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Entry  Mon Jul 18 22:26:43 2011, Nicole, Update, SAFETY, Painful Voltage Regulator and Circuit-Testing Lessons P7180423.JPGP7180424.JPG
    Reply  Tue Jul 19 13:45:22 2011, Suresh, Update, SAFETY, Painful Voltage Regulator and Circuit-Testing Lessons 
Message ID: 4986     Entry time: Mon Jul 18 22:26:43 2011     Reply to this: 4990
Author: Nicole 
Type: Update 
Category: SAFETY 
Subject: Painful Voltage Regulator and Circuit-Testing Lessons 

Today I learned some important circuit-building lessons while testing my photosensor circuit box (i.e. how NOT to test a circuit and, conversely, things that should be done instead). 

I blew my first circuit today. The victim is in the photo below (bottom 7805 voltage regulator). The plastic covering fell off after I removed the fried regulator.  After checking various components, I figured out that I blew the circuit because I had forgotten to ground the regulator.  Although this was very unfortunate, I did make an important discovery. While testing the voltage output of the 7805 voltage regulator (I put a new one), I discovered that contrary to the claims of the datasheet, an input voltage of 5V will not produce a steady 5V supply. I found that at 5V, my regulator was only producing 4.117 V. I was using a 5 V supply because I wanted to use only 1 power supply (I was using a two-channel power supply that had a fixed 5V output to produce the +15, -15, ground, and 5 V I need for my photosensor circuit box).  After seeing this, I got a second power supply and am now using 10V to as an input for the regulator to produce 4.961V. I found that from a voltage range of 10V to 15 V, the regulator produced a steady  4.961 V supply. I have decided to use 10V as an input. My newly-grounded voltage regulator did not smoke or get hot at 10V.

After several more debugging trials (my LED was still not lighting up, according to the infared viewer), I learned another painful lesson. I learned DO NOT USE CLIP LEADS TO TEST CIRCUITS!!!! Initally, I was powering my circuit and making all of my connections between the photosensor head (2 photodiodes and 1 LED) with clip leads. This was a BAD IDEA BECAUSE CLIP LEADS ARE UNSTABLE AND IT IS VERY EASY TO SHORT A CIRCUIT IF THEY ACCIDENTALLY TOUCH! I did not realize this important lesson until my photosensor circuit was once again burning. Confused as to why my circuit was once again burning, I foolishly touched the voltage regulator. As you can see on the top voltage regulator in the photo below, my finger left its mark on the smoldering voltage regulator. As you cannot see the wincing on my face as I try to type this long elog, I will painfully type that the voltage regulator left its own mark on my finger (an ugly sore little welt).  Suresh has taught me a valuable lesson: WHEN DEALING WITH SOMETHING OF QUESTIONABLE/UNKNOWN TEMPERATURE, USE YOUR NOSE, NOT YOUR FINGER TO DETERMINE IF THAT COMPONENT IS HOT!!!! 



To make my circuit-testing safer, upon the suggestion of Suresh, I have since removed the clip leads and inserted a 12 pin IDC component (pictured below). There are 12 pins for the 6 inputs I will get from each of the 2 photosensor heads. I have requested orders for a 16 pin IDC connector, 15 pin Dsub male part, 15 pin Dsub feed-thru, 9 pin Dsub male part (2), and 9 pin Dsub feed-thru (2). After receiving these components, I should be able to safely test my circuit.


 In the meanwhile, I can explore SimMechanics and try to figure out how to use the accelerometer

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