Wondering about this circuit

[RissViss]

Building a power supply, and wanted to power a fan off of the 19v 7amp laptop brick. Wanted to drop the voltage down to between 10 - 9 volts.

In picture 1 I used a 75 ohm 3w 5% axial resister and had a voltage of 10.37.
In picture 2 I used the 75 ohm resister and a Zener Diode 9.1v 5w axial and had a voltage of 10.00.

Is there an advantage to using the Zener over just the resister?
And any other info would help.

[RissViss]

extra pics

[StillTrying]

"In picture 1 I used a 75 ohm 3w 5% axial resister and had a voltage of 10.37.
In picture 2 I used the 75 ohm resister and a Zener Diode 9.1v 5w axial and had a voltage of 10.00."

You must have the zener the wrong way around.

Is there an advantage to using the Zener over just the resister?

The voltage at the fan would be more constant,  if the fan needs a high start current the volt drop across the resistor can sometimes be enough to stop it starting.

[RissViss]

When I flipped the zener it was reading around 5 volts

When I used a 100 ohm 1/4 watt resister in place of the 75 the zener was right on the 9.1 volts, but the resister got hot.

[RissViss]

could i put the 100 ohm resister after the zener and lower the temps on the resistor.

[Nerull]

Zeners don't provide perfect regulation, its normal for their voltage to vary with current.

Increasing the resistance of a shunt regulator will reduce the current in the shunt, but it will also reduce the current you can supply. Essentially, the zener is trying to keep the current through the resistor constant, and will take up any 'extra' current not being used by the load. If the load draws more current than this you will fail to regulate and output voltage will drop.

You should figure out how much current your load requires and pick a resistance that produces a voltage drop with that current that is low enough for the zener to regulate. If you're supplying 19V and need 9V out than the voltage drop across the resistor should be less than 10V at max current.

[StillTrying]

From the 75 ohm 3w 5% axial resister and voltage of 10.37 at the fan, the fan is drawing (19-10.37)/75 = 8.63/75 = 0.115A.

So the resistance is dissipating about 8.63*0.115 = 0.992 W, so a 1/4 W resistor would get quite warm.

Anyway you do it there will be 1W to dissipate, for ~10V at the fan. Use the fan to cool the dropper!

@Nerull
I think the 9V1 zener or resistor is in series with the fan to drop the 19V to ~10V.

[RissViss]

Thanks for the info.

Will use the 75 ohm resistor and the zener.

[digsys]

could i put the 100 ohm resister after the zener and lower the temps on the resistor.

12V 0.13A is equivalent of a 92R resistor, total power = ~1.6W. With any series solution, something has to dissipate the remaining power.
First, a 12V fan can handle a reasonable range of voltage eg 6-8V low (fan speed will be affected) > 13-14V (fan will run a bit faster and maybe hotter),
BUT there won't be any appreciable degradation at the slightly higher voltage. They will also handle a short spike of even higher.
IF you are not concerned about the EXACT fan speed, then a resistor is well good enough. Any rating from 1W to 2W is fine.
The advantage of using a resistor instead of a zener to lose the ~6V is - when a zener fails, it usually goes short cct, a resistor usually goes open.
Both will have the SAME heat to get rid of. If you need an exact voltage (=speed?), then a resistor / zener regulator, or just use a LM7812 (xx) regulator.
If you also want near zero heat, then there are plenty of dc-dc regulator modules for ~3 bucks+ out there. Depends on your criteria.

[RissViss]

The resistor puts the fan speed down low enough that it is almost silent.
I made a mistake when I order the resistors they should have been higher 85 - 90 ohms.
The speed with the 75 ohms is good enough and gets the job done.
There is little to no heat on the 75 ohm 3 watt resistor.

Thanks for the help

[StillTrying]

"Will use the 75 ohm resistor and the zener."

LOL
The "Zener Diode 9.1v 5w axial" will do it just by itself, when connected to drop 9V and not just 0.7V.

[RissViss]

I was using this as my information.

https://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/zener-diodes/

all the circuits had resistors.

this is not about be practical, for me this is about learning different things.

[RissViss]

Just tested, using the zener diode, and the voltage is down to 8.9 volts.

thanks for the help.

more experimentation is better, never hurts to learn something new.

[StillTrying]

"I was using this as my information.
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/zener-diodes"

They're all shunt voltage regulation, which needs a resistor, yours is in series volt dropping rather than regulation.

"Just tested, using the zener diode, and the voltage is down to 8.9 volts."

If your 9V1 zener is really a 5W it won't mind dissipating the ~1W of heat at all.

[RissViss]

For me this is extra information, that I can use.

Much better for me to learn with building and testing then just reading.

Now I have a way to get the voltage anywhere from 8.9 to 10.37

Electronics is way more fun when you have a power supply.

Thanks everyone for the info, if you have any other things I can do with this would be great.

[digsys]

For "fine tuning" the voltage, if you need to keep the supply fixed, you simply add series diodes, at 0.6V each, as many as you like. In fact, technically,
you can drop the entire amount with just "forward biased" diodes - 10X at 0.6V = 6.0V approx. The heat also get "shared". Or zener + diode combo
Lots of options