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Topic: LM2917 Frequency to Voltage Converter (Read 8600 times) previous topic - next topic

matinzk

Hi everyone.

I have made use of a frequency to voltage converter (LM2917, 8pin version) to convert the frequency from my car's RPM tach signal wire to voltage to then read it with my Arduino and make a digital RPM readout.

Here is the circuit I have done:


The resistor, capacitor and the diode connected to the tach signal wire is to get rid of the DC offset.

There are 2 problems at the moment:-
1) The chip doesn't convert the signal properly, the higher the RPM from the car the less accurate the output voltage, hence a very off RPM compared with the RPM dial in the car. For example when car's RPM dial reads 3000 RPM the digital output reads 2400 RPM.

2) The chip itself gets very hot even if there is no signal into it and even when nothing is connected to the output voltage pin.

Has anyone got any experience with this chip at all?

Any help is appreciated.

Thanks

winner10920

A quick look at the datasheet suggests the diode on the input isn't necessary, it seems desined to have the tach ac input, no experience with the ic tho

matinzk

I've checked the current that the circuit takes and it's about 65mA. It's supposed to take less than 5mA. This is with the input signal frequency and the output voltage disconnected.

Anyone?

wanderson

In your circuit have you connected pins 4 & 7 to pin 6?  That schematic indicates they shouldn't be, but with that style it would be easy to misread that.
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Far-seeker

#4
Aug 23, 2012, 10:56 pm Last Edit: Aug 23, 2012, 11:04 pm by Far-seeker Reason: 1

In your circuit have you connected pins 4 & 7 to pin 6?  That schematic indicates they shouldn't be, but with that style it would be easy to misread that.


I think in the style used by matinzk only dots or filled circles indicate connected lines.

Edit: Added missing word I thought was there.

wanderson



In your circuit have you connected pins 4 & 7 to pin 6?  That schematic indicates they shouldn't be, but with that style it would be easy to misread that.


I think in the style used by matinzk only dots or filled circles indicate connected lines.

Edit: Added missing word I thought was there.


Yep, that was why I asked.  I suspect the OP might have wired those connections by misreading the schematic.
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Current version 1.0.1

matinzk

Nope I don't have pins 4 & 7 connected to pin 6. Only pin 4 & 7 are connected and 5 & 6 connected together.

Docedison

Although not the high current issue, The reason for the discrepancy in the speed is due to that series capacitor and diode... at some point probably around 2000 - 2200 rpm that capacitor is going to be charged high enough to start to 'block' the input pulses... connect a meg or so from the diode capacitor junction to ground... Not enough to affect accuracy, just enough to make sure the diode operates as a diode. What happens is that the diode is cut off by the dc generated from the input pulses. At some input condition the diode becomes reverse biased and cut off except for the diode and capacitor leakage and the load resistance which is the reason it 'sorta' works above 2 - 2.5K rpm's.

Doc
--> WA7EMS <--
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matinzk

Thanks again for the replies.

I connected an oscilloscope to the RPM pulse wire, with the DC offset circuit (resistor, capacitor & diode) also connected and did some test with the pulse going through the DC offset circuit and the frequencies seems to be fine.

Another thing I've found is that the reason why at higher RPMs it lows lower, is that, since the chip was getting hot the accuracy was dropping, so it seemed it was reading incorrectly. So if I leave the circuit off before it's hot, and then inject 100Hz (3000RPM) to it using an Arduino, it will read the way it should. Then of course drops when the chip gets hot =(

Docedison

I think (Know) you will find a negative voltage present on pin 1 when the chip gets hot... and there is no path to "bleed" off the positive voltage, Under "No Signal" conditions?.
If you look at the series diode capacitor connection and think about it carefully you will see that at some input condition (pulses in) the cathode of the diode must go positive as the diode prevents the capacitor from discharging and at some point the diode is biased off... it's Basic electronics theory... Electronics 101... try something, measure the current without any input whatsoever (disconnect everything from pin 1) if it is still @ the 60? - 70? mA... If so the device is defective. I've read the attached LM2917 data sheet 3 times and I can't find a schematic even close to your drawing... and pin 1 is the + input to an Op-Amp... The non inverting input to any op-amp is a very high impedance point... There is no discharge path for that 2R2uF cap... which may have damaged the op-amp input...

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

matinzk

The diode, cap and resistor aside and completely disconnected from the circuit and the pin 1 of the chip, the circuit pulls that much current.

I've tested this with a few of these chips that I've got. All the same.

winner10920

Did you try all of these ics in the previous circuit? Perhaps you have burned them all,

matinzk

No,

These have all been brand new with the above disconnected.

Docedison

Did you look at the data sheet I included? and see if perhaps one of the other 'suggestions' might be more applicable. What happens, If you Ground pin 1, to the current drain... as I pointed out before that is a very high impedance node... The next likelyhood in my experience is that you don't have wired what you think you do... usually when devices draw inordinate amounts of current there is a reason for the behavior... In my experience it is something Assumed or Overlooked (looked over) so many times that you simply can't see it any more... that was what my techie was for... double checking me. As much as I hate to say it I still make similar mistakes and I have to stop and think about what I had.... I once tore a circuit up and re-built it ... Three Times... The Same way. My Employer walked up behind me and asked me why did I have an output grounded... I had swapped two wires in a cable harness and had "Checked it" every time... the ground pin was adjacent the output pin and both went to a connector.

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

retrolefty

Quote
I once tore a circuit up and re-built it ... Three Times... The Same way. My Employer walked up behind me and asked me why did I have an output grounded... I had swapped two wires in a cable harness and had "Checked it" every time... the ground pin was adjacent the output pin and both went to a connector.


That's been my experience also, there is often nothing smarter then a second set of eyeballs. ;)

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