In our automation project, we'll be using a buck converter to convert the DC voltage from 72V to 12V. This method of conversion is quite expensive!! We are converting the voltage to power the raspberry pi 3 B+ and some sensors. So first, we need to charge our power bank through a car charger.
I was thinking if we can use a potential divider circuit for this purpose or a cheaper way?
What are the drawbacks of doing this?
it seems like not possible or having critical drawback 🙂
Best Answer
In essense: no.
You will need to use very large resistor values, elsewise the current draw from the 72V source and heat dissipation on the voltage divider will be too large. By making a potential divider with huge resistor values, you make a power supply with a ridiculous output resistance, since the output resistance of a potential divider is equal to R1*R2/(R1+R2), where R1 and R2 are the resistor values in the potential divider.
Let's try to design a potential divider for your case. The current drawn by the potential divider itself will be 72V/(R1+R2), while the heat dissipation will be (72V)^2 / (R1+R2). 1W power losses is already very bad, but let's stay with that value for reduced complexity. From (72V)^2 / (R1+R2) = 1W we see that R1+R2 = 5184 ohm. Since you need 12V output, R2/(R1+R2) = 12/72. From there two equations we get that R1 = 4320 Ohm, R2 = 864 Ohm and the output resistance R1*R2/(R1+R2) = 720 Ohm.
720Ohm output resistance means that this potential divider PSU could handle no more than 12V/720Ohm current, which is equal to 16mA. This is way too low for the Pi to even turn on. Furthermore, any current ripple will kill the power from the Raspberry Pi, since the voltage ripple will be equal to 720 Ohm * current ripple.
You may try to increase the power dissipation. Then, for example, for 0.5A current draw you can reverse-solve the equations I gave before and get a power dissipation of (72V - 12V) * 0.5A = 30W.
Even a linear regulator won't suite you here, since 60V drop is just humongous (I think I've never even seen one that could handle such a high voltage). The power dissipation (and loss) will be the same: (72V-12V)*0.5A = 30W.
So, you need to use a DC-DC converter. And they cost a lot because handling 72V is no easy task in power electronics.