Hello! This is my first post to the forum. Today is the day that Math will save my ears! (long intro, scroll down for actual problem titled:

My "desktop" pulls close to 1200 Watts (with all 5 monitors, laptops, and gaming rig under full load), according to the two UPS' I've burned out. So, I got my hands on a big powerful APC 2200VA unit allowing up to 1980 Watts (on a 30 Amp circuit). Since 1200 Watts is under the 15 Amps, I've installed a normal 15A plug on to it.

Powering the UPS unit on, I was greeted to the jet engines of the 24 Volt 6000rpm fans the unit has in it. Needless to say, this is unacceptable. Some searches on APC forums indicate that all APC Smart UPS units these days all have extremely loud fans in them, always running, even when powered off and unplugged from wall! This should be a bug! But, APC claims it's "normal". Considering my previous APC SmartUPS 2000VA I used in an old office didn't even have a fan running unless it was on battery, this incidated to me that there is something else going on (such as APC not caring about fan selection but wanting to keep the units from failing early).

Over the weekend, I dissassembled the unit and found the two 24V 80mm fans. I disconnected them, and closely monitored the temps inside the case as I stress-tested my equipment to around 1080 Watts. Turns out, my theory was correct: the fans aren't needed! The temps only rose 10C over the course of 2 hours of ~1000 Watts of power. I measured it in two places: next to the "heatsinks" that had 1 fan dedicated to, and the ambient air around the reducer coils on the other side of the unit. As a comparison, the heat being expelled from my desktop's radiators during gaming is 30C over room temp! Hot!

So, I am replacing the fans with some quality "quieter" rubber-isolated fans rated at about 2/3rds of the CFM the original unnecessarily-load fans are. Problem is, the fans are 12 Volts - not 24 Volts.

And finally, I get to the point of my post...

First problem, I need to reduce the voltage from 24 Volts down to 12 Volts. Doing some searches online, I'm reminded of the Volt Divider rule. Simple math, but I remember the case where if I introduce a load in parallel to R2, then things change a bit...

Please check my math:

R1 = Resistor in series of the circuit.

RL = The resistance supplied by the load.

Rb = The bleeder resistance which is the resistance supplied by the resistor in parallel with RL.

Formulas:

RL = Volts / Amps

Rb = Volts / (Amps * .1)

It (Impendence Total) = Amps + (Amps * .1)

Rb is based on the theory that we want 10% of the load current to pass through it. Found an old post that mentioned that listed here.

Total volts in circuit: 24 V

RL is rated for 12 V at 0.17 Amps.

RL resistance = 12 V / 0.17 Amps =

Rb = 12 V / 0.017 Amps =

Impendence Total = 0.17 Amps + 0.017 Amps =

Req = 12 V / 0.187 Amps =

R1 = 12 V (to be dropped) /

Checking my math:

Total Power = 24 V * 0.187 Amps = 4.488 Watts

R1 64 Ωsees 12 V, which is 12 V * 0.187 Amps = 2.244 Watts

Rb 705 Ω sees 12 V * 0.017 Amps (bleed) = 0.204 Watts

R1 = 64 Ω needs to be rated at 2.25 Watts

Rb = 705 Ω needs to be rated at 1/4 Watt

Could you remind me: it is ok to place resistors in series to obtain the total reistance I need, correct?

I plan on combing five (5) 12 Ω rated at 3 Watts in series, plus three (3) 1.5 Ω 2 Watts to achieve the 64.5 Ω I need for R1. Though, I am calculating 2.25 Watts of load, the three smaller "Power" resistors are only rated at 2 Watts. It's all I can find...

I also plan on combining a 680 Ω with a 24 ohm to achieve 704 Ω at 1/4 watt for Rb, to be in parallel with the load (Fan).

Finally, just need to confirm... The math still works if the APC unit lowers the voltage down to 16 Volts, correct? That seems to be what it idles at at all times (even when powered off, and unplugged from wall!).

Thank you advance! I know, it was a VERY long post... Let me know if I need to sum it up, and I'll remove the "bacl story" to the project.

**24 Volts -> 12 Volts**)My "desktop" pulls close to 1200 Watts (with all 5 monitors, laptops, and gaming rig under full load), according to the two UPS' I've burned out. So, I got my hands on a big powerful APC 2200VA unit allowing up to 1980 Watts (on a 30 Amp circuit). Since 1200 Watts is under the 15 Amps, I've installed a normal 15A plug on to it.

Powering the UPS unit on, I was greeted to the jet engines of the 24 Volt 6000rpm fans the unit has in it. Needless to say, this is unacceptable. Some searches on APC forums indicate that all APC Smart UPS units these days all have extremely loud fans in them, always running, even when powered off and unplugged from wall! This should be a bug! But, APC claims it's "normal". Considering my previous APC SmartUPS 2000VA I used in an old office didn't even have a fan running unless it was on battery, this incidated to me that there is something else going on (such as APC not caring about fan selection but wanting to keep the units from failing early).

Over the weekend, I dissassembled the unit and found the two 24V 80mm fans. I disconnected them, and closely monitored the temps inside the case as I stress-tested my equipment to around 1080 Watts. Turns out, my theory was correct: the fans aren't needed! The temps only rose 10C over the course of 2 hours of ~1000 Watts of power. I measured it in two places: next to the "heatsinks" that had 1 fan dedicated to, and the ambient air around the reducer coils on the other side of the unit. As a comparison, the heat being expelled from my desktop's radiators during gaming is 30C over room temp! Hot!

So, I am replacing the fans with some quality "quieter" rubber-isolated fans rated at about 2/3rds of the CFM the original unnecessarily-load fans are. Problem is, the fans are 12 Volts - not 24 Volts.

**24 Volts - 12 Volts**And finally, I get to the point of my post...

First problem, I need to reduce the voltage from 24 Volts down to 12 Volts. Doing some searches online, I'm reminded of the Volt Divider rule. Simple math, but I remember the case where if I introduce a load in parallel to R2, then things change a bit...

Please check my math:

R1 = Resistor in series of the circuit.

RL = The resistance supplied by the load.

Rb = The bleeder resistance which is the resistance supplied by the resistor in parallel with RL.

Formulas:

RL = Volts / Amps

Rb = Volts / (Amps * .1)

It (Impendence Total) = Amps + (Amps * .1)

Rb is based on the theory that we want 10% of the load current to pass through it. Found an old post that mentioned that listed here.

Total volts in circuit: 24 V

RL is rated for 12 V at 0.17 Amps.

RL resistance = 12 V / 0.17 Amps =

**70.59 Ω**Rb = 12 V / 0.017 Amps =

**705.88 Ω (10% of 0.17 Amps)**Impendence Total = 0.17 Amps + 0.017 Amps =

**0.187 Amps**Req = 12 V / 0.187 Amps =

**64.17 Ω**R1 = 12 V (to be dropped) /

**0.187**=**64.17 Ω**Checking my math:

**0.187**Amps * (**64.17**Ω +**64.17**Ω) = 24 VTotal Power = 24 V * 0.187 Amps = 4.488 Watts

R1 64 Ωsees 12 V, which is 12 V * 0.187 Amps = 2.244 Watts

Rb 705 Ω sees 12 V * 0.017 Amps (bleed) = 0.204 Watts

R1 = 64 Ω needs to be rated at 2.25 Watts

Rb = 705 Ω needs to be rated at 1/4 Watt

**Combining resistors**Could you remind me: it is ok to place resistors in series to obtain the total reistance I need, correct?

I plan on combing five (5) 12 Ω rated at 3 Watts in series, plus three (3) 1.5 Ω 2 Watts to achieve the 64.5 Ω I need for R1. Though, I am calculating 2.25 Watts of load, the three smaller "Power" resistors are only rated at 2 Watts. It's all I can find...

I also plan on combining a 680 Ω with a 24 ohm to achieve 704 Ω at 1/4 watt for Rb, to be in parallel with the load (Fan).

**Voltage Reducing**Finally, just need to confirm... The math still works if the APC unit lowers the voltage down to 16 Volts, correct? That seems to be what it idles at at all times (even when powered off, and unplugged from wall!).

Thank you advance! I know, it was a VERY long post... Let me know if I need to sum it up, and I'll remove the "bacl story" to the project.

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