| Electronics > Beginners |
| What's the difference between a dead short and a high current draw? |
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| aheid:
--- Quote from: ataradov on November 03, 2018, 04:29:24 am ---The obvious answer - the dead short is a load that consumes infinite amps. --- End quote --- I feel that's the wrong way to look at it. Instead you should think of the actual load as having nearly zero resistance. In that case, the internal resistance of the PSU (PCB traces, capacitor ESR, cabling etc etc), is the effective load. As mentioned, what happens to the power supply depends on the design. |
| ArthurDent:
--- Quote from: ataradov on November 03, 2018, 04:29:24 am ---The obvious answer - the dead short is a load that consumes infinite amps. --- End quote --- The obvious answer - the dead short is a load that TRIES TO consumes infinite amps. (fixed it) :) The original post might need some further explanation. My take on the question is based on my having several lab type bench supplies that are adjustable voltage and current and the way to set the current is to short the output and adjust the current limit. I could also set the voltage to 10 VDC unloaded, connect a 2 ohm, and slowly adjust the current knob up until the current read 5 A and the voltage returned to 10 VDC. If you had the typical SMPS that is built into equipment rated 10 VDC 5 A, you probably wouldn't be asking the question because the current limit, if it has one, would be fixed just slightly over the maximum current of 5 A. |
| macboy:
The short answer is: the difference is the voltage across the load, and hence, the power delivered to it. Your ideal maximum load of 2 ohms will have the output voltage at 10 V and 5 A of current delivered, consuming 50 Watt. The power supply is operating normally and is in voltage-controlled mode (not current limiting). Assuming it's a current limited supply, if you apply a dead short (let's say a length of wire having 0.01 ohm resistance), then the supply goes into current-limiting (current-controlled) mode, and the current delivered will still be around 5 A, but the voltage across your 0.01 ohm "load" will be 0.05 V. The power delivered will be only 0.25 Watt. Note also the maximum output current will be higher than the rating, usually 5% to 10% higher. Also, remember this assumes that the power supply has active current limiting of some kind, but if it is not current limited then all bets are off. A traditional power transformer can deliver several times its rated current into a short, for example. Also note that power supplies usually have relatively large capacitors on their output, and in the case of suddenly applying the "dead short", the spike of current delivered from the capacitor alone will be significant, and can be high enough to damage the connectors/contacts which completed the connection. It can also cause small wires or thin PCB traces to open. Another consideration, especially for a linear supply, is power dissipated in the supply itself. A 10 V linear supply may have a transformer & rectifier that makes about 15 VDC unregulated (at full load), to generate the 10 V regulated output. At full load then, about 5 V is dropped by the regulator, so at 5 A output, that's 25 W of heat dissipated in the regulator pass transistors. With a (current limited) short circuit, all 15 V are dropped across the regulator, so about 75 W is dissipated; that's 3 x as much. This could damage the supply if it is not designed for continuous operation into a short circuit. For this reason, some supplies have "foldback" current limiting, which will allow the full rated current when the output voltage is normal, but limits the current to a lower value when the output voltage drops (due to a low resistance load like a short). This is also why a traditional lab or bench supply with a "current limit" control should generally not be used as a current source or current-controlled power supply. They are designed for low output impedance, which is the opposite of what's required for a current source. |
| pelule:
I am a bit confused - why asking same item in several differents posts? https://www.eevblog.com/forum/beginners/whats-the-difference-between-a-dead-short-and-a-high-current-draw/msg1936558/#msg1936558 https://www.eevblog.com/forum/beginners/safe-to-test-stability-of-power-supply-board-by-shorting-output/msg1941466/#msg1941466 https://www.eevblog.com/forum/beginners/how-to-measure-power-in-watts-in-a-power-supply-with-current-limiting/msg1941697/#msg1941697 /PeLuLe |
| KL27x:
^ Despite all the answers already posted, it is possible the OP still doesn't have the answer to his question. |
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