Lowering a resistance range across the board? - Page 1
Forum section: electronics / projects

author: Superkryptonite
author_rank: Contributor
timestamp: June 11, 2016, 01:03:19 am
content: Hi guysI have a fuel sender for a project I'm working on and it's resistance range is ~30ohms to ~105ohms.What I'd like to do is bring it down around ~10-15 ohms across the board so that the 30ohms is between 0-10ohms, and the 105ohms is down in the 90-100 range.30-105 ->> ~~5-95Is this possible to do easily? I've looked into the possibiltiy of using a uC and a digital pot, but I fear the gauge that reads the resistance will put too much current through the chips. Also looked at using another resistor in parallel, which it seems would only lower one of the extremes (either 30 or 105) to the value I want, and not both.

author: amspire
author_rank: Super Contributor
timestamp: June 11, 2016, 02:12:27 am
content: It is very easy working with a 30 - 105 ohms range with either a micro circuit or an opamp circuit.If you put a 10mA current through the sensor, you will get 0.3v to 1.05V out. It would still be fine to run the sensor at 1mA current which would give 0.105V maximum output. It is easy to set up a LM358 opamp circuit to turn the 0.3V to 1.05V (or 0.03 to 0.105V) into, say, 0 - 2.5V or whatever you need.You might need to plot the resistance versus fuel curve since it is probably not a linear curve and you may want to correct for this. It makes sense that when the sensor says the tank is half full, it actually is half full.

author: Rerouter
author_rank: Super Contributor
timestamp: June 11, 2016, 02:58:43 am
content: Your generally looking at up to 250mA through the sensor, so no luck there,Your going to need a constant resistance load (They are suprisingly easy with a ladder DAC)You will also need a lift up resistor on the input,Attached is the circuit I'm planning to use to do the exact same thing, (Untested), and the vref pin probably needs a protection zener,

author: amspire
author_rank: Super Contributor
timestamp: June 11, 2016, 03:31:56 am
content: Quote from: Rerouter on June 11, 2016, 02:58:43 amIs there a reason you need to run fuel sensors at 250mA? Sounds high to me. I was assuming that the sender is a simple float + potentiometer combo. And I do not get why you would want a circuit that acts as a constant resistance load.Is the idea to drive a conventional car's heater/bimetalic strip petrol guage from an electronic circuit?

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 11, 2016, 03:44:24 am
content: Maybe it's a PTC level sensor that relies on self heating to work?

author: Rerouter
author_rank: Super Contributor
timestamp: June 11, 2016, 05:11:49 am
content: It depends on the type, but i was trying to cover all cases, there are a number of "bimetallic" gauges that run those currents, and even most automotive cross coil gauges are up at mid 10's to 150ish,I built my thing to sweep gauges on the bench, so i built it to those specs.In the bimetallic case they have a bimetallic regulator so it is connected with very low resistance across 12V for a short period at high current, then the contact breaks and its off for about 0.7 seconds,

author: Superkryptonite
author_rank: Contributor
timestamp: June 11, 2016, 12:48:27 pm
content: Thanks guys...eesh...a bit more complicated than I was hoping for...I'm more of a newbie when it comes to these types of electronicsAnd as Rerouter mentioned, I'm not sure of the voltage/current that is sent to get the resistance back from the sender, but I believe it's too high for the small digital pots/etc. I'm actually planning on buying a universal style gauge cluster and unfortunately the voltage/current data on that is unknown until I buy it and test it myself. Was just hoping to get some ideas before I go that route...if it's too complicated to do I might not even buy the new gauges (they run 0-90 ohm for the gauge).The style I have is one of these:Rerouter do you think the gauges read by constant current or constant voltage methods?Quote from: amspire on June 11, 2016, 02:12:27 amI've looked into opamps more and I understand the basics of them now. I've seen these types of fuel gauges online that have what I believe to be opamps installed on them with 2 pots, one for the baseline and one for the gain. How do I wire in the two pots exactly? I know one goes between the inverting input and the output of the opamp, but what about the other one?

author: Rerouter
author_rank: Super Contributor
timestamp: June 11, 2016, 10:36:08 pm
content: There are places that sell those resistance cardsHowever take a better photo of the card (lift the arm up), on those you should be able to run a tiny bit of mod wore past the first black blob, (its literally a fixed resistor)This would be out of a Toyota land cruiser at a guess, he is a cross coil gauge at about 45mA i think,To be honest i thought it was the older style with a wirewound card

author: Superkryptonite
author_rank: Contributor
timestamp: June 12, 2016, 02:04:45 am
content: Quote from: Rerouter on June 11, 2016, 10:36:08 pmThis sender unit is actually from a 2000 Audi A6. It's being put into a Honda motorcycle.

author: cs.dk
author_rank: Unknown
timestamp: June 13, 2016, 03:34:09 am
content: Why not use an original Honda fuelsensor, and modify it's lenght to match full/empty in your custom tank?

author: Rerouter
author_rank: Super Contributor
timestamp: June 13, 2016, 05:18:55 am
content: Well to that extent there is an aftermarket 5-90 ohm fuel sender produced by VDO, with the same card, if he had to replace it would probably be cheaper to grab one of those (length and shape adjustable)Only it uses the normal 5 hole pattern not the 4 hole like the one shown,

author: Superkryptonite
author_rank: Contributor
timestamp: June 13, 2016, 10:49:08 am
content: Quote from: cs.dk on June 13, 2016, 03:34:09 amWell the bike is from 1984 so parts are basically non existent anymore.As for getting a different VDO card...I guess it's possible...but for now the sender is sealed in the tank because it's installed on the bottom, and because it was bigger it was a PITA to get into the tank...I don't think I'll be taking it out again which is why I was hoping to find another option.

author: Rerouter
author_rank: Super Contributor
timestamp: June 13, 2016, 11:48:35 am
content: well from my tests, i can confirm my circuit does indeed work, (better than i thought it would to be honest)so an attiny or similar bolted on and using one of its ADC's to measure the original,There probably is some magic way to do it all analog, but unknown to me,

author: Superkryptonite
author_rank: Contributor
timestamp: June 13, 2016, 04:35:17 pm
content: Dang cool. I wish I could understand your schematic more. Are you using an ATTiny/Arduino in the end?

author: Rerouter
author_rank: Super Contributor
timestamp: June 14, 2016, 07:04:56 am
content: an SAM3X8E, technically the same processor as an arduino due, but in my own project board,The circuit works by using the buffered option on the vref pin, so the input to the ladder dac is the voltage at the input (through a divider to get in range), e.g. as more current is sunk, the voltage across the dac decreases lowering the setpoint, and lowering the amount of current that is sunk, this keeps the feedback loop at a constant resistance,Compared to say a constant current load where the input voltage would be fixed (e.g. setting the dac to use VCC for its vref),Only thing i haven't done the math on is if i can use the internal band gap without issues (vref becomes an ouptu when you select it)So any micro of your choosing that can talk via I2C can control him,