Need resistor value on Tripp Lite PV750FC inverter

Thread Starter

Jack of some trades

Joined Mar 16, 2020
12
Hi,
We have a gifted Tripp Lite PV750FC, 750 watt inverter. A maybe two-watt resistor in it burned open. Most of the color code vaporized and what was left, well, who knows what the colors were originally. The only things it connects to are two #12 or 14 wires coming out of the transformer. The unit seems to run okay without the resistor but how well for how long? Tripp Lite won't help; they are probably scared of liability. Anybody got a line on a schematic for this beast? Couldn't find one googling. Thanks for any help you can provide.
 

Thread Starter

Jack of some trades

Joined Mar 16, 2020
12
Take pictures of it..
Not sure what you wanted pictures of so here's everything. Unfortunately, all I have is a flip phone camera. In my post I was working from memory which isn't too good. So the resistor is definitely 2-5-something (the first band looks orange in the pic but is definitely red). The last band is actually gold. The multiplier is anybody's guess because that's where all the heat was. It might be black and it might not. And that's the good side of the resistor. There was also a capacitor which is also fried. It says 0-melted mass-mfd, which really narrows it down. The series combo of the resistor and cap were across the two heavy bare wires sticking out in the picture of the transformer. The red stub was apparently NC from the factory. If you google the model # of the inverter you will see a physically larger unit with a cooling fan, so this must be an older version.
 

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dl324

Joined Mar 30, 2015
11,317
all I have is a flip phone camera. In my post I was working from memory which isn't too good. So the resistor is definitely 2-5-something (the first band looks orange in the pic but is definitely red).
I'm sure you realize that we can't help with poorly focused pictures and shifted colors.
1584502874847.png
First band looks orange as you said. Third band looks like it could be gold or brown; definitely not black.

E24 doesn't have red,green or orange,green for the first two bands. The only values with a green second band are 15x and 75x.
1584505343636.png
 
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Thread Starter

Jack of some trades

Joined Mar 16, 2020
12
Thanks, Dennis. Looking at it again, yes I guess it could be brown rather than red. See the scan comparing Burnie with actual brown (top) and actual red (bottom) on similar resistors. However, apparently red green for the first two bands on a resistor are not unknown, e.g. https://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10001&productId=2274223&catalogId=10001&langId=-1&CID=BINGMC&msclkid=f5d3fa53a946180254bfa421ea896d80.
And since it was overheated, the colors might have changed. Actually, I wasn't expecting anybody to read the color code on a fried part. Probably even a crime lab couldn't do that. I just thought there was an off chance somebody might be familiar with this inverter or have a line on a schematic. I'm going to google some other numbers from the inside of the thing and anything comes up. Will let you know.
 

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dl324

Joined Mar 30, 2015
11,317
However, apparently red green for the first two bands on a resistor are not unknown
That isn't valid for any 5% resistor (gold tolerance band). A 1 or 2% resistor requires 5 bands. Only 1% or 0.5% tolerance resistors have values starting with 25.

I'm not sure I believe that listing. This is from the datasheet on that page:
1584556144977.png
It mentions E24 and I posted standard values earlier.
 

Thread Starter

Jack of some trades

Joined Mar 16, 2020
12
Okay, I'll go with brown-green-whatever. But unless we know what "whatever" is it's kind of a moot point. Even then we'd have to know the value of the fried capacitor. I googled every number I could find on and in the inverter but nothing. So guess we'll just run it till something blows or use it for a paperweight in a hurricane. Thanks for your efforts.
 

Thread Starter

Jack of some trades

Joined Mar 16, 2020
12
Happy ending to the story: I figured the RC combo was for wave shaping and/or transient suppression. I hooked an oscope up to the output. Wave shaping? The output was your basic inverter pseudosine, a two-step square wave. There isn't much two parts can do for that. Transients? Got em, either with no load or an inductive load. I tried various combinations of R and C across that winding but nothing conclusive happened. Finally tried a 1K pot with various capacitors. Turns out there's an optimum value for R. Too high gives you one kind of spike and too low gives you a different kind. The Goldilocks point was 367 ohms. C wasn't so critical but it looked like the more the better. I stopped at 0.25 mf. That's probably a lot higher than what was in there originally but the waveform looked good and there was no smoke, so whatever works.
 

Thread Starter

Jack of some trades

Joined Mar 16, 2020
12
Thanks but yours looks like a completely different animal. It's new school high frequency; mine is 60 Hz start to finish, with a big old gorilla transformer. They probably have few if any parts in common. And BTW 6000 watts--wow.
 
Happy ending to the story: I figured the RC combo was for wave shaping and/or transient suppression. I hooked an oscope up to the output. Wave shaping? The output was your basic inverter pseudosine, a two-step square wave. There isn't much two parts can do for that. Transients? Got em, either with no load or an inductive load. I tried various combinations of R and C across that winding but nothing conclusive happened. Finally tried a 1K pot with various capacitors. Turns out there's an optimum value for R. Too high gives you one kind of spike and too low gives you a different kind. The Goldilocks point was 367 ohms. C wasn't so critical but it looked like the more the better. I stopped at 0.25 mf. That's probably a lot higher than what was in there originally but the waveform looked good and there was no smoke, so whatever works.


Here you go.....the original resistor is a 150 ohm 5% tolerance. and the cap is a 250V .47mfd +/- 10%.Inverter 1.jpgInverter 2.jpgInverter 3.jpg
 
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