Trying to read correct PWM signal .

Discussion in 'General Electronics Chat' started by LETITROLL, Mar 22, 2014.

  1. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    Hi all , am new to this board .

    So what got in hands is a MMA inverter welder 160 A , where the problem is mainly on the chopper area , i have found that the chopper IGBTs are shorted so i have replaced them with new ones and BAM , they shorted again and the unit shut down .

    Now i have removed the shorted transistors and the usnit turned on , and started to analyse deeply where the problem is hanging .

    I started this mysterious mission by analyzing the control section that drives the IGBTs , this is done by a UC2845 PWM ic .

    So i have checked the VCC pin of it and i see a 15 Volts dc , good input so far , but when it comes to the output , the unit is working with a 60 Khz signal , with a 65% Duty cycle .

    So in order to read that signal i used an old HUNG CHANG 5502 20 Mhz oschilloscope using two methodes :

    1) connecting CRO GND to IC GND pin and probing the ic output :
    result : High signal ( 12 V dc ) with no oscillations

    2) without using the GND i only connected the CRO (+) probe to the IC output and here is the signal i get :

    http://www.youtube.com/watch?feature=player_embedded&v=EZjPo86UsKs

    So what you think of this results ? i don't see any square wave signal , is this considered bad IC or am not reading the signal the right way ?
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,762
    1,099
    1) Looks like the IC, or something connected to it, is not as it should be.
    2) If you are using only one probe of the CRO you will not get any meaningful measurements. The CRO needs a reference (such as ground).
     
  3. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    Hi alec , i used two methods , the one using GND gives a dc signal with no oscillations at the output , i have used the IC's pin 5 as GND reference .
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,963
    743
    sound like your signal is just picking up noise, you need to connect the ground probe to pin 5(8pin dil) or 9(14pin dil)

    datasheet UC2845
     
  5. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    Hi dodgydave , i have connected it as you mentioned and i don't get a square wave on;y DC signal
     
  6. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
    2
    So is this considered bad functioning IC ?
     
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