Tachometer using LM2917N-8 - Increasing output

[hump_shunter]

Hi folks

I have a requirement to drive an old (1960's) moving coil voltmeter tachometer from a modern magnetic impulse sensor. I had the idea to use an LM2917N-8 frequency to voltage converter and after reading through the data knocked up a circuit. After experimenting with C and R values I was finally getting the 450hz/volt output that I was after, but the output stopped at 6.6v and about 30Khz input. I have now read that the output of this chip is limited to this ammount so I need some ideas please on how to boost the output so it can reach 16v DC at around 7200Hz input.

My supply is 24V DC unregulated (alternator) so I am guessing that an op-amp solution will not be possible as Vcc/2 < 16v. Do I need a transistor amplifier ? or are there frequency to voltage converters that cover 0-16v output as I couldn't find one ?

Thanks for any help

[Simon (Webmaster)]

Hi,

Is the limitation of the V/F converter only the output voltage, or also the max input frequency? If it is just the output voltage that is low you might be OK with an op-amp on a single supply - after all the output doesn't need to swing negative. Op-amps such as the LM324 are intended for single-supply operation (LM324 is quad, not sure if there is a single version). Only problem is that you don't get a linear response near 0V. Is there any way of applying a positive offset of say 0.5V to the output of the V/F to overcome this?

Another option, given that a negative output isn't needed, is to generate an asymmetric split supply, rather than Vcc/2. Set the split to give at least 16V on the positive side, and a lower negative side (hopefully enough to give linear response from the amp down to 0V). You need to experiment to see if this would work though - it's not something I've had the need to try. Alternatively a simple transistor amplifier might work.

Obviously if you could find a V/F converter with a higher output it would be simpler, I'm not very familiar with them.

[hump_shunter]

Thanks for your thoughts Simon

The problem with the chip I have chosen is that Vcc is brought down to 7.56v by an internal zener diode, despite using a +24v supply rail and a 470R dropper resistor. There is another version of the same chip LM2907N-8 which doesn't have this zener so I will try one of those first I think, Vcc will be 24v to the chip so higher output voltage should be possible. The NS datasheet is poor at explaining this difference but at least one of the example circuits using the 2907 has a 0-10v output with a 15v supply. According to the formulas I should be good up to 30K Hz input (it previously stopped increasing at 3Khz (not 30 as I had written previously).

I chose the zener version because it offers output stability with supply rail fluctuations which I will have off an alternator, but I think I will try and limit those with a 7824 regulator on the supply instead now.

Cheers
Andy

[Simon (Webmaster)]

That sounds like the way forward - get a stable supply first then try the 0-10V output example. Remember that the drop-out voltage of a standard 78x regulator is 2-3V, so a 7824 probably won't work. You might have to use a lower voltage regulator, a low drop-out type or an LM317 or similar adjustable regulator set to around 20V or so.

[hump_shunter]

You are right of course, I was idling wondering what might happen if the supply went below 24V and of course a 7824 will drop out before then. I've hooked up an LM317 now to supply 20v as you suggested. Just waiting for my new 2907 chip and I'll report on the results.

Andy

[hump_shunter]

I'm pleased to report that the new chip LM2907N-8 worked a treat, with a supply of +20v DC and suitably adjusted timing components, the tachometer output is now 0-16v DC for a varying frequency input of between 0-7200Hz. I installed it all on a diesel shunting locomotive yesterday and it now has a working tacho for the first time in many years. The only thing that might kill it now is low temperatures as the LM317 regulator I had on hand was a 'T' version which is only good down to 0 degrees C, I may have to upgrade it to an 'AT' version at a later date (good for -40 degs).