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Reducing Voltage from 29 TO 28 Volts

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aKO

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Jan 3, 2022
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My system is 24 volt nominal 29 volt actual and i want to fit a small Computer type fan to turn on when a piece of equipment reaches a certain temperature . The only thing i am having problems with is finding a 8cm fan that will operate at this voltage , 5 , 12 , 24 and 48 are al no problem however none of those nominal voltage fans will operate at 29 volts , the range they need is either higher or lower than i require . 30 Volt fans are available but here in Spain getting anything is a problem , Amazon is the best option most of us have but none there either .

A friend suggested i bought a 24 volt fan and reduce the voltage to suite , the voltage required on these fans is around 27.5 volts and they are all around 2 watts . I have been told that as i plan to install a light bulb to show when the fan is on that if wired in series it will reduce the voltage but i know very little about electronics so dont know by how much and what size bulb i would need .

If anyone can help then i would be most grateful .
 
aKO said:
My system is 24 volt nominal 29 volt actual and i want to fit a small Computer type fan to turn on when a piece of equipment reaches a certain temperature . The only thing i am having problems with is finding a 8cm fan that will operate at this voltage , 5 , 12 , 24 and 48 are al no problem however none of those nominal voltage fans will operate at 29 volts , the range they need is either higher or lower than i require . 30 Volt fans are available but here in Spain getting anything is a problem , Amazon is the best option most of us have but none there either .

A friend suggested i bought a 24 volt fan and reduce the voltage to suite , the voltage required on these fans is around 27.5 volts and they are all around 2 watts . I have been told that as i plan to install a light bulb to show when the fan is on that if wired in series it will reduce the voltage but i know very little about electronics so dont know by how much and what size bulb i would need .

If anyone can help then i would be most grateful .
Click to expand...
With a bulb in series it’s very likely the fan would see a voltage it is happy with, and will not burn out. The other simple thing you could do is just extend the input cables between the fan and the power source with some thin wire which will have a voltage drop within it when the current flows. Thus the fan might only see 27.5V even if the power source is 29V (depending how thin / long the wire is).

But anyhow, I would expect the volt drop across the bulb will save you in this case, but if you have a multimeter, just connect it up and measure the voltage at the terminals of the fan itself. Briefly connecting it will cause no problem even if it transpires that the voltage is a little too high according to the fan spec sheet.
 
A few silicon diodes in series will drop about 0.6 volts per diode.
 
Now had chance to try it but at the moment i only have a 12 volt motor and 12 volt x 20 watt halogen bulb but connecting just the motor i had 13.8 volts on Multimeter , with bulb in series that dropped to 12.8 which would be ideal except that a 20 watt bulb is far to big just as a small illumination light to show fan is on .
 
You are Over charging & Maintaining too high actually. 29V = 3.625 Volts per cell - Too High !
Ideally you should Charge at 27.4V (3.425 Volts per cell) and Floating at 27.3 (3.415 Volts per cell)
Working Voltage for LFP is 3.000-3.400.
The ALLOWABLE Voltage Range is 2.500-3.650 but that is NOT for daily use.
The Allowable Range is what LFP will Tolerate but that is not the voltage it is designed for to be used.

Your best option to reduce voltage for 12V to run a PC Type fan is a small Buck Converter that handles 30V to 12V.
One like this shown below takes 24-30V Input and outputs a regulated 12V. You NEED one that can handle up to 30V Input for when you are charging.

24V Step Down to 12V 30A 360W DC/DC Converter Voltage Regulator Reducer Adapter Waterproof Buck Transformer Power Supply : Amazon.ca: Automotive

24V Step Down to 12V 30A 360W DC/DC Converter Voltage Regulator Reducer Adapter Waterproof Buck Transformer Power Supply : Amazon.ca: Automotive
www.amazon.ca

Price is also relative to how many Amps/Watts output you want.

These shown Below are often "exactly 24V to 12V" potted step-down converter, they DO FAIL if you go over 25V... I've melted a few & when they fail they pass through the 24V. Avoid this type.
1662641729520.png
 
You are totally wrong with your charging voltages Steve if your talking about Lead Acid battery which i have . Look at manufacture data sheets and you will be better informed . Rolls for instance , a leading make set Absorption voltages at over 29 volts . I have Trojan AGMs and charging voltage for them is 28.8 volts with 27 volts for float . You also show a difference of only 1/10 of a volt between Absorption and Float , i have never heard of any battery Lead Acid battery with a float voltage as close to absorption as that , in fact it would be almost pointless .
 
Sorry, I assumed you were using LFP / LiFePo4 not Lead Based.
I transitioned from Rolls Surette S-550's to LFP a while ago.
 
A lamp is not a very good way since the initial resistance is much lower when initially powered and then it increases. A string of diodes is the way to go as these would have no failure rate compared to converters, I often lower fan voltage to reduce noise and computer fans are not designed to work at higher battery voltages.
 
I wouldn't use a converter , had them before and each one caught fire . The place they would need to go is inside a small plastic box with no ventilation or airflow so i think i would be inviting another fire .
 
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