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Forum Thread Details
  • State Verified Answer
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  • current pickup
  • comparator
  • scits
Related

Project PITS Problem

three-phase
three-phase over 6 years ago

I have been trying to build up a Pick-up Indicator Test System (PITS) to detect the overcurrent indicator on protection relays I test, for the project14 test instrumentation.

 

Once  I figured out that there are some photo diodes that respond better to green LEDs than others, I managed to get the circuit working. However, I then decided to add an output indicator off the negated output of the AD8561 comparator chip I was using. After adding this, both outputs of the comparator remain low and the LED is on all of the time.

 

When I drop out R6, the 10k resistor to the pnp transistor base, the circuit starts to work again.

 

Anybody have any ideas what I am doing wrong? The only thing I can think of doing is to increase the size of R6, I had originally calculated this as 14k for a 1mA base current, but from my limited selection, 10k was the closest I had.

 

I also find that when turned on, the MOSFET has a resistance of 780 ohms, which seems a little high to me?

 

image

 

image

 

Kind regards

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  • jc2048
    jc2048 over 6 years ago +9 verified
    Your transistor is upside down - you've got the collector and emitter swapped. A transistor will work like that, but the gain is very much less than if you use it the right way round so you'd only ever…
  • Dudley
    Dudley over 6 years ago in reply to dougw +5 suggested
    better now?
  • three-phase
    three-phase over 6 years ago in reply to 14rhb +4 suggested
    Hello Rod, Many thanks for your help. I have read through the data sheet for the AD8561 and it does not say much about the outputs and if they are differential or inverse from one another. I have just…
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  • three-phase
    0 three-phase over 6 years ago

    Now I am stuck, can I mark two answers as correct as Rod and Jon have both helped me out?

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  • jc2048
    0 jc2048 over 6 years ago in reply to three-phase

    I think it can only be one. I'd give it to Rod - he got there first.

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  • 14rhb
    0 14rhb over 6 years ago in reply to jc2048

    Very kind but I'd give it to Jon for his great schematic and circuit knowledge image

     

    Or if you still can't decide, perhaps just forget marking any comments as correct, most readers should be able to follow the thread.

     

    Rod

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  • 14rhb
    0 14rhb over 6 years ago in reply to jc2048

    Very kind but I'd give it to Jon for his great schematic and circuit knowledge image

     

    Or if you still can't decide, perhaps just forget marking any comments as correct, most readers should be able to follow the thread.

     

    Rod

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