EveryCircuit
Contact
Reviews
Home
psteichen
modified 9 years ago

Will it burn.....

2
7
121
01:27:09
Millions of amps on a trace! When a capacitor is charged without a resistor a large current is transmitted through the trace, but only for a fraction of a millisecond. Of course some native resistance exists in the trace/capacitor/switch/battery that EC doesn't account for. But let's just say for the sake of this question that a million amps did flow. If it were to occur for such a small time period, the heat buildup in the trace would be minimal. So my question is: Is there a rule of thumb as to the maximum size capacitor that can be charged without a limiting resistor on a given sized trace without resultant heat damage/fusing?
published 9 years ago
hurz
9 years ago
It depends on the connected component to the cap. The cap itself mostlikely will survive, cuz as you already said there is current limiting resistance (e.g. ESR). But a transistor or diode might get damaged.
thebugger
9 years ago
Depends on the source mostly. The cap is made so it'll withstand current peaks, but the source may not be. For instance tube rectifier manufacturers always give a recommended limiting resistor before the first capacitor in their datasheet, and also a recommended capacitor value, which you should exceed at your own peril. But tubes are usually high impedance sources, and they have similar limitations. With transistors though, where the transformer can expect sudden peaks up to a few amps, it's usually not a problem when the initial surge is demanded. The transformer voltage will kneel to some degree in the beginning, and the internal resistance of the diodes will further limit the short circuit before the cap energises completely. The more important thing to consider is not whether the transformer will withstand the surge, but whether the diode bridge will. Usually when you choose a diode bridge you take one with a peak rated power at least 5 times the average one ( I usually take it with 10 times the capability). Other than that, I don't think you'll have a problem. Capacitors have internal resistance which will further dampen the surge.
thebugger
9 years ago
Ah and about the rule of thumb you asked, it varies a lot, but for all intends of purposes I choose a capacitor with 1500uF for every amp of current. So for 5A of average current it'd choose 7.5mF, the closest value being 10mF (I usually round up to the next available value above the one I need)
hurz
9 years ago
And as bonus check this ;-) http://everycircuit.com/circuit/5236088859787264
thebugger
9 years ago
Yes hurz is right with the bonus. Moreover capacitors have special places in the circuit, and I don't mean where they're just needed, I mean in the physical layout. Since they draw current in a very spikey manner, they can cause ground loop problems and interfere with the correct operation of some more sensitive parts of a circuit like a preamp for instance, where the noise is desirable to be kept very low. People usually try and eliminate these problems by using star shaped ground topologies, or increasing the ground surface area on the board.
faceblast
9 years ago
hurz invented the word "niddle"
hurz
9 years ago
Shit, hope “niddle“ is not to offensive in your language ;-)

EveryCircuit is an easy to use, highly interactive circuit simulator and schematic capture tool. Real-time circuit simulation, interactivity, and dynamic visualization make it a must have application for professionals and academia. EveryCircuit user community has collaboratively created the largest searchable library of circuit designs. EveryCircuit app runs online in popular browsers and on mobile phones and tablets, enabling you to capture design ideas and learn electronics on the go.

Copyright © 2026 by MuseMaze, Inc.     Terms of use     Privacy policy