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post #81 of 92 (permalink) Old 18-04-2017, 23:43
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Incony

Who advised that that chip would work in the application you plan?

the data sheet is not the easiest to understand that I have worked with...but.....

I see a few problems, as this is a display driver chip, and not an Analogue to digital converter!

The chip is a constant current sink. The current it sinks is fixed by the ratio of R2 in you diagram, as in 12.5/R2. This is a fixed ratio, so will give a fixed current sink.

Does the temperature gauge work by sensing voltage, or current? (Is it measuring the voltage on the wire to it (the temperature sensor is one leg of a potential divider), or measuring the current flow through the gauge (gauge is an Amp-meter)).

If it is measuring current:-

It does not matter what value of resistor you select for each step, the chip will sink the same current, so the gauge will read the same value!


If temperature gauge measures voltage, then you have a chance..... but:-

You are assuming a fixed source voltage from the gauge. However applying different resistances to ground will change the gauge source voltage.
The value of the resistors needs determining in conjunction with the equivalent internal resistance of the chip (giving the current sink). Actual resistor value = measured resistor value - equivalent internal resistance

To complicate this more, the temperature gauge voltage will be changing as each individual resistor is switched in (via ohms law) I = constant R = changing, so V=IR Voltage must change with each resistor switched in.

However it is more complicated than that, as the effective internal resistance of the chip is not fixed, and will depend upon the voltage from the temperature gauge (it is a current sink so (ohms law) R=V/I as I is fixed, then as V changes R must also change) The chip is a current sink, not a fixed internal resistor.

As said the data sheet is not clear (gives no internal circuit representation), but a Comparitor used to sink current, as shown, will have constant current characteristics.

Build it and prove me wrong

I think to use this chip you will need to use it as a driver for 10 transistor switches (or JFET much lower ON resistance) or CMOS analogue switches. These will work fine with the constant current characteristics of the 3914, and still switch the resistances you require.

Peter
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post #82 of 92 (permalink) Old 19-04-2017, 16:33 Thread Starter
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you are right gOrsq and though the idea looks ok it wont work like ive shown... there will always be a set current flow..

and so i must add an isolator for each output of the 3914... which adds more components.. i havent got a spare temp gauge to play with... so i just my two digatal meters ...

i do know that the resistor values are good enough to make the temp gauge stable enough... ive had those in place in the boot... to get the resistance values
needed to ground...

its just getting the positive rising volt input to give declining resistor values in sequence... and why at first i thought the 3914 could solve two problems at once, because though i am using the 3914 to react to the positive rising volt input.. its outputs go low... and so i thought i could juast use resistance values alone with a common output ...

i cant..because the low.. isnt really low...

and like you say.. cant do exactly what i want.. yet...

so yes.. i beats me this mo... despite my hopefull aspirations...

i am just going to give up on the idea this mo.. i have the pressure gauge in the boot.. measuring the pressure quite happilly.. its been there a couple of weeks now since i plumbed it in... and tells me all i need to know.


if i can find something that will give me falling resistance values for a rising voltage input then that would be good... one could use the temp gauge to do two things...


laughs a stepper motor turning a pot would be easier to deal with right now,
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post #83 of 92 (permalink) Old 19-04-2017, 18:30 Thread Starter
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in effect anything ive said so far about using the 3914 wont work... it needs additional components... opto isolators might be a way to go. but the turn on and off point needs to be finite... so frustrated because i thought the 3914 would do it alone... but i like the idea of using the 3914 to enable two completely different sources to combine in one... i cant think of a less component and less complicated and cheap method of making a rising positive voltage become a falling negative resistance.. ( sorry for the choice of words its how i see it in my head )

i will either source some negative turn on transistors, like the 2N3906`s or maybe SSR`s the better the isolation, and less components needed and the simpler method is where i want to go.

i might delete this whole post because of that... its wasted effort so far, as far as using the temp gauge for two purposes is concerned.

Last edited by Incony; 19-04-2017 at 19:42.
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post #84 of 92 (permalink) Old 19-04-2017, 20:08
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PNP transistors as switches should work fed by the 3914.

There is no such transducer that takes voltage and converts to resistance! The chip you specify is the best solution to this problem I have seen, though only in finite steps.

A simpler way, is to just use a transistor or fet to control the current being sunk through the temp gauge, a bit like a simple transistor amplifier, Base to the transducer, via a resistor to control base current, collector to the gauge, and emitter to ground. A few resistors will help scale the gauge.

Instead of deciding what resistance you need, measure the temp gauge current per division reading, then bias the transistor to give the range you want.
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post #85 of 92 (permalink) Old 22-04-2017, 19:15 Thread Starter
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ive got the range... the resistors decide the scale to drive the gauge... i just need to switch them in and out , in the right order, and the gauge will show what i want it to..


i am going to use probably, 2N3906`s... their drive current is @ 20ma, well within the 3914`s ability to provide..

you still dont really understand what i want to do...


use a single pole C/O switch to switch in and out on demand two completely different inputs... to one gauge on demand... to do that i must provide the gauge with inputs it can use... one i already have.. the temp... and one need to make fit.. the pressure...

when i have the time... and the bits to do it i will post again... but not this mo..
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post #86 of 92 (permalink) Old 23-04-2017, 20:04
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Quote:
Originally Posted by Incony View Post

you still dont really understand what i want to do...

Why do you think this?

This is the circuit I recommend:-

As the pressure transducer voltage rises, then the transistor will conduct more current (Range resistor needs selecting to give required current flow through the Gauge).

The upper limit and lower limit resistors will dictate the maximum current and minimum current through the gauge so you can "span" the transducer to useful transducer voltages. They might, or might not, be required. Range resistor will be very large, about 100K to convert the transducer voltage to base current.

Basically the transistor is used to simulate the temperature sensor.


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post #87 of 92 (permalink) Old Yesterday, 17:13
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Last night I had a chance to get some testing done on my 0-30 PSI transducer and got the voltage outputs over a range from 20-0 PSI.
They are on this table below, Mr Incony, these may be useful to your calculations on your resistor networks for your own circuit.

I also got a chance to power up my digital display and calibrate it accurately to the output of the transducer. My original calculations were close, by only being out by 0.4 PSI over the range, but with some fine tuning I managed to get this to 0.1 PSI over 0-20 PSI range. I capped off my tank with a plain unventing cap to allow me to get a steady pressure of 20 PSI built up.
I did not fancy going over this, even though the sensor is rated to 30 PSI.
The figures I got back should allow my max pressure of 30
PSI to be reasonably accurate anyway.
Here is the meter working with the transducer and my digital pressure switch acting as a checking device to make sure my readings are correct.
The multimeter is displaying the voltage output from the transducer.

Here is the full set up,

And finally a close up of the transducer. 0-30 PSI 0.45-4.5v linear output

Now all I have to do is think of a way to mount my meter that is not obtrusive and get it plumbed in and working.
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post #88 of 92 (permalink) Old Today, 07:32 Thread Starter
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refreshing, , ty Sundance. its good to see, and i appreciate your posts.. my reckoning then, wasnt far out.. 2.5 volts for 15 psi..

as you know ive put mine on hold for a while.. and i have been pursuing pwm control of the motor blower...

G0rsq you too give excellent advice.. and that circuit looks brill. ty..

if it can combine it with the transducer Sundance and i are using, it looks like a much better and simpler solution than mine.. and that is my aim, cheap and simple..


what transistor would you recommend G0rsq ? and now i have Sundances excellent volt scale map, your circuit is a much better road to go...
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post #89 of 92 (permalink) Old Today, 07:44 Thread Starter
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using my resistance scale v gauge reading. and Sundances volt map,

i can map the volt range to the gauge now.. and it gives me some idea of the resistance values to use in G0rsq`s circuit.

Last edited by Incony; Today at 07:49.
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post #90 of 92 (permalink) Old Today, 08:04 Thread Starter
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as an aside, G0rsq, the pressure transducer requires 5V not 12. its a three wire device, plus - minus and output v.
its range is 0.45 to 5 volts, of which i want 0.45 to 2.5 full scale. or perhaps a little more...

see later posts from sundance and myself.
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post #91 of 92 (permalink) Old Today, 14:32
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I have not given any solutions, just an idea.

The simple transistor circuit does not need to know any resistor value!

A transistor is a current control device. A small current flowing through the base makes a bigger current flow from collector to emitter.

The increase in current is the hfe (gain) of the transistor.

The transducer gives voltage, so you need to use this voltage to control the base current, so you need the "range resistor" This will be about 10k ohm or 100kohm, and will depend upon the hfe of the transistor, and the current flow through the temp gauge to indicate. You need to measure the gauge current at the two ends of its display, or find range resistor value by trial and error.Transistor will depend upon required current flow.

The other two resistors may be required to reduce the span of the transducer to the desired range, and keep this within the range of the temp gauge. Again all trial and error, as they will interact with the range resistor.

Set it up with three 100k variable resistors and a few hours playing should show if this is a viable solution.
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post #92 of 92 (permalink) Old Today, 15:34 Thread Starter
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i dont need that G0rsq if its your suggestion i need values
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