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Resistor wattage confusion

MrPolarBear

Feb 25, 2023
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Feb 25, 2023
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Hi all
I recently picked up a kingshill 36v5c power supply. I got it working from a previous zombie state, exept now i have an issue with the wattage of a resistor. It has two 1k resistors in parralel that helps drop voltage to create a negative bias for the opamp.

The issue arises in that the manual says the two resistors should be 1/2w, so 1w in total. Exept someone else has replaced these with two 1w resistor, so 2w in total, but the are burning hot when running and killed a transistor that was situated next to it, i replaced it with a 3w resistor but it is still very hot. The measured voltage across the resistor is 24.5v with 0.05A running through it. P=V x I = 1.2w so why is it still hot?
 

danadak

Feb 19, 2021
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All depends on the free air (if no blower in system) radiating surface area of the resistor,
hence its thermal R value. I know 1W in a TO92 transistor package will not let you hold
the body very long, unless you are trying for a tattoo....


Regards, Dana.
 

Externet

Aug 24, 2009
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Aug 24, 2009
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Welcome.
Seems your supply is still in some zombie state. Use a voltmeter to probe the output, and the voltage across the resistors.
Can the voltage be adjusted or is fixed at some value ?
 

MrPolarBear

Feb 25, 2023
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lol Dana

I Hear what you are saying Extranet, but the psu is fully functional and presents no more problems after its revival. I have highlighted the resistors that i am talking about, its seems that it is part of regulating the negative supply voltage of the opamp, so no it is not adjustable and the negative voltage presented is completely stable and ripple free. In retrospect there probably is nothing wrong. But I just dont understand why the resistor are heating up so much, even though their current rating is way above the manuals spec.
kingshill_v_series_model_c_power_supply_service_manual_removed (1)1024_1.jpg
 

Harald Kapp

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Nov 17, 2011
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Using 2 W resistors instead of 1 W resistor is technically not an issue. On the contrary.
But if these resistors are still scorching hot, I agree with @Externet that something is still not working as it should be.
Check zener diodes MR9, MR11 and MR12 for example.
 

MrPolarBear

Feb 25, 2023
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@Harald Kapp, i did replace the 1w with 2w resistors and still it was burning hot. I replaced majority of the zener diodes ( all of which you mentioned) and all the transistors. They had become very unstable, tapping them would either cause a short or for them to go open, i did not replace VT6 and VT7. The psu came from some sort of marine biology lab from my understanding and the high moisture probably caused some internal oxidation on the pins.

The only thing left now that may cause a issue might be that I replaced MR5 AND MR3 with 8.7v zener diodes instead of the specified 9v zener. The lead time for high wattage 9v zeners on RS was 2 weeks. But in my defence i dont think that it can be the root of the problem right?

and thank you @bertus, that is where i received the manual in the fist place
 

Bluejets

Oct 5, 2014
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and killed a transistor that was situated next to it, i replaced it with a 3w resistor but it is still very hot.
How did you determine the correct replacement transistor.?
The way you wrote this, it appears like you replaced the transistor with a 3w resistor.
 

bertus

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Nov 8, 2019
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Hello,

Here is also the partlist for the kingshill 36v5c:
kingshill 36v5c partlist.png

Bertus
 

MrPolarBear

Feb 25, 2023
4
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lol Im sorry, yes it does appear that i wrote it to look like i replaced the transistor with a resistor. Writing is not my best talent, im a guy that rather prefers working with numbers as thats my profession.
As for my way of determining suitable replacements, I usually do a google search, look through a couple of forums to see what others have used and then its to the datasheets for double checking and not to mention what i actually have on hand.

originals on pcbreplacement
VT3-5 BC109BC547
VT8 BCY712n2907a
VT9 BFY51BC140
 

crutschow

May 7, 2021
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resistor is 24.5v with 0.05A running through it. P=V x I = 1.2w so why is it still hot?
1.2W still must be dissipated no matter the rating of the resistors, so they will get hot.
If they are heating a close-by transistor, then you might mount the resistors with long leads above the board so they are not so close and can more freely dissipate the heat.
 

Alec_t

Jul 7, 2015
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There seems to be a connection from the OUT- rail to a tapping on R23. What's that all about?
 

bertus

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Nov 8, 2019
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Hello,

There seems to be a connection from the OUT- rail to a tapping on R23. What's that all about?

That is not a tapping but a staple of the booklet.
kingshill 36v5c binder.png
Bertus
 
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