kentmagpie
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| posted on 12/3/15 at 09:25 AM |
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pull up resistor
Hey guys,
I'm currently configuring my race technologies dash2 and I'm having a few niggles. My fuel gauge wasn't reading but is wired in.
After speaking to race technologies themselves they say I have to run a wire using a pull up resistor of around 250ohms. I've rung a couple of
places that specialise in car electrics and they didn't have a clue what I was on about. To give you a heads up ill input the wiring diagram so
it makes more sense. Any info on where I can pick one of these up would be much appreciated.
http://www.race-technology.com/wiki/index.php/HowDoI/Connect1Or2WireSensorToDL1
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loggyboy
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| posted on 12/3/15 at 09:34 AM |
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IIRC a pull up resistor isnt a type of resistor, its just a resistor being used to perform a certain function in a circuit.
Mistral Motorsport
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kentmagpie
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| posted on 12/3/15 at 09:40 AM |
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quote:
Originally posted by loggyboy
IIRC a pull up resistor isnt a type of resistor, its just a resistor being used to perform a certain function in a circuit.
Thanks for ur help so basically i just need a resistor 
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HowardB
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| posted on 12/3/15 at 09:44 AM |
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the value of the resistor is typically high enough to pull the voltage and to conduct low current. Values are normally greater than 1k ohm
Howard
Fisher Fury was 2000 Zetec - now a 1600 (it Lives again and goes zoom)
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DW100
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| posted on 12/3/15 at 09:50 AM |
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Yeah most car electrics places will have no idea what you are talking about.
But this is like what you'll need
http://www.maplin.co.uk/p/metal-film-06w-240-ohm-resistor-m240r
This is the closest common value to 250 Ohm if that is what is specified. You are making a voltage divider circuit looking at the diagram
[Edited on 12/3/15 by DW100]
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kentmagpie
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| posted on 12/3/15 at 10:21 AM |
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Guys thanks for all your help. I'll be popping up maplin later then
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kentmagpie
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| posted on 12/3/15 at 07:20 PM |
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Ended up getting a 270 ohm version in the end but it did the job guys. Thanks again
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MikeRJ
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| posted on 13/3/15 at 08:23 AM |
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It's a bit late, but I would have gone for something a bit bigger than a 0.6 watt resistor.
The Race Technology website says that it uses a 12v reference voltage (probably just system voltage), so when the fuel level sensor is close to zero
ohms (i.e. very full tank) the pull-up resistor will be dropping 12v. This equates to a power of (12*12)/270 = 0.53 watts, almost on the limit for
that resistor and probably over the limit once all the de-rating parameters have been accounted for (max power dissipation reduces as ambient
temperature increases). Irrespective, that resistor will be getting very hot under this condition.
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peter030371
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| posted on 13/3/15 at 08:43 AM |
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quote:
Originally posted by MikeRJ
It's a bit late, but I would have gone for something a bit bigger than a 0.6 watt resistor.
The Race Technology website says that it uses a 12v reference voltage (probably just system voltage), so when the fuel level sensor is close to zero
ohms (i.e. very full tank) the pull-up resistor will be dropping 12v. This equates to a power of (12*12)/270 = 0.53 watts, almost on the limit for
that resistor and probably over the limit once all the de-rating parameters have been accounted for (max power dissipation reduces as ambient
temperature increases). Irrespective, that resistor will be getting very hot under this condition.
I would connect the divider across the reference 5V output for better stability as advised by them on the examples they show....this will mean you
have a maximum draw of 100mA with a 270ohm resistor.
Unfortunately this is twice the power the 5V on a Dash2 can provide so you really need a 510ohm resistor so why did they advise 270? If you want to
try a 510ohm resistor on the 5V I can stick one in the post to you as we have thousands at work and they cost bugger all 
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kentmagpie
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| posted on 13/3/15 at 09:28 AM |
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quote:
Originally posted by peter030371
quote:
Originally posted by MikeRJ
It's a bit late, but I would have gone for something a bit bigger than a 0.6 watt resistor.
The Race Technology website says that it uses a 12v reference voltage (probably just system voltage), so when the fuel level sensor is close to zero
ohms (i.e. very full tank) the pull-up resistor will be dropping 12v. This equates to a power of (12*12)/270 = 0.53 watts, almost on the limit for
that resistor and probably over the limit once all the de-rating parameters have been accounted for (max power dissipation reduces as ambient
temperature increases). Irrespective, that resistor will be getting very hot under this condition.
I would connect the divider across the reference 5V output for better stability as advised by them on the examples they show....this will mean you
have a maximum draw of 100mA with a 270ohm resistor.
Unfortunately this is twice the power the 5V on a Dash2 can provide so you really need a 510ohm resistor so why did they advise 270? If you want to
try a 510ohm resistor on the 5V I can stick one in the post to you as we have thousands at work and they cost bugger all
Hang on. I bought 2 resistors yesterday just in case I damaged the first etc. Can I run the second in parallel??? would that create 540ohms and 1.2
watts??? otherwise I also have a 240 ohm resistor which im theory would create the 510 u suggest.
Correct me if im wrong but im not great with electrics
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peter030371
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| posted on 13/3/15 at 09:35 AM |
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If you have two 270ohm then you can wire them in series to make 540ohm 
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kentmagpie
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| posted on 13/3/15 at 09:42 AM |
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quote:
Originally posted by peter030371
If you have two 270ohm then you can wire them in series to make 540ohm
Ok nice one thanks
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MikeRJ
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| posted on 13/3/15 at 12:04 PM |
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quote:
Originally posted by kentmagpie
Hang on. I bought 2 resistors yesterday just in case I damaged the first etc. Can I run the second in parallel??? would that create 540ohms and 1.2
watts??? otherwise I also have a 240 ohm resistor which im theory would create the 510 u suggest.
Correct me if im wrong but im not great with electrics
Parallel connection gets you 135 ohms and 1.2 watts. Series connection gets you 540 Ohms and 1.2 watts.
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loggyboy
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| posted on 13/3/15 at 12:36 PM |
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quote:
Originally posted by peter030371
If you have two 270ohm then you can wire them in series to make 540ohm
Pretty sure it would be 270 still.
the first resistor would drop the current, the 2nd has no effect.
Its like having 2 taps in series., which ever is set lowest is the flow you get, regardless of the other one..
[Edited on 13-3-15 by loggyboy]
Mistral Motorsport
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BaileyPerformance
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| posted on 13/3/15 at 12:50 PM |
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quote:
Originally posted by loggyboy
quote:
Originally posted by peter030371
If you have two 270ohm then you can wire them in series to make 540ohm
Pretty sure it would be 270 still.
the first resistor would drop the current, the 2nd has no effect.
Its like having 2 taps in series., which ever is set lowest is the flow you get, regardless of the other one..
[Edited on 13-3-15 by loggyboy]
Wrong, two 270 res in series gives 540 ohms
two 270 res in parallel gives 135 ohms
two resistors in series of the same value will drop the same voltage - if connected across a 5v supply the middle of the two resistors would read half
the supply voltage (2.5V).
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loggyboy
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| posted on 13/3/15 at 01:56 PM |
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Thanks for the clarification - no idea what made me think that... electrickery!
Mistral Motorsport
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