Re: Pulse Width Modulation Help
From: Al Borowski (al.borowski_at_EraseThis.gmail.com)
Date: 10/19/04
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Date: Tue, 19 Oct 2004 10:58:13 +1000
Hi,
[...]
>
> I thought it would be impossible to find the actual capacitor, but the
> leads
> are fairly large near the header, and can be traced with the naked
> eye. There
> is indeed one sitting right before each header, and it is 47uf @ 25v.
Rats. That makes thing harder.
[...]
>
> And given that the capacitance is now known (47uf@25v=47uf@12v here
> right?)
It doesn't work like that I'm afraid. The '25V' on the capacitor means
the capacitor is rated for a maximum of 25V. It still has the valur of 47uf.
> based on that how would one calculate the speed at which it discharges
> with
> a given resistor value @ 12v?
This page may help -
http://www.play-hookey.com/dc_theory/rc_circuits.html - or just google
"rc discharge equation". Seeing there is a real capacitor there, the
calculations become a bit trickier. I'd crunch the numbers myself but i
don't have time right now.
>
> And in this scenario, I guess we also need to know how fast the signal
> is
> pulsated?
Yes.
As I'm assuming if the capacitor is not discharged fast
> enough,
> and the transistor has a certain threshold before it switches off, the
> pulses coming off the transistor will be longer. Logic correct?
Yes. One danger is that the transistor may operate in the linear region,
where it is neither fully on nor fully off. This means it will get much
hotter then if it was switched fully on or fully off. In a sealed
plastic box, this could cause problems.
>
>>What kind of box? Metal?
>
>
> Plastic, 1" x 1" x 2".
I'm not expert, but it sounds like that could be asking for trouble.
Maybe a cheap metal box would be the way to go?
>>4) hook up the circuit as follows
>>
>>
>>
>>
>> 12V
>> |
>> |
>> |
>> FAN +
>>
>> FAN -
>> |
>> |
>> |
>> |
>> |
>> 120 ohms | Collector
>> ___ |/
>> +12V from MB------|___|-----|
>> Base |>
>> | Emitter
>> |
>> |
>> |
>>
>> 0V
>>
>
>
> Please bear with me for a second, and this might sound silly, but
> shouldn’t the resistor connect with one end to the 12v MB and
> the other end to 0V? (I think 0v is common ground in computers, that
> is the case and every other component shares that ground.)
It effectivly does!
There is a (small) vltage drop accross the Base-Emitter pins, but
otherwise its connected to 0V through the transistor.
>
>
> Power Supply
> | 12v+
> |
> |
> |
> |
> |
> Motherboard Header |/
> 12v+ ----------------------| NPN
> | |> Transistor
> .-. |
> | | |
> | | |
> '-' |
> Resistor | |
> | |
> | | +
> | |
> | .-.
> | (FAN)
> | '-'
> | | -
> | |
> | |
> ___ ___
> _ Ground _
> | |
>
>
>
> Would this schematic work? (Modulating (+) instead of ground?)
Possibly, but its asking for trouble:
1) Too much base current may flow. The resistor in the first circuit
served to limit the bse current, but also to discharge the capacitor.
2) since you just want to use the NPN transistor as a switch, you are
better off placing the fan between 12V and the collector. I don't have
time to explain why right now, but google "common emitter" "emitter
follower" and "transistor switch".
I can give more details later if needed.
cheers,
Al
>
> Thanks,
> Mike
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