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# Program launch: "Get Paid to Publish your Project!"

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Hi Hero999,

It works but only with a duty cycle higher than 70%, ib = 12mA. Tomorrow I'm going to change the resistors, let you know.

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Does the transistor get hot?

How did you measure Ib?

You need to use a 'scope with either a current probe or to measure the voltage across R3 and working out the current using Ohm's law. If you use a meter, you can only accurately measure it when the duty cycle is 100% because the  meter will only accurately measure DC or 50/60Hz AC.

Here's another idea using a PNP transistor to switch the positive side of the fan's power supply.

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"How did you measure Ib?"

I didn

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How do did you calculate it?

I reckon IB wil be between 17mA and 21mA, my calculations are shown below:

IB(Q1) = (VDD -  VBE(Q2) - VBE(Q1))/R3

With VBE = 0.7V

IB = (3.3 - 0.7 - 0.7)/100 = 0.019 = 19mA

If VBE is lower IB will be higher and vice versa.

I didn't account for R4 because the current though it will be tiny, 0.6/1000 so won't effect the calculation.

Do you think I've gone wrong somewhere?

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Hi Hero999,

I change R3 for 67 Ohm and I have ib of 24mA and vbe = 1.6V and it works with a min duty cycle of 3.13%.
I will send the file to Mixos.

Thank you for that it will have a faster switching time and it's cheaper to make.

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Your calculations are Roth, I was using a different resistor 160 Ohm as R3 when I was doing I only notice now.

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What do you mean by Roth?

I'm glad you got it working but give 68R in the parts list as 67R will be hard if not impossible to get hold of because it's not a standard value.

I don't see why it should limit the minimum duty cycle though, the transistors should be able to turn off continuously, enabling a duty cycle of zero.

Does the fan draw a hight peak current when the duty cycle is low? You'd need a 'scope to measure it but that would make sense.

What transistors are you using?

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I mean Right. I'm using the ones you told me BC338. I was think the same, that the specs in the fan would affect the way it works with dutycycle. It's working nice, but i will try different fans to know why it happens.

Thank you

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• 5 weeks later...

Has the program been stopped?
I was checking new projects on the website and the add seems to be missing...

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The program has ended. Stay tuned for future launch of this program.

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• 2 months later...

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