Buck converter will power LED's but voltage divider will not....

  • Thread starter kerryh
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Summary
Am seeking help understanding why when I take my battery (50V max 44 nominal; 15aH capacity; 12s5p) and attempt to step down the voltage that sometimes it works and sometimes not...
Summary: Am seeking help understanding why when I take my battery (50V max 44 nominal; 15aH capacity; 12s5p) and attempt to step down the voltage that sometimes it works and sometimes not...

Hi all,

New to forum. Chemical engineer seeks help from electrical engineers.... Thanks for reading this. I am an active DIY-er who has built my own custom electric skateboard. The board is powered by a 12s5p li-ion battery with a max charge of about 50v and 44 nominal. I am trying to add LED lights to the board for night time safety reasons. When I use a buck converter to step down the voltage to 12v, most of the time it works (for the other times that the buck converter doesn't flat out burn up). The rating for the buck converter is input 5-36V (output 1.5-30v) and realize that this is out of range for the supply voltage but a couple of the converters can handle it for some reason. As an idea to stop burning up converters I stumbled across what is called a voltage divider so that I can drop voltage before it ever reaches the converter. First let me back up, before I try connecting it to the converter, I fabricated a voltage divider with r1= 910k ohms and r2=350k ohms delivering a output voltage of 12.5v as measured with a multi meter, and connected it straight to the LED supply.

FYI- When I fabricated my first voltage divider with equal resistors (100 ohm) on both r1 & r2, voltage should be half, which it is when I connect it to a 12v small motorcycle battery, or any other small capacity battery. But when I connect it to my skateboard battery (48v) the voltage does not drop at all, except if I use the really large resistors.

When I use a straight up 12v battery (12.7v), or a buck converter that doesn't actually smoke and blow up, to power on the LED's they work fine. When I use the second voltage divider (the kilo ohms resistors, the 910k and the 350k) which is putting out exactly 12.55v, the desired voltage, the LED's will not light. (The minuscule .15v lower voltage difference is not the issue, I checked via other means).

I hope I have provided enough detailed explanation. Can anyone tell me what I'm not seeing?
 

Baluncore

Science Advisor
6,858
2,101
50 volts is too high for the majority of buck converters.
You must find one that will work safely at higher voltages, or use a pre-regulator to drop the converter input voltage to safe levels.

Buck converters can be very efficient. A divider or series resistor to drop voltage before the buck converter will get hot and waste energy. If the resistance is high, insufficient current will reach the converter. With a series resistor, if the LEDs are disconnected, or not turned on, the buck converter will get full voltage.
 

Baluncore

Science Advisor
6,858
2,101
Search for 'High-Voltage EBike DC-DC Converter Buck Module'.
 
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Thanks for the reply guys. I have already ordered a converter with a higher rating. What I was trying to gather from asking this question was to understand why, when I use a converter dropped to the proper voltage, the LED's will light but when I drop to the same voltage using a divider then the LED's do not light up. Still hoping someone can answer this. Thanks!

Btw- I have attempted to drop the voltage (via divider) before connecting it to the converter however, the same problem exists. The converter will not power on just like the LED's will not power on despite it being at the required voltage.
 
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Regarding voltage dividers: they work only if the load current is significantly lower than the current through the divider itself. With the mentioned big resistors, that's around some uA. Any load which try to draw higher current will significantly modify (lower) the output voltage. The LEDs (or the converter) you tried most likely belongs to this category...

Since the requirement of the higher 'idle' current compared to the 'useful' current voltage dividers are mostly used only when power considerations are not relevant.
 

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