Electronics > Projects, Designs, and Technical Stuff

What is the difference in a 24V application vs 12V where power is constant?

<< < (3/3)

james_s:

--- Quote from: DW1961 on July 22, 2020, 04:45:54 am ---OK, so I think I understand. I was talking about apples to apples, that is, a DC light for both 24 and 12V. I didn't mean to mix constant and nonconstant power devices. (At least not yet, and probably never! :) )

I'm starting to understand now given your explanation about regardless of 24v Amps or 12V amps, amps are amps, and so the 1A 24V supply would still be called upon to deliver 2A, and thus exceed the supply's rating, with the above consequences.

"Usually there should be no problem running the filament at a higher voltage (within reason), but would not advise trying to run a 12V lamp at 240V with PWM, for other reasons."

I appreciate you going into details bout how to limit power draw from the device (light) because you are explaining to me that you can use a 24  on a 12 device if you could limit the devices power needs. Question: Could you run a resistor in-between the 24V device to limit power to the device, while still running 24V instead of 12V? I'm assuming that would be inefficient because of heat loss?

So, it's not about volts, but as I assumed, it's about getting the correct power to the device. It's about heat destroying things, not anything inherently different about 12 vs 24V? In other words, it's about how one delivers power vs the other, and power = heat?

--- End quote ---


Yes, you can absolutely use a resistor in series with a resistive load like an incandescent light bulb. The problem is if you have a bulb that draws 1A at 12V (so it consumes 12 watts) and you want to run it from a 24V supply you need a resistor that drops 12V across it leaving 12V remaining for the bulb. This resistor will be carrying the same 1A as described by Kirchoff's laws, meaning that the resistor will be burning up another 12 watts, consuming the same amount of power as the bulb but turning it into useless heat. There are active devices that do this too, they're called voltage regulators. A linear regulator is essentially an automatic resistor, it adjusts to whatever load is connected to keep the voltage constant, but it still burns up the excess voltage as heat. A switchmode regulator is more complex but can transform voltage to current or vice versa.

Current causes heat. Power is a product of voltage and current together, so if you double the voltage you can carry the same amount of power with half the current. Half the current means half the heat, wires can be smaller for the same amount of power, this is why power is transmitted long distances at very high voltages, 80,000V or more and then stepped down by substations closer to the point of use.

It may be more clear to think it terms of mechanical systems. Power is a product of torque and speed. The strength required of a shaft depends on the torque, so if you want to transmit a lot of power at a low RPM you need to use a big strong shaft that can take the torque without twisting into a pretzel. If you spin the shaft at a much higher speed you can use a gearbox at the other end to exchange some of that speed for torque, same amount of power but now you can use a much smaller and lighter shaft, smaller joints, less support structure, everything can be smaller, lighter and cheaper which becomes more significant as the distance you want to transmit the power increases. Voltage is a lot like speed, current is a lot like torque. A clutch that slips to regulate the output speed is like a resistor or linear regulator, a gearbox is like a switchmode regulator.

DW1961:

--- Quote from: james_s on July 22, 2020, 05:06:51 am ---
--- Quote from: DW1961 on July 22, 2020, 04:45:54 am ---OK, so I think I understand. I was talking about apples to apples, that is, a DC light for both 24 and 12V. I didn't mean to mix constant and nonconstant power devices. (At least not yet, and probably never! :) )

I'm starting to understand now given your explanation about regardless of 24v Amps or 12V amps, amps are amps, and so the 1A 24V supply would still be called upon to deliver 2A, and thus exceed the supply's rating, with the above consequences.

"Usually there should be no problem running the filament at a higher voltage (within reason), but would not advise trying to run a 12V lamp at 240V with PWM, for other reasons."

I appreciate you going into details bout how to limit power draw from the device (light) because you are explaining to me that you can use a 24  on a 12 device if you could limit the devices power needs. Question: Could you run a resistor in-between the 24V device to limit power to the device, while still running 24V instead of 12V? I'm assuming that would be inefficient because of heat loss?

So, it's not about volts, but as I assumed, it's about getting the correct power to the device. It's about heat destroying things, not anything inherently different about 12 vs 24V? In other words, it's about how one delivers power vs the other, and power = heat?

--- End quote ---


Yes, you can absolutely use a resistor in series with a resistive load like an incandescent light bulb. The problem is if you have a bulb that draws 1A at 12V (so it consumes 12 watts) and you want to run it from a 24V supply you need a resistor that drops 12V across it leaving 12V remaining for the bulb. This resistor will be carrying the same 1A as described by Kirchoff's laws, meaning that the resistor will be burning up another 12 watts, consuming the same amount of power as the bulb but turning it into useless heat. There are active devices that do this too, they're called voltage regulators. A linear regulator is essentially an automatic resistor, it adjusts to whatever load is connected to keep the voltage constant, but it still burns up the excess voltage as heat. A switchmode regulator is more complex but can transform voltage to current or vice versa.

Current causes heat. Power is a product of voltage and current together, so if you double the voltage you can carry the same amount of power with half the current. Half the current means half the heat, wires can be smaller for the same amount of power, this is why power is transmitted long distances at very high voltages, 80,000V or more and then stepped down by substations closer to the point of use.

It may be more clear to think it terms of mechanical systems. Power is a product of torque and speed. The strength required of a shaft depends on the torque, so if you want to transmit a lot of power at a low RPM you need to use a big strong shaft that can take the torque without twisting into a pretzel. If you spin the shaft at a much higher speed you can use a gearbox at the other end to exchange some of that speed for torque, same amount of power but now you can use a much smaller and lighter shaft, smaller joints, less support structure, everything can be smaller, lighter and cheaper which becomes more significant as the distance you want to transmit the power increases. Voltage is a lot like speed, current is a lot like torque. A clutch that slips to regulate the output speed is like a resistor or linear regulator, a gearbox is like a switchmode regulator.

--- End quote ---

Very good explanation. I understand.

Navigation

[0] Message Index

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod