EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: JanJansen on June 09, 2017, 04:13:25 pm
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Hi, i use these zero ohm links 1/4 watt for 12v and - .
I have a row cards made they take together 44mAh, the power to the rails for the cards goes via these 1/4 zero ohm links.
Now i went searching : http://www.electronics-tutorials.ws/resistor/res_7.html (http://www.electronics-tutorials.ws/resistor/res_7.html)
According to Resistor Power Rating Example No1 i dont have enoug with 1 link, i need 3 instead.
Now i read Wikipedia : https://en.wikipedia.org/wiki/Zero-ohm_link (https://en.wikipedia.org/wiki/Zero-ohm_link)
They say : For example, a surface-mounted 0805 size resistor of 0.003 ohms, rated at 1/2 watt can, in theory, safely pass up to 12.9 amperes of current.
That is a lot more.
If these 1/4 watt links are not good enough, can someone suggest me what to use instead ?
i have these :
http://nl.mouser.com/ProductDetail/Yageo/MFR-25FTE52-0R/?qs=sGAEpiMZZMu61qfTUdNhG0IXHLFuiNndEiango%2ft3K4%3d (http://nl.mouser.com/ProductDetail/Yageo/MFR-25FTE52-0R/?qs=sGAEpiMZZMu61qfTUdNhG0IXHLFuiNndEiango%2ft3K4%3d)
they also have these with a bit higher watt rating ( 400 mW ):
http://nl.mouser.com/ProductDetail/Vishay/SFR25000Z0000ZA100/?qs=oyz6OjtwqKSFE2PwBzTDHg%3d%3d (http://nl.mouser.com/ProductDetail/Vishay/SFR25000Z0000ZA100/?qs=oyz6OjtwqKSFE2PwBzTDHg%3d%3d)
Thanks in advance
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Check these 1/4 ones out at Mouser:
71-FRJ50-0-E3 - 25A @ 25 degrees C, 10 mohm
603-ZOR-25-R-520R - 10A @ 25 degrees C, 20 mohm
Why are you concerned? You are only using 44 mAh per card. Even if you have 100 cards in a row, that is only 4.4 amps.
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According to Resistor Power Rating Example No1 i dont have enoug with 1 link, i need 3 instead.
It looks like you did the calculation incorrectly.
Resistor Power Rating Example No1
What is the maximum power rating in watts of a fixed resistor which has a voltage of 12 volts across its terminals and a current of 50 milliamperes flowing through it.
There is not 12V across your resistor, there is 12V on one side and very slightly less than 12V on the other. So the actual voltage across is on the order of microvolts (0.0001V).
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Assuming you are talking about thru-hole (vs. SMD):
Remember that "1/4 watt" only refers to the SIZE of the body.
It has NOTHING to do with the actual electrical rating of what amounts to a solid piece of wire.
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Assuming you are talking about thru-hole (vs. SMD):
Remember that "1/4 watt" only refers to the SIZE of the body.
It has NOTHING to do with the actual electrical rating of what amounts to a solid piece of wire.
Strictly speaking - it actually does refer to the power dissipation capability of the device.
However, considering that it IS pretty much the equivalent of a solid piece of wire, you'll be having other issues with the magnitude of the current required to get to that power level ... like frying PCB tracks.
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Thanks, so the voltage difference only counts for selecting parts ?, nice to know.
So 1 is enough for the 44 milliampere, i was worried about nothing, next time i let em fry before asking.
btw : i have these ones to be exact : http://nl.mouser.com/ProductDetail/TE-Connectivity/1623934-1/?qs=%2fha2pyFadugkd5x9VElL9rBQfoUflu8yomTrfzEP61BzKubyve0uGw%3d%3d (http://nl.mouser.com/ProductDetail/TE-Connectivity/1623934-1/?qs=%2fha2pyFadugkd5x9VElL9rBQfoUflu8yomTrfzEP61BzKubyve0uGw%3d%3d)
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71-FRJ50-0-E3 - 25A @ 25 degrees C, 10 mohm
603-ZOR-25-R-520R - 10A @ 25 degrees C, 20 mohm
Does it not matters for the ( 0v ) ground to have 20mohm ?
Does mine also have 20mohm ?
http://nl.mouser.com/ProductDetail/TE-Connectivity/1623934-1/?qs=%2fha2pyFadugkd5x9VElL9rBQfoUflu8yomTrfzEP61BzKubyve0uGw%3d%3d (http://nl.mouser.com/ProductDetail/TE-Connectivity/1623934-1/?qs=%2fha2pyFadugkd5x9VElL9rBQfoUflu8yomTrfzEP61BzKubyve0uGw%3d%3d)
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.... considering that it IS pretty much the equivalent of a solid piece of wire,
No, it isn't "the equivalent", it is ACTUALLY a solid piece of wire. It just has a "body" molded around it for convenience of handling (automated pick-n-place, etc.)
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I also think it's quaint how they are also sometimes specified at 0 ohm +- 5% :-DD
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Thanks, so the voltage difference only counts for selecting parts ?, nice to know.
Yes, that is essentially correct. The power rating tells you how much power the resistor can dissipate (i.e. turn into heat inside the resistor), not how much power it can transmit to a load downstream. Because your resistor value is very small in this case, the dissipated power will be very small: P = R * I².