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Dynon Fuel Level & Vans Fuel gauges problem

A

ArjanPlomp

Active Member
Hi all,
Our RV-8 has the resistive type fuel level senders and the standard Vans fuel gauges. They indicate fuel levels just fine. Last weekend I installed a Dynon EMS D10 and hooked up its two (left/right) fuel level wires to the signal line on the Vans fuel gauges (so it is also connected to the fuel level senders as they go into the same signal input). Interestingly enough, as soon as the Dynon is hooked up, the Vans gauges show incorrect (too low) fuel amounts. Dynon support (great support by the way) tells me that parallel hook up may not work since the fuel level input ports on the Dynon have a pull-up resistor effecting the voltage and therefore the fuel level indication in the Vans gauges. Anyone seen or tried the same and come up with a solution?
Thanks,
Arjan Plomp
N83SE.
 
I don?t see how using multiple in parallel readouts, analog meters or EMS would ever work properly. The meter or EMS typically provides a constant current source accross the resistive sender. The meter then simply displays the voltage developed across the sender as a result of the constant current, same principle as an OHM meter. The problem with hooking multiple parallel readouts to the same sender is that each readout device will try to provide a constant current to the sender and they will fight each other doing it. End result is difficult to predict because of the many types of constant current source circuits. Bottom line, don?t hook multiple resistive readouts in parallel. At best you will get inaccurate readings, at worst you will damage the readouts.
 
Hi, fully understood. I was hoping someone would be able to provide an easy solution. I.e. convert the signal going into the Vans fuel gauge input to something the Dynon would be able to read and convert into a fuel level.
We'll stick with the analog gauges for now.
 
Dutchman in the USA

Tonny,
Yes, correct. I'm Dutch and have been in the USA exactly 10 years this month.
Met vriendelijke groet,
Arjan
 
You could do it with a high impedance OP Amp and some biasing circuitry but not sure you want to go that far to make this work. The high impedance OP amp's input could hitch a ride on the signal from the senders to the Van's guages. A high impedance would stop the tap from screwing with the signal. The OP amp could be biased to supply the signal to the Dynon.
 
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Try a series resistor...

...of quite a high value.... like 47K ohms.

If the Dynon input is a high impedance voltage input (seems like it from the installation manual) this should minimize the interaction.

The resistive fuel sensor is in the approx. range of 24 to 330 ohms.

It's worth a try, and won't blow anything up....:)

The better (maybe...:)...) option might be to use the resistor above, but attach the fuel level sense terminals to two of the GP inputs on the Dynon and configure those GP inputs as fuel level (without the 1k resistor to +5) mentioned in the manual. This option by-passes the bias current into the resistive sender.

PS ... I'm reading the D-180 manual, but I believe the electronics are similar.
 
Not the same application or problem.

The diode would do nothing to prevent the source signal from being drug around by the input of the Dynon. Also you would have to overcome the forward voltage drop of the diode.
 
Ok, here is another possibility. Don't use the guage set up in the dynon. Which brings up a question. Can you not use the guage set up and still use the fuel flow option in the dynon.
 
I would be surprised if a high impedance op-amp would work as described. If there is no-load on the Dynon fuel sender line, the Dynon (and any other type of resistive measuring device) will try to supply it’s designed current level to the sender. Since there is no load or a high impedance load, the result will be a maximum voltage on the sender signal line at the Dynon unit which will be interpreted as either a full or empty reading (can’t remember is resistance goes up or down with the fuel level). I would still be very hesitant to hook anything up to the Dynon fuel level inputs unless it simulates very closely a resistive load in the range of a fuel sender and I would be extremely cautious of connecting anything that provides an external voltage or bias to this signal. We really don’t know the circuitry in the Dynon and whether this signal is protected or not. If the Dynon constant current source is only capable of generating currents that will result in a voltage range of 0 to 3 volts, what will happen if the sender or op-amp for that matter outputs 6 volts to the Dynon? Are you willing to bet your $2,400 Dynon that it has internal over voltage protection on the signal, I’m certainly not?
 
The Dynon can be set to voltage input which is used with the capacitive to voltage converters that they sell.

This input accepts 0-5V

The OP amp could be setup to mirror the voltage drop being created by the resistive fuel level sensor of which is being created by the constant current source of the Van's guage. With some minor tweaks it would work and work very well.

An OP amps output is not high impedance, its input is. The output is typically low impedance. A simple biasing circuit on the output could take care of current limiting and scaling.

By the way, their general purpose inputs are not constant current but instead single ended voltage inputs. I suspect the same for the fuel level inputs when in resistive sender mode but can't be sure.
 
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Brian,

Sounds like you have a well thought out design, good luck with it, might even be a market for it.
 
Naaa,

Not many people wanting dual fuel guages...Not me anyway so most likely won't take the time to work it out.

It can be done though. The risk to the Dynon would be nil since one could test without it hooked up to the Dynon. Once you had the output spanning o-5v, then it could be connected.

The Dynon is most likely protected on its input for anything between 0-5v due to the fact that it has both resistive and voltage sensor modes. If someone hooked up one of the cap to volt converters while the unit was set for resistive, that would be bad if they did not have it protected.

Also one could imitate a resistive sensor with an op amp and a cheap transistor so even that mode could be configured to work.
 
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Yeah,
It reminds me of the guy with two watches not really knowing what time it is.

I have limited experience using a Dynon FD180. When you fuel you input the fuel amount in the computer and the fuel flow subtracts from this giving a computed quantity. After calibrating the Dynon gages they read a max of 18+ gallons, since this is the quantity where the floats top out in my aircraft (ie float sensor value doesn't change anymore). I've been logging the difference between the computed fuel and the gage fuel remaining. In level flight they differ normally by less than 1 gallon. I haven't finished dialing in the exact "K" factor yet, but it is close. The Dynon uses computed fuel remaining for other computations such as fuel remaining at next waypoint and maximum range based on current trend. The quantity gages just display what the floats are seeing and are influenced by the aircraft attitude. The nice feature about the quantity gages is that they set off alarms that the user pre-sets for low quantity. The computed fuel remaining has no alarm function. I think using both computed and gage fuel remaining is the way to go.
 
The Dynon Fuel Level inputs and GP inputs have the same (or very similar) circuitry behind it (according Dynon support) with the same pull up resistor. So switching between fuel level inputs and GP inputs will not matter.
Since the Dynon can be switches to read voltages for capacitive fuel senders, it should be very easy to use an Op Amp and a few resistors. If anyone can provide me with the technical information on the Vans fuel gauges and fuel senders, that would help me design a simple circuit. Risk to the Dynon is negligable.
The Dynon will show fuel flow, regardless of using the fuel level inputs.
Thanks for all your input so far,
Arjan Plomp
 
I just realized something, but I could be wrong:
The pull up resistor in the Dynon circuitry must have a very high value compared to the resistance in the Vans fuel level circuit and therefore not influence the Vans reading just by itself. The fact that the Dynon is to resistive and therefore provides a current probably causes the change in reading on the Vans instruments. Therefore, if the Dynon is set to resistive to capacitive and only reads a voltage, it should not effect the reading on the Vans gauges. My assumption is only correct when:
- the pull-up resistor in the Dynon is very high compared to the resistance in the Vans fuel level circuitry
- the Dynon has a circuitry to provides a constant current which is switched off when changing to capacitive readings.
Any thoughts on this?
The solution could be as simple as changing the Dynon to capacitive readings and calibrate the fuel levels.
Arjan Plomp
 
I suspect that they do not use a constant current source on this input but instead just a pullup resistor creating a voltage divider when connected to a resistive sender and the unit is in resistive mode.

I do also suspect that they turn off this pullup voltage when in capacitive mode. Just ask them, maybe they will tell you????

If this is true, changing to the capacitive mode might work only if the voltage input circuit is high impedance. Not sure it is but it might be. Ask them this as well???

Sure can't hurt to try it as long as you do not put more than 5 volts on that input, the Dynon should be safe.
 
Resistors

Brantel said:
I suspect that they do not use a constant current source on this input but instead just a pullup resistor creating a voltage divider when connected to a resistive sender and the unit is in resistive mode.

......
Click to expand...

Brian, the fuel inputs and the GP inputs are different.

If you want to use any GP inputs for a resistive fuel sensor, then the IM calls for a 1 K resistor to the Dynon +5 line. With the 33 to 240 ohm range of the sensor (approx. - numbers from memory) this makes sense.

I still think a high value resistor from the Vans fuel sensor line to a GP input would do the trick. If the Vans instrument gives a voltage higher than 5 volts, then use two 10 K resistors in series to ground, and connect the center point to the Dynon GP input.

Should be easy enough to test, and certainly won't do any harm to the Dynon since the lines have already been hooked up directly....:)

An isolating op-amp is probably overkill here...
 
Exactly my point! However, Dynon says that the GP and Fuel level inputs are very similar. They will call me back with more details later today. Two 10K resistors will have no effect to the 33-240Ohm circuitry, so that should work or 10K and 14K (15K) to really keep it down in the 0-5V range. I currently have one full and one almost empty tank in the RV, so that will help me get the voltages on the Fuel Gauge inputs.
 
Please post solution in TIPS

ArjanPlomp said:
I just realized something, but I could be wrong:
The pull up resistor in the Dynon circuitry must have a very high value compared to the resistance in the Vans fuel level circuit and therefore not influence the Vans reading just by itself. The fact that the Dynon is to resistive and therefore provides a current probably causes the change in reading on the Vans instruments. Therefore, if the Dynon is set to resistive to capacitive and only reads a voltage, it should not effect the reading on the Vans gauges. My assumption is only correct when:
- the pull-up resistor in the Dynon is very high compared to the resistance in the Vans fuel level circuitry
- the Dynon has a circuitry to provides a constant current which is switched off when changing to capacitive readings.
Any thoughts on this?
The solution could be as simple as changing the Dynon to capacitive readings and calibrate the fuel levels.
Arjan Plomp
Click to expand...

Once you find a solution that works, could you please post this under TIPS. I plan to use Dynon and will use the Van's Gauges. If you could add pictures that would be greatly appreciated, I am sure with the popularity of Dynon more will go this route.

Thanks
 
ArjanPlomp said:
Exactly my point! However, Dynon says that the GP and Fuel level inputs are very similar. They will call me back with more details later today. Two 10K resistors will have no effect to the 33-240Ohm circuitry, so that should work or 10K and 14K (15K) to really keep it down in the 0-5V range. I currently have one full and one almost empty tank in the RV, so that will help me get the voltages on the Fuel Gauge inputs.
Click to expand...

The purpose of the two 10k in series would be to create a voltage divider which would in turn 1/2 the voltage seen across the sender. If you use disimilar values, you will get something other than 1/2 the voltage.

What you really need to know is how much current the Van's gauge is providing to the sender. Once you know that, it is easy to see what your voltage range will be to the Dynon. Once you know that you can select the best resistor ratio to scale the output to the Dynon as close as you can get it to 0-5V that way you use the AD converter efficiently and have more resolution.

To find this current, just put an amp meter inline with the sender and Van's fuel guage.

Hopefully the input impedance on the Dynon input when set to Capacitive mode is high so you won't need the OP amp.
 
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Dynon just confirmed that when set to Capacitive input, it reads voltage (as indicated in the EMS-D10 manual) and has a high impedance.
Maybe I'm wrong, but knowing the current thru the Vans fuel level sensor is not needed, as long as one knows the max voltage at the Vans fuel gauge input. (since I have one full tank and one empty tank at the moment, it will be easy for me to find out later today). I.e. if that voltage is 12V maximum:
Vans fuel gauge input: (assumed 12V maximum, if indeed the maximum value at this input is lower, the resistors below can be changed as to use the A/D converter efficiently)
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47K Ohm
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Dynon Fuel level input (now maximum 4.95V)
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33K Ohm
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Ground.

Maximum current 12V/80K=0.15mA (Max). If the impedance on the Dynon is not high enough, one could try 4.7K and 3.3K (1.5mA Max)
 
Sorry we missed this until today.

As has been mentioned, if you put us in capacitive mode, we turn off our internal pull-up, and just measure voltage. But we are limited to 5V. Put more in, and you won't damage stuff permanently, but we won't read right when the voltage is above 5V either.

So you could use a simple voltage divider to get 5V from your 12V gauge. We are very high impedance on that input (meghoms) when in capacitive mode, and we calibrate for your actual tank, so even if you happened to give us 1.87V to 3.62V, we'd still be OK.

Remember though, in this setup, there is NO redundancy. The power and current for the float sender comes from your analog gauge. So if that gauge fails, changes resistance, doesn't regulate master power voltage changes out, varies with temperature, or just goes wonky, the reading on your EFIS is toast. So basically, when hooked up this way, always be wary of the reading on the EFIS, since we can no longer calibrate our reading well.

When you use the internal pull-up, it's tied to a regulated, referenced, and often battery backed up source, which is how we give you better than 1G accuracy across your tank.

If you do this, please cal your tanks with the engine running, since I bet the current the analog gauge sends to the sender varies when master power goes from 12V to 14.4V.
 
The constant current supply in the gauge will vary the voltage needed to get the current thru the sender to remain constant.

Knowing this exact current would allow you to calculate what you need on paper but if you do it your way, it will serve the same purpose so it does not matter.

Your dead on what you need to do with the resistor network.

Now that Dynon has confirmed what we were all speculating (simple voltage input when set to capacitance) and that the input is very high impedance, you won't need the OP amp.

This should work fine but keep in mind what they told you about the redundancy issue if that was your goal.

ArjanPlomp said:
Dynon just confirmed that when set to Capacitive input, it reads voltage (as indicated in the EMS-D10 manual) and has a high impedance.
Maybe I'm wrong, but knowing the current thru the Vans fuel level sensor is not needed, as long as one knows the max voltage at the Vans fuel gauge input. (since I have one full tank and one empty tank at the moment, it will be easy for me to find out later today). I.e. if that voltage is 12V maximum:
Vans fuel gauge input: (assumed 12V maximum, if indeed the maximum value at this input is lower, the resistors below can be changed as to use the A/D converter efficiently)
|
|
47K Ohm
|
|
|
Dynon Fuel level input (now maximum 4.95V)
|
|
33K Ohm
|
|
Ground.

Maximum current 12V/80K=0.15mA (Max). If the impedance on the Dynon is not high enough, one could try 4.7K and 3.3K (1.5mA Max)
Click to expand...
 
Dynon posted their reply and they told me the same over the phone. I can confirm that it works this way. A voltage divider is not even necessary as the voltages were reading 0.5 Volts with a full tank and 2.1 with an empty tank. This means that only roughly about 40% of the A/D convertors accurary will be used, but oh well.......

I calibrated the right tank with aircraft level, engine not running and all seems to work well. However I did not fly the right tank enough yet to cross check the readings with the Van's gauge or Dynon fuel flow. However, I'm pretty convinced it is now working just fine.

Thanks for Dynon for their great support.
Arjan (Alexander) Plomp
 
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