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LM2901: Protection of LM2901

Vladimir Naumenkov
Expert 2970 points
Part Number: LM2901
Other Parts Discussed in Thread: TLV1704

Dear Sir, please answer

We use LM2901 in power channel of traffic light controller. Please see attach.

LM2901 is used to monitor the signal level on a R9 resistor.

If there is a short circuit, there can be a large voltage - 40....80 V on a R9 for a time 10 mS - (until the protection goes off and the triac control signal is not turned off)

The LM2901 often breaks down during short circuit - protection in the form of VD5, VD6, VD13  does not help. Rather helps, but badly. I want to clarify that the voltage control on R9 is not done to determine the short circuit. If everything is normal, then a current of 50...200 mA flows through the R9.

Can you advise how to improve the protection?

TIACircuit LM2901.docx

Sincerely,

Vladimir Naumenkov

www.agat.by

  • 0
    TI__Mastermind 24850 points

    Vladimir

    Add a resistor (10k) between pin 4 and the connection between R21 and VD6. VD6 is probably not fast enough to protect the input, so adding this resistor will limit the input current during these fault condition.

    Chuck

  • 0
    Guru 140200 points

    Hi Vladimir,

    I would increase R13 and put a Schottky diode in parallel to VD5 to clamp the negative going voltages.

    Kai

  • 0 in reply to Chuck Sins
    Expert 2970 points

    Dear Chuck, dear Kai, thank you

    Dear Chuck, please see attach - you recommend to add R24  (10k, red colour in the drawing) ?

    May be to set R21=R24=5.1k ?

    Please, report, how do you think, is it possible in this circuit to detect 7...10 mV variations on R9 in the temperature range from minus 30 to plus 50 C? The AC current 50 Hz flows via R9. 

    The circuit is designed to detect a broken lamp.

    Please, report, what do you think about "to increase R13 and put a Schottky diode in parallel to VD5 to clamp the negative going voltages."

    TIA

    Sincerely

    Vladimir Naumenkov

    Circuit LM2901 _2.docx

  • 0 in reply to kai klaas69
    Expert 2970 points

    Dear Kai, dear Chuck, thank you

    Dear Chuck, please see attach - you recommend to add R24 (10k, red colour in the drawing) ?

    May be to set R21=R24=5.1k ?

    Please, report, how do you think, is it possible in this circuit to detect 7...10 mV variations on R9 in the temperature range from minus 30 to plus 50 C? The AC current 50 Hz flows via R9.

    The circuit is designed to detect a broken lamp.

    As to put a Schottky diode in parallel to VD5 and icrease R13, I will make this revision in my circuit. Although, frankly, I would just add a Schottky diode and not increase the R13 resistor( if to use a  Schottky diode for 2-3 amperes) ?  Or still to increase? 

    TIA

    Sincerely

    Vladimir Naumenkov

    4130.Circuit LM2901 _2.docx

  • 0 in reply to Vladimir Naumenkov
    Guru 140200 points
    Hi Vladimir,

    what is the exact voltage you apply to pin 5 of LM2901?

    Kai
  • 0 in reply to kai klaas69
    Expert 2970 points

    Good day, Kai!

    20...50 mV, but I would like to try 7...10 mV for smaller currents. Temperature from minus 30 to plus 50 C! ( but it would be good to ensure a minus 40 С).  I use LM2901AVQDRG4.   

    There are traffic lights now with very small currents - this scheme was developed 15 years ago to work with filament  lamps - there the currents were very large and everything was easier

  • 0 in reply to Vladimir Naumenkov
    Guru 140200 points
    Hi Vladimir,

    adapting the circuit to the lower currents is very difficult, because the LM2901 has big input bias current and input offset voltage. Also, VD5 and VD6 add huge leakage currents. Could you increase R9? This would also increase your wanted signal...

    Kai
  • 0 in reply to kai klaas69
    Expert 2970 points

    yes, I can increase R9 within reasonable limits. I think, that I can increase to 2.7... 3 Ohm. But at the same time, when there will be a short circuit, it will be even worse ? Short-circuiting happens all the time at traffic lights.   

    If we talk about protection against short-circuiting, all the same to put a Shottky diode and increase R13?

  • 0 in reply to Vladimir Naumenkov
    TI__Mastermind 24850 points

    All of Kai’s comments are very good. One thing I was thinking is for you to match the impedance seen by each input pin in order to minimize the impact of the input bias current.

  • 0 in reply to Chuck Sins
    TI__Mastermind 24850 points
    Another option would be to switch to TLV1704 which has much lower input bias current, low offset voltage, and traditional ESD protection diodes on the input. Limiting the input current as I suggested with series resistance should solve your problem and adding external Schottky diode to the supply pin to divert current away from the device as Kai suggests is probably your best solution. Cost is a little higher but this device is better suited for your application.
    Chuck
  • 0 in reply to Chuck Sins
    Expert 2970 points
    Dear Chuk, thank you
    Are you an employee of TI? No one else from TI will read my question? Can I mark "resolved"?