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## 2018년 09월 04일

### 원문 게시자: Minho , 2018년 09월 04일

#### 본문:

@morticool

I see this question every once in a while. So if you have a DCPS and you set it to 4.2V and you connect it to something that is essentially a dead short, such as a logic board with a short on VCC_MAIN and the DCPS is delivering 4A, what do you think is really happening?

The answer is…not much. Most bench or lab power supplies are current limited. That means that when the current draw reaches the limit, either the limit you set or the max limit the DPCS can provide, the voltage starts dropping to compensate and maintain the set current.

You can actually simulate this directly with your power supply. Set a voltage and a max current and then short the leads. What you’ll see on the display is that the voltage drops to near zero while the current hits the top end. You will feel some heat in the leads because there is some resistance. On my DCPS, if I set the current limit to 1A and the output voltage to 4.2V, what I see on the display when shorted is 1A and 130mV. That means the DCPS is “seeing” a resistance of 130 mOhms (R=E/I —> 0.130V/1A=130 mOhms) and the power dissipated by the leads under full short conditions is 130mW (P=V*I —> 0.130mV * 1A=0.130W), which is less than the old school 1/4 watt through-hole resistors of yesteryear. Because the leads are long, you just feel a warm touch along the lead wire.

The same applies to troubleshooting. If a main cap is a dead short, let’s say 0.1 Ohms or less and you apply 4.2V, in theory you should see 42A (!!) and the power dissipated by this component would be 176W (P=VI —> 4.2 * 42). That’s enough to get the whole board glowing red hot. But that’s not what happens…is it? No, if you’re connected to a current limited power source like a DCPS, the voltage drops and the current maxes out and at best, the DCPS is delivering a fraction of a Watt…and most of that is being dissipated by the leads. To get a part glowing hot, it either needs to be slightly higher resistance, say 1 Ohm or you have to be able to pump current through without limit.

That’s where a battery comes in handy ;>) but that can also be dangerous as it may blow components that are otherwise good but are now operating outside their specs. Most components will catastrophically fail “open” under extreme currents but in some cases, they can actually fuse inside and cause an inrush of current along the rest of the circuit.

open