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  • current pickup
  • comparator
  • scits
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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…
Parents
  • jc2048
    0 jc2048 over 6 years ago

    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 do it in a circuit if you specifically wanted the very low saturation voltage it gives [in the old days, people used to build chopper amplifiers with transistors in 'inverse mode' for that reason].

     

    The comparator outputs are designed to drive TTL inputs where the threshold would be around 1.5V and the high output only needs to be (at least) 2.8V.

     

    If you've got a high-brightness white, green or blue LED, where the forward voltage would be a little over 3V and it would illuminate on a couple of milliamps, you could consider something like this

     

    image

     

    The comparator is very fast. Make sure you have the decoupling they recommend, close in to the part's supply pins, and be careful with the layout. You might also want some hysteresis - the input will only move slowly when there's no photodiode current [the reverse capacitance of the diode is quite high and there's only 5M to charge it], so when you slowly transition past the switching point there's a real possibility that noise will cause the output to chatter.

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

    Many thanks for the information, I believe the led I fitted was high intensity so I can look to rework the circuit and get the transistor connected the correct way around.

     

    The circuit shouldn't transition slowly. The photodiode will sit enclosed over an LED on the protection relay and conducts when that LED is lit. I haven't noticed any chatter whilst testing the circuit.

     

    Unfortunately, I haven't given all the details of how it will be used, so you wouldn't have known any of this.

     

    Kind regards.

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

    Many thanks for the information, I believe the led I fitted was high intensity so I can look to rework the circuit and get the transistor connected the correct way around.

     

    The circuit shouldn't transition slowly. The photodiode will sit enclosed over an LED on the protection relay and conducts when that LED is lit. I haven't noticed any chatter whilst testing the circuit.

     

    Unfortunately, I haven't given all the details of how it will be used, so you wouldn't have known any of this.

     

    Kind regards.

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

    If it turned out you did need the hysteresis, the datasheet has a section explaining how to implement it.

     

    BTW there's nothing wrong with having the LED drive and MOSFET drive separate, indeed from a design point of view it's probably better - I just thought it was neat that with a 5V rail you could use the 3V drop of the LED to do the level translation up to the transistor and combine the two.

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