Custom Voltage in a Laptop

[caspian]

Dear Forum members,

I need to create a custom voltage of 3.8v in a laptop to drive the CPU fan.
It is not available in laptops and I have to create it. I can think of a simple voltage divider depicted in the photo.
but I have doubt that the voltage divider may consume extra power from the +5v_Run rail and make this rail burn after a while.
the fan consumes 0.16A when the input voltage is 3.8v.
the laptop is an ordinary 90W laptop called Dell Inspiron 1564.
the fan has no PWM input and the original FAN Driving Circuit on the mainboard is defective.

So I have got the questions that:

- Is it possible that this voltage divider damage something on the mainboard?
- Is such voltage divider suitable to create a custom voltage in a laptop?
- If not so, what kind of circuit does suite this purpose such that it can be easily made on the mainboard?
- What values should be selected for R1 and R2 such the voltage divider consumes very low power?

Thanks

[piernov]

Re: Custom Voltage in a Laptop

Unfortunaltey a voltage divider will not work in this case. You're trying to drive an inductive load that'll be drawing a fair amount of current especially at startup.
If you only want a constant voltage with no control over it (which isn't really the best idea for a laptop fan), you can simply use a voltage regulator. An LDO (like an 1117) can do the job, it'll also require a couple of capacitors and a resistor or 2 to set the output voltage.
You could also connect it directly to the 3.3V rail, it'll run a bit slower.
Anyway, you're better off fixing the fan circuit on the motherboard.

[ktmmotocross]

Re: Custom Voltage in a Laptop

if u dont have fan circuit, 3.3V rail is good for that purpose

[peste]

Re: Custom Voltage in a Laptop

you can use that

https://fr.aliexpress.com/af/buck%25...wCP=y&jump=afs

but better fix the problem on the motherboard

[caspian]

Re: Custom Voltage in a Laptop

Dear ktmmotocross,
I have tested 3.3v, but the fan spins at a low rate using 3.3v.

Dear peste,
the buck converter modules are really interesting to use in sensitive parts of mainboards. Since it is just a FAN, I thought a simple voltage regulator might be enough.

If I use high resistance values for R1 and R2 in the kilo ohm range, does this voltage regulator burn the +5v_Run rail?

[peste]

Re: Custom Voltage in a Laptop

I do not think it will burn, because I see on the schema that the fan is powered by the 5v voltage, but you never know ... here ... you can make a test if you want ... but I do not think it will be stable ... in order to be more stable instead of R2 it should be a zener diode .. but for that should be other calculations ... another problem is that the fan will turn around all the time .. to fix it safely motherboard .. but if you want to play ... it's ok ..

[caspian]

Re: Custom Voltage in a Laptop

thank you. I will try it. The laptop owner said: no matter if the fan spins continuously.

[ktmmotocross]

Re: Custom Voltage in a Laptop

download electrodroid or another app for calculating all this circuits.

is good current on that fan? fan i have here tells 0.4A

[ala_borbe]

Re: Custom Voltage in a Laptop

take 5v from usb and put two zener diodes in series (or three if it spins to fast)... every diode makes ~0.7v voltage drop

works like a charm... done 500 times at least

[mcplslg123]

Re: Custom Voltage in a Laptop

take 5v from usb and put two zener diodes in series (or three if it spins to fast)... every diode makes ~0.7v voltage drop

works like a charm... done 500 times at least

I too follow ala borbe's method and in some cases taking load directly from 3.3V rail.

[String2016]

Re: Custom Voltage in a Laptop

@Alaborbe what type of zener diode to use please

[caspian]

Re: Custom Voltage in a Laptop

I searched the net about using Zener diodes for voltage regulation. and I found out many things are involved in such a circuit. For example, Voltage and Power properties of the Zener diode and R_Load which is the resistance of the load. they say a resistor is required between the voltage source and the Zener diode to restrict the current passing the Zener diode.
that means lots of calculations are required.

[caspian]

Re: Custom Voltage in a Laptop

the other thing which is important is R_Load. Ideally, the output of the voltage regulator should not depend on R_Load. there are two kinds of R_Load: one when the load component is on and the other when the load component is off.
Thus, the calculations get complicated a bit. I am not an Electronics expert. So I cannot do these calculations for Zener Diodes.

[ala_borbe]

Re: Custom Voltage in a Laptop

@Alaborbe what type of zener diode to use please

1n4007
cheapest available
https://components101.com/1n4007-diode

[caspian]

Re: Custom Voltage in a Laptop

I discovered that the circuit of Post#1 is not correct.
How should the two serial Zener diodes be used so that the output voltage does not depend on R_load?

[peste]

Re: Custom Voltage in a Laptop

Do not complicate yourself, what ala_borbe said in post # 9, is the simplest.

[caspian]

Re: Custom Voltage in a Laptop

That is right. Thank you.