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Fusible resistors on output emitter
Fusible resistors on output emitter
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Old 10th January 2018, 12:45 AM   #1
joaoveludo is offline joaoveludo  Portugal
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Default Fusible resistors on output emitter

Hello .

What are the pros and cons of using fusible resistors on emitter of BJT outputs (EF), provided they have the recommended wattage for the circuit? (Ex.0r22 3watt)

Do fusible resistors drift more in value than other types when used in nominal conditions? Are they unsuitable for this position?

thank you.

Last edited by joaoveludo; 10th January 2018 at 10:38 PM.
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Old 10th January 2018, 10:51 PM   #2
joaoveludo is offline joaoveludo  Portugal
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bumpity?
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Old 11th January 2018, 09:56 AM   #3
AndrewT is offline AndrewT  Scotland
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fuseable will probably take too long to rupture to do any good.

Use 3W (or higher) wirewounds, or metal oxide.

Fuseables need heat, lot's of heat to rupture. That probably means they use a high tempco resistance element to get that temperature up during the incident. High tempco is not what you need. Even metal oxide is highish at around 100 to 300ppm/C
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Old 11th January 2018, 10:29 AM   #4
ilimzn is offline ilimzn  Croatia
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AndrewT has it exactly right. These are mostly designed for power supplies and points where the resistance can be low enough for the nonlinearity not to matter. As an emitter resistor they are not to be recommended, too non-linear with temperature/current (they need to be to provide semi-decent fusing action).
It should be noted that they were developed as a means to guard against fire. Regular carbon film resistors WILL catch fire when overheated sufficiently - carbon burns in air. However, fusibles have fallen out of favor in many places, because using metal film will prevent combustion, yet provide a cheap but stable 'sacrificial' element that will fail instead of, say, a PCB track or worse. Older type fusibles (remains to be seen with newer ones, manufacturers claim to have solved that problem) tend to increase resistance or even go open with age, without the current ever approaching limit values. If I had a $ for every one I replaced in vintage equipment, well... there would be a lot of $
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Old 11th January 2018, 10:36 AM   #5
AndrewT is offline AndrewT  Scotland
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One of our Members uses many paralleled resistors in most of his designs.
Using lots of 1r0 600mW 1% 50ppm/C is one way of getting good power/current handling and accuracy at your required value.
eg. 4off 1r0 gives 2.4W & 0r25, 5off gives 3W & 0r20, 18off in series/parallel gives 10.4W & 0r222
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Last edited by AndrewT; 11th January 2018 at 10:39 AM.
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Old 11th January 2018, 11:07 AM   #6
joaoveludo is offline joaoveludo  Portugal
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Ok, got it.

Maybe i'm missing something but when i searched for fusibles, and went trough a lot of datasheets, they range from around 100 to 200 ppm, just one yageo was 300ppm.
When i searched for wirewounds 5% they range from 80 to 600 ppm.
So maybe they are not so terrible in that aspect, or are there other things involved?

Edit: that parallel trick is neat.
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Old 11th January 2018, 11:14 AM   #7
AndrewT is offline AndrewT  Scotland
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Saves having to hold stock of odd values. I just bought a thousand off 1r0 from Rapidonline.

Power Diode offcuts make good leads that are pre-soldered and usually fit the bigger holes for Power Resistors.
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Old 11th January 2018, 01:14 PM   #8
ilimzn is offline ilimzn  Croatia
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Not sure how important it is to get drift low in emitter resistors - since the internal emitter resistance (not to mention the current gain of the transistor) vary far more with temperature and it follows high instantaneous currents.
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Old 11th January 2018, 11:19 PM   #9
joaoveludo is offline joaoveludo  Portugal
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And how does TCR behave in parallel resistors? Is it better?
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Old 12th January 2018, 07:59 AM   #10
AndrewT is offline AndrewT  Scotland
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proportion the currents and use that to determine the temperature rise in each resistor.
That gives you the change in resistance of each resistor. Then go back through the proportioning stage since you have a better handle on the resistances.
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