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Building voltage controlled PWM generator - problem with voltage conversion

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New Member
Feb 10, 2016
Posted this on an electronics forum but having seen the excellent work on a similar project, the 4 pin fan controller, here.
Thought i'd post it here too and see if anyone can help.

I'm working on a project to take the variable 12v produced by a pc motherboard header (the fake 4 pin kind) to control fan speed and convert this to a PWM signal to control a device (PWM Laing D5) which has a greater current draw than the motherboard can handle. (max 1A provided by the header and up to 2A required by the pump)
The motherboard then takes the resulting RPM from the tach wire and plots against input voltage, this can be used to control the pump based on temperature providing flow when needed and slience when its not.

I have a circuit which is working well in simulation based on a 556 timer design largely inspired by this video (and the second part):
Was unable to copy the schematic as the quality was rubbish :/

The circuit runs off the 5v supply provided by the PSU and must be able to provide an output with frequency between 21kHz and 28kHz, max current of 5mA, logic low of <0.8V and max voltage of 5.25V in line with the intel standard.
In simulation it satisfies these criteria.

My problem is with converting the variable 12v signal produced by the motherboard and reduce this to 5v whilst maintaining the variability.
I have looked at using DC-DC converters and voltage regulators however these all seem to fix the output at a certain level rather than allowing for any variation in voltage, this would obviously remove the control element.

The best that I have been able to come up with is a simple resistor based voltage divider taking into account the load of the internal PWM control circuit of the pump to be controlled.

I was wondering if anyone has suggestions regarding a better way of scaling this 0-12v to 0-5 whilst maintining the control element and does not rely on a potentiometer (the circuit will be inside a case and therefore inaccessible)

Also, any suggestions for the current source IC able to produce current of >12.2mA, 12.1mA is the minimum for this capacitor on the monostable element of the circuit to function correctly.
So far I have found the PSSI2021SAY which seems to fit the criteria or to produce one from component parts...

Thanks in advance!

Schematic of the circuit is here... View attachment Pwm generator.pdf
Don't have the time to go through everything you posted.

The answer for dropping the variable 12 V is an op-amp circuit. You set the op-amp up for unity gain (connect the output to the "-" input). To the "+" input, you connect the output of a voltage divider that scales the 12 V signal to 5 V. The op-amp has a high impedance input, so you'll get the proper scaling from the voltage divider. Also, the op-amp provides isolation from input to output so it can drive the next stage effectively.
Use a voltage divider. Note that you'll probably need to low pass filter the output. And in some cases, add a dummy load like an actual fan.
The op amp to isolate the voltage divider looks to be exactly what was needed.
Hadn't known much about these but done some research following your post and it makes complete sense.

Tested it out in simulation and it works perfectly.
I've modified the design slightly too, dropped the current source using a standard RC instead and now run the variable input at 0-3.33v which gives good results.

Next up is to get the components together and get one built.

Hopefully it will be of some use to others who want to convert a 3 pin voltage controlled header to true 4 pin pwm...

I don't know why motherboard manufacturers use 4 pin headers if they are only going to make 3 of the pins work... Only causes confusion in component selection!
if i use this one with an 4017 i could control the output duty cicle from 25% to 100% in a stage mode with a single click of a button?