I am looking for a way to convert a + 24V square wave coming from a cnc controller, to -5V. The frequency of a square wave will vary from 15kHz to 35kHz.
What methods would you suggest?
Thanks for your attention.
Electrical – Positive to negative PWM converter
pwm
Related Solutions
Use an op-amp, powered from +/-15 V rails to both amplify and level shift the 0 to 5V square wave: -
The circuit above is called an op-amp summer (or adder or mixer) in that two analogue values are mathematically added. If V1 is the input waveform centred at 2.5 volts, the action of applying - 2.5 volts to V2 will result in the output, Vout being level shifted to make a square wave symettrical about 0V.
Choosing Rf appropriately will ensure the square wave amplitude is 24 volts p-p. Use a fast op-amp to preserve the speeds of the edges of the sq wave.
One more thing, the circuit above inverts the square wave so if duty cycle is important, this will appear inverted too.
If you really need to operate that slowly (usually faster is better, at least up to some kHz for PWM) then you have a couple choices- use a slower clock frequency (much lower, so maybe not desirable- but slower clock means less power consumption). It also does not give you fine resolution on the PWM period (because normally nobody cares that much as long as it is fast enough).
Or you can use the timer compare function to provide sub-microsecond accurate transitions since it can toggle the pin with the hardware compare function- you then set it up for the next compare (and, optionally, toggle the output pin). That would be the preferred approach, in my opinion.
Some of the 16F series parts have a numerically controlled oscillator peripheral (NCO), but I don't think it's available in the 18F series (could be wrong).
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Best Answer
Take your 24V pulses and strongly differentiate them with an RC high pass filter: -
Make sure they are properly differentiated like example (a) below: -
The resistor shown above needs to be connected to -2.5 volts - maybe use two equal value resistors - one to 0V and one to -5V
Reduce the amplitude to 5Vp-p (resistors and/or diode clamps can do this)
Set up a schmitt trigger running from a -5V rail and feed the clamped signal to its input. The output from the schmitt trigger will be the PWM signal you want. It needs to be a schmitt trigger so that it only changes state when a spike occurs: -