# PWM hall effect throttle problem

#### maker_f7qflnb8_1663278496

Sep 15, 2022
4
Hiya.
Hoping someone can give me some advice. I have swapped a potentiometer on a pwm controller with a hall effect throttle pedal on a kids electric car build (I flipped the magnet in the pedal so it outputs the way the pwm is expecting)

The problem I am having is that the minimum output the throttle gives is 0.8v which makes the motor run at 17% load constantly, as opposed to the pot which can go all the way to 0v. I can't think of a simple way to reduce the voltage at zero throttle, but maintain the full voltage at full throttle (as this is already only 4.2v max, as opposed to the pots 5v max).

Feb 19, 2021
205
Send output/signal thru a silicon diode, depending on current that will drop
V by .7 or more.

Regards, Dana.

#### Bluejets

Oct 5, 2014
5,995
You'll more than likely have to feed the signal through a rail to rail op amp amplifier.

#### maker_f7qflnb8_1663278496

Sep 15, 2022
4
Thankyou. I considered a diode but that would drop the voltage of the max throttle too. Can a diode survive breakdown voltage over and over again? I.e. if I insert it into the circuit backwards it would resist the initial 0.7v but when throttle is applied it would give up and allow the full voltage through?

I'll look into op amps now

Feb 19, 2021
205
Not sure of you currents and power, but a near zero V diode.....

Regards, Dana.

#### Alec_t

Jul 7, 2015
3,246
If you presently have 0.8V to 4.2V but want 0V to 5V then you need a rail-to-rail op-amp as Bluejets mentioned. Just using a diode won't do what you want.

Feb 19, 2021
205
This will help calculate the necessary gains and offsets needed for OpAmp solution :

Regards, Dana.

#### maker_f7qflnb8_1663278496

Sep 15, 2022
4
This will help calculate the necessary gains and offsets needed for OpAmp solution :

Regards, Dana.
Thankyou Dana for that. There's a lot to digest there for me, my electronics education stopped at age 13! I'll see what I can do.

#### crutschow

May 7, 2021
520
I can't think of a simple way to reduce the voltage at zero throttle, but maintain the full voltage at full throttle (as this is already only 4.2v max, as opposed to the pots 5v max).
That's because there likely is none.

Below is the LTspice simulation of a basic differential op amp circuit that should do what you want.
It generates a 0 to 5V output for a 0.8V to 4.2V input.
The op amp must be a rail-rail type (inputs and outputs can go from ground to the plus supply).

The simulation shows the output (yellow trace) going from 0V to 5V for an input (green trace) of 0.8V to 4.2V.
The circuit uses a 5V supply.
The comments show how the calculations are done for a desired negative offset (0.8V here) and circuit gain (5 / 3.4 = 1.47 here).

Last edited:

#### maker_f7qflnb8_1663278496

Sep 15, 2022
4
This will help calculate the necessary gains and offsets needed for OpAmp solution :

Regards, Dana.
Thankyou Dana for t. There's a lot to digest there for me, my electronics education stopped at age 13! I'll see what I can do.
That's because there likely is none.

Below is the LTspice simulation of a basic differential op amp circuit that should do what you want.
It generates a 0 to 5V output for a 0.8V to 4.2V input.
The op amp must be a rail-rail type (inputs and outputs can go from ground to the plus supply).

The simulation shows the output (yellow trace) going from 0V to 5V for an input (green trace) of 0.8V to 4.2V.
The circuit uses a 5V supply.
The comments show how the calculations are done for a desired negative offset (0.8V here) and circuit gain (5 / 3.4 = 1.47 here).

View attachment 56249
Thankyou that's a big help

#### Bluejets

Oct 5, 2014
5,995
Another option is to use say, an ATtiny85 and map the new input to your required output level.

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