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> Sorry for the late reply - I'm traveling. No problem! > A constant-current power supply is the preferred way to charge a capacitor wihtout destroying it. S

> Sorry for the late reply - I'm traveling. No problem! > A constant-current power supply is the preferred way to charge a capacitor wihtout destroying it. Sure, I agree. I just wanted to use a simpler circuit to keep my explanation simple. >As to circuits other than ideal ones, I draw your attention to the behavior of superconductors, in which currents flow endlessly until interrupted. I think you have cause and effect mixed up. How do you get this current started in the super conductor? What is the magnitude of this current? Infinite amps? You will still need an initial jolt (voltage source) to get the current started. >For a constant-current source, the rise is linear You do realize there no such thing as a practical constant current source? A current source is just a voltage source which can adjust its terminal voltage to keep it's current output constant with changing load? Like I said you need to understand the cause and effect relationships here. > Capacitors have an upper limit to inrush current Sure practical capacitors, conductors, resistors, ... everything has an Imax. I did say raise the voltage as long as you are within the safe operating area. SOAs are defined in terms of Imax, Vmax and Pmax. >I will apply 10 amperes of constant current As I have already explained a constant current source is just a voltage source with current feedback. You are actually changing the terminal voltage to maintain your 10A. I agree charging at Imax is the fastest way of charging a capacitor but my circuit was simpler (constant voltage source with varying current vs your constant current source with varying terminal voltage). > Will I be able to charge the capacitor faster from a source of 200 volts than from a source of 50 volts? This is a resounding yes. A constant voltage source rated at 200V will charge a capacitor faster than a source rated at 50V by virtue of the capacitor charging equation (assuming internal resistances are comparable). > Surely you realize one doesn't attach capacitors to voltage sources, or is this still not understood? There is no reason why you cannot attach a voltage source to a capacitor. See: http://imgur.com/gallery/sOZ1F http://imgur.com/gallery/sOZ1F. Let me remind you a so called current source is also a voltage source. Current sources are a theoretical construct. Internal resistances have to be considered in a practical circuit otherwise you will have to deal with infinite currents at t=0.

lutusp 10y ago
> You will still need an initial jolt (voltage source) to get the current started. Yes, but that wasn't what I replied to. Your claim was this: > There is no current without EMF (measured in volts). That's false. There can be massive currents without any potential difference. Again, superconductors show this behavior. >> For a constant-current source, the rise is linear > You do realize there no such thing as a practical constant current source? Oh, really? So all those power supplies I designed for NASA were not really what they seemed? Constant-current sources are a normal part of electrical engineering practice and have been for decades. https://en.wikipedia.org/wiki/Current_source https://en.wikipedia.org/wiki/Current_source A typical, simple schematic of a constant current source: http://www.allaboutcircuits.com/technical-articles/the-basic-mosfet-constant-current-source/ http://www.allaboutcircuits.com/technical-articles/the-basic... Also, by using a switching inverter and manipulating the relationship between voltage and current in reactive elements, a constant-current source can be made very efficient. This is the ideal way to charge a supercapacitor, and it is standard practice. But I think I already said that. > I did say raise the voltage as long as you are within the safe operating area. Yes, but that's not how you charge a capacitor -- you use a controlled current, not a voltage. The voltage follows the current, not the other way around. >> Will I be able to charge the capacitor faster from a source of 200 volts than from a source of 50 volts? > This is a resounding yes. Quite false. For a given current that a capacitor can tolerate, the charging rate is the same regardless of the source voltage. The reason? You don't charge capacitors with voltage, you charge them with current. The example I gave earlier was a capacitor rated at 50 volts and two current sources -- one that can deliver 200 volts and one that can deliver 50 volts. The charging rate is the same. The reason? You don't charge capacitors with voltage, you charge them with current. > There is no reason why you cannot attach a voltage source to a capacitor. No reason at all. But don't be in the same room with a large capacitor and a voltage source that will supply substantial current to maintain a specified voltage. But, you know what? I already said that. I've been designing power supply circuits for 40 years. My man-rated designs flew on the NASA Space Shuttle. I hold several patents. You're arguing with the wrong person. > Current sources are a theoretical construct. Current sources are an everyday, trivial design task that all competent designers must learn to be regarded as employable. See the linked schematic above. > Internal resistances have to be considered in a practical circuit otherwise you will have to deal with infinite currents at t=0. That is true for a voltage source. It is not true for a current source, and this is by design.

lutusp 10y ago

> You will still need an initial jolt (voltage source) to get the current started. Yes, but that wasn't what I replied to. Your claim was this: > There is no current without EMF (measured in volts). That's false. There can be massive currents without any potential difference. Again, superconductors show this behavior. >> For a constant-current source, the rise is linear > You do realize there no such thing as a practical constant current source? Oh, really? So all those power supplies I designed for NASA were not really what they seemed? Constant-current sources are a normal part of electrical engineering practice and have been for decades. https://en.wikipedia.org/wiki/Current_source https://en.wikipedia.org/wiki/Current_source A typical, simple schematic of a constant current source: http://www.allaboutcircuits.com/technical-articles/the-basic-mosfet-constant-current-source/ http://www.allaboutcircuits.com/technical-articles/the-basic... Also, by using a switching inverter and manipulating the relationship between voltage and current in reactive elements, a constant-current source can be made very efficient. This is the ideal way to charge a supercapacitor, and it is standard practice. But I think I already said that. > I did say raise the voltage as long as you are within the safe operating area. Yes, but that's not how you charge a capacitor -- you use a controlled current, not a voltage. The voltage follows the current, not the other way around. >> Will I be able to charge the capacitor faster from a source of 200 volts than from a source of 50 volts? > This is a resounding yes. Quite false. For a given current that a capacitor can tolerate, the charging rate is the same regardless of the source voltage. The reason? You don't charge capacitors with voltage, you charge them with current. The example I gave earlier was a capacitor rated at 50 volts and two current sources -- one that can deliver 200 volts and one that can deliver 50 volts. The charging rate is the same. The reason? You don't charge capacitors with voltage, you charge them with current. > There is no reason why you cannot attach a voltage source to a capacitor. No reason at all. But don't be in the same room with a large capacitor and a voltage source that will supply substantial current to maintain a specified voltage. But, you know what? I already said that. I've been designing power supply circuits for 40 years. My man-rated designs flew on the NASA Space Shuttle. I hold several patents. You're arguing with the wrong person. > Current sources are a theoretical construct. Current sources are an everyday, trivial design task that all competent designers must learn to be regarded as employable. See the linked schematic above. > Internal resistances have to be considered in a practical circuit otherwise you will have to deal with infinite currents at t=0. That is true for a voltage source. It is not true for a current source, and this is by design.