Text on resistor not visible - Page 1
Forum section: electronics / beginners

author: Shivram10
author_rank: Contributor
timestamp: July 17, 2013, 09:20:28 am
content: Hi everyone,I am an hobbyist and new in electronics, my friend had an unbranded UPS which he gave me to check, when I opened the UPS I saw a black spot on the top left corner of PCBa close up of the black spotthere were 2 - 1N1007 diodes, one was fine and other was faulty and third component there was a resistor which doesn't have any color band on it but there was some text written on it which is not visible, after using the magnifier it was barely readable, it readxxPLxxJ VPA2 0809xxPLxxJ (2 letters not visible) VPA2 (letters VP are not clear but slight print is left so I am guessing here) 0809I measured the resistor with my DMM it read 1.6ohm, also there are 2 cracks developed on the resistoris the resistor ok/usable ??So now my question is -1. Can the reading be accurate? can the cracks on the resistor change its value??2. What can be the value and wattage of the resistor?I googled online with the text on resistor but no result.The resistor on left (red) is the one I want the power rating and resistor on the right is 1/4w just for comparison.any help will be appreciated,Regards,Shivram10

author: Andy Watson
author_rank: Super Contributor
timestamp: July 17, 2013, 09:56:12 am
content: Quote from: Shivram10 on July 17, 2013, 09:20:28 amDoes this value change when you heat it up? I suspect it is a thermistor that limits the inrush current.

author: Shivram10
author_rank: Contributor
timestamp: July 17, 2013, 10:16:20 am
content: @Andy WatsonThanks for the reply,I unsoldered the resistor from the circuit and then checked the values, secondly I am an beginner in electronics so I didn't understand what you meant byQuoteI just unsoldered the components when I opened the UPS, never turned the UPS on. My friend said that the UPS was tripping the Circuit Breaker of his house when he turned the UPS on. So I guessed the UPS had a short circuit inside.

author: amyk
author_rank: Super Contributor
timestamp: July 17, 2013, 10:43:25 am
content: Looks like a flameproof resistor, maybe acting as a fuse. Trace the schematic to figure out its use.

author: madires
author_rank: Super Contributor
timestamp: July 17, 2013, 10:51:57 am
content: Quote from: Shivram10 on July 17, 2013, 10:16:20 amHe meant that you should measure the resistance while heating up the resistor (hair dryer, soldering iron) to determine if it's a resistor or a thermistor.I'd guess it's a 2W wirewound resistor.

author: AndrejaKo
author_rank: Frequent Contributor
timestamp: July 17, 2013, 10:54:06 am
content: First, connect the part to a multimeter in resistance measurement mode to the part and measure the value. Write it down, which you actually already did do.Next, use a soldering iron on one of the leads to heat up the part. Check if the resistance is changing and if it is, is it going up or down. Next, if you can, put the part in a freezer for a a couple of minutes and measure the resistance. See if it changed and if it did, did it increase or decrease.I expect the part to be a negative temperature coefficient thermistor. They operate by decreasing their resistance as the temperature increases, so when the unit just turns on, the resistance will be high, limiting the in-rush current which is often caused by charging of input capacitors. After a while, the part will heat up and resistance will decrease. At that point, the capacitors should have be charged and the circuit should be ready for normal operation.In catalogs, look for phrase NTC Thermistor or just NTC (if that is indeed a thermistor) and you might be lucky and find appropriate component.On the other hand, it might be a fusible resistor used instead of a real fuse, but I don't think that such resistors in the pictured format would have printed text on them. Instead, ring marks seem to be more common.I definitely recommend replacing that part, once it's identified.Also note that there's a pretty high chance that this part isn't the only thing that died. I suspect that its failure was caused by failure of some downstream component.

author: Shivram10
author_rank: Contributor
timestamp: July 17, 2013, 11:03:07 am
content: Quote from: amyk on July 17, 2013, 10:43:25 amAs I said earlier that is an unbranded UPS, there is no number or any marking on PCB to trace the schematic,the length they went to keep the schematic secret- as you can see in the first image at the center of the PCB - right side of the buzzer there is an IC whose number is scratched off and is painted with white ink and "OP" is hand written on it.@madires & @AndrejaKook thanks for clearing my doubts I will do as you said and get back with you with my result.

author: Andy Watson
author_rank: Super Contributor
timestamp: July 17, 2013, 11:21:01 am
content: As the others have indicated, a negative temperature coefficient thermistor is often used to limit the inrush current when you switch the UPS on. However, the more I look at that package the more I am becoming convinced it is a straight forward resistor.Try this linkhttp://www.variturn.com/pdf/axiallead/vpaseries.pdf

author: amyk
author_rank: Super Contributor
timestamp: July 17, 2013, 11:54:24 am
content: Quote from: Shivram10 on July 17, 2013, 11:03:07 amAre the traces on the other side painted over too?

author: Shivram10
author_rank: Contributor
timestamp: July 17, 2013, 12:14:04 pm
content: Quote from: amyk on July 17, 2013, 11:54:24 amOn bottom of the PCB there is no text just solder joints and traces.@madires & @AndrejaKoI just checked the resistor with following -1. Normal (room temp) - 1.5ohm to 1.6ohm2. Heated (using hot air gun) - 2.8ohm to 3.0ohm3. Frozen (kept in freezer) - 1.4ohm to 1.6ohm(When I first heated the resistor measured the value and left it for some time to let it cool down to room temp and afterwards I kept it in the freezer for 10 minutes)@Andy WatsonThanks a lot, on my resistor VPA2 is written that makes it a 2W resistor as show in the data sheet.Finally thanks everyone for your replies, you just made my day.I will update the status of my repair in coming days.Regards,Shivram10

author: AndrejaKo
author_rank: Frequent Contributor
timestamp: July 17, 2013, 12:29:41 pm
content: Don't forget what I wrote: The real issue is probably somewhere further along, so make sure you get few of those resistors, if you plan to replace it and then power up the UPS.

author: SeanB
author_rank: Super Contributor
timestamp: July 17, 2013, 06:07:19 pm
content: Pretty sure what happenned is the battery died with shorted cells and the charge circuit cooked itself trying to charge it. Replace the resistor with a 1R8 5W wirewound unit ( it will be in a white ceramic boat so will be bigger) and replace the diodes with some 1N5404 3A diodes, they will be cooler running and a bit fatter so you will probably have to mount them a little above the board, the resistor will need this as well.