# Voltage controlled switch?

So I am in the process of working on a rather strange and specific project. I am looking to capture as much energy as possible from a small steam turbine using a DC motor and then use that to power another DC motor.

Essentially my idea was to charge a capacitor using the first dc motor until a specific voltage and then discharge it as fast as possible into the other dc motor. The only part I am not sure of is creating the circuit that will allow me to flip the switch when a certain voltage is reached.

Also as a secondary question what would be the advantages and disadvantages of using a battery vs a capacitor in this situation?

I apologies if this is a simple question, I am still new to circuit design so any recommendations would be super appreciated.

• Are you looking to store the captured energy for a specific amount of time? Or are you just needing to reach a threshold voltage before releasing it. Because this would be the primary determining factor between using a capacitor vs a battery. Also the amount of charge you need to store is another important factor. I apologize for not directly answering the question, but these are some additional considerations that would be at play before I could give a more direct answer. Feb 4, 2019 at 19:12

I believe you are describing a charge pump.

You don't give any specifics on what you're trying to do, though, so I can't really give meaningful feedback on anything else. Generally speaking, though:

I am looking to capture as much energy as possible from a small steam turbine using a DC motor and then use that to power another DC motor.

If you're driving it, it's a generator. Not all DC motors are the same, so you can't just grab any DC motor and spin the output shaft and expect to get voltage at the terminals. Shunt motors, brushless motors, etc. will require extra inputs to get a useful output.

Essentially my idea was to charge a capacitor using the first dc motor until a specific voltage and then discharge it as fast as possible into the other dc motor.

You don't get to pick how fast a capacitor discharges unless you can control the load resistance/impedance. The motor will determine how quickly the capacitor discharges. And again, you haven't stated anything about the drive cycle, supply/consumption voltages, etc., so I'm not clear why you're not just driving the motor directly with the steam generator.

Also as a secondary question what would be the advantages and disadvantages of using a battery vs a capacitor in this situation?

Capacity/energy density, but also inherent voltage regulation. Battery voltages are determined by their chemistries; a fully discharged lead-acid battery may still read about 10.8 volts. A capacitor still has a charge at 1 V, but that may not be useful energy because the voltage isn't in a useful band.

Don't re-invent the wheel. Don't re-invent the switching mode converter. If you need to change the bus voltage, use a buck/boost converter to a voltage that is suitable for your output, and then use a battery float if you need surge capacity (where the instantaneous power demand might be greater than the source can generate, even if your source is capable of delivering >= the average power demands.)

• You might want to add that the energy storage in a standard capacitor is several orders of magnitude less than that of a similarly sized battery.
– Drew
Nov 17, 2023 at 19:53
• @Drew do you mean to say energy density? Or specific energy? OP doesn't mention needing a specific storage quantity, the question just uses "storage" as a way to "cache" electricity until some higher voltage is achieved. As I mention in my answer, this kind of "boost" converter already exists as a mature product. I think it's highly unlikely anyone is going to design their own boost converter that's more efficient, unless boost converter development is their specific field of work. Nov 20, 2023 at 4:39
• I mean specific energy. They're so different it's rare to have an application where one or the other is not clearly preferable.
– Drew
Nov 20, 2023 at 14:54
• @Drew maybe, but it's not clear to me how a battery (or capacitor) relates to OP's question or my answer. OP never clarified what they're trying to do, so it's hard to tell what's relevant. Nov 24, 2023 at 1:40