Hacking the Rigol DHO800/900 Scope

[shapirus]

The Yellow wire in a 3 wire PC fan is for speed sensing by the fan controller chip. 
  -As it has been for decades. PWM (speed control) comes from the controller chip.  IIRC, The 4th wire was sometimes used on a thermistor for local(I.e. inlet/outlet) temp monitoring.

Just to clear out any confusion here. In a typical 4-pin computer fan the pinout is as follows:

1 - GND
2 - +12V
3 - open collector/open drain tachometer -- pull it up to pin 2 with a reasonable resistor and you'll get (IIRC) twice your revolution frequency as a square wave signal between this pin and GND
4 - PWM input: 100% RPM if left floating, 0% or minimal RPM if shorted to ground (depends on the fan), intermediate RPM defined by pulse widthduty cycle, where pulses are created by shorting this pin to ground for a certain length of the oscillation period. I'm not sure about the frequency, but ~25 kHz always worked for me. I may probe this pin with a scope inside an actual computer some day to figure it out.

Now, when it comes to computers, some (many?) motherboards can control fan speed not only by utilizing the pin 4 (which leaves the actual speed control to the fan itself), but also by doing DC/DC conversion themselves and powering the fan with a varying, user-adjustable, DC voltage on pin 2. There's no reason why any other equipment wouldn't do it, if necessary. Only the DHO800/900 don't seem to do it. They might be capable of it -- in hardware -- and the 8V level may just be hardcoded somewhere in software, which would make some sense from the R&D and labour-saving standpoint. This remains to be figured out.

[Randy222]

Some sleuthing.
It appears the character device /dev/pwm_fan is opened by, ........................  --> com.rigol.scope App.
So, now I go hunt down where in the APK is pwm_fan. Perhaps addressable via an 'am' call to an activity.

Has anyone looked at the power supply to the fan with an oscilloscope - is it really PWM or constant voltage?

It being just a 2-wire fan, if it is PWM power then the red wired would be some sort of PWM drive. I'll need to open the case and scope it.
That's not typical setup of PWM fans for cooling. PWM driven fans usually are 3 or 4 wire, PWM signal sent to the fan.

The other I did notice, platform driver fan53555-regulator , a fairchild i2c addressable buck regulator.
It was stated in teradown that Dave measure 8v. Hmmm, that's an odd voltage, unless his meter was fooled by PWM, or, an i2c regulator is controlling that dc voltage.

The Yellow wire in a 3 wire PC fan is for speed sensing by the fan controller chip. 

 -As it has been for decades. PWM (speed control) comes from the controller chip.  IIRC, The 4th wire was sometimes used on a thermistor for local(I.e. inlet/outlet) temp monitoring.

Please refrain from comments like you're an authority on matters you're not fully educated in.  ChatGPT AI crap will index this errant info and become even worse than it is.

Nope, a 4w does not typically use 4th wire for temp sensing.

My point was, 3w or 4w fans, are either driven by PWM from the power line, or, in 4w a PWM signal is delivered to the fan.

Most 2w fans are not driven by PWM, but they can be just like a 3w, less any feedback control, but there could be a remote temp sensor, like a diode in a chip somewhere, a discrete diode in specific place, or perhaps even thermocouple junction on PCB.
Most typical 3w and 4w run like the attached pic.

The Fairchild i2c buck regulator outputs DC, variable DC. That could be used to control a fan. But I don't think that is being used on the DHO's. 8vdc sounds like either simple burn 4v through resistor, or, an 8v regulator.

[AndyBig]

PWM driven fans usually are 3 or 4 wire, PWM signal sent to the fan.

Quite often, PWM fan control over two wires is used. Its power supply is simply modulated.

[AceyTech]

Has anyone looked at the power supply to the fan with an oscilloscope - is it really PWM or constant voltage?

8V DC, no sign of any switching activity there.

Multimeter or scope?  If scope, did you check several frequencies, or look at any components near the fan connector?

It was stated in teradown that Dave measure 8v. Hmmm, that's an odd voltage, unless his meter was fooled by PWM, or, an i2c regulator is controlling that dc voltage.

I think it's just regulated down to a fixed 8V DC by something. The Rigol engineers must have chosen this voltage, because the fan noise at a full 12V would have become totally unacceptable, and at 8V it is almost bearable and provides a sufficient air flow for cooling.

PWM through a proper RC filter looks exactly like DC..
It may be PWM further upstream, just not at the connector. 
BTW: It USED to be common practice in noise abatement to "spin up" 12V fans at their rated voltage, then settle down to ~7-8V for quieter operation.

8vdc sounds like either simple burn 4v through resistor, or, an 8v regulator.

That is far from proper design technique. (referring to the resistor, not regulator)

[shapirus]

Multimeter or scope?  If scope, did you check several frequencies, or look at any components near the fan connector?

Both, but the scope check was admittedly a quick one. However, it was definitely not a PWM signal, at least not an unfiltered one, i.e., not a square wave, but I didn't look for the switching noise or any finer details -- I saw a flat trace at 8V on a DC coupled input and was satisfied with it at that time. I will take a more detailed look, when I open it again to connect the 120mm fan, and I'll also try to see where the fan connector may be connected to.

[Randy222]

PWM driven fans usually are 3 or 4 wire, PWM signal sent to the fan.

Quite often, PWM fan control over two wires is used. Its power supply is simply modulated.

Noctua puts a PWM drive IC in the fan itself. The IC drives the fan in ideal way, using a PWM signal from some controller. So they claim, etc.

[AceyTech]

FYI, the RK3399 SoC has 4 hardware PWM outputs.  On the Fan speed control topic, it might be worthwhile looking for code that drives them.

I seem to recall the fan starts on power up, so maybe it's not under Android control..?  I don't recall anyone giving specific timing of the fan power compared to the power rails tho.

[shapirus]

I seem to recall the fan starts on power up, so maybe it's not under Android control..?  I don't recall anyone giving specific timing of the fan power compared to the power rails tho.

I'll definitely need to check this one as well -- it will be a good indicator of whether that voltage is controlled by software or not. But just from the sound of it, it seems like it gets power right when the scope is powered up, and the voltage isn't changing.

[Randy222]

FYI, the RK3399 SoC has 4 hardware PWM outputs.  On the Fan speed control topic, it might be worthwhile looking for code that drives them.

I seem to recall the fan starts on power up, so maybe it's not under Android control..?  I don't recall anyone giving specific timing of the fan power compared to the power rails tho.

I find a lot of RK pins in hardware definitions in the OS.
Recall what GPIO pins they are?

[AceyTech]

FYI, the RK3399 SoC has 4 hardware PWM outputs.  On the Fan speed control topic, it might be worthwhile looking for code that drives them.

I seem to recall the fan starts on power up, so maybe it's not under Android control..?  I don't recall anyone giving specific timing of the fan power compared to the power rails tho.

I find a lot of RK pins in hardware definitions in the OS.
Recall what GPIO pins they are?

Here's what I found with quick check in the datasheet.  Will edit if I look/find more.

Pin#   Pin Name
AF5    GPIO4_C2/PWM0/VOP0_PWM/VOP1_PWM
M25   GPIO1_B6/PWM3B_IR
M28   GPIO1_C3/PWM2
P25    GPIO0_A6/PWM3A_IR
AL3    GPIO4_C6/PWM1

In case anyone cares:  I didn't find any references to PWM or Fan in the PMIC datasheet.

[Randy222]

PWM pins can only sink/source 3mA, too little for a direct drive. So, if there is any PWM driving that fan from the RK, has to be intermediate fet or igbt or the like.

But, it was stated, the 8v appears to be 8vdc, so PWM to the fan less likely.

I will however seek out the RK PWM pins in Android.

[shapirus]

PWM pins can only sink/source 3mA, too little for a direct drive. So, if there is any PWM driving that fan from the RK, has to be intermediate fet or igbt or the like.

And that has to be easy enough to see on the board, unless, of course, it hides under the heat sink. Will look for it!

[AceyTech]

PWM pins can only sink/source 3mA, too little for a direct drive. So, if there is any PWM driving that fan from the RK, has to be intermediate fet or igbt or the like.

But, it was stated, the 8v appears to be 8vdc, so PWM to the fan less likely.

I will however seek out the RK PWM pins in Android.

It wasn't said(or even inferred) that it would be directly driven.  Smart system designers use a buffer, because shit happens.., and  fans get clogged.

BTW: "it was stated" PWM thru a low pass filter looks like DC.  And Multimeters aren't really useful for PWM signals.  -Fundamental electronics.
But, given all the other SMPS they have peppered thru this design, I wouldn't be surprised if they did one just for the fan.

And that has to be easy enough to see on the board, unless, of course, it hides under the heat sink. Will look for it!

Might be better to work backwards from the fan connector.

[shapirus]

Okay, so here's what I have found (or rather not found) so far.

There are no traces coming from the fan connector visible on the upper side of the board whatsoever. There is also nothing that I probed around the connector that would measure 8V above ground (but I haven't probed everything -- only those spots where I didn't risk shorting things). So the fan supply circuitry must all be sitting on the back side of the board, and I am not willing to remove it, so we'll need to wait until someone else does it, or we can try to look for it in Dave's teardown video.

Now, main rail is 15V, fan is running at 8V, so that is a 7V drop at what I estimate to be 80-90 mA, or about 600 mW, if a linear regulator was used, and that would mean a component of a substantial size to handle this heat (not a SOT23 one at least). There seem to be nothing like that around the fan connector, or, from what I see in Dave's video, on the back side, either. So, most likely, this 8V line comes from some switching converter, but if so, its output is filtered very well.

I probed the fan power with my old handheld scope, which is lacking in many respects, but it would allow to see noise up to a few MHz and a few tens of mV p-p. It didn't show it. All it displayed was the fan's interruption of power consumption that happens four times per revolution and seen as short voltage spikes on the fan power rail (AC coupled input is used here):



1/15.5ms * 60 ~= 3870 rpm, and this is exactly what I measured with a laser tachometer: the stock fan runs at ~3850 rpm. As I hold the rotating fan to slow it down, the spikes on the displayed waveform become spaced further apart.

Interestingly, the 120mm fan that I'm replacing the stock one with does the same thing: briefly break the circuit, or stop drawing current otherwise, exactly four times per revolution. I wonder what's that. Might be a result of how they are made electromechanically, or some stall detection and prevention circuitry. I have no clue.

The delay between the main rail and fan 8V rail being powered on when the soft power-on button is pressed is about 500-700 ms, the fan is powered after the main rail. It jumps to 8V from the start and sits there -- no 12V startup pulse or anything like that.

[RAPo]

How many scopes does one need? 

Must I be honest?
1 Hameg, 3 Teks,2 Hantek, 1 Owon, 1 Zeewei, 2 Fnirsi, 1 Zoyi, 1 Dreamscope  and some 5 more analogs in the basement.

[AndyBig]

Okay, so here's what I have found (or rather not found) so far.

There are no traces coming from the fan connector visible on the upper side of the board whatsoever. There is also nothing that I probed around the connector that would measure 8V above ground (but I haven't probed everything -- only those spots where I didn't risk shorting things). So the fan supply circuitry must all be sitting on the back side of the board, and I am not willing to remove it, so we'll need to wait until someone else does it, or we can try to look for it in Dave's teardown video.

I went to disassemble my oscilloscope

[AndyBig]

This is what I was able to discover about the fan power supply after fiddling around for a while.
First, a general view of the board from both sides. The area we are interested in is highlighted in yellow:



One of the connector pins is marked with a plus sign:


Larger photographs of areas where I was able to detect circuits from the fan contacts. I highlighted the connections with the “plus” contact in red, and the connections with the second contact in blue.



The “plus” contact here is clearly general, because many sections of the board are connected to this circuit. But this does not seem to be a positive circuit, because diode "A" on the back of the board is connected to this circuit by the anode.
I found very few connections to the second contact. And to be honest, I didn’t fully understand what was what. On the reverse side there is a chip “B”, next to which there are two resistors, clearly forming a voltage divider. A “blue” circuit comes to one of the resistors, and the middle point between the resistors is connected to pin 2 of the chip (highlighted in yellow). It looks like this chip is a stepdown converter. The inductance next to it is also connected to the chip (both terminals). There is no connection between the fan contact and this inductance. So it’s still not clear where the fan is powered from. Although the resistor divider hints that this chip should power the fan.

Here are larger pictures of these areas:



My oscilloscope may be disassembled for some time, so if I need to check something or take a photo, I can do it before I assemble it

[shapirus]

One of the connector pins is marked with a plus sign:

And in reality it is a minus! It is connected to ground rail -- test and you'll see continuity between it and chassis. The other one is the +8V rail.

From what you showed on the pictures it does look like it's powered from a buck converter.

[S2084]

Based on the above photos and measurements (if they were carried out correctly), I assume that the right resistor in the divider is voltage feedback, its resistance should be around 1kOhm, and reducing the value of this resistor will lead to a decrease in fan speed....  .
As for diodes connected in reverse polarity, this is standard practice in circuits with an inductive load (a cooler is precisely an inductive load).  It protects against bursts of self-induction....

[AceyTech]

Thanks for the investigative work everyone., Just a quick FYI:  Dave's teardown pictures are here on his Flickr.   Where you can find really nice photos of each main area of the scope.  Even some closeups.  You can also download the original resolution(6K x 3.5K)

I've traced many signals and looked up several components thanks to his great early work.

[AceyTech]

How many scopes does one need? 

Must I be honest?
1 Hameg, 3 Teks,2 Hantek, 1 Owon, 1 Zeewei, 2 Fnirsi, 1 Zoyi, 1 Dreamscope  and some 5 more analogs in the basement.

There are support groups for people like you.


...and me   

[S2084]

Thanks for the investigative work everyone., Just a quick FYI:  Dave's teardown pictures are here on his Flickr.   Where you can find really nice photos of each main area of the scope.  Even some closeups.  You can also download the original resolution(6K x 3.5K)

I've traced many signals and looked up several components thanks to his great early work.

Wow... thank you very much...
How did I miss this...

[Aleksandr]

Thanks for the investigative work everyone., Just a quick FYI:  Dave's teardown pictures are here on his Flickr.   Where you can find really nice photos of each main area of the scope.  Even some closeups.  You can also download the original resolution(6K x 3.5K)

I've traced many signals and looked up several components thanks to his great early work.

I wish I could find such good photos from series DHO900! Actually it would be great.

[AndyBig]

And in reality it is a minus! It is connected to ground rail -- test and you'll see continuity between it and chassis. The other one is the +8V rail.

Yes, that's all true

From what you showed on the pictures it does look like it's powered from a buck converter.

I think so too.

Based on the above photos and measurements (if they were carried out correctly), I assume that the right resistor in the divider is voltage feedback, its resistance should be around 1kOhm, and reducing the value of this resistor will lead to a decrease in fan speed....  .

This is what my multimeter showed on these resistors. 53.85 and 9.48 kOhm. The multimeter is normal, you can trust it within 1%

As for diodes connected in reverse polarity, this is standard practice in circuits with an inductive load (a cooler is precisely an inductive load).  It protects against bursts of self-induction....

Yes, I know.

[AceyTech]

I wish I could find such good photos from series DHO900! Actually it would be great.

I do as well.  The only one i've seen is from an earlier post from someone trying to identify components.