Need a resistance multiplier...

Discussion in 'The Projects Forum' started by RASelkirk, May 4, 2008.

  1. RASelkirk

    Thread Starter New Member

    May 4, 2008
    6
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    Hi All,

    Newbie here! I'm retrofitting a new fuel level gauge in my hotrod project. The new gauge requires a sender (variable resistor) that ranges from 240Ω empty to 33Ω full. The current sender has a range of 70Ω empty to 10Ω full. It looks simple enough, somewhere around a 3.3:1 "step up" in resistance would make the gauge read correctly throughout it's range. Could this be done and what would my circuit look like?

    Thanks!

    Russ
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Can you post a schematic please. It would be helpful.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    mik3,
    He doesn't have a schematic.

    Basically, the fuel gage is normally powered by the ACC circuit. The fuel gauge contains either an ammeter-style d'Arsonval movement or a nichrome-wound bimetal strip clockwork; as the current changes through the gauge, the needle moves.

    Seems like it might be easier to use a voltage divider & Vreg ahead of the fuel gauge rather than trying to multiply the resistance.
     
  4. RASelkirk

    Thread Starter New Member

    May 4, 2008
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    0
    Yes, the gauge takes 12V power and moves the needle according to the resistance it sees from the sender...

    Tell me more! I'm pretty much unfamiliar with circuits, even though I can spout off the various pieces with no problem...

    Russ
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    It's gonna cost ya - I want to see a pic of your hot rod ;)

    Anyway, have a look at the attached. On the left is a schematic representation of your original level sender and fuel gage, the latter being represented by an ammeter.
    On the right, a voltage regulator has been added. The 12 Ohm resistor absorbs much of the power of dropping the voltage down. R7 is the "fine adjust" for the voltage level. You can use this to "tweak" your gauge so that when the tank is empty, your gauge really reads "E".

    Note that "C1" should actually be two separate capacitors; one 10uF 25v electrolytic and one 0.1uF (100nF) tantalum or ceramic. The larger capacitor helps to supress low-frequency transient noise. The small capacitor takes care of high frequency noise.

    You'll have to click on the image several times in order to see it full size. They recently made a change in the forum software. Images are re-scaled to fit in a frame, but that leaves much to be desired with schematic images.
     
    Last edited: May 5, 2008
  6. RASelkirk

    Thread Starter New Member

    May 4, 2008
    6
    0
    Done! Here are the new gauges, along with a before and progress shot.

    I can build this, I actually have all the parts in my kit. Do the caps go in series or parallel?

    Thanks!

    Russ
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Cool - what is that, a '70 or '71 Maverick? What's the engine, a re-done 4.6L? Some of the Mavs came with a 302 CID in 'em.

    Yes, the caps go in parallel.
    Rule of thumb for the voltage rating of electrolytic capacitors is twice what you might expect to see in the circuit. Although the output from the Vreg will be somewhere around 4v to 4.5v, you might as well use higher voltage rating caps if you have them.

    But, it looks like your gauges (oil, fuel, volts, water) are all in one unit - do they have separate voltage supplies? Or is there just one main V+ input for all of them, and then each gauge has a separate connector to go to it's individual function/sensor/whatever?
     
  8. RASelkirk

    Thread Starter New Member

    May 4, 2008
    6
    0
    Yes, it's a '72 Mav and the engine is a 2003 Corvette LS-1. I've always had a thing for GM engines...

    DOH! My apologies, there is a common 12V and ground, then individual inputs for each (gauge) sender. Guess that'll throw a kink in the plan!

    It may just be easier to buy the proper range sender, cut off that resistor unit, and reattach it to the factory sender assembly after removing the old one. To use the "new" sender as-is, I'd need to cut another hole in my new tank (don't want to do that).

    Russ
     
  9. Jack_K

    Active Member

    May 13, 2009
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    He said he needs to step UP the resistance. Your circuit steps it down, right?

    Jack
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
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    Jack K,

    The thread is a year old. The OP has gone elsewhere.
     
  11. RASelkirk

    Thread Starter New Member

    May 4, 2008
    6
    0
    I'm still here, I do appreciate all of your inputs. I ended up "grafting" a sender with the correct resistance to my original fuel pickup tube.

    Thanks!

    Russ
     
  12. Pyrotech20

    New Member

    Jan 21, 2011
    1
    0
    Wholly thread revival batman! I have virtually the exact same issue as the OP, except in my case it would be easier to cut into the gauge voltage supply rather than trying to extract the sender from the tank (it was a lot of work to get it!). My sender has a resistance of 5ohm full -> 82ohm empty. The gauge however is expecting 5ohm full -> 102ohm empty. Not a lot in it, but just enough to trick someone into thinking they had enough gas to get down the road when in fact they don't! :)

    My question is what would the values of the resistors in the circuit prepared by sgtwookie be for my application?

    Thanks Wes
     
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