Author Topic: Help choosing a "low frequency" (under 1GHz) resistor  (Read 1047 times)

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Offline Pat PendingTopic starter

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Help choosing a "low frequency" (under 1GHz) resistor
« on: January 25, 2020, 10:05:48 am »
A termination resistor for a (DC-300MHz) differential signal shows a 120R 1% 1206 resistor rated for 0.5W.
The differential voltage across the resistor is ~0.4V and there is no direct DC path that would create the conditions for a large thermal dissipation.

Much of the literature online indicates that thin film resistors in much smaller package sizes are useful out to 1GHz before the effect of parasitics significantly kick in.
And other online discussions on DIY VNA calibration standards that extend the useful frequency range to several gigahertz.

I am tempted to replace it with a 0603 package to simplify layout but my question is why specify such a large package size and large thermal rating when the signal of interest is low power and band-limited to under 300MHz?

 

Offline Leo Bodnar

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Re: Help choosing a "low frequency" (under 1GHz) resistor
« Reply #1 on: January 25, 2020, 11:55:05 am »
Is it a field bus termination and has exposed green terminal connectors?
Then expect an electrician to stick 12V or 24V down it from time to time.
Leo
 
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Offline Kalin

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Re: Help choosing a "low frequency" (under 1GHz) resistor
« Reply #2 on: January 25, 2020, 04:28:17 pm »
Then expect an electrician to stick 12V or 24V down it from time to time.
As an electrician I feel personally attacked. Usually we stick 120V down it! No messing about with that extra low voltage rubbish.
 
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Online ConKbot

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Re: Help choosing a "low frequency" (under 1GHz) resistor
« Reply #3 on: January 27, 2020, 02:08:58 pm »
Then expect an electrician to stick 12V or 24V down it from time to time.
As an electrician I feel personally attacked. Usually we stick 120V down it! No messing about with that extra low voltage rubbish.

I've done an optoisolated digital input into a DAQ before, put a current source to drive the LED that could take up to 340V forwards, and a similar reverse diode.  This paid off within a week of putting it to use when it got hooked to a packaged system with multiple pieces of equipment in a pelican case powered via a 240V single phase generator (EU grounded neutral and 240V phase style, not US split phase style) and a ground wire broke, putting 120V with more than a couple mA of current behind it from power supply emi cap leakage on all the 'grounds'. If you can allow for it, expect mains everywhere :P

But yeah, using an oversized resistor on user accessible terminals is insurance against dumb end users to to a degree.
« Last Edit: January 27, 2020, 02:12:35 pm by ConKbot »
 

Offline tkamiya

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Re: Help choosing a "low frequency" (under 1GHz) resistor
« Reply #4 on: January 30, 2020, 12:04:22 am »
Then expect an electrician to stick 12V or 24V down it from time to time.
As an electrician I feel personally attacked. Usually we stick 120V down it! No messing about with that extra low voltage rubbish.

As yet another electrician, I AGREE!  How about 3 phase 480V?  ;D ;D
 


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