voltage zero when load is connected

Discussion in 'The Projects Forum' started by ksp1717, Mar 19, 2012.

  1. ksp1717

    Thread Starter Member

    Oct 30, 2008
    13
    0
    Hi all

    I have the circuit as shown in the schematic. There are two voltage regulators 5V and -5V both of 1.5A. I have a D/A converter(MCP4725) which takes works between 0-5V. I am using it in bipolar operation, so I have an opamp UA741IDT the output of which is connected to a pair of PNP/NPN transistors to amplify current in both polarities. The final voltage is passed through a current sensor (ACS712) and finally to a load. (arm of a hard drive). I have two power sources a 12V, 1.5A which is connected to voltage regulators and a 5V, 30mA source powering only the D/A and current sensor.

    The D/A is damaged so, to test the remaining circuit I have connected the output of a working MCP4725 (which I have separately) to the one on the board. I did not give the 5V input so the current sensor is not powered.

    Now my problem is when I measure the voltage at the output terminals(named "Coil" in the schematic) it is behaving the way it is supposed to (sine wave -1.5 to 1.5 V). But when I connect the load I measure only 0V. I tried connecting just a simple 10K resistance across the terminals and it is still the same.

    I am not sure what the problem is. Is it because I have a current sensor and it is not powered up. I can try powering it up but I have to either power both the sensor and the faulty D/A or none.

    Any suggestions are greatly appreciated.

    -ksp
     
  2. ksp1717

    Thread Starter Member

    Oct 30, 2008
    13
    0
    I have got past that particular problem apparently some of the components are burned. So I had to replace them and now things are working and the voltage does not die when I connect a load.

    However now I am having different problem. The output is not behaving like it is supposed to. My intention is to have the output voltage to follow a sine wave but the output is being random (erratic voltages above and below zero)

    Can some one please look at the schematic I have attached and tell me what is wrong with it or why is it behaving like that ?

    Thank you for your help

    --ksp
     
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    Can you please bring your schematic into a more readable format?

    Also, use the right symbols (OPAMP 741 for example), not the IC package symbols.

    Where is the collector of the right transistor connected to?
     
  4. ksp1717

    Thread Starter Member

    Oct 30, 2008
    13
    0
    I am sorry, I have updated the schematic with some text in it. The collector right transistor is connected to +5V,1.5A voltage regulator.

    Thanks

    --ksp
     
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    I meant more like this:
    [​IMG]
    but it's still confusing. It's not unreadable what you did, it just takes more time to read it.

    Normally signal flow is from left to right, positive voltages at the top/pointing up, gnd at the bottom/pointing down.

    What did you draw it with? Maybe you can post the original schematic file? :rolleyes:

    I don't have time now , maybe later.
     
  6. ksp1717

    Thread Starter Member

    Oct 30, 2008
    13
    0
    Hi thanks for your reply. I am not used to making circuits and drawing schematics. I did it in Diptrace. I am also attaching the related schematic.

    Thanks

    --ksp
     
  7. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    Hi,

    I redrew your schematic. See the attached pdf.

    Maybe you can tell us what this circuit is supposed to do.

    I did not have a deeper look into it but at first glance there are missing base resistors and a connection to pin 4 of the negative voltage regulator. (the resistor)

    I hope someone else can help you more with this because my time is quite limited now.
     
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