Raising a resistnace 4x through a range?

[Norcal02]

Alright guys, I'm a serious newb here. I used to be into all sorts of odd little electronic projects, but it's sort of all left me and I can't get my head around this problem I have.

In the real world, it's adapting a fuel sender that reads 60hm: empty, and 8.25hm: full, and need to use a gauge, that conveniently enough reads 240hm: empty, and 33hm: full. I need some serious help to figure out some way to raise the resistance at every point along the range of 60hm: to 8.25hm: -exactly- four times.

Possibile without anything too complicated?
TIA for any ideas you all can come up with.

 

[Pommie]

Gauges aren't measured in hm: s. By "33hm: full", do you mean that it reads full when connected to 12V via a 33hm: resistor? If this is the case then the gauge needs 400mA for full deflection. Your sender being 8.25hm: would require 3V to deliver 400mA. Try running it on 3V and see if that works.

Mike.

 

[Norcal02]

Sorry about that, the voltage is what could be changed easiest to get the desired result. I was just trying to think of it in terms of resistance only. The bigger picture sure brings in quite probably the proper way to solve it. Dur dur.

Good point though, I see where that's coming from. Unfortunately I do not have the gauge yet, as I've been trying to figure out a solution that I know would work before committing to buy them as the kit costs ~400 bucks for speedo and a cluster of 4 other gauges in one (including the fuel level) plus a tach that I'd need to buy to round out a new instrument panel.

That sounds logical. I could go ahead and buy the gauges, it is only the fuel gauge that I'm a little skeptical on. From there it shouldn't be difficult to work out.

 

[Pommie]

The other way to "fix" this would be to put a shunt on the gauge. Do not do this as this will increase the current through the sender and possibly cause heating or arcing. Maybe someone with more experience with car electrics would care to comment.

Mike.

 

[Norcal02]

Yeah, that could turn out pretty interesting considering the sender is swimming in gasoline ....but I couldn't be sure what that may do. I think I'll end up buying the gauges, it seems as though there are a few ways to go about this "problem" anyway. I appreciate the help. If anybody else has ideas, or comments on the ones above, post away.

Almost..............but just found this. =/

Hmm, one thing I just realized. The gauge in particular here that I'm looking at has 4 gauges in one. Easy enough right? (FWIW - https://www.electro-tech-online.com/custompdfs/2007/10/1118.pdf) Problem is, there is one +12v, one GND, then inputs for the other gauges that will also use the +12v. This sort of complicates things for any hope in changing the +12v to +3v. The +12v and GND terminals also happen to be used for a voltage gauge, one of the four. No screwing around with that.

What I think the diagram will end up looking like for the fuel sender would be +12v to the back of the multi gauge, it runs through the fuel level gauge, and out to the changing-resistance fuel sender. Connecting to one of the two pins that change resistance when the float is at different levels, the other pin is connected to ground.

Now that I found that PDF, I still like the idea of the quad gauge, but damn. It doesnt look like I can do much about the input voltage. The shunt would work pretty much anywhere else, but I'd rather not risk a fire, or worse

I suppose I could crack open the gauge, cut a trace, and run a separate +12v in to that individual gauge and reduce the voltage too if it worked in a trial run without any modifications, but beyond not knowing if that is even possible, it seem's a little extreme.

I could be stepping out on a limb here, but is there any way to change anything on the fuel sender side of things, or somewhere in between? Such as maybe even a little box (that would need +12v) with a circuit that intercepts the changing resistance from the sender and spits out an entirely different one? Perhaps adjustable with pots or done up with a PIC?

Unlikely, and even if so, way more work than is necessary, but would something as bizarre as that be possible? Seems I just let my mind wander, but what the heck. I'm open to ideas.

 

[Norcal02]

Yeah, that could turn out pretty interesting considering the sender is swimming in gasoline ....but I couldn't be sure what that may do. I think I'll end up buying the gauges, it seems as though there are a few ways to go about this "problem" anyway. I appreciate the help. If anybody else has ideas, or comments on the ones above, post away.

Almost..............but just found this. =/

Hmm, one thing I just realized. The gauge in particular here that I'm looking at has 4 gauges in one. Easy enough right? (FWIW - https://www.electro-tech-online.com/custompdfs/2007/10/1118-1.pdf) Problem is, there is one +12v, one GND, then inputs for the other gauges that will also use the +12v. This sort of complicates things for any hope in changing the +12v to +3v. The +12v and GND terminals also happen to be used for a voltage gauge, one of the four. No screwing around with that.

What I think the diagram will end up looking like for the fuel sender would be +12v to the back of the multi gauge, it runs through the fuel level gauge, and out to the changing-resistance fuel sender. Connecting to one of the two pins that change resistance when the float is at different levels, the other pin is connected to ground.

Now that I found that PDF, I still like the idea of the quad gauge, but damn. It doesnt look like I can do much about the input voltage. The shunt would work pretty much anywhere else, but I'd rather not risk a fire, or worse

I suppose I could crack open the gauge, cut a trace, and run a separate +12v in to that individual gauge and reduce the voltage too if it worked in a trial run without any modifications, but beyond not knowing if that is even possible, it seem's a little extreme. (Edit-AutoMeters are sealed gauges.. no go. =x)

I could be stepping out on a limb here, but is there any way to change anything on the fuel sender side of things, or somewhere in between? Such as maybe even a little box (that would need +12v) with a circuit that intercepts the line from the sender and spits out something that will allow the gauge to work properly?

Unlikely, and even if so, way more work than is necessary, but would something as bizarre as that be possible? Seems I just let my mind wander, but what the heck. I'm open to ideas.

 

[Pommie]

A 9V zener in the line from gauge to sender should fix it. It would need to be rated at >3.5W so probably 5W.

Mike.

 

[Norcal02]

If that actually would do it, that could be the ticket there. Simple solutions are always best, and I like that. If it works properly, I don't see how it could be much simpler than that. Thanks a bunch.
Justin

 

[Diver300]

The 9V zenner is a good idea. However, there are two possible problems with it.

Firstly, if the supply voltage changes, for example from 12V with the engine stopped to 14V with it running, then the voltage remaining after the zenner (12-9 = 3 up to 14-9 = 5) will change a lot.

Secondly, the guage probably has resistance that is similar to the minimum resistance of the sender. For one thing, if the sender short out, it is a good idea not to have a lot of current and sparks inside the fuel tank.......

So if the guage needs more than 3V accross it to read full you won't be able to get to the top of the scale.

You should try to keep the resistance in series with the sender the same as what it was before.

I suggest:-



R1, the zenner and the Base-emmiter drop of the transistor provide 3V to the sender. R2 is the fake guage, so that the sender has the same resistance in series with it as it was meant for. Try to find out the resistance of the original guage that the sender worked with before, and use a resistor of that value.

Because the pair is fed by 3V not 12V, the current should be 1/4 of original. The transitor has a current gain of 1 in this configuration so that current is fed to the guage.

This circuit will not be affected by the supply voltage.

You should check the collector voltage when the tank is full. If it is less than 4V the transistor is saturating and you might need a more complicated circuit.

You can adjust the zenner voltage and R2 to get the guage to read correctly over the whole range.

 

[ronsimpson]

Good idea Diver300.
I am working on the same problem with my old Ford truck. You can replace the gas tank sender to a different type!
My gauges have a 6 volt regulator. The 12-14 volts get regulated down to 6 volts before the gauge.

 

[Norcal02]

Thanks for the replies, in fact I haven't read the 2nd post up yet, but regarding changing the fuel sender to another type, as far as I am concerned, that is out of the picture. At least if I can avoid it with some fancy work between the sender and gauge. reason being, it's a BMW sender that is integrated into the pump that is twist-locked onto the tank. The oval shaped tube style sender slides through the top of the in tank fuel pump and is secured with a few nuts. If I go to a different sender, which it's looking very unlikely, not only would I have to either leave the original sender in place (or fabricate a quick and handy block off plate), I would need to drill another hole into my freshly restored gas tank. Then I would probably have to somehow secure a flange on it in order for it to have some threaded area for bolts to go through in the typical 5 bolt pattern I've seen.

No thanks I'll be doing my best to avoid having extra junk.

 

[Norcal02]

Hmm. My internet went down or I would have followed up a lot sooner. Anyway --

The 9V zenner is a good idea. However, there are two possible problems with it.

Firstly, if the supply voltage changes, for example from 12V with the engine stopped to 14V with it running, then the voltage remaining after the zenner (12-9 = 3 up to 14-9 = 5) will change a lot.

I was kinda wondering about how that would work....

Secondly, the guage probably has resistance that is similar to the minimum resistance of the sender. For one thing, if the sender short out, it is a good idea not to have a lot of current and sparks inside the fuel tank.......

How'd ya guess hehee

So if the guage needs more than 3V accross it to read full you won't be able to get to the top of the scale.

You should try to keep the resistance in series with the sender the same as what it was before.

I suggest:-

View attachment 15102

R1, the zenner and the Base-emmiter drop of the transistor provide 3V to the sender. R2 is the fake guage, so that the sender has the same resistance in series with it as it was meant for. Try to find out the resistance of the original guage that the sender worked with before, and use a resistor of that value.

Because the pair is fed by 3V not 12V, the current should be 1/4 of original. The transitor has a current gain of 1 in this configuration so that current is fed to the guage.

This circuit will not be affected by the supply voltage.

I like that last bit!

You should check the collector voltage when the tank is full. If it is less than 4V the transistor is saturating and you might need a more complicated circuit.

I sure hope not! Ill try your suggestion and report back...

You can adjust the zenner voltage and R2 to get the guage to read correctly over the whole range.

Thanks.

---

A couple of notes though, you mentioned R2, there are two R1's. Did you mean to label one of them R2?
What wattage would you suggest for the zener?
Any suggestions on a particular transistor that would be suitable? NPN right? Would any old NPN transistor work?
Last thing I can think of for now is should the ground on the sender be the same ground that is used for the zener, or should that not matter?

I hope the 3.6v on the zener is good, because I don't exactly have a big box of electronics parts, I would have to order anything if I change it up... =/

It's been too long since I've even had electronics on my mind, please forgive my extremely basic questions . . .

 

[Diver300]

A couple of notes though, you mentioned R2, there are two R1's. Did you mean to label one of them R2?

Yes. The one in series with the sender is R2

What wattage would you suggest for the zener?
Any suggestions on a particular transistor that would be suitable? NPN right? Would any old NPN transistor work?

There could be quite a lot of heat generated in the transistor so you need one that can take the power.

If R2 is 0 hm: then th.e maximum current is 3/8.25 or 0.36A. If the guage has little resistance, there will be 9V across the transistor, so the power will be 3.3W, which is large enough that you need a heatsink.

If the transistor's gain is 100, you need 3.6mA to the base, so allow 4mA, so R1 should be (12-3.6)/0.004 = 2100 hm:

If you use a 1.8 khm: then the current will be 4.7mA so there will only be 0.0047*3.6 = 17 mW dissipated in the zenner, maximum, so any will do.

You can use any method you like to generate 3.6V at the base of the transistor.

Last thing I can think of for now is should the ground on the sender be the same ground that is used for the zener, or should that not matter?

It shouldn't matter where the earths are. We are not looking at tiny voltages here.

As another thought, some BMWs have two senders, one each side of the tank and they are wired in series. There is a pump one side of the tank, and a sender + pipe assembly connected to it on the other side of the tank, fitted to the tank with the same sort of fitting. The senders are in series.

The tanks in the BMWs have a lump in the middle where the prop shaft and exhaust pipe goes. As a result, the fuel from one side can't flow to the pump. A venturi is used to get fuel to the pump side, so the levels don't stay the same on both sides, once the fuel is below the lump. That is why they have two senders.

I drive a Rover 75, and my wife drives a new Mini. Both were made by BMW and both are front wheel drive, but they both have that same fuel tank arrangement.

 

[Norcal02]

Thanks for the reply, I'll look into it a little more in depth when I get home and actually read all of it. I just scanned through it in my email. But FWIW, this is a 1971 2002, and the tank is in the trunk taking up the rear passenger side, so its not like a later model car that has the tank under the rear seat taking up both sides. The pump/sender is out of an 1990 318i with that style tank, but that's the only sender it has
^^
Uhh.. wrong LOL. Next post down..

 

[Norcal02]

Umm, I botched that. For whatever reason I didn't think it had two senders, but in fact it did. I'm going to edit this post here in a minute with the work shop manual diagram, and tomorrow I will check my daily driver car (same tank/senders as I have one of to use) to see what the deal is with the 2nd one. Whether it has the same resistances or not. It should, if it doesn't that wouldn't make any sense.

My sender has 3 pins on it, whereas the other sender only has two. One pin on the sender that I will be using has infinite resistance between the other two at any point in the travel of the float. I think the switch for low fuel is broken in it. In fact, I am positive of it. That useless pin should have a closed circuit and no resistance when the float is at the bottom. The other two function as the variable resistance between 60hm: and 4.5hm: (but again as far as we care, its 60hm: to 8.25hm. One of which goes to ground, and the other goes to the other sender, which then passes on through to the gauge.

For those of you who have contributed, please continue to keep an eye on this thread. I have ordered the gauges, and they should arrive here probably some time next week, at which point I can really test this stuff out.

Here's the official diagram of the fuel level system -
**broken link removed**

It's on the right side there. The bottom sender is the one that goes into the pump that I can use, and therefore the sender that I'm using. the one pin represented on the top right is the one I presume is broken, but that doesn't have anything to do with our circuit to get my gauge working.

IF they are both the same resistance, side to side, what does that mean for me on R2 then to replicate the original fuel gauge to the individual sender? Even that would change +/- ~60hm: due to the missing sender, assuming it has the same range. Is that critical, or can I just take the resistance of the original gauge, then add 30hm: to sort of cut it in the middle? The only reason to do that is to avoid too much current going through the sender, right?

Also, in order to gather the resistance through the gauge, do you think it would be better to measure from the red circle to red circle terminals, orrr, the blue to blue, but should I then subtract the 68hm: or does that seem like it's an integral part for the gauge to work, and not just something between the gauge and speedo (more probable to me, leaning towards subtracting it)? I'm exhausted, and just going by the idea that it doesn't hurt to ask before I go try it tomorrow. The problem is, I'd have to take my instrument cluster out of my working car, and its a PITA.

Thanks so much guys, I'd be really lost without your help. We're close to the end of this project, I promise

 

[Diver300]

If you have a working car with a dual sender guage, I would do the following:-

Disconnect where you have the red and blue circles. You might be able to do that at the back of the car, where the wire goes to the two-wire sender.

Turn on the ignition.

One side ofthe break will have a voltage on it. That is the one to look at for now.

Measure the voltage to ground. Write it down.

Measure the short circuit current to ground. Write it down.

Put everything back together.

The voltage devided by the current is the effective resistance that the pair of guages is supplied through. For one guage there should be half the resistance.

You can adjust the zenner voltage to give any ratio of resistance.

However, there is no guarantee that the guage will match the sensor at half tank even if it matches at full and empty.

 

[Norcal02]

Ill try that tomorrow, I was going to measure the gauge in the cluster itself tomorrow and reply, but I might as well update with what I know for now.

Although it isn't much, I took both of the senders out of my daily driver yesterday and measured them at full and empty. They're both roughly 4hm: to 60hm:. I think theyre close enough to the same for my purposes.
The actual lowest measurements I got from each of them were -
Primary - 4.0hmF) 59.0hmE)
Secondary - 3.4hmF) 65.7hmE)

Like I said, Ill try what you suggested tomorrow and see how it goes and post back.

Thanks

 

[Norcal02]

Okay, well I did a considerable amount of extra work till I went back and looked at your post again. I'll chalk it up to a brain fart and not thinking hard enough

Well, this is what I found-
V- 7.12 off
8.10 idling
A- 88mA off
99mA idling
Resistance through the gauge- ~81hm:
Since I'll only be using one sender, it should be 40hm:

As far as the senders go, they're linear, so I'd think they should match.

This is what I'm thinking, lemme know if I'm on the right track.
Reverting back to your schematic-
**broken link removed**
-Bottom R1 (R2) should be a 40hm: resistor.
-The 3.6v zener should stay as far as we know because I'm cutting the voltage in one fourth the supply voltage from a straight +12-14v on my project car, not cutting the 7-8v that I saw *at the sender* in my running car.
-The transistor should be a TO-220 transistor in order to be able to add a heat sink. A TIP31? I actually have one of those so that would be good.
-What should I be looking at for R1?

Thanks!

 

[Diver300]

Well done doing all the hard work. I would agree that the sender pair see 80 hm: and that one should see 40hm:

As the lower resistor is 40hm: and there will only be 3V available, the current will be about 75mA

There will be up to about 9V across the transistor so about 0.7W of heat in the transistor, assuming low guage resistance. If the guage resistance is high, there will be less heat.

The TIP31 has a dissipation of 62.5 °C/W on its own, so that only gives a rise of 44 °C so you would probably get away without a heat sink. I would bolt it to a little bit of metal on the circuit board, but more to stop vibration breaking the legs, which is likely as they are in a straight line.

The gain of a TIP31 is poor, but better at low currents. https://www.electro-tech-online.com/custompdfs/2007/11/TIP31A-DPDF.pdf page 5 says that 2 mA is enough base current for 0.3A collector current, so aim for that.

That gives the upper resistor as (12-3.6)/0.002 = 4200hm: but anywhere around there would be fine.

I hope it all works.

 

[Norcal02]

Much appreciated! If everything is on time I think I should be getting my gauges today or tomorrow. In the mean time I'll find the parts and put together the circuit. Since I have a running car and know that with both senders together it works as it should with the stock gauge, I plan to just have a secondary circuit to plug into the primary sender to test the new gauge. If the stock gauge and my new autometer one read the same, then we're golden.

FWIW, I'll post back with results when I get them. Thanks.