Jul 13, 2008
    850
    215
    Hi Sarge ! Long time no talk...
    Curiosity. I have yet to encounter the problem in anything I've built, tho' they mention a component oscillating.

    Is this a physical vibration that can " shake the component apart " or only something that shows up on an o'scope ?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hiya PRK; I've been real busy. I'm still busy!

    It's not a physical vibration; these oscillations are much higher in frequency, and you'd need an O-scope to see it.
     
    PackratKing likes this.
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    I had a similar problem running DC fans off a 7812 and it turns out the load characteristics of the fan are squirrely and make the regulator oscillate. I put a fat electrolytic cap on the reg output and it cured the problem.

    Anything like 100uF or larger is good.

    Look at the output with a SCOPE not a meter and see if there is some kind of hash on it.
     
    PackratKing likes this.
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