PWM to Analog converter circuit

I get an error(You do not have permission to login). In the forum FAQ sections says I will need to be a member for 180 days and have 1000 posts to use the Trust system.

I am not sure if you happen to post on any other tech forums forums and am hesitant to post others links here as some forums are very against it(even more so for new users.).

I will message a moderator and see if they have anything they can do, but chances are rules are rules. This is why this forum manages to not get piled with spammers like many others.
 
@pete910

I built something about a month ago and then whilst evaluating it my oscilloscope decided to fry its EHT transformer so I got sidetracked looking at that. I didn't know it was the transformer originally but I had several days of fun and learning along the way trying to fix it and the replacement scope I got in to fix it.

I designed a circuit to switch a 10v mosfet fully on using just a 3.3v-5v PWM signal while I waited for the original logic mosfet I planned on using and this looked good.

It has no rpm feedback, or if it will have some it'll be unreliable. It should be able to drive 10s of amperes with only minimal heating as the mosfet I chose has a fairly low on resistance of 40mOhms.

I've obviously got no pump to test it on but I should be able to find a high power inductive load of some kind around here, or just slap all my fans and a few power resistors on it and see what it draws.

Now that I've got at least one working scope I was hoping to get back to it over the next few days and will post up some findings. I am also building a circuit for someone on the forums so it'll probably be done around the same time.
 
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Right. Here's a quick video showing the high current development circuit in use. I don't have quite as much as 2A easily available, either on my power supply or as a load so 1A will have to do for the moment.

The following circuit is based around a few small signal transistors to convert a <5v PWM signal into a >10v PWM signal that can drive a power mosfet, which in turn is to drive the fan/pump array.


The biggest problem with this circuit is the losses across the mosfet which can be as much as 1.3v. The second issue is the speed of the pump/fan can go down to zero with this circuit so it's not really safe to use unless you really know how to set up your fan curve. I feel what it needs is a lower limit that it can't go beyond.

I think there are likely to be better, cheaper and safer off the shelf solutions to anything I could put together. :)
 
Tealc said:
The biggest problem with this circuit is the losses across the mosfet which can be as much as 1.3v. The second issue is the speed of the pump/fan can go down to zero with this circuit so it's not really safe to use unless you really know how to set up your fan curve. I feel what it needs is a lower limit that it can't go beyond.
Click to expand...

I don't know much about water pumps but don't they all run from SATA/MOLEX?

My thought is that they have a 5v supply. Would it not be possible to build a sort of fail-safe into the circuit so that if the PWM controlled supply dips under that voltage, it takes over making it so that the voltage for the pump can only be between 5v and 12v

I feel that won't make much sense as it's 1am ramblings and I've been awake a long time. Hopefully the gist of what I'm saying will come over.
 
I think pumps usually have a nominal 12v requirement, although they could be overvolted if desired.

Yesterday I did put together another circuit using a P channel mosfet that can be trimmed with a resistor, or potentiomter so that it doesn't drop below, say 5v or 7v depending on a resistor selected. The circuit worked well, ramping the speed of the fan up and down in response to my PWM signal.

The only issue I'm having is that I'm dropping 1.2v across the mosfet and it is getting warm, something which you wouldn't expect with a fet as it is being used as a switch and not in it's linear region. Mind you the only p-channel fet I have on hand is a high voltage one and they tend to have higher on resistance, need a higher gate charge and voltage to switch them on fully.

I'm considering purchasing a different p-ch mosfet that has a lower on resistance, maybe an order of magnitude lower than the one I have. I can live with a small voltage drop. :) The main problem is that there are literally many thousands of the things to choose from. :/
 
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Some progress on the higher current circuit.

I got some new P-channel logic level mosfets from CPC today (they are still doing free delivery on any order). They are a FQP27P06 mosfet with lower on resistance than the IRF9640 I was using previously. The result is a 11.6v voltage across 1.2A of fans at 97% duty cycle.


This circuit should allow the use of fan feedback as it maintains a lower threshold which is controllable by a resistor/potentiometer.

Heat may be an issue but larger heatsinks can be employed to cope with that. The heat is only an issue at low duty cycle as the transistor is then in its linear zone so acts like a resistor itself.

In a few days I will try out a logic level N channel mosfet with silly low on resistance used in high side configuraion and maybe low side as well to see the differences and benefits in real life. I expect this to allow the fet to run ultra cool and control around even more fans, certainly more than I have and can power.
 
Here's an interesting variation.

Bow asked for a kit of parts for producing a combination of existing PWM fans, some 3 pin fans off a graphics card header.

k2iv83.jpg


The kit contains:-

1x PWM converter circuit with 3 fan output.
1x 4 pin PWM connector
1x 3 pin fan connector
2x 4 pin PWm headers
Some small heatshrink
Some medium sized heatshrink
Some large heatshrink
Some terminals for fan connectors.
Some black wire
A Molex connector (I left the SATA on so the length of wire can be cut perfect)

The PWM converter is one I built and tested two weeks ago. It makes sense to send as is and let Bow modify to his own needs.

I could have thrown in a graphics card fan connector as well but it wasn't needed for this build.
 
Awesome :)
I guess I have to pinch off two cables of the 4-pin header and connect them to the Molex, while the PWM and RPM signal cables go into the VGA header?
 
bow said:
Awesome :)
I guess I have to pinch off two cables of the 4-pin header and connect them to the Molex, while the PWM and RPM signal cables go into the VGA header?
Click to expand...

That's the basic idea. You might as well leave the tach wire in the connector and run off to the spare motherboard header you mentioned, or swap it for the 3 way connector if you would rather use that.
 
Hey Tealc,

Do you think this converter will enable to control a 36W watercooling pump (12V/3A) off a PWM header on a motherboard header or a PWM header of a Aquaero 6 XT?

Thanks,
deFi
 
deFiniLoGy said:
Hey Tealc,

Do you think this converter will enable to control a 36W watercooling pump (12V/3A) off a PWM header on a motherboard header or a PWM header of a Aquaero 6 XT?

Thanks,
deFi
Click to expand...

It isn't designed for that sort of load. The limit of the shown circuit is around 1A.

Using a logic level MOSFET in high side configuration will allow switching of a 3A load but affords no tach feedback and no limits on PWM duty cycle, so if set up by the user incorrectly could lead to pumps stalling.
 
Hello,

I am trying to build this adapter on my own. I did not find any scheme in the previous 16 pages. I tried to discover the wiring fromt the pictures posted in this thread and made this schema :

Image removed according to Tealc request.

I am not sure if this schema is corect. I was also not be able to realize where the PWM and GNd wire are conncted to the PCB. I am also unsure about the valuef ot he R1, R2, R3 resistors and abot the value of the trimmer. I also hope I got the interruption of the traces right. (marked red) Could you please check my schema and correct my mistakes ? Thank you - K
 
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I'm afraid you are a bit off with your routing there.

R1 is 1k
1k -> I usually go with 330 Ohm 1/2w but 1k is fine but will limit your power output.
R3 is 10k
R2 is 470k
Your PNP is the wrong way around.

The GND goes to the top trace.

The Capacitor needs to be swapped around as it'll go bang that way around.
You also need to isolate the emitter of the PNP so it doesn't feed the capacitor, bridging the collector to the capacitor line.

PWM feeds in to R1.
 
Ok, second attemp :)

Image removed according to Tealc request.

I hope I switched the right IC (the BS170). Or should I rotate the 2sB772 ? I did not fully understand the end of your comment.

Tealc said:
I'm afraid you are a bit off with your routing there.

The Capacitor needs to be swapped around as it'll go bang that way around.
You also need to isolate the emitter of the PNP so it doesn't feed the capacitor, bridging the collector to the capacitor line.
Click to expand...

I interrupted the line between the emmitor of the 2sB772 and the capacitator (1µF right ?) I need to bridge the collector to the capacitator ? Also what is the value of the poti / trimmer ?

Thank you very much - greetings K
 
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There are no ICs here, just two transistors. You did swap the wrong one. The n channel MOSFET is the bs170, the PNP bjt is the b772. Turn the bjt around so that the it is base, collector, emitter from bottom to top. The bs170 needs to have its source pin on the ground rail.

The cap I find to be good around 22uF. 1uF is a little low. 10uF will work ok if its all you have.

I use a 50k trimmer but you might be alright with a lower value. All that happens is that you lose some control over ultra low power fans.
 
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Ok thank you very much. I removed the images from my posts. I did not want to reveal your secrets, there were lot of pictures of your work so I thought I can post the wiring.

Going out from my first schema : I rotated the B772 so its B-C-E from the top, interrupted the line between the base and the lower pin of the capacitator and bridged the emitter to the lower pin of the capacitator.

Is the rest of the wiring Ok now ?
 
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