Trying to amplify PWM signal from P1


I’m trying to increase the PWM signal to 10V.

But I must have done something wrong in my schematic, because it doesn’t work.

Can somebody see how I could improve this?

The output is just 7.4V, no PWM

Just for completeness: Your schema doesn’t show whether your micro is actually connected to GND.
Also what PWM frequency are you going for?

I’m also not entirely sure why you have R30 || (D11 + R32).

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yes, its connected to the same GND

here is the complete schematic. (at bottom of page)

I have just noticed this slightly dodgy layout on my PCB. i will check tomorrow if perhaps there is no proper connection here

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I found out that these 8 points did not have ground.
Could that be a problem?

(I already corrected it in this photo)

I don’t really understand why that schematic is setup the way it is–normally you would not need the parts that @ScruffR mentioned above. Look for a BJT level shifter schematic to see what I mean.

Another thing with this type of circuit is that it inverts to PWM signal–when the micro’s PWM output is high, the fan PWM output is low. This has the effecting of making the PWM work in reverse so that when set the micro for 80% high time, you get 20% high time at the fan. Now the fan could have another inversion or the software could account for this, but we don’t know about from what’s shown here. Just thought I’d mention it.

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R29 + R30 is a Voltage Divider. Bringing 24V to 12V.
D11 was added to protect the PCB from someone connecting something wrong to FAN_PWM_OUT
Not sure what R32 does.


Yes, that is reversed. I corrected it in Software.

Ok, I have just ordered another PCB with the GND issues corrected.
I will test how that works,


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(EDIT) R32 is in parallel with R30 when Q1 is “off”, so it acts to decrease the positive output to around 8V depending on the diode drop.


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