Probe problem?

Discussion in 'General Electronics Chat' started by KLillie, Feb 17, 2015.

  1. KLillie

    Thread Starter Member

    May 31, 2014
    126
    14
    Earlier this week I posted this:
    http://forum.allaboutcircuits.com/threads/555-voltage-output-low.107087/
    I think my problem the whole time was an attenuating probe (10x). I was reading my expected output as about 1 volt (.7 volts? I don't remember) when I expected 8v - the 1.7v? drop approx 6.3. Attenuated that would look like .63 volts. Does this sound feasible?
     
  2. JohnInTX

    Moderator

    Jun 26, 2012
    2,345
    1,028
    A 10X probe will present a 1/10 signal to the scope input. Many scopes have automatic sensing and automatically adjust the vertical scale accordingly. If your scope doesn't have auto sense inputs and/or your probe is not compatible with auto-sensing, yes your signal will be 1/10 of what you might expect. If this is the case and you read .63V, the actual was 6.3V - not unreasonable for 8Vcc on the 555.

    A 10x probe also requires input compensation to present a true signal. If its not right, the signal will show a peaked or rolled off signal depending on which way the compensation is off. Most scopes have a probe test pin that issues a square wave at some convenient frequency (1KHz is common.) To adjust the 10x probe, connect it to this signal and adjust the trimmer screw on the probe for a good square wave. Then assign the probe to that input only.
     
  3. KLillie

    Thread Starter Member

    May 31, 2014
    126
    14
    Thanks JohnlnTx. Just what I wanted to hear.
     
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