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Misc Electronic Questions

Discussion in 'General Electronics Chat' started by Billy Mayo, Apr 4, 2013.

  1. Billy Mayo

    Billy Mayo Member

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    So the swinging is asymetrical? Or the range is from +1 volt to +11 volts?

    But the swing from +5 volts to -15 volts would be in a range but i don't get why the negative is so low, any reason why so much?

    I have seen in logic circuits is they put an RC network only for the high state but for the low st state its straight to ground, why would u want the high states to have a delay time?
     
  2. Billy Mayo

    Billy Mayo Member

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    There is only an RC time delay only on the HIGH logic state , the low logic state goes through a bypass diode straight direct to ground

    Why are these RC time delays used for logic HIGH states?

    Logic High RC network#1.jpg
    Logic HIGH RC time pic#2.jpg
    Logic HIGH RC time pic#3.jpg
     
  3. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    It very much depends on each individual chip.

    As I said, my only guess is that they needed -15 or so for something else. Turning on MOSFETS for example. Early stuff usually had +24, +-15 and 5V supplies. Now logic is 5, 3.3, 1.8 and 0.8 V, but a lot of stuff still requires TTL compatible signals to work.

    +24 would be for relays and solenoids, +-15 for analog stuff and 5V for logic. Many things required linear operation from +-10V , so 15 V was used because of the OP amp limitations.

    I have seen in logic circuits is they put an RC network only for the high state but for the low st state its straight to ground, why would u want the high states to have a delay time?
     
  4. dave

    Dave New Member

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  5. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I've used the first circuit before. Basically I wanted over current protection for a motor for a model railroad (not mine). When a motor turns on there is a large current, so the RC time constant levels that out a bit. but once the motor was below the threshold, C would discharge immediately because of the diode. Now it would work if the motor slammed into the side.

    This is a lot simpler than the logic to ignore the motor start pulse for 100 mS.

    PS: What is your general background?
     
  6. Billy Mayo

    Billy Mayo Member

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    Here is the RC networks , only is for the HIGH logic state and the LOW logic state goes to the diode

    What I don't understand is why does the HIGH and only the HIGH logic state need a RC time delay?

    One of the diodes goes to ground and the other diode in pic#3 doesn't, it bypasses the RC network, how are they different?

    RC network for HIGH logic state pic#1.jpg
    RC network for HIGH logic state pic#2.jpg
    RC network for HIGH logic state pic#3.jpg
     
  7. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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  8. Billy Mayo

    Billy Mayo Member

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    Yes the logic HIGH goes through the differentiator but not the LOW state goes through the diode
     
  9. Billy Mayo

    Billy Mayo Member

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    Those RC networks "waveshape" the logic high state going to a GATE , why would they want a shape trigger or waveshape going to a AND or OR GATE?
     
  10. Billy Mayo

    Billy Mayo Member

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    What is a Diode OR mean?
     
  11. kubeek

    kubeek Well-Known Member

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    OR gate made from diodes.
     
  12. Billy Mayo

    Billy Mayo Member

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    But how does that work?
     
  13. kubeek

    kubeek Well-Known Member

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    Take a look at this: high level on any of the inputs causes the output to be high. If you have low on all the inputs, then the resistor maintains logic low.
     

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  14. Billy Mayo

    Billy Mayo Member

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    ok thanks

    When I'm troubleshooting a short , there is components that will measure a short, making the DVM meter BEEP, but when I take out the component out of circuit it will measure FINE

    In circuit the component will measure a short and make my DVM meter BEEP

    So What is this called? a false short? or ghost short? a false continuity or ghost continuity

    And why does the GOOD component make my DVM meter BEEP and have a shorted Continuity? is it because the GOOD component is in parallel or in series with the short?
     
  15. kubeek

    kubeek Well-Known Member

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    I think you are still missing the point on what IS a short. First of all, a DVM doesn´t measure shorts, it is an ohm meter that beeps when the resistance measured is lower than some magic number like 50ohms. So if you try it between two points in a circuit that are connected by a 33ohm resistor it will beep even though the circuit is perfectly ok. Basically a short is just what the name says - a shortcut between parts of circuits that shouldn´t be there. It doesn´t have to be a straight away zero ohm connection between two points.

    So to answer the rest of your quiestions, when "In circuit the component will measure a short and make my DVM meter BEEP" it is called wrong interpretation of what your meter tells you. Nothing less, nothing more. If you don´t know how to use the functions of the meter and what they actually do with the circuit you will get flawed and incorrect results.
     
  16. Billy Mayo

    Billy Mayo Member

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    Yes , but if my DVM meter is Measuring low resistance and beeping my meter on certain components , the only way to verify if those components are the short or not is to take them out of circuit?

    Because any of the components can be the short, so i have to take them out of circuit to verify that

    I just thought when measuring a short it only happened for the BAD component that is shorted

    I never knew that other GOOD components can make my meter BEEP or measure 0.2 ohms on my meter when the components are in circuit, but out of circuit the components measure GOOD

    The GOOD components measure 0.2 ohms in circuit , but why do they? is it because they are in parallel or in series with the short cut / short circuit?

    What is this called when GOOD components measure a Continuity in circuit? or very low resistance

    These resistance are 10K, 100K, in circuit they measure 10K , 100K , but when a short happens it will measure 0.2 ohms in circuit
     
  17. kubeek

    kubeek Well-Known Member

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    You basically got two choices, take the components one after another out of the circuit and measure separately, or cut the traces that are going to them. With through-hole components like resistors it is enough to take just one of the pins out of the circuit.
    As you know the total resistance of a parallel circuit is allways lower then the lowest resistance, so yes when there is a short somewhere this will shift the resistances measured to low figures.

    If you have more parts in parallel then you will measure the same resistance across any of them.
    Say you have a 100ohm resistor parallel with 0.2ohm, you will measure aprox. 0.2ohm even though there is no short.
    Really you need to start with the schematic and look at what parts are around the one you are measuring and determine what effect they will have on the circuit. Or just replace all the parts straight away and hope for the best (that the short is not in the pcb itself for example).
     
  18. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    The ohmmeter can unintentionally turn on a transistor junction therefore you have to know what the surroundings are to use the ohmmeter safely. The diode test is usually your friend in-circuit.

    Tube/valve gear relied on resistance measurements for troubleshooting.
     
  19. Billy Mayo

    Billy Mayo Member

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    I thought the Diode tester can turn on the transistor junction not the ohmmeter setting

    Yes I had to take each component out one by one until I found the component that was causing the short

    I found one resistor that was a 5.6 ohm that measured 180K out of circuit

    I have no idea how a BAD 5.6 ohm resistor can measure 180K out of circuit, I guess it is HALF open HALF short internally, but the resistance went higher
     
  20. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Both the resistance and the diode setting can turn on a junction because of the voltage, so you can get different resistance values depending on polarity because of nearby PN junctions. Capacitors cause a charging effect.
     
  21. Billy Mayo

    Billy Mayo Member

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    So I can use the Diode setting to find shorts?

    A shorted or Open Capacitor would measure what using the Diode setting?
     

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