Short circuit accident.

Thread Starter

cheddy

Joined Oct 19, 2007
87
Hey guys. I was feeling a little reckless today and thought it would be interesting to measure the amperage of the outlet on my potentiometer controlled soldering iron on different settings.

I've never measured high voltages or amperages before so I just did what I thought was right. I changed the multimeter probes to test Amps with the the 10A unfused holes. Out of sheer stupidity I forgot to switch the meter to read from direct current to alternating current.

Well I put the test probes in the outlet and started at maximum resistance and slowly starting turning the resistance down on my soldering iron base station (turning the current up). I got about half way through and started wondering why the amperage wasn't reading until POW! I saw and heard a big spark inside the base station and quickly shut all the power off.

I opened it up and looked at the circuitry inside which is very simple. Has a .5Mohm potentiometer hooked up to a 15Kohm resistor, 2 diodes, a 250uF capacitor and a really big transistor.

Anyone know what the transistor and capacitor are for?

Well I looked for what exploded and it turns out it was just some solder exploded and left and open circuit right were the live wire was attached.

I re soldered where there was an open circuit and put it back together and it works fine again.

I was wondering if the only reason it exploded was because I had the direct current measurement on by accident? If I tried the experiment again with the correct alternating current set on the multimeter do I run a risk of another fateful accident or should I get results (as long as I don't go over 10A)?
 

beenthere

Joined Apr 20, 2004
15,819
At a guess,the function of the transistor is to pass current to the soldering iron heater. The capacitor is the filter for the DC supply inside the base.

With no current limiting element in series with the ammeter, the result will always be the same, no matter the setting. An ammeter is supposed to have minimal resistance so it disturbs the circuit as little as possible. By itself, it resembles a short circuit.
 

bloguetronica

Joined Apr 27, 2007
1,372
You should connect an Ammeter in series with the load. It you used as you would use a voltmeter, probably you short circuited the outlet of the soldering station (if I got it right).
 

Thread Starter

cheddy

Joined Oct 19, 2007
87
You should connect an Ammeter in series with the load. It you used as you would use a voltmeter, probably you short circuited the outlet of the soldering station (if I got it right).
I put the red probe in the positive outlet hole and the black probe in the neutral outlet hole and *ZAP!*... opps. Yeah I guess that's how you should measure a voltage and not current.

So if I wanted to measure the current in a soldering iron I would have to cut an opening in the live wire of the soldering iron and put the ammeter in series with the base station and the soldering iron while it's turned on?
 

Mazaag

Joined Oct 23, 2004
255
or you could use a current probe which you could hook up around the live wire and which uses the magnetic field to determine the current
 

bloguetronica

Joined Apr 27, 2007
1,372
I put the red probe in the positive outlet hole and the black probe in the neutral outlet hole and *ZAP!*... opps. Yeah I guess that's how you should measure a voltage and not current.

So if I wanted to measure the current in a soldering iron I would have to cut an opening in the live wire of the soldering iron and put the ammeter in series with the base station and the soldering iron while it's turned on?
You don't need to ruin a wire. If you have one aligator clip with a bananas you can measure the current. Put the banana plug into the neutral outlet hole and connect the aligator clip to soldering station's plug. Then put one ammeter probe in the other hole of the outlet and the other to the other plug's connector. Although you have the aligator clip connected to the neutral you can get a shock. So wear rubber gloves. It is dangerous to measure current that way.
Also, when measuring current use the multimeter set at Amps AC. Amps DC doesn't give a measure.
 
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