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 Post subject: Resistor wattage value?
PostPosted: Dec Mon 04, 2017 11:46 pm 
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This resistor (open) is from Treadmill motor controller MC 70A and I'm curious about the wattage....It's in series with what I believe are 2 watt Zeners one of which has shorted.( I do plan to replace it (R57) and the zeners with a higher wattage but it would be nice to know what the original wattage was...A 7 watt wire wound would fit in there nicely..but I could use a 10 watt if I wanted to...Seems to be a common failure on these boards

The only markings on it it are 1K ohm 10%.

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DSCN0186.JPG
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At the top is a 10 watt and below that a 7 watt resistor for comparison

Attachment:
DSCN0201.JPG
DSCN0201.JPG [ 127.56 KiB | Viewed 287 times ]


Ken


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 Post subject: Re: Resistor wattage value?
PostPosted: Dec Mon 04, 2017 11:57 pm 
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Hi Ken,

You need to find the original parts list or a datasheet from the resistor manufacturer.

If you know the voltage drop across the resistor, the actual dissipation (P) is:
P = E * E / R
(where R is the 1K value).

The rated power should be a few times that calculated value.
It never hurts to use a part with a higher power rating (if it fits).

The manufacturer's power rating is in 25C (77F) air with free air circulation.
If ambient temp is higher, the achievable dissipation is lower.

- Leigh

On edit: Corrected value of R to 1K

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Last edited by Leigh on Dec Tue 05, 2017 1:58 am, edited 1 time in total.

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 Post subject: Re: Resistor wattage value?
PostPosted: Dec Tue 05, 2017 1:16 am 
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Thanks Leigh for the quick reply....The treadmill belongs to a friend and although I couldn't find a schematic for it I've tested most of the components and caps(ESR)on the board and really only found a shorted zener and the open R57 resistor. The boards on Epay go for way over a 100 repaired so I thought I'd give it a shot since he really doesn't want to spend that kind of money .....Below is the replacement resistor that they used in the circuit board that they sell on their repaired boards.

Attachment:
ARF Wire Wounds 1k (2).jpg
ARF Wire Wounds 1k (2).jpg [ 59.08 KiB | Viewed 267 times ]


Thanks
Ken


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 Post subject: Re: Resistor wattage value?
PostPosted: Dec Tue 05, 2017 2:01 am 
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Hi Ken,

I'm wondering if that resistor is intentionally under-rated so it acts as a fuse if a Zener shorts.

That sounds like a high probability.

The R in the photo __seems__ to be a 10-watt part, but that's only a guess.
Don't take it as gospel.

- Leigh

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 Post subject: Re: Resistor wattage value?
PostPosted: Dec Tue 05, 2017 2:39 am 
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Leigh wrote:
Hi Ken,

I'm wondering if that resistor is intentionally under-rated so it acts as a fuse if a Zener shorts.

That sounds like a high probability.

The R in the photo __seems__ to be a 10-watt part, but that's only a guess.
Don't take it as gospel.

- Leigh



Good point about the resistor in relation to the Zener..Didn't think about that way fuse wise but it does makes sense.. I was actually wondering why they failed thinking the real problem may be down the line since it does control the motor.... From what I've I read (there's little info out there online and seen no mention of the resistor except below) the zeners are the most common failures on these boards ...I am going to increase the wattage on two of the zeners from two to 5 watt...The few companies that repair them add the resistor but I think the original manufacturer actually gives you a whole different board with a totally different set up than the original.


Below is a part of the ad for one of the outfits that repair the board.....That white resistor is where the part came from.
Attachment:
A mc70-treadmill_ad (Z).jpg
A mc70-treadmill_ad (Z).jpg [ 140.17 KiB | Viewed 247 times ]


Regards
Ken


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 Post subject: Re: Resistor wattage value?
PostPosted: Dec Tue 05, 2017 3:02 am 
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Tarpawns wrote:
...the zeners are the most common failures on these boards ...
I am going to increase the wattage on two of the zeners from two to 5 watt...
Make sure your idle current is correct for the higher-power Zener.

You may be using a part outside its proper operating region.

- Leigh

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 Post subject: Re: Resistor wattage value?
PostPosted: Dec Tue 05, 2017 3:20 am 
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Leigh wrote:
Tarpawns wrote:
...the zeners are the most common failures on these boards ...
I am going to increase the wattage on two of the zeners from two to 5 watt...
Make sure your idle current is correct for the higher-power Zener.

You may be using a part outside its proper operating region.

- Leigh


Thanks for the tip ...I'll have to check out the circuit out a little better and get back to you.

Ken


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 Post subject: Re: Resistor wattage value?
PostPosted: Dec Fri 15, 2017 4:38 pm 
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I imagine you checked this already, but sometimes the more common failure is the output devices that actually drive the motor. Check those too before powering up .. lots of times a cascade failure (zener-resistor) had it's origins further down the line.


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 Post subject: Re: Resistor wattage value?
PostPosted: Dec Fri 15, 2017 5:26 pm 
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I'd agree that the somewhat different construction of the original part was because it has special characteristics for safety reasons. Unless you are able to verify exactly what it was, any replacement is only guesswork and could very well be quite dangerous from a safety standpoint. It's common to use resistors as fuses, and that could easily be one.

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