Muddle color bands on burnt resistor, how to determine value?

[straitsboy]

Hi. I'm trying to replace a blown resistor on the control board of an electric oven. Unfortunately the color bands are partially darkened. Testing it with a multimeter gives an open-circuit in the resistor. I don't have the circuit diagram or the service manual for the appliance.

It looks like the colors are: Brown, Black, Brown, Gold. (100 Ohms, +/-5%), but I'm not sure.

I'm hoping someone experienced with resistors and color bands could confirm my guess from the images below, and whether a 100 Ohm resistor is likely to be found in an oven's control board.

Thanks much.

 

[Baluncore]

Welcome to PF.
I do not think we can be much help without the circuit. Can you trace out the connections for the components pictured on the PCB. If the grey block capacitor, (value ?), was in series with the resistor then, as a snubber network, 100 ohms would be a reasonable value.
I would then question if the resistor failed because the capacitor failed short circuit?

 

[straitsboy]

Thanks for the reply. The blown resistor is connected in series with two diodes and then to a 4.7uF/400V capacitor.

The diodes tested OK in-circuit (ranges between 0.45-0.55). The capacitor looks OK physically, no bulging or burn marks. So maybe only the resistor needs to be replaced?

 

[Averagesupernova]

Resistors don't just blow for no reason. I would venture a guess that it is a resistor being used as a fuse based on how it is placed in the circuit. I will assume that the circuit described simply rectifies and filters line voltage. If I troubleshot this I would find an incandescent low wattage bulb and replace this blown resistor with this and see what happens. If the bulb lights full brightness then there is obviously a problem down-stream. I have troubleshot automotive electrical systems in this manner when the fuse/breaker immediately blows. Just unhook things while watching the light. It goes without saying to be careful here. You are messing with voltages that can do you harm.

 

[davenn]

The grey thing looks like a relay and the diode to the right of it will be the snubber diode

where is the other diode you were talking about ??

take a photo of the other side of the board please and another pic of the component side looking directly down on it

Dave

 

[straitsboy]

Here's another pair of pics, front and back (vertically inverted). Hopefully it's clearer.

Appreciate the replies.

 

[davenn]

OK that clarifies thing lots :) thanks

Now the point you have 240V marked ... I assume that's going to phase ?
where is the Neutral on the mains power connected to ?

Dave

 

[davenn]

Here's a quick cct of that area

it begs some more questions...

Is that really 240V on that connector pin ...
Is it AC or DC ?
I don't believe an electrolytic cap negative pin would be connected to 240AC Phase (HOT)
If it's 240V DC, It would probably have to be the negative rail

Ohhh and what is that IC ? I cannot quite make out the number

UK305 ? LIK305 ? something else ?
cannot find any google reference in google to my 2 guesses



Dave

 

[straitsboy]

Hi and thanks for the circuit diagram. I'm just done measuring the top terminal (in your diagram) and can confirm it is 240V AC and goes to Live. The bottom two terminals goes to Neutral. The voltage across the burnt resistor measures 130V.

That IC is labeled LNK305GN (an AC/DC converter, from google?)

Are we any closer to getting the resistor value?

 

[Baluncore]

The LNK305GN is a Lowest Component Count, Energy-Efficient Off-Line Switcher IC.
http://www.digchip.com/datasheets/download_datasheet.php?id=3810261&part-number=LNK305GN

Page 4 and 5 gives a similar circuit diagram, with a design guide.

“The input stage comprises fusible resistor RF1, diodes D3 and D4, capacitors C4 and C5, and inductor L2. Resistor RF1 is a flame proof, fusible, wire wound resistor. It accomplishes several functions: a) Inrush current limitation to safe levels for rectifiers D3 and D4; b) Differential mode noise attenuation; c) Input fuse should any other component fail short-circuit (component fails safely open-circuit without emitting smoke, fire or incandescent material).

RF1, 100 ohm ? , 2 watt.
http://www.digikey.com/catalog/en/partgroup/fkn-series/6149
http://www.digikey.com/product-detail/en/FKN2WSJR-73-100R/100DYCT-ND/2813221

 

[davenn]

an updated circuit with more of the components

Everything ... the way the circuit is wired and the data sheet info, tell me strongly that either you
1) have misidentified the Phase and the neutral, or
2) that it was wired incorrectly

I'm really sure that the positive terminals of the electrolytics would be connected directly to the phase line

The datasheet comments on the input resistor ...

Resistor RF1 is
a flame proof, fusible, wire wound resistor. It accomplishes several functions:
a) Inrush current limitation to safe levels for rectifiers D3 and D4;
b) Differential mode noise attenuation;
c) Input fuse should any other component fail short-circuit (component fails safely open-circuit without emitting smoke, fire or incandescent material).

Looking at that resistor, I would say it failed part c)

the resistor value in the datasheet is 8.2 Ohms 2W fusible and wirewound

cheers
Dave

 

[davenn]

Baluncore posted whilst I was busy drawing the circuit update and looking through the datasheet :)

Tho the datasheet said 8.2 Ohms. Its impossible, due to the damage, to tell what the value was of the one on your board. Any colours are likely to have been severely discoloured due to the heat/burning
The only way to find out for sure would be to see another working board

The next thing to keep in mind if that ... why did the fusible resistor fuse ?

1) did it fail because the phase and neutral were reversed ?
2) did it fail because of a mains voltage spike ?
3) did it fail because maybe the IC has failed ?

They would be the 3 most likely. There may be other reasons

Dave

 

[Averagesupernova]

I see no reason to suspect that phase and neutral would be reversed. At least not so far. It is possible that the resistor in question takes a hit every time the power is turned on and eventually gave out. I have seen this happen. However, it is unlikely since the power is never switched. Once plugged in, it stays on. I would not worry about sizing the resistor until it is determined what blew it out and that has been fixed. I think it is a given that the resistor failed as if it were a fuse so trouble shoot it in that manner. Hook up a bulb as I described, if that lights brightly, try a 100 ohm resistor. If there is a major short even a 100 ohm will go up in smoke. My guess is a capacitor.

 

[straitsboy]

Sorry to be a noob, regarding the identification of Phase and Neutral lines, according to my multimeter there is electrical continuity from the pin that I marked 240V to the Live terminal on the main power cable. Similarly for the pair of Neutral pins. Does that not confirm they are from Phase and Neutral? Also the colors of the connecting wire matches that of the the power cables (Blue for Live, Black for Neutral).

Regarding the thing being wired incorrectly before the thing blew, sure that's possible, after all it's a french oven. :)

Anyway, I'm going to try out what Averagesupernova suggested, even though the light bulb thing sounds dangerous. I'm also going to desolder one leg off each capacitor and test them off-circuit. So.. if I don't come back and post my findings, you'd know what happened to me...

 

[jim hardy]

Could be you have an outlet that's wired backward. I fixed one in my old house just a couple days ago.. Still cleaning up after a previous handyman(?not very) owner...

Do they have little outlet testers like this where you live ? They're $4 at Walmart here.
https://www.amazon.com/dp/B002LZTKIA/?tag=pfamazon01-20

old jim

 

[dlgoff]

Anyway, I'm going to try out what Averagesupernova suggested, even though the light bulb thing sounds dangerous.

Quick and dirty: Solder a couple of lead wires to the bulb like this (using a 120 volt light bulb instead) and solder them into the board's resistor holes.

image from http://www.robotroom.com/Model-Rocket-Launch-Controller-3.html

You'll come back safe and sound if you double check that the board isn't powered while doing the work.

 

[davenn]

I see no reason to suspect that phase and neutral would be reversed. At least not so far.

Specifically, why do you say that ?

All I see is a bunch of electros with their negative legs going to what would be the phase rail

tell me why that's not a bad thing Dave

 

[Averagesupernova]

Specifically, why do you say that ?

All I see is a bunch of electros with their negative legs going to what would be the phase rail

tell me why that's not a bad thing


Dave

Tell me why it is a bad thing. Everything is relative and here I see nothing that will hurt the capacitors as long as the diodes are working correctly. Do you remember hot chassis radios back in the day? Same thing.

 

[Baluncore]

I believe polarisation is a distraction here as the reported colours cannot be certain.
I agree that, where possible, the bulk of the floating electronics should be connected to the neutral side.
The appliance should be designed to be safe and work with either polarisation.

A fusible resistor can be destroyed by an intermittent plug or an on-off switch that arcs.

 

[jim hardy]

The 0.4 inch body length looks like a 1 watt .

Peaky current at that point in circuit has high heating value. And with 800 uf there'll be a lot of inrush.

I'd search on the model number of that oven with phrase "burnt resistor"
could be a design oversight , resistor not up to its job.

I'd use a 240 volt lamp for your lamp test.

Here's Digikey's resistor color code decoder for four and five band resistors.
My guess is it's between one and ten ohms.
http://www.digikey.com/en/resources...version-calculator-resistor-color-code-4-band


EDIT Add:

Can you identify the rectifiers? What is their surge current rating? That'll give another clue to desirable value of that resistor. It protects the diodes against overcurrent when power is applied at the line peak and capacitors are discharged.
240√2/R < surge rating of the diode rectifiers.

 

[Gordon]

It looks like a 100 ohm resistor to me + or - 5 percent, I've scorched many, I could tell you for sure if you had taken your picture on the side of the resistor with the lesser damage. Take a few 100 ohm resisters, and smoke them, and compare the outcome.

 

[straitsboy]

Hi all, I've tried the lamp test. Not only did the bulb (240v/40w) lit up brightly, the LCD screen turned on as well :). The interior oven lights also works, but that's about all it. When I pressed the power-on button, I get a long beep and an "Err" on the LCD. Ammeter indicates a current of 0.03-0.05A and voltmeter indicates 100v in this setup.

I'm going to buy a 100 ohm resistor later today and try that out.

@jimhardy: How do I measure the surge current rating on the diodes?

 

[jim hardy]

Can't measure it, got to read their part number and look it up.

I don't know whether this circuit powers the inverter for a magnetron in a microwave or just the computer board for a toaster oven. Those two applications woould use drastically different diodes. Twin 400 uf capacitors seems pretty big..

That bulb lit brightly says something was trying to draw substantial current.

Anyhow , here's a link to a datasheet for 1N4004, a typical 1 amp rectifier.
http://www.vishay.com/docs/88503/1n4001.pdf

The line of interest is in "Maximum ratings" section

Peak forward surge current 8.3 ms single half
sine-wave superimposed on rated load

and it's 30 amps. That's the one-half cycle surge rating for those 1 amp diodes.
So an 8 ohm resistor would protect them. (240 volts/8ohms = 30 amps).

If you really have 800 uf of filter capacitors
i suspect that supply is designed for at least three amps. Its diodes are probably bigger than 1N4000 series.

How many watts was your test lamp?

You have proved that the computer and its power supply function ... good work !

 

[straitsboy]

Urgh! Went back to inspect the diodes and realized I didn't hook up one of the cable. With all cables connected, and the light bulb over the blown resistor, the thing actually works! Heat and fan and everything... :)

Measuring the resistance of the light bulb, I get about 120 ohms. So I'm guessing a 100 ohm resistor should be close enough?

The appliance is a 2900w grill oven by Rowenta. This one:
http://www.appliancist.com/small_appliances/rowenta-gourmet-pro-grill-oven-oc-7868.html

 

[jim hardy]

Hmmm good work. Don's lamp idea is great !


That's a 2.9KW oven, over ten amps at 240 volts.

I think it's a safe bet the electrolytic caps you saw do NOT power the heating elements, there'll be a relay on your board for that.

Measuring the resistance of the light bulb, I get about 120 ohms...

It'll be more when it's hot.

If your new 100 ohm doesn't get hot and char the board you're probably okay.
A highly technical test is this:
Run the oven for several minutes; if the resistor "fries spit" then it's running too hot.

You might buy a ten ohm too, just in case.