Turbo speed divider circuit

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Turbo speed divider circuit

Postby bdinnerv on Tue Dec 11, 2018 3:17 pm


I have a EFR Turbo with speed sensor that I want to log on a CDL3 using a digital speed channel input.

My understanding is that the CDL3 dig inputs have a max rate of 3200 hz. The speed sensor has a 8x divider circuit built in and is a 12 blade impeller with a max speed of 154,000 rpm meaning a peak frequency of 3850 - over the max supported on a CDL3 dig input. I am most interested in logging max speed so really need that space beyond 3200 hz.

I have looked at using a 4017 flip/flop IC circuit as a divider which will knock the signal down again by a factor of 2 bringing my max frequency from the sensor down to sub 2khz (costing me some resolution) which is supported on the CDL3 but there may be some signal noise issues with using this circuit - can anyone confirm if this is possible, if something more complex is needed or if there is a way to get the CDL3 to work with this sensor as a digital input.


Its not the speed that kills, its the sudden stop at the end
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Re: Turbo speed divider circuit

Postby David Ferguson on Wed Dec 12, 2018 2:29 am

I would be tempted to wire the sensor directly, but use an intermediate connector that would have the necessary voltages to allow you to add a divider if necessary.

at 3200Hz maximum, you can still measure turbine speeds up to 128,000 rpm.

If you find that is not enough, then you go back and add the pulse divider.
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Re: Turbo speed divider circuit

Postby adrian on Wed Dec 12, 2018 9:25 am

The 3200Hz limitation is on the output, not the input. This means it will read up to approx 192000 rpm.

Just put in a custom calibration of:
0 - 0,
3850 - 154000

and it will work fine.
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Re: Turbo speed divider circuit

Postby bdinnerv on Wed Dec 12, 2018 11:35 am

Thanks Adrian, that makes it much easier!
Its not the speed that kills, its the sudden stop at the end
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Re: Turbo speed divider circuit

Postby Scott@FP on Fri Dec 14, 2018 12:07 pm

You can also buy the same fitment sensor with a 1/10 divider. More expensive.

BTW these plastic body sensors are not good for extended high PR's (temp), they get heat damaged pretty easily.

This is the limitation of the 16 bit input in some of the devices, positive numbers of 0-32767, the digital inputs in freq mode only use the positive side and divide by ten internally (3276.7Hz = 32767) so you have an upper limit of 3276.7 Hz. There are a couple different ways around this such as using the RPM function and its calibration number.
Set digital input to speed measure, RPM, for 12 blade wheel with 1/8 divider (12/8) use calibration number of 150 (only whole numbers allowed can't use 1.5) then 0=0 RPM, 2000= 200.0 RPM in a user channel, this will display speed in thousands with a tenth.
100000 RPM is displayed as 100.0 and no 3276.66 Hz limitation.
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