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Short on pp_vcc_main that produces no heat

So Iv had this twice now in the last three days and its driving me insane. It goes as follows.... Customer drops in a 6s that's been water damaged. Won't charge, won't power on, changing Tigris, tristar, active diode makes no difference. 3v and 1.8v at tristar surge up to operating voltage, drop to zero then back to operating voltage again. Short is detected on pp_vvc_main.

Iv tried plugging in a power supply and giving it 4 volts 2.2 amps and no heat, not a %#*@ thing. I even tried putting the board in isopropanol and then putting it in front of a fan and then using my flir camera to look for heat, the isopropanol and fan make the board really cold so even the lowest resolution camera can see a heated area on the board, but nothing, even after leaving it 5 mins.

Once I'd given it up as totally dead I put 8 volts into it too see if anything would heat up then,it was dissipating 4.4amps but all that was happening was the power supply leads were heating up, the board was still totally cold.

So I'm totally confused now, how is it possible for a board to sink that much current and not get hot in the slightest ?.

Has anyone ends had this problem with a 6s ?, as Iv had the same problem with same symtoms to the letter in 3 days.

The thing is Iv had shorted caps and ic's before and they generally get very hot when you ram 4.4amps through them, this on the other hand does nothing.

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Hey, I know this topic is quite old now... but did you finally find a solution? I have exactly the same problem... My iPhone 6S handles about 4 ampere without getting hot somewhere expect the leads themselves. That must be a big short. I also tried different sports on the board but nothing gets hot

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the last liquid damaged iphone 6s i repaired there was a shorted cap below the pmic. the 2 solder ends of the cap looked like they had risin up. i removed the 2 vcc_main caps beside the wifi chip first before looking anywhere else

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@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.

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