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Trying to figure out a specific cement resistor

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billymac4511

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Hi there! I'm looking for assistance in identifying a part I have. Let me offer a little background first.

I have a 3kw/6300w-peak inverter in my Jeep.

Due to vibration (Jeep is off-roaded most weekends) two of the cement resistors have fallen off the board.

The resistors in question actually total 4 that were wired onto the board, but were stacked 2-high in two separate spots. The wires pulled out of the end of the cement on the top resistor on each 'stack'.

I THINK they are 15W 1ohm resistors, but other than that, I've got no idea. This is from info I've googled. Trust me, I've googled off and on for about three weeks now and I can't seem to find a 100% sure answer, so I thought I'd ask some folks that obviously know what they're doing! LOL :)

Radio shack sells 10W 1 ohm and 15W 10ohm, but neither are the same as what I think I've got. I really don't know the voltage that they are...

The 'top' resistor's (the ones that fell off) wires were simply twisted around the bottom resistors (still on the board with the wires wrapped around them and securely soldered to the printed board). So, because of this, I'm fairly confident that if I can find the exact replacement parts, I can just untwist the old wires and remove them, and re-twist CAREFULLY the new parts on to the board. Then I hope to brace them some how so this doesn't happen again... I haven't figured that part out yet. I still need to find the replacement parts!

Anyway, here are some pictures of the application and of the specific resistors I've got with the wires broken off just inside the ceramic.

**broken link removed**
**broken link removed**

in the first pic you can see the specific part. In the second, you can see the two locations where they came from, the wires that came out of the cement, still sticking up from the two that are still attached to the board.

Any help with identifying these two resistors (they're the same, so... if you can identify one of them LOL...) I'd really appreciate it... if you can point me in the direction of where I can buy them (radio shack doesn't look good...) I'd GREATLY appreciate it! :)

Lastly, would it be possible to replace the 'double' resistor setup at each spot to a single resistor that did essentially the same thing, lessening the chance that they are going to vibrate off again?

I'm decent with a soldering iron, but definitely not a master... I think I could pull off this solder job, it's actually pretty sizable on the printed side of the board so there's lots of room for errors by a novice (like me!)
 

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With the R preceding the 1, I would expect it to be 0.1 ohm. Have you tried to measure the undamaged ones? Should be able to do that without removing them.
 
Caddock - MP821-1.00-1% - Allied Electronics
Take a look at these resistors, you can mount them to the side of your inverter and run leads to the board. looks like you have 15 watt 1 ohm resistors in parallel giving you 30 watts at .5 ohms. I would replace both sets with this type of resistor that is mounted to the metal sides of your inverter. get 2 20 watt, .25 ohm resistors and hook them up in series to get your .5 ohms (easier to wire than parallel) and if you need to, you can mount a small aluminum plate to the outside of the box to carry the heat away.
Kinarfi

If mneary is correct, and I think he is, Here's what would work, looking at your photo, the board doesn't appear to have gotten hot, so one 25 watt mounted to heat sink would probably work instead of 2 15s.

Caddock
Resistor; Thick Film; Res 0.05 Ohms; Pwr-Rtg 25 W; Tol 1%; Radial; TO-126; Heat Sink
Mfr's Part#: MP825-0.05-1%
Allied Stk#: 524-0104
View Datasheet PDF
 
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Thank you for the quick responses!

I'm pretty sure it hasn't overheated. I have used it to capacity a few times, but it is mounted to the roll bar behind the passenger seat. Usually when I have used it (the times I have pushed it to capacity too) the hard top has been off, so there has been NO air restriction around it anywhere.

Also, it has two fans at the bottom that push air up and out the top. I mounted it like this just in case one or both of the fans malfunctioned some how... since heat rises, I thought this was the best mounting setup (plus the plugs are at the top this way.)

If I am to understand correctly, kinarfi, you are suggesting that I remove the remaining two resistors and replace them with the ones you linked in your post? (One of the Caddock - MP821-1.00-1% instead of two of the cement resistors at each spot? or two of the MP821's at each spot making 4 total?) Just soldering in one of each of these resistors in place of the pairs at each point? Am I correct? I DO like the idea of having them mounted with a screw to an aluminum mounting plate. That will most certainly prevent any further vibration issues!

I have some thermal grease (from mounting a heat sink/water-block on a CPU on a liquid cooled computer I built). Should I apply that between the tab on the resistor when mounting it to the aluminum bracket? Would it help or will just the mounting pressure of a sheet-metal screw be sufficient for heat transfer?

I'm sorry for my ignorance on the specifics! This is exactly why I asked folks who know! :)

Lastly, I did test one still on the board just now. I'm not 100% sure about the meter reading, but I think it's .5 ohm. I attached a picture of the meter reading just so you folks could confirm it. :)
 

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The original resistors are marked R1 which is 0.1 ohms and J which is 5% tolerance. They are 15W.
Kinarfi is wrong.

Your meter is measuring the resistance of its leads plus the resistance of the resistor. Short the leads together and see the reading of 0.4 ohms.
 
Caddock - MP825-0.05-1% - Allied Electronics
This is the one you want 2 of, yes, use the thermal grease,
Aavid Thermalloy - 530613B00000G - Allied Electronics
2 of these might be a good idea also. You'll need 4-40 screws, washers, lock washers nuts.
Solder wires from the board to the leads and silicone them down so they don't vibrate. put some silicone around the resistor leads to stabilize them also. the leads will break off if bent more than a few times so be careful with that.
Good luck, wish I could bring my rig up to Maine and go for a ride with you.
Kinarfi
 
audioguru, please confirm that the use of the "R" represents the decimal point on resistor and caps.
I'm wasn't wrong, :) the correct info at the bottom just didn't hyperlink, except for that, thanks for explaining OP's meter readings, he should short the leads and adjust to zero prior to measuring, and if he did, I would still go with the print on the resistor.
kinarfi
 
Europeans replace the decimal point with the letter "R" so it is marked R1. Everybody else marks "0.1 Ohms".
 
Ok, I checked the tare setting on the meter. Unfortunately, it's zeroed as low as it will go... which gives the resistance in the leads as about .3 When I test the resistor it goes up a hair... to about .4ish.... so I'm guessing he's right... it's 0.1 ohms.

Do I still replace each 'bank' of 2 cement resistors with 1 of the linked parts?
 
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Yes,
Caddock - MP825-0.05-1% - Allied Electronics
This is the one you want 2 of, yes, use the thermal grease,
Aavid Thermalloy - 530613B00000G - Allied Electronics
It will insulate itself from the metal, so just bolt it to the side as discussed earlier.
You'll probably get nailed for a $5.00 handling fee for being under $25.00, so if you can think of any thing else you need, order it and it may be essentially free.
Kinarfi
 
Kinarfi has the right idea to use silicone RTV to anchor the parts on the board. We did this at a company I once worked at to keep the components from vibrating off the boards. We made radar equipment used in jet fighters.
 
Kinarfi has the right idea to use silicone RTV to anchor the parts on the board. We did this at a company I once worked at to keep the components from vibrating off the boards. We made radar equipment used in jet fighters.
Brevor, who did you work for? I used to work for Hughes Air Craft making Radars, I was an Electronic, Electrician, maintenance worker. Neat job, got to see satellites being put together and got to play with liquid nitrogen, fun stuff.
Kinarfi
 
Awesome! Thanks for the tips guys! I will put an order in shortly and reply with the results. Let's hope this gets my inverter back up and running again!
 
Hi Kinarfi,
I worked for a small company called Micron in Chicago, we made a lot of subassembles for Hughes that went into airborne and shipboard radars. We also made stuff for Teledyne, Boeing and General Dynamics. Once a month a very nice man from Hughes would drop in on us and check our test results on new units and qualify the items for acceptance.
 
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they look to be 0.1 ohm 15 watt resistors and should be readily available.
Use silicon to hold them together and to the board, as this will dampen the vibration
 
Ok, so I wanted to update folks on this... it's been a little bit but I finally pulled the trigger and purchased the recommended resistors and heat sinks. (The ones linked above)

I carefully removed the remaining 2 cement resistors and soldered in two wires about 6" long at each resistor spot and soldered the other ends to the resistor contacts. Coated the ends/solder spots with liquid electrical tape to give it a nice clean finish and to keep them from shorting on anything. I put some Artic Silver 5 on the backs of the resistors and attached them to the heat sinks, then mounted the sinks to the plastic end of the case (was much easier than drilling out the aluminum sides plus, no errant metal bits floating around.)

Put 'er all back together and hooked it up... and nothing. Gah!

Well, while I was soldering I noticed what appeared to be a burn mark on the bottom of the board... I hadn't seen this before as the board slid into the C-shaped aluminum case with this side of the board hidden. The burns were (From my uneducated guess) pretty minor, but... they still suggest something else went wrong...

The burns were around the two contact points for a capacitor... and a pretty hefty looking one at that. Sooo... I carefully removed the resistor (I learned at a young age with a disposable camera that had been taken apart, capacitors can be mean if you're not careful!)

I got some pictures of the burn mark and the capacitor... suggestions? Is there a way to test this bugger? ... no, I'm not going to put a 9V to it, then touch it to my tongue! LOL :)

Once again, I appreciate the continued help with this project! I really don't want to toss my $200 inverter if it's just a few inexpensive parts!.... especially now that I've got another $20 invested into it... not to mention I bought a new soldering iron since my old one was really weak... so, make that $40 invested... even though I've got a decent soldering iron now. ;)

Ok, so here's the pictures I got of the burn and the cap:
 

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That particular burn looks more like something fell on it or (more likely) The solder pad was disconnecting from the copper underneath and this is the arcing smoke, the nearly microscopic, cure would be just resolder it. Even better would be to scratch some of the green off to make a larger stronger solder pad. Did the tongue test pass? what volt meter do you have.
Kinari PM sent
 
Well, for what it's worth, the spots where it solders in is actually pretty large. This is nothing like a computer circuit board by any means. Most of the solder points are at least 3 MM or larger. There was a decent amount of solder where the cap was attached and it wasn't loose or anything.

Close inspection of the cap doesn't reveal that it's damaged... but then again, appearance isn't everything :eek:

My volt meeter is this lil guy **broken link removed**.
I have been looking at a nicer looking digital one but this analog one seems to do me ok for most of the stuff I do. The nicer ones I've been looking at are around $20-$30 from wal-mart/home depot.

They have three ports on the front for the leads. One is a common ground and the other two are for the positive, depending on what tests your running. Also, one of them has a voltage-presence test?... something like that... you wave it at a wall-socket and it will tell you if it's live or not... and an IR thermometer. Two nice additions for working on my Jeep. :)

I really don't want to spend much more than that on a voltmeter... I probably will never use all the other fancy gadgets they have. This analog one I have... I initially purchased as a glorified battery tester! LOL
 
Good way to check a capacitor is to measure it's resistance, I should start out as a low ohm resistor and the needle will climb to open resistor. the fast the climb, the smaller the capacitor & vise versa.
 
Good way to check a capacitor is to measure it's resistance, I should start out as a low ohm resistor and the needle will climb to open resistor. the fast the climb, the smaller the capacitor & vise versa.

Well I put a long post up listing what meter I had and some other stuff, but it never appeared... then I tried to repost it and the forum told me that I had all ready posted that same thing. /shrug/

I'm going to try again.

I've got a cheapo analog meter. Couldn't justify buying a bigger one when this one was initially purchased just to test batteries! lol
 
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