Wire wrapped Digital voltmeter project

Discussion in 'The Projects Forum' started by mentaaal, Mar 19, 2008.

  1. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Hey guys, we have been given this project to do for an assignment this semester. It is a digital panel meter which can display a unipolar voltage between 0 and 30V DC.

    The info on the project as well as a schematic diagram are included in this post.

    This is the problem that i am having:
    When the two ICs are removed from their holders, if i put 7V DC into the circuit, the output voltage from the 78M05CT regulator is exactly 5 volts which is normal but when i plug back in the ICs (well the one that causes this problem in particular is the CA3162E.) the voltage falls to 3.2 VOlts. I am not sure if i was supposed to do this but i increased the supply voltage to say 12 volts (my lecturer did say that this was safe enough) and then the output voltage from the regulator started pulsing around 4 volts. The current meter on my power supply was also pulsing in time with this. The Leds were also flashing in the same fashion with garbage outputs. I have checked the wiring maybe 5 times now and cant seem to figure out whats going on.

    Can anyone hazard a guess as to why the regulator's voltage is dropping like this? When the output voltage falls the current meter is still saying that there is very little current out so i dont think there is a short? (unless the regulator has an inbuilt short circuit protection?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    There is a possibility that your regulator may be having problems due to no bypass capacitor. For stability, a cap should be right there at the output pin to ground. About .1 uF should be ok. Placing 2 uF right at the input won't hurt, either. Three terminal regulators need caps coming and going, placed as close to the input and output pins as can be managed.

    Doing some heat sinking on the 7805 won't hurt, either. Those displays draw current, and could cause the 7805 to heat and go into shutdown.

    When the regulated voltage drops, is the source voltage stable? If it's falling below 7, then that might be the cause of the problem. If the 7 volts is holding, and the 7805 gets hot, then the load is probably too much. That 3162 may be defective.

    Are you certain there is no error in the wiring? Especially around the 3162 - some bridge between two pins that would cause a large current flow?
     
  3. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Hi beenthere, thanks for the speedy reply, well i have bypass capacitors on both sides of the regulator. I do have however, the input bypass capacitor connected to one of the diode pins which is connected to the voltage regulator. The capacitor is a few cenitmetres away from the regulator. Could this make a difference?

    In answer to your question about possible bridging, this is what i cant understand. I was thinking that for the output voltage to drop as it does, perhaps there is a short somewhere in the circuit. But the current meter only starts fluctuating when i increase the voltage to over 10 volts. Does the regulator have short circuit protection?

    I will check the supply voltage and let you know.

    Thanks for your help so far ;-)
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Yes, 78XX regulator do have short circuit protection. What is the RMS voltage feeding the input, and what is the size of the filter capacitor?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    What current are you drawing from the 7V supply?
    The 78M05 is rated for 500mA, but regulation will be somewhat dicey above 200mA, and the practical limit is around 350mA.

    It's a bit difficult to tell, but it looks like you may have the 78M05 wired backwards.
    With the lettering on the 78M05 facing you so that you can read it, Pin 1 (IN) is on the left, then Pin 2 (GND) and Pin 3 (OUT)
    (c101 and C102 connections are OK for the moment - found D101; was hard to see in the photo)
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Ok, double-check the connections on the output of D101. (I finally found it :rolleyes:)

    There is a wire going to C101, and also a wire roaming off somewhere. It's difficult to tell; is that roaming wire actually going to pin 1 of IC101?
    It would be better if there was a direct connection between + C101 and IC101 pin 1 rather than depending on the output connection of D101. There is a small amount of inductance involved; although really too small to make much difference here, it'll come into play when you start building high speed stuff.
     
  7. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    The rms voltage going into the regulator is between 7-8 volts. (I have a 0-30 volt variable voltage supply so whatever i select it too really.)

    Ok i measured the current coming out of the supply and it is around 17mA (That is with the supply feeding in around 7 volts. This current was measured at the input supply.).

    Just let me know if the images i posted arent sufficient or clear. If you can see, there are two cables coming from the negative pin of the diode. One goes to an electrolytic 10uF capacitor and the other cable goes to the voltage in of the regulator. The white cable coming from the vout of the regulator goes to another capacitor of the same type as the previously mentioned one.

    As said in the first post, the current starts to oscillate and the outputs of the leds flash when the voltage is increased to maybe 12 volts or more. Thinking that the regulator was damaged, i tried feeding in 5 volts directly from the supply and the same thing occurred, the leds flashing out nonsence.

    Apart from the fact that this is a school project, this is just irritating me because it makes no sence to me!! grrrrrrr
     
  8. Pich

    Active Member

    Mar 11, 2008
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    Your drawing has two limiting resistors with a total of 112 ohms at the power input. If you circuit draws 100 ma lets say, that is a voltage drop of 11.2v.
    Check the voltage at the input pin of the 7805 it needs to be at least 7 volts if not you could do the following, increase the input voltage at the power supply or remove the the 2 56 ohm resistors one at the time and check the voltage at the 7805 input pin.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Well, the OP says they're getting about 17mA of current out of the regulator. Even if it were 20mA, the Vdrop across R101 & R102 would be 2.24V. Since the 78M05 has a dropout of around 1.8v, his power supply could provide enough current to give a 5V output at around 9V.

    One of the purposes of R101 & R102 is to protect the other elements in the circuit in case something is miswired. The other would be to act as a filter (in conjunction with C101) to remove any noise from the power supply.
     
  10. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Ok, this is strange then, particularly in light of what the two of you guys who last posted here said: i measure the voltage at the voltage in of the regulator. (That is after the two resistors) If the supply voltage is 7.2, then the voltage into the regulator is 4.6.

    If i increase the supply voltage to 9 volts, then the voltage into the regulator is nearly 4.5.

    At a supply voltage of 12V then the input to the regulator is 4.7.

    At a supply of 16V the regulator input is 4.8. If I increase it above this point

    then the leds start pulsing and the current starts fluctuating as explained earlier. (Before this voltage usually one 7 segment display is steady on some number. )

    The displays continue to flash even when i reduce the voltage to zero and drive it back up again. (Even if i turn the supply off!) The only way i can seem to "reset" the thing is if i reduce the current dial on the supply to limit the current. I'M CONFUSED!

    The capacitors used before and after the regulator are 10uF electrolytic. I have removed them and checked them with a multimeter.
     
  11. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    When i put remove the analogue to digital converter chip, the votlage out of teh regulator is always stable at 5 volts no matter the supply voltage (7-17 volts) there is practically no voltage drop acros the two series resistors either. Is this idicative of a short somewhere from the chip to ground because when i put in the ADC chip, there is always a fair voltge drop across the series resistors?
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    I haven't reviewed the datasheet for your ADC. It would be a good idea for you to determine what the normal current utilization is.
     
  13. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    I shall! I'd do anything at this stage. (That includes rewirewrapping the whole bloody thing!) I'm that cheesed off!
     
  14. Pich

    Active Member

    Mar 11, 2008
    119
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    Relook at the advice given on the second post by "beenthere" By your voltage readings it seems like you may have a short somewhere.
    You can try removing the 3161 so that the LEDs are not powered if it still draws down the power supply you have problems around the ADC.
    You should understand that when the circuit is working properly the LEDS will draw in eccess of 150ma the decimal point alone will draw 20ma.
    Is your 0-30V supply AC or DC? if it is an AC source you should boost C101 to an minimum of 100uf if DC C101 OK.
     
  15. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Ok the supply current for the chip is 17mA and when the supply voltage for this circuit is around 10 volts, the curent is 20mA. The output voltage of the regulator is 3.17.

    Any more insights or suggestions as to what i could try? Thanks for all the help you guys have given me so far though!
     
  16. Pich

    Active Member

    Mar 11, 2008
    119
    4
    Lets start form beginning
    Check regulator.....remove both ICs and power the board starting at 10 volts,
    check the voltage at the output of the 7805 it should be 4.95 to 5.05 volts. Now increase the power supply voltage to 20 volts the 5 vlts output should remain constant. If you are getting less that 5 volts or drawing heavy current look at the circuit around the regulator or change the regulator.
    if the 5 volts output proves OK install ADC and go through the same steps as above. when you have proven the ADC IC ok then install the second IC and do the same checks, at this point the circuit will draw more current so keep an eye on the ampmeter.
    Hope this will help
     
  17. Pich

    Active Member

    Mar 11, 2008
    119
    4
    One more thing.... at 20 volts and displays on the 7805 may get very hot. if so lower the input voltage. In the end you may need to add a small heat sink.
     
  18. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
    0
    Ok new developments, i did what you sugested pic and started from scratch:
    Removing both chips showed the regulator to be aok. I put the ADC chip in and at 20 volts, the regulator was stable with an output at 5 volts. I removed the 3 7 segment displays and put in the display driver chip and the regulator is still stable at 5 volts but the voltages across the series resistors are fluctuating between round 2.6 and .4 volts, and the current drawn is fluctuating as well, although not by as much as the last time....

    Would it be safe to say that the chip is faulty?

    I also havent put in the decimal point resistor as we have not been given it yet...
     
  19. Pich

    Active Member

    Mar 11, 2008
    119
    4
    Not yet you can continue with installing the displays. Your current will go up quite a bit (what is it now). I don't understand why there is so much resistance at the input of the power supply unless you were ment to use a higher voltage
     
  20. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
    0
    With the displays removed, as said, the voltage across the series resistors fluctuates between 2.6 and .4 volts. The .4 volts i think is what is due to the ADC chip as that is what it what fluctuating to with the displays and the display driver removed. But anyway, to answer your question the current is then 2.6/112 = 23mA. I'll try putting in the displays one at a time and see how the regulator responds and let you know.
     
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