Frequence to Voltage Converter?

[blueEnergy]

We are currently logging current and voltage on a HOBO U12 and wind data on an NRG wind explorer from one of our turbines.  We've attached a bicycle computer up to our turbine to track the RPMs. It would be nice to take the pulsed output from the bicycle computer wire (it is just a magnet and sensor that we mounted on the rotor plate of the turbine), convert it to a voltage between 0-2.5V and feed it into our HOBO.  We've been looking at parts on the web, but aren't sure about the linearity or frequency range of some of the schematics we've found.  Our RPMs won't exceed 500.  

Any ideas?

Thanks,

bE

[Tom in NH]

I used to use a chip called the 9400 voltage-to-frequency converter. It would also convert frequency to voltage. It is a great little chip that works from dc to about 100 khz. It's been a long time since I've bought one. See if you can find it at Jameco or Digikey. Or maybe they will have newer better ones now. --tom

[Nando]

WHAT IS A HOBO ?. In detail ?.

What accuracy are you looking for ?.

One problem is the low frequency generated that if the Frequency to voltage is analog the DC output may have a lot of ripple, giving in-accuracies

A digital frequency counter may be better but it is a bit more complicated.

A digital counter measures the time between pulses and calculates the equivalent RPM or RPS that may present, the output is a digital value or a DC equivalent voltage.

There are commercially available for GRID frequency meters that are RPS ( Revolution per Second) like 50 or 50 HZ that if the sensor is properly "massaged" the meter may be usable for your aims -- some do present a DC voltage as you want.

How many pulse per rotation does the sensor give ?.

You may take advantage to increase the number of pulses by placing several other magnets for the sensor to give, let's say 10 times X pulses the RPM of the wheel, this way a simpler analog counter may help.

Placing enough additional magnets and the circuit calculated for the top MAXIMUM RPM.

There are several integrated circuits designed for such conversion that give a DC equivalent voltage.

You say 500 RPM if 10 pulses then the equivalent is 5000 RPM divided by 60 seconds is 83.3 RPS OR HERTZ, readable by those commercial units.

You worry would be the lowest frequency capabilities.

Nando

Nando

[Flux]

I don't think that one pulse per rev at that low speed will give a decent signal.

If you sensor senses a magnet then you may be able to mount it so that it senses the magnets on the rotor.

If that doesn't work then your best option is to take a couple of leads before the rectifier and measure the frequency of the alternator.

If you use a 2917 tacho chip you may need a small transformer to feed the input . A small 240 to 12 or 120 to 6 v transformer of about 3 va will be adequate.

If you go to this link it will give you a lot of information on the chip. Feed your little transformer in where it shows the inductive pickup. You may need a bit of filtering to clean up the waveform but most likely it will work without it.

http://www.national.com/pf/LM/LM2917.html

Flux

[scottsAI]

Hello BlueEnergy,

What you want to do is both easy and hard to do.

I expect you don't want to have a big power supply connected to the circuit.

Most of the chips out there will draw 4-8ma at 5v, something like that.

Use a CD4538 one shot. Current 55ua at 3v.

Check the cross reference, and data sheet confirm 3v operation.

Use the pulsed output from the bicycle computer as the trigger to the 4538.

Check the pulse is not too large or small. (needs to be > 1/2 Vcc)

Magnet with inductor pulse size is frequency dependent.

Use a zener on the input if to big.

Set the 4538 pulse output "on time" to equal the on time at the Max RPM.

If your supply is 3v then adjust the on time to give 2.5v at 500RPM.

Filter the output pulse. You now have a Freq to volt output.

See what I mean, easy if you understood what I said, else hard!

Have fun,

Scott.

[amiklic1]

You can see at:

http://www.fieldlines.com/story/2006/2/1/135839/9171

I tried this kind of measurement with hand-spinning method of 60 and 120 rpm, and it works great. If try to use the same device and let's say put 8 magnets, the reed switch is a bit "confused", because it cannot perform so fast switching as my generator ir turning.

Just my 2$

[blueEnergy]

I don't quite follow.  The CD4538 is a monostable mulivibrator which has a fixed voltage output triggered by an input voltage, no?  So I don't follow how the output voltage will vary due to the input...

bE

[commanda]

The output will be a pulse train. The "ON" period is fixed. The "OFF" period varies with the frequency of the mill. The pulse train is integrated with a resistor & capacitor. This gives a DC voltage proportional to the mark/space ratio.

_____|__________|____________ integrates to a low dc voltage

__|__|__|__|__|__|__|__|__  integrates to a higher dc voltage

It's still easier with an LM2917 which is expressly designed for the job. Use a resistive divider on the input, and hang it off the AC before the bridge rectifier.

Circuit in my diary.

Amanda

[scottsAI]

Hello BlueEnergy,

commanda has it right.

The LM2917 draws 3.8ma.

Battery operation then reconsider the CD4538.

It can run on two AA batteries for a year or so.

No battery then use the 2917.

The charge pump in the 2917 is the same thing as 4538.

The 2917 does have a nice front end interfacing to the sensor.

The 4538 needs a filter which is an RC filter (as does the 2917), it's frequency should be lower than the RPM you would like to see. I'm guessing but 1 RPM is not interesting to you.

I have found using a two stage RC to RC works best, if you have spice try the frequency plot to get what you want. Very difficult to figure out the response doing the math.

Have fun,

Scott.

[commanda]

The LM2917 draws 3.8ma.

3.8 mA for 24 hours = 91.2 milliampHours per day.

Even a single 12 volt deep cycle battery of 80AHrs (I have two of these here for 24 volts) would run the LM2917 for 80/0.0912 = 877 Days.

I fail to see where the current draw of the LM2917 is relevant; sorry.

And yes, you run the 2917 off the same battery you're charging; you don't need a separate supply.

Amanda

[scottsAI]

Hello commanda,

The HOBO U12 is a tinny battery operated logging device.

Next, this is where assuming something may not be right.

"we mounted on the rotor plate of the turbine), convert it to a voltage between 0-2.5V and feed it into our HOBO"

I took this to mean at the tower. No battery near by.

So battery operation would be a desired feature.

Thus how much power it draws is important.

I did spell out if no battery then use 2917.

I specialize in designing very low power instrumentation.

3.8ma is 69 times more power than 55ua! (not counting the volts:)

As you know a carpenter sees all problems solved by a hammer.

When I see low power, micro amperes are important.

P.S. in you power calculations you forgot self discharge.

A larger term than the load!

Have fun,

Scott.

[commanda]

P.S. in you(r) power calculations you forgot self discharge.

A larger term than the load!

Didn't forget. Just didn't think it relevant in terms of the conversation.

You may assume he meant at the tower. I would assume he has more common sense than to stand out in the middle of the paddock :-)

And if it's meant for long-term use, then it's definitely going to be inside. If it's only for short-term use, even alkaline batteries will be more than sufficient.

Anyway, between us we have presented several options. Assuming he has enough black belts in electrickery, either will do what he requires. Otherwise, it's a moot point.

When I see low power

I don't see "Low Power" anywhere in the original post.

Amanda

Still having fun & avoiding wayward hammers.

[blueEnergy]

Great!  Thanks for explanation....we are waiting on the parts