Troubleshooting

Troubleshooting: It's time for me to share an embarasing story.

I had a machine that was not working. I assumed that the Solenoid valve had fried.
Ok I disconnected the Coil from the PLC output. It's a 110V coil coming from an old SLC100. I went back to the PLC and measured the voltage on the output, and it measured 107V. but the light wasn't on! How can this output be on?
I incorrectly assumed that when the coil went south, the output also went south.

Boy that was a bad assumption!
Well I had an old SLC100 laying around, so I actually changed out the PLC.
Then as soon as I measured the output voltage it hit me.
You have to have a load on this type of output contact in order for the potential difference to be measured.
Dog barking up the wrong tree.

The man who hasnt done something wrong or blown something up hasnt done much.

In the game we are in, we have and will continue to drop clangers. We are the people who dare to tread where others fear.

About three months ago I was programming a small machine on the fly. The first four motors needed to start in auto sequence with a delay between each starting.

I decided to try the code so far. All four outputs came on in sequence and all four contactors came in, in sequence.

Shortly afterwards I noticed smoke bellowing out from behind the machine. You know the feeling; dive for the isolator quick.

A mechanic had left a spanner in the gear box, jamming the gears. And the bright programmer forgot to include the overloads in his code.
One brand new motor baked to a crisp

Right you Are

I agree..... We've all been there! Some of us many times. I could be there again tomorrow.

been there

I used an exercise in the old days, for apprentice electricians,
to learn all about 'source impedence'. You can do this
experiment yourself.

1.)Attach a carbon resistor to the (+) terminal of a DC power supply. Leave one side of the resistor un-connected.

2.) Energize the power supply, and read the voltage across the
(+) and (-) terminals.

3.) Read the voltage between the un-connected resistor lead and
the (-) terminal.

4.) Repeat steps 2 and 3 for both a VOM (analog multimeter) and
a digital multimeter.

Explain the differences in readings.

Do this, and you'll never forget

Especially, when you get your fingers across about a 2000mfd cap. I still remember my first lesson. ouch!

This is where I sparked a debate about whether or not to use a tester with a solenoid in it to to check the triac outputs. I personally need mine on a daily basis, and my outputs have never fried.......

Well I reckon I am confused, whats new?

An unloaded resistor should by all rights read the potential of the power supply whether you use an analog or digital meter.

BTW where I come from a VOM is a voltage and ohm meter, not an analog multimeter. A multimeter implies that it can also do other testing such as current. There are digital VOM ..ie digital voltage/ohm meters AND digital multimeters which implies multiple capabilities...ohm, volatage ac or dc, current (possibly ac or dc), frequency maybe, diode etc depending on the DMM (digital multi meter).

The problem with the digital multimeters is their accuracy? ACCURACY? yes accuracy, they are designed to read ANY voltage / potential difference. With many solid state devices its possible that the leakage current will produce a valid value on a DMM. An analog or D'Arsonval movement type analog meter isnt susceptible to the leakage current voltage.

My fallacy when dealing with electricity is the NEED to KNOW. I dont care for neon etc type testers because they do NOT tell me what may be present. A light to me is not enough, I want to know a numeric value, if there is a question that number is wrong then I will investigate. It is possibe to make some non meter/light type testers to produce a light from a carpet.

To each their own. If you feel comfortable with your techniques then use them. I spend as much time using a scope as I do a multimeter, both analog and digital. My favorite meter is a Fluke 87 but I use a BK oscilloscope and a Fluke 97 scopemeter just as much. This doesnt include the times I have to use an amp probe for higher AC.

As it is now for me I dont have to do much of the preliminary troubleshooting anymore. When I am called its usually more involved and more detailed instruments are needed for the troubleshooting. I work with 2 electricians that are a grade lower than I am, they both use the pocket style neon testers to determine if voltage MAY be present. I have no problem with this at all. It may offer a safety aspect for them. I do not believe it is a good tool for ltroubleshooting in detail.

To each their own, the objective is to learn and know when to use what instrument that applies to the situation.

BTW gbradley, BTDT (been there done that), as many have before us and will do after us. It is a common, I hate to use the word mistake BUT, mistake we all make at some time.

The other thing that I find creates total confusion is the different input impedance values between an analog and digital meter. Can give some very different readings in certain circumstances.
A very embarrasing moment occurred when commissioning an 11kV fully automatic live power station. Using Woodward AGLC (Automatic Generator Load Controller) load ramp controllers. These are old fashioned analog devices with transistor outputs to indicate that the load is now low enough to open the circuit breaker.
We had 1 x 4.1mW set and 1 x 2.8mW set on line with a load of approx 5.7mW of load. I dug out my trusty digital multimeter with a very high impedance input and decided that I wanted to see what the output transistor was doing. Surely with that high an input impedance there would be no problem
WRONG WRONG WRONG. Measuring the transistor output totally confused the controller. 2.8mW set turbo was screaming merrily along doing about 22,000 RPM and, after checking the transistor, started to scream a little less, then a little less etc, in other words the AGLC was unloading the set. 5.7mW into 4.1mW does not equate. Instant blackout. 3000 people in the dark more than usual. Heard it coming, knew it was coming and just had to sit and wait till it happened.
The electrical engineer from the power authority calmly walked over, after we got the sets back on line, and quietly asked what I had done so that he never repeated the error.
mddr

Nearest icon I can find to a very red face.

flame guard - pre-emptive

Before somebody points this out, I'd like to say that I
am aware that I used a DC example (power supply and resistor),
to an apparent AC related question.

FET leakage and transistor biasing is indeed different than
internal resistance of a DC power supply.

And, rsd..., check your PM for a longer, off-board, explaination.