Analogue circuit question -advice needed please !

I should know how to do this, but please treat as a Newbie query

I have a simple circuit question for any analogue experts out there !
I'd be delighted if someone can suggest what I suspect is a simple
solution. I have no problem with digital circuitry, or making the
solution. My problem is a lack of understanding of analogue bits n
pieces and lack of time to learn due to work pressures


THE PROBLEM

- I have to link a very modern fuel-tank level probe to an old
fashioned car fuel gauge, via a simple circuit and it all runs of the
cars 12v supply.


THE DETAILS

- The supply rail will be typically 12-14vs
- The fuel probe is an expensive dedicated hyper modern capacitive
device with a pic chip type device in it. Once calibrated it generates
a high impedance output voltage of 0-5v linearly. Linearity isn't
important to me. It cant source more than a milliamp.
- The old fuel gauge is a fixed resistance 45ohm device with internal
mechanical damping. It isn't very linear but this isn't important.
The gauge reads empty to full with a voltage change across it of 0 -
10volts, and current consumption of 0-230ma or so.
- The 0 -> 5v input signal is pretty much a steady DC value so I cant
have a capacitor on the input as in a conventional simple amplifier
circuit.


THE QUESTION

- What simple FET based (or transistor!) circuit would link these two
devices ? Ideally I'd like someone to advise on a schematic, with
approx component values, that I can go off and build and tweak.


Help !! If you want to email me offlist use amremote at hotmail dot
com

Thanks in advance.
Andy / UK.

 

[kell]

- The supply rail will be typically 12-14vs
- The fuel probe is an expensive dedicated hyper modern capacitive
device with a pic chip type device in it. Once calibrated it generates
a high impedance output voltage of 0-5v linearly. Linearity isn't
important to me. It cant source more than a milliamp.
- The old fuel gauge is a fixed resistance 45ohm device with internal
mechanical damping. It isn't very linear but this isn't important.
The gauge reads empty to full with a voltage change across it of 0 -
10volts, and current consumption of 0-230ma or so.

You should cross-post to sci.electronics.design
I imagine you will get suggestions to use comparators or op-amps.
Here's an idea using transistors:

Vcc n-channel mosfet out
_____________c e_________________________________________
| |___| |
| | /
|_/\/\/\_____|_________c \
2k2 \ / about 50K
\ \
| /
npn |----------------|
| |
probe 100 / /
signal-->____/\/\/\_____/e \
| / 100K
| \ adjustable pot
| /
| \
| |
| |
|__________________|
|
|
ground

After you finish twiddling the resistor divider on the right it will
impress a certain fraction (here, you will end up setting it at about
half) of the output voltage on the base of the transistor. This gets
compared to the probe signal voltage by means of the base-emitter
junction of the bjt. If this (half) the output is .6 volt (that's the
b/e drop) more than the probe signal, the bjt pulls down the bias on
the mosfet gate. So the output voltage is regulated at (input +
..6volts) X (divider ratio). Linear, though not quite a direct
proportion.
You may have to amplify the input.
If so: get another npn, connect its collector to Vcc, its base to the
probe signal, and you will get amplified current out the emitter at a
voltage one diode drop below the probe signal. This would be a good
thing because the two diode drops we get in this circuit would
thereupon cancel each other! Cute.

 

[kell]

- The supply rail will be typically 12-14vs
- The fuel probe is an expensive dedicated hyper modern capacitive
device with a pic chip type device in it. Once calibrated it generates
a high impedance output voltage of 0-5v linearly. Linearity isn't
important to me. It cant source more than a milliamp.
- The old fuel gauge is a fixed resistance 45ohm device with internal
mechanical damping. It isn't very linear but this isn't important.
The gauge reads empty to full with a voltage change across it of 0 -
10volts, and current consumption of 0-230ma or so.

I forgot to mention: in my post with the ASCII schematic, use a
proportional font to view it. If you are in Google, click options and
"view original."

 

[kell]

Vcc n-channel mosfet out
_____________c e_________________________________________
| |___| |
| | /
|_/\/\/\_____|_________c \
2k2 \ / about 50K
\ \
| /
npn |----------------|
| |
probe 100 / /
signal-->____/\/\/\_____/e \
| / 100K
| \ adjustable pot
| /
| \
| |
| |
|__________________|
|
|
ground

After you finish twiddling the resistor divider on the right it will
impress a certain fraction (here, you will end up setting it at about
half) of the output voltage on the base of the transistor. This gets
compared to the probe signal voltage by means of the base-emitter
junction of the bjt. If this (half) the output is .6 volt (that's the
b/e drop) more than the probe signal, the bjt pulls down the bias on
the mosfet gate. So the output voltage is regulated at (input +
.6volts) X (divider ratio). Linear, though not quite a direct
proportion.
You may have to amplify the input.
If so: get another npn, connect its collector to Vcc, its base to the
probe signal, and you will get amplified current out the emitter at a
voltage one diode drop below the probe signal. This would be a good
thing because the two diode drops we get in this circuit would
thereupon cancel each other! Cute.

D'oh! I labeled the mosfet like a bjt. Instead of c and e, it should
be labeled d for drain (in place of the c) and s for source (in place
of the e).

 

[kell]

Vcc out
_____________c e_________________________________________
| \___/ |
| | /
|_/\/\/\_____|_________c \
1k \ / about 50K
\ \
| /
npn |----------------|
| |
probe 100 / /
signal-->____/\/\/\_____/e \
| / 100K
| \ adjustable pot
| /
| \
| |
| |
|__________________|
|
|
ground

Edit: you need a bipolar as shown above instead of the mosfet. The

2k2 resistor changes to a 1k 1/2 watt resistor or a few hundred ohms, 1
watt.

 

Thanks to all for the suggestion so far.

Couple of answers to the above :- I am not too concerned with absolute
accuracy due to the horribly non linear fuel guage. But yes, when the
tank is heading towards empty the probes output tends towards zero
volts. (This is for a restored classic car with a new petrol tank and
fuel level sensing, and I merely need some indication of the tanks
contents, using the original dial ! )

One thing I am wary of is loading the probes output, and the first
suggestion above has a 100ohms to ground, where as that would have to
be a few K to avoid dragging down the probes output. Maybe an input
amplifier will be needed.

Anyone else got ideas/comments ? I'll have a crack at trying it out
tommorow.
Thanks.

 

Thanks to all for the suggestion so far.

Couple of answers to the above :- I am not too concerned with absolute
accuracy due to the horribly non linear fuel guage. But yes, when the
tank is heading towards empty the probes output tends towards zero
volts. (This is for a restored classic car with a new petrol tank and
fuel level sensing, and I merely need some indication of the tanks
contents, using the original dial ! )

One thing I am wary of is loading the probes output, and the first
suggestion above has a 100ohms to ground, where as that would have to
be a few K to avoid dragging down the probes output. Maybe an input
amplifier will be needed.

Anyone else got ideas/comments ? I'll have a crack at trying it out
tommorow.
Thanks.

 

[Chris]

amremote@hotmail.com wrote:

Thanks to all for the suggestion so far.

Couple of answers to the above :- I am not too concerned with absolute
accuracy due to the horribly non linear fuel guage. But yes, when the
tank is heading towards empty the probes output tends towards zero
volts. (This is for a restored classic car with a new petrol tank and
fuel level sensing, and I merely need some indication of the tanks
contents, using the original dial ! )

One thing I am wary of is loading the probes output, and the first
suggestion above has a 100ohms to ground, where as that would have to
be a few K to avoid dragging down the probes output. Maybe an input
amplifier will be needed.

Anyone else got ideas/comments ? I'll have a crack at trying it out
tommorow.
Thanks.

Mr. Holme has provided a good, workable circuit for what you want. It
might be able to use some input protection for the op amp, but other
than that, it should work fine.

Here's a redrawing with a bit of input protection and his in-line fuse
(view in fixed font or M$ Notepad):
` VCC
` +
` |
` VCC o
` + 1/2 LM358 ( FU1
` | ) 1/2A
` D - VCC o
` ^ + |
` Vin ___ | ___ |\| |
` o----o-|___|-o-|___|--|+\ ___ |/
` 0-5V | 1.8K | 3.3K | >--|___|--|TIP31C
` .-. - .--|-/ 100 |>
` R| | D ^ | |/| | Vout 0-10V @ 0-250mA
` | | | | === o---------o-----.
` '-' === | GND | |
` | GND | .-. |
` === | 10K| | |
` GND | | | / \
` | '-' (M 1) 45 ohms
` | | \_/
` '-----------------o |
` | |
` .-. ===
` 10K| | GND
` | |
` '-'
` |
` ===
` GND
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

This should work well with a 13.8V supply. With low battery or with
the key on ACC, a full tank may look just a little low, but that
shouldn't be a problem. Choose whatever value for R that you feel you
need for circuit input impedance, or leave it out altogether. The 1.8K
and 3.3K input resistors and the diodes (use 1N4002 or better) provide
protection for the non-inverting op amp input (use an LM358 or other
single supply op amp).

The op amp, TIP31C power transistor and resistors provide a gain block
with a precise, linear voltage gain of 2, and the ability to source up
to 1/4A into your 45 ohm load.

Try Mr. Holme's circuit, you should like the results. An op amp
circuit is much easier, more accurate and less expensive than cobbling
together a circuit with discrete transistors.

Good luck
Chris

 

[kell]

Chris wrote:

Try Mr. Holme's circuit, you should like the results. An op amp
circuit is much easier, more accurate and less expensive than cobbling
together a circuit with discrete transistors.

Chris got it right. The op-amp circuit is much better. Piece o' cake,
prolly work first time out.

Kell

 

[arem_29]

hi!...i hope im not too late, try checking this link regarding your
problem,
http://www.tpub.com/electronics.htm
well, if ever you already solved it, this still can help you, in soe
other case!.. not just for electronics, also other topics like
engineering and others!..




amremote@hotmail.com wrote:

I should know how to do this, but please treat as a Newbie query

I have a simple circuit question for any analogue experts out there !
I'd be delighted if someone can suggest what I suspect is a simple
solution. I have no problem with digital circuitry, or making the
solution. My problem is a lack of understanding of analogue bits n
pieces and lack of time to learn due to work pressures


THE PROBLEM

- I have to link a very modern fuel-tank level probe to an old
fashioned car fuel gauge, via a simple circuit and it all runs of the
cars 12v supply.


THE DETAILS

- The supply rail will be typically 12-14vs
- The fuel probe is an expensive dedicated hyper modern capacitive
device with a pic chip type device in it. Once calibrated it generates
a high impedance output voltage of 0-5v linearly. Linearity isn't
important to me. It cant source more than a milliamp.
- The old fuel gauge is a fixed resistance 45ohm device with internal
mechanical damping. It isn't very linear but this isn't important.
The gauge reads empty to full with a voltage change across it of 0 -
10volts, and current consumption of 0-230ma or so.
- The 0 -> 5v input signal is pretty much a steady DC value so I cant
have a capacitor on the input as in a conventional simple amplifier
circuit.


THE QUESTION

- What simple FET based (or transistor!) circuit would link these two
devices ? Ideally I'd like someone to advise on a schematic, with
approx component values, that I can go off and build and tweak.


Help !! If you want to email me offlist use amremote at hotmail dot
com

Thanks in advance.
Andy / UK.

 

Follow up :-

Many thanks for all the ideas submitted. I now have a fully working
solution based on the op-amp idea and it works a treat.

I only wish my knowledge of analogue electronics was better so I could
contribute something back to this particular forum.

Cheers !



Chris wrote:

amremote@hotmail.com wrote:
Thanks to all for the suggestion so far.

Couple of answers to the above :- I am not too concerned with absolute
accuracy due to the horribly non linear fuel guage. But yes, when the
tank is heading towards empty the probes output tends towards zero
volts. (This is for a restored classic car with a new petrol tank and
fuel level sensing, and I merely need some indication of the tanks
contents, using the original dial ! )

One thing I am wary of is loading the probes output, and the first
suggestion above has a 100ohms to ground, where as that would have to
be a few K to avoid dragging down the probes output. Maybe an input
amplifier will be needed.

Anyone else got ideas/comments ? I'll have a crack at trying it out
tommorow.
Thanks.

Mr. Holme has provided a good, workable circuit for what you want. It
might be able to use some input protection for the op amp, but other
than that, it should work fine.

Here's a redrawing with a bit of input protection and his in-line fuse
(view in fixed font or M$ Notepad):
` VCC
` +
` |
` VCC o
` + 1/2 LM358 ( FU1
` | ) 1/2A
` D - VCC o
` ^ + |
` Vin ___ | ___ |\| |
` o----o-|___|-o-|___|--|+\ ___ |/
` 0-5V | 1.8K | 3.3K | >--|___|--|TIP31C
` .-. - .--|-/ 100 |
` R| | D ^ | |/| | Vout 0-10V @ 0-250mA
` | | | | === o---------o-----.
` '-' === | GND | |
` | GND | .-. |
` === | 10K| | |
` GND | | | / \
` | '-' (M 1) 45 ohms
` | | \_/
` '-----------------o |
` | |
` .-. ===
` 10K| | GND
` | |
` '-'
` |
` ===
` GND
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

This should work well with a 13.8V supply. With low battery or with
the key on ACC, a full tank may look just a little low, but that
shouldn't be a problem. Choose whatever value for R that you feel you
need for circuit input impedance, or leave it out altogether. The 1.8K
and 3.3K input resistors and the diodes (use 1N4002 or better) provide
protection for the non-inverting op amp input (use an LM358 or other
single supply op amp).

The op amp, TIP31C power transistor and resistors provide a gain block
with a precise, linear voltage gain of 2, and the ability to source up
to 1/4A into your 45 ohm load.

Try Mr. Holme's circuit, you should like the results. An op amp
circuit is much easier, more accurate and less expensive than cobbling
together a circuit with discrete transistors.

Good luck
Chris

 

[Chris]

amremote@hotmail.com wrote:

Follow up :-

Many thanks for all the ideas submitted. I now have a fully working
solution based on the op-amp idea and it works a treat.

I only wish my knowledge of analogue electronics was better so I could
contribute something back to this particular forum.

Cheers !

One other small thing -- you _will_ need a heat sink for your pass
transistor, if you don't have one. With a 14V supply, about 2 watts of
power will be dissipated by that transistor when the gas tank is about
half full. That's well into the "groan zone" for a TO-220 package by
itself.

Just about any heat sink made for a TO-220 package will do, but the
bigger, the better -- especially in an automotive environment, where
you can get elevated temperatures. A heat sink rated for 4 watts or
more is probably a good idea. A thin coat of silicone heat sink
compound (so you can just barely see through it when applied) is
optional.

Thanks for an interesting question, and taking the time to give
feedback. That's contribution enough.

Good luck
Chris

 

[Rich Grise]

On Fri, 08 Jul 2005 05:50:02 -0700, Chris wrote:

amremote@hotmail.com wrote:

Many thanks for all the ideas submitted. I now have a fully working
solution based on the op-amp idea and it works a treat.

Cool!

I only wish my knowledge of analogue electronics was better so I could
contribute something back to this particular forum.

Ah, contribute whatever you've got - we run the gamut.

One other small thing -- you _will_ need a heat sink for your pass
transistor, if you don't have one. With a 14V supply, about 2 watts of
power will be dissipated by that transistor when the gas tank is about
half full. That's well into the "groan zone" for a TO-220 package by
itself.

Hopefully, it'll be under the dash, and not out in the crud.

Just about any heat sink made for a TO-220 package will do, but the
bigger, the better -- especially in an automotive environment, where
you can get elevated temperatures. A heat sink rated for 4 watts or
more is probably a good idea. A thin coat of silicone heat sink
compound (so you can just barely see through it when applied) is
optional.

No, it's NOT! That's almost as bad as not using a heat sink at
all. Personally, I prefer that pasty white stuff, but probably
only because I "inherited" about a 16-oz jug of it from the
Air Force. ;-)

Cheers!
Rich