Ohm's Question

[rijo1]

I am working on a circuit board out of a mig welder. The problem is a
resister burnt and I can't read the some of the color bands. This
circuit board controls wire speed, arc stabilizer, etc. There is also
high voltage on this board . How would be the best way to determine the
proper resistance of the burnt resister ? Thanks for any help in advance
..

 

[Sam Goldwasser]

I am working on a circuit board out of a mig welder. The problem is a
resister burnt and I can't read the some of the color bands. This
circuit board controls wire speed, arc stabilizer, etc. There is also
high voltage on this board . How would be the best way to determine the
proper resistance of the burnt resister ? Thanks for any help in advance

You really need to figure out how the resistor is used. What part of
the circuit, how much voltage/current it has on it, etc.

That is, of course, if someone doesn't have the schematic!

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[Charles Schuler]

I am working on a circuit board out of a mig welder. The problem is a
resister burnt and I can't read the some of the color bands. This
circuit board controls wire speed, arc stabilizer, etc. There is also
high voltage on this board . How would be the best way to determine the
proper resistance of the burnt resister ? Thanks for any help in advance

Have you contacted the manufacturer of the welder? Also, realize that if
another part failed and caused this, then that too must be resolved. For
example, a capacitor or solid state device might be shorted.

 

[N Cook]

I am working on a circuit board out of a mig welder. The problem is a
resister burnt and I can't read the some of the color bands. This
circuit board controls wire speed, arc stabilizer, etc. There is also
high voltage on this board . How would be the best way to determine the
proper resistance of the burnt resister ? Thanks for any help in advance
.

Resistors catastrophically failing are a bit like fuses in this sense that
they tend to fail because something else has failed first. So replace
whatever is suspect connected to this R first. Then starting high value R
suck it and see. With the main welding transformer disconnected so just
powering the controller board power up
via a variac initially.

electronic hints and repair briefs , schematics/manuals list on
http://homepages.tcp.co.uk/~diverse

 

[Asimov]

"rijo1" bravely wrote to "All" (19 Nov 04 15:30:55)
--- on the heady topic of "Ohm's Question"

ri> From: rijo1 <[email protected]>
ri> I am working on a circuit board out of a mig welder. The problem is a
ri> resister burnt and I can't read the some of the color bands. This
ri> circuit board controls wire speed, arc stabilizer, etc. There is also
ri> high voltage on this board . How would be the best way to determine
ri> the proper resistance of the burnt resister ? Thanks for any help in
ri> advance .

Just to add to what has been said that sometimes using a hand
microscrope with a strong light to view the underside of the resistor
body side which sustained the least burn can sometimes turn up flecks
of pigment which still survive.

Another trick with unknown resistors is to measure the resistance of
whatever is left and estimate the value from the physical dimensions.
This involves using needle probes, etc.

Another strategy is to try a ballpark value. For example a small
resistor value is 1 ohm and a large value is 1 Meg, so pick the
geometric mean or 1K ohms. Generally 1K ohms is a middle value for
many circuits. Consider that if 1K is too large it won't damage much
and if it is too small it also won't damage much either.

But you should try figuring out what the resistor does first!

A*s*i*m*o*v

.... Thomas Edison invented the "Light Emitting Resistor"

 

[James Sweet]

I am working on a circuit board out of a mig welder. The problem is a
resister burnt and I can't read the some of the color bands. This
circuit board controls wire speed, arc stabilizer, etc. There is also
high voltage on this board . How would be the best way to determine the
proper resistance of the burnt resister ? Thanks for any help in advance
.

Post the model of the welder, someone else with the same unit can have a
look.

If the resistor burned up though something else is also wrong.

 

[BOB URZ]

I am working on a circuit board out of a mig welder. The problem is a
resister burnt and I can't read the some of the color bands. This
circuit board controls wire speed, arc stabilizer, etc. There is also
high voltage on this board . How would be the best way to determine the
proper resistance of the burnt resister ? Thanks for any help in advance
.

What kind of resistor?
Form factor?
Wire wound?
Metal film?
How did it fail?
Blown in half, or just open?

You first have to ID the type and wattage.
If its a metal film or wire wound, you may be able to take
resistance readings from each end to the bad point and add
them together to determine the approximate value of the
resistor. This might get you in the ballpark.

Bob

 

[rijo1]

Thanks Everyone , this is a Telwin multi Mig welder : 90090 model ... The
resister is a corbon 1/8 watt resister that durnt . On one end there is a
switching transister : MSP A92 and on the other end connection is a BUV64 .
The origional resister had a brown stripe on the first color band and Gold
tollerence . The rest of the 2 olor bands were burnt up and could not be
identified . Thanks for the help .

 

[rijo1]

OK , I will try to be more specific with the problem of the circuit . The carbon
resistor that is burnt is connected on one end to the base of a MPS A92 silicon high
voltage, general purpose amplifier transistor . On the other end of the burnt carbon
resister is connected to the base of a BUV46 silicon NPN power transistor for switching
power applications .
If someone would please look up the 2 transistors and calculate the proper resistance
for the resistor that is burnt would be greatly appreciated . Thanks for all the help ,
Rick

 

[Charles Schuler]

OK , I will try to be more specific with the problem of the circuit . The
carbon
resistor that is burnt is connected on one end to the base of a MPS A92
silicon high
voltage, general purpose amplifier transistor . On the other end of the
burnt carbon
resister is connected to the base of a BUV46 silicon NPN power transistor
for switching
power applications .
If someone would please look up the 2 transistors and calculate the proper
resistance
for the resistor that is burnt would be greatly appreciated . Thanks for
all the help ,

If you want to experiment, replace both transistors and put in a 1000 ohm
resistor. Probably not a base connection on both ends, by the way.

 

[rijo1]

Hi Charles , according to my data sheets on the transistors and the connections
of the resister on the circuit board , both ends of the resister connects to the
base of both transistors . Thanks , I will try changing out the transistors .