suspect products

Discussion in 'The Projects Forum' started by malcolm, Mar 6, 2010.

  1. malcolm

    Thread Starter New Member

    Mar 6, 2010
    7
    0
    Having designed a 6th order sallen key butterworth low pass I'm trying to build it, so far with limited success.

    I suspect faulty dual rail quad op amps. How do I test for functionality.
    If I plug in a 14 pin DIL chip to a +/-ve 12v supply completely open circuit I would expect the power rails to remain. They don't, however, the negative rail goes 12v positive, differential at or virtually zero. Whats going on?

    Please appreciate I'm a total novice at this.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Does the DIP in question, probably a dual or quad op amp, get hot with power applied? Are you certain that your circuit is assembled properly, with no component leads touching or solder bridges? A hot op amp may point to a shorted output or a power supply in oscillation.

    What happens if you put a spare op amp in the socket?

    I have seen a problem arise with some nonamo regulators. Sometimes one of the 7X12 regulators will not start up when the other does, leading to a condition similar to the one you describe. If swapping the op amp and verifying the circuit does not help, change the regulators.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Do both the +12 and -12v rails have capacitors to ground, at both the regulator outputs and at the opamp Vcc/Vee inputs? Generally, 0.1uF (100nF) caps are used.

    If you are using 7x12 regulators, beware that they may oscillate at high frequency if you do not use the recommended input and output capacitors, generally 0.33uF (330nF) on the input, and 0.1uF (100nF) on the output.

    It is generally not a good idea to leave the inputs of opamps floating (ie: no path to ground) as that can cause them to oscillate at high frequency. This causes high power consumption and heating.

    One easy way to keep that from happening is to connect the output of each unused opamp to it's respective inverting (-) input, and connect the non-inverting (+) input to a reference voltage halfway between Vcc and Vee (the positive and negative rails). In your case, you have ground available - so connect the + inputs to ground.

    If you were using a single-supply opamp, you could use two equal-value resistors to make a voltage divider between Vcc/+V and ground. The resistive divider should have roughly 0.1mA to 1mA current flowing through it, so divide your supply voltage by 0.5mA to get an approximate value to use for each resistor. 12v/0.5mA = 24k Ohms.

    This turns the opamp into a voltage follower, also known as a unity-gain buffer. It is also a handy way to find out what the input offset voltage of your particular opamp is. The difference between the + input and the output is your input offset. For common hobbyist-type opamps, this can be several millivolts.
     
    Last edited: Mar 6, 2010
  4. malcolm

    Thread Starter New Member

    Mar 6, 2010
    7
    0
    Thankyou guys.

    1 voltage regulators are fine except for the -ve regulator which despite carrying the correct product code turned out to yield -18v. I fixed that with a voltage splitter.
    Both regulators are grounded through the correct capacitors, as advised.
    2 my soldering skills are appaling. But the track board I'm using is thouroughly cleaned, there are no cross tracks to my knowledge
    3 the chip isnt hot or even vaguely warm.
    4 I have 8 no. chips altogether. Only one of them maintains +/-12ve with all outputs and inputs unconnected.
    Question is. Is this inability to maintain +/-12ve with everything open circuit an indication that the product is wacked.
     
  5. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    I have to second SgtWookie's suggestion.

    It is a much better way to do a simple check of an opamp. I don't think you can guarantee results without connecting the opamp inputs.
     
    malcolm likes this.
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Good soldering is an acquired skill that takes a fair amount of practice. If I haven't done any soldering for awhile, I'm truly appalled at the results. :rolleyes:

    "Cleanliness is next to Godliness" when you're soldering. I keep a 3M Scotchbrite(tm) pad handy to prepare component leads and board surfaces. You can get them at any grocery store near the cleaning stuff. It'll strip corrosion off without embedding particles, like steel wool will.

    Use isopropyl alcohol (90% or better) and an acid brush (rolled metal handle, black nylon bristles, available at auto parts stores) to clean your board and components right before soldering. You might find 99% or better isopropyl in hardware stores. Some Wal-Mart stores carry 90% near the pharmacy area. Don't use the 70%; too much water in it. You need to get your finger oils off the parts; otherwise the solder won't flow properly, that's why the isopropyl alcohol is used. It is flammable, and burns with an almost invisible blue flame - so keep it away from items that may produce sparks or heat.

    If your parts/board are really and truly clean, you don't need to use flux. If you DO use flux, I suggest rosin flux. Sn63/Pb37 solder is the easiest to use; it is "eutectic" meaning that it has no plastic state, it goes directly from liquid to solid. Sn60/Pb40 is OK, but requires a steady hand.

    Keep the tip of your iron clean and tinned. I use a stainless steel scrub pad (also found in the grocery store) to clean up the tip. Temperature regulated irons are the best, but a selectable 15w/30w pencil iron works pretty well. Unplug a non-regulated iron if you won't be using it within the next 5 minutes, or you will wear it out much more quickly.

    Even joints that look OK might not stand up under close scrutiny. Having a microscope available is grand, but they are pricey. I find a Jeweler's loupe to be a good compromise. You might be surprised what you find. On a recent project, I had some hair-like solder bridges shorting some traces; I simply couldn't see them without magnification.

    A good solder joint will look like it's still wet; the surface bright and shiny. If you have blobs, the solder didn't flow properly.
     
    Last edited: Mar 6, 2010
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