Hello all and good day. Here is a quick rundown of what I'm working on. I have a reluctor wheel with 40 teeth being used with a pm generator. I need help converting/conditioning the output AC voltage into an analog DC voltage for use as input for a microcontroller project. Keeping the frequency intact is vital to operation. I know I'll have to use a rectifier to convert from ac to dc. I'm just unsure of how to keep the frequency accurate as an input and then condition the dc signal for the microcontroller. As you have probably guessed, I am a novice so please be gentle. lol. Thank you in advance for any input you can provide.

Bud

 

If you are trying to measure the frequency, do not rectify. Rectifying gets rid of the ac signal. The resulting dc signal has no frequency.

If you need to rectify the signal for something else, the ac signal is still there before the rectifier.

You may need to amplify, and you may need to offset the voltage so that the resulting signal is in the 0 - 5 V.

 

Ok, so how do I convert the ac voltage to dc without losing the frequency signal that I need?

 

Is the reluctor wheel used in the generator proper, or is it just part of the tachometer?

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Ron

 

It is for a vehicle speed sensor project I am working on. they are production vehicle components, so yes they r proper.

 

You can't. You can split the signal, measure the frequency of the one part, and rectify the other part. You don't need much in the signal to measure its frequency.

In some mains powered devices, the mains frequency is used as a frequency reference. In that case the ac from the transformer is rectified to produce the dc that runs the device. The ac from the transformer is also taken from before the rectifier.

Some cars run the rev counter from the alternator. The alternator output is rectified to charge the battery. The rev counter needs a separate connection, one that is not rectified, so it is still ac, and has the frequency, which the dc on the battery does not have.

Why are you converting the the ac to dc? Are you wanting to measure the voltage or are you wanting to use the power?

 

I need a vehicle speed input using an unspecified automakers automatic transmission output shaft speed sensor for a microcontroller project. The pm generator produces ac voltage that ranges from approx 0-100vAc. Since I don't think the mcu could accept that sort of ac voltage I need to figure out an interface of some sort between the two.

 

It all depends on how close to 0V you want to go.

You could just put a 10 kΩ resistor in series and connect straight to the microntoller. That would limit the current to about 10 mA which the input diodes of the microcontroller would probably handle.

The minimum voltage would be about 1.5 V

If you offset the voltage by about 1.5 V it would work down to about 0.2 V.

A 4.7 V zenner diode after the resistor would mean you weren't relying on the input diodes of the micrcontroller.

You can then read the frequency as though it had come from a logic level signal.

 

Thanks Diver, I'll give it a try. If it fries my mcu, I'll be looking for you pal. lol