Electronics > Beginners
Completely discharge a capacitor on an active load
raff5184:
Hi,
is it possible, only using passive components or components that absorb a minimum amount of energy, to completely discharge a capacitor on an active load that needs a minimum voltage of say 2V to operate?
For example, if the capacitor has a voltage rating of 5V, can I use only the energy stored when the voltage across the capacitor is between 2V and 5V (leaving the energy from 0V to 2V unused) or can I do better?
T3sl4co1l:
Sure, use nothing. Let the "active load" discharge the capacitor as it will. Presumably the load will continue draining it below 2V, so I don't really know how anything should go "unused"...
If you have more specific scales of time and current in mind here, in terms of what counts as "absorb", "minimum energy", "completely", "discharge", "minimum voltage" and so on, a more useful answer may be possible.
Tim
raff5184:
Thanks.
I think that I failed to mention that the active load (for example a PCB) has to keep working even when the voltage across the capacitor goes below the minimum voltage (2V) of the active load. What I see is that the capacitor gets drained but the active load does not work because the voltage is not enough. Is there something to "push up" that remaining charge in the capacitor so that the load can actually use it?
In terms of "absorption" I was thinking of something passive (not an amplifier or similar) because I don't have a supply (e.g. a battery) that absorbs currents in the order of few microAmps. My only supply is the capacitor
Mr Evil:
You can't do that with passive components, but it's possible for a buck-boost converter to do it, powered from the capacitor itself. For instance the LTC3119 could output a constant 2V from the initial 5V input all the way down to 0.25V.
Is it really worth it though? Once the capacitor has drained from 5V to 2V, it has only 16% of its energy left.
Berni:
--- Quote from: raff5184 on January 30, 2020, 05:11:24 pm ---is it possible, only using passive components or components that absorb a minimum amount of energy, to completely discharge a capacitor on an active load that needs a minimum voltage of say 2V to operate?
--- End quote ---
Yep put a large value resistor across said capacitor. Its a passive component that absorbs a minimum amount of energy (high resistance value) and operates down to 0V
But yeah all electronic loads have a minimum voltage. In practice this issue is solved by introducing a additonal power supply in series to effectively bring the ground terminal of the electronic load down to a negative voltage below ground. So it doesn't matter if the load can only drop a minimum of 2V if the load terminal is held at -3V for example. But yes this is an active component. It must be active because the load dropping extra voltage causes extra power to be absorbed by it. This power has to come from somewhere, hence a power supply to provide it.
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