M4-ATX and an attempt to find/mod a supply that generates less or no spurious noise (interference).
Since I’m rebuilding the case and PC layout I decided to take another look at the M4 and locate an audio noise that is “absolutely” coming from the M4, however due to other issues I have with the firmware of this device I decided to look at an alternative first.
During the course of my initial tests, when trying to make the M4-ATX usable near a GPS and DAB radio in weak signal locations, I tried a small low cost supply from Jaycar.
This is a 205 watt unit, it’s nicely made and it did indeed produce very little RF interference.
However it had at that time two limitations:
1. The 12v input is not regulated or boosted – and is a major reason for its low noise.
2. It has no start-up or shutdown controller.
The second problem is not really a problem as the M4-ATX PC interface and shutdown controller sucks!
It will take me all of a day to make a microprocessor interface with voltage monitoring and controller settings available in a small windows application that blows the M4 dumbness out of the water.
The first problem allowed me to find a way to regulate the +12v supply using some form of linear regulation – NO switching regulator to generate noise. I will not be boosting the 12v rail as I run the PC from a small aux battery AND boosting the rail requires some form of switching convertor.
Yes I know +5v and +3.3v rails use a switching regulator, however the voltage is low and the power is low in relative terms, it’s that switching +12v @10A (or more) regulator/booster that is the real noise maker.
There are problems using a standard linear regulator due to the voltage drop across the device when the input voltage approaches or drops below the design regulated output voltage. And NO, I’m not interested in some exotic circuit that you may have to overcome that.
I had an idea for a simple low cost (some would call it crude) system to keep the voltage at around +12v +- 5% from an input of 11.5v to 15v. However I wondered just what tolerance my MB would take before it spat the dummy (Signalled power good failure or just stopped).
The CPU and most circuitry in a modern MB are 5v or below. I assume that the input to the switching regulators for most of these devices is usually the +12v supply, therefore I could see no reason why it could not run at much lower supply. Searching the NET was a waste of time with usual folk law and totally dumb/wrong information trotted out as fact.
The test on my MB was surprising, I don’t know how low I can take the +12V rail but I decided to stop at +8.4 volts. The PC boots and runs faultlessly at that voltage.
NOTE: I should point out that there could be a “Major problem” in taking the 12v rail low. The CPU and other circuitry can be drawing high currents and the switching regulators for these devices will be drawing more and more current as the input voltage drops. It’s possible to go outside the design limitations of the MB and damage it.
I reasoned that if I kept the +12v to a max of +12.4v and I feel I could safely run the MB down to 11v. Actually I think it would be fine down to 10.5 volts.
This relaxed the very fine regulation requirements and allowed me to try an idea for a controller consisting of 3 x 30A diodes in series, each shunted by a power MOSFET. The FET’s are controlled by a small voltage monitor and switched progressively on or off as the input voltage changes.
Each diode has a forward drop of almost 800mv for a total drop of 2.4 volts in three 800mv steps.
With this circuit the output voltage swings between 11.7 and 12.4 as the input voltage varies from 11.8 to 15v at currents from 2A to 20A.
At an input voltage of 10.9v the output is 10.7v.
This circuit causes absolutely no noise, the total dissipation at 15v input and 10A is around 24 watts, at the normal running current of 3.3A its 8 watts. This is with an input of 15V which it will never see due to wiring length and the Vehicle regulator.
At a nominal 13.6V input at 10A the dissipation is around 16 watts. At 12v @ 3.3A output for 13.6v input (PC runs at that all day) power dissipation in the circuit is around 5 Watts.
This is a very simple low cost circuit and should be almost indestructible. Combined with the Jaycar PSU, it results in a complete absence of interference in the Audio, FM, DAB and GPS spectrums.
It has been running the Car PC all day at 14v input and is cold to touch.