0
\$\begingroup\$

I recently repaired a vending machine that had coca-cola spilled on a terminal strip. A 9.5V source from the strip measured correctly on a DMM when there was no load on it but dropped to 2V with its normal load (less than 1 amp). Cleaning the strip of hardened cola fixed the issue. There was corrrosion damage noted after cleaning. Why would the 9.5 volts behave this way? (I know cola is corrosive).

\$\endgroup\$
3
  • 2
    \$\begingroup\$ the corrosion would break the connection and insert itself as a resistor \$\endgroup\$
    – jsotola
    Nov 3 '17 at 3:42
  • \$\begingroup\$ Why does it matter? You fixed it I presume... \$\endgroup\$
    – Voltage Spike
    Nov 3 '17 at 3:42
  • 2
    \$\begingroup\$ Why does understanding something matter? If this is indeed a site for, "For electronics and electrical engineering professionals, students, and enthusiasts", the question would seem to be on topic. I can see how "enthusiast" questions could be boring and tedious for "engineering professionals", but that's an issue to take up with meta. Depending on the temperament of the OP, anything less than encouraging, positive feedback could be taken as negative criticism. If these type of questions suck, move on, and let noobs like me answer them. Plus, a user with 2,085 rep did give a helpful answer. \$\endgroup\$
    – user160409
    Nov 3 '17 at 6:43
2
\$\begingroup\$

To elaborate on Passerby's answer, my mental model of the situation would look something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

The cola caused some resistance to develop on one of the connections in the strip. When the load is not there, no current flows through the resistance so there is no voltage drop. When the load is applied, there is a voltage drop.

\$\endgroup\$
0
\$\begingroup\$

The first reading you made from the terminal strip was, "when there was no load". I think you meant that the usual load was not there. Since a circuit was completed when your test probes made contact with the terminals, and current flowed through your DMM, your meter became the load (although an extremely tiny one). enter image description hereSo the resistance of the cola is in series with the resistance of your DMM. And together they are the (tiny) load. DMMs by design, have extremely high input impedances (many tens of MegaOhms, IIRC) so that their impact on the circuit they interact with is negligible. In a series circuit, the larger resistance drops most of the voltage. Taking the value of Rcola given in Houston's answer, makes your DMM's resistance is many millions of times greater than Rcola. That makes the DMM the entire load (virtually), and the measurement you got was the voltage being dropped by itself!

Now when you made another measurement with the usual load attached to the terminal strip (as shown in Houston's schematic), this time the DMM is in parallel with the load. And in a parallel circuit, the smaller resistance drops most of the voltage. This time, your DMM's super high resistance makes it the one to ignore. So you're back to a series circuit: Rcola in series with Rusual, and the measurement of 2 Volts you got was all that was being dropped by the (usual) load. And this time Rcola was greater and so it dropped most of the voltage.

One last thing to note is that the corrosion you noted might not be entirely from the cola. One project I made used two wires to sense the water level in a fountain. Being even more of a novice than I am now, I had current constantly flowing from one wire through the water to the other wire. Seemed fine, everything worked... AT FIRST! But in less than 24 hours, the wires completely disintegrated causing complete circuit failure! I later learned what I had going on was called "electrolytic corrosion" (I really hope I remembered that correctly!). Perhaps some of that corrosion you noticed was from electricity?! I don't know, I'm just taking a wild guess!

\$\endgroup\$
-2
\$\begingroup\$

Hazard a guess, high resistance plus large load = voltage drop.

\$\endgroup\$
6
  • \$\begingroup\$ It is SE policy "to be nice". If this is indeed a site for, "For electronics and electrical engineering professionals, students, and enthusiasts", the question would seem to be on topic. I can see how "enthusiast" questions could be boring and tedious for "engineering professionals", but that's an issue to take up with meta. Depending on the temperament of the OP, anything less than encouraging, positive feedback could be taken as negative criticism. If these type of questions suck, move on, and let noobs like me answer them. Plus, a user with 2,085 rep did give a helpful answer. \$\endgroup\$
    – user160409
    Nov 3 '17 at 6:47
  • \$\begingroup\$ Literally a brown out! \$\endgroup\$
    – user98663
    Nov 3 '17 at 10:33
  • \$\begingroup\$ Thanks guys, I figured it was something like this but I never encountered this situation before. Getting that correct DMM reading really sidetracked me because I assumed there was no voltage issue. Lesson learned. Pepsi Syndrome! \$\endgroup\$
    – Walter
    Nov 3 '17 at 14:03
  • 1
    \$\begingroup\$ @Ray simple question, simple straight forward factual answer. Your attack on me is a "be nice" fail. \$\endgroup\$
    – Passerby
    Nov 3 '17 at 14:48
  • \$\begingroup\$ @Passerby You called my comment an "attack" on you. The words "complaint" or "protest" would be some of my choices. The list of relevant words could get quite lengthy. You described your post as being a, "simple straight forward factual answer." But do you think it was helpful? I read your rep history in your profile. I was very impressed and a little embarrassed; SE needs more people like you. I'll never be able to contribute at that level. I was hoping to help with noob perspective. There's always room for improvement, maybe lighten up on noobs. You said the question was "simple". I read \$\endgroup\$
    – user160409
    Apr 9 '18 at 22:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.