Need for testing short something draw 5,10 or 15 amp

[Agent24]

Re: Need for testing short something draw 5,10 or 15 amp

so a milliohms meter is too obvious then?

I assume he doesn't have one.

[macattack600]

Re: Need for testing short something draw 5,10 or 15 amp

something like this

http://www.amazon.com/Lampux-Flexibl...led+tape+light


3528 smd leds runs off 12v the tape light seem to have positive and ground ,not on the rope light. In the event I try to limit current to 0.5amps on the psu what's my best route? I just think it will bring the voltage down too much is why I wanted a better solution.

10hm 10watt resistor will burn it right ? on 12v rail?

[stj]

Re: Need for testing short something draw 5,10 or 15 amp

damn that strip is cheap!

btw, a big reostat and an ameter from a car dashboard will do what your after.

[macattack600]

Re: Need for testing short something draw 5,10 or 15 amp

damn that strip is cheap!

btw, a big reostat and an ameter from a car dashboard will do what your after.

so reostat and ameter together back to psu instead of leds? I think i'm going to buy 2 strips of leds. If I went the current limiting route don't you think a 10ohm 10w resistor would burn up on me ? I need something like 24w?

[Agent24]

Re: Need for testing short something draw 5,10 or 15 amp

I think a 24 Ohm 10 Watt resistor is a better choice

[macattack600]

Re: Need for testing short something draw 5,10 or 15 amp

I think a 24 Ohm 10 Watt resistor is a better choice

What about a buck regulator like this?


sorry had to update did some research that won't regulate amps just the voltage my bad

[dellxps15]

Re: Need for testing short something draw 5,10 or 15 amp

i saw the video for finding short, but for me is even better to look on short at coil point.

since your pc doesnt boot and does drain voltage even if powered off, i think all problem are related to 3.3 volt. 5volt should come on after pressing the power botton on macbooks

on macforum i spotted a short by this way. ull find my post looking backwards. for me it was a fet q7055 (if i remember well) and spotted it by just only multimeter. problem now is even if i removed short, i dont have the 12.5 output voltage, and i have to investigate and see why that.

so on the schematics u have for your board, look for those Lxxxx and see if short in in any of them. then look what they power and then try remove something at once, but this is a step u can do later after first spot shorted coil.

[SteveNielsen]

Re: Need for testing short something draw 5,10 or 15 amp

I'd like to know how you guys plan to get around Ohm's Law, or what am I missing here?

It will take a 2.4 ohm, 60 watt resistor to draw 5 amps at 12 volts.

[stj]

Re: Need for testing short something draw 5,10 or 15 amp

it's about creating a current limiter to put between a psu and *something shorted* to heat or burnout the short.
or just smoke the whole pcb!

[SteveNielsen]

Re: Need for testing short something draw 5,10 or 15 amp

I know that. There's no magical exception to Ohm's Law.

[eccerr0r]

Re: Need for testing short something draw 5,10 or 15 amp

That strip probably has more closer to 300 LEDs not 3000 LEDs. At 20mA per 3, that at most would take 2A or so, but the thing is that the current will be voltage dependent only after the LEDs turn on...

A real resistor or properly used current/voltage limiting PSU is still better. Or if you have a LM317 and a 2.4 ohm 1W resistor you could make a quick current limiting ~500mA circuit that is voltage independent up until you fry the LM317...

[Agent24]

Re: Need for testing short something draw 5,10 or 15 amp

I know that. There's no magical exception to Ohm's Law.

What specifically do you see as being wrong?

That strip probably has more closer to 300 LEDs not 3000 LEDs.

Yeah, it says 300 LEDs on it. Each LED is in a 3528 SMD Package.

[stj]

Re: Need for testing short something draw 5,10 or 15 amp

this is getting boring, just use a welders mask and a pair of jump leads

[SteveNielsen]

Re: Need for testing short something draw 5,10 or 15 amp

[QUOTE=Agent24;495457]What specifically do you see as being wrong?/QUOTE]

You are trying to get a specific current at a specific voltage, it takes a specific resistance and will dissipate a specific amount of power and it seems like you're skirting around that. You aren't going to get 5 amps flowing through that light rope.

[macattack600]

Re: Need for testing short something draw 5,10 or 15 amp

What specifically do you see as being wrong?



Yeah, it says 300 LEDs on it. Each LED is in a 3528 SMD Package.

Agent I sent you a PM let me know what you think. I sent one to you to stj thanks.

[Agent24]

Re: Need for testing short something draw 5,10 or 15 amp

What specifically do you see as being wrong?

You aren't going to get 5 amps flowing through that light rope.

I never said he would. According to the reviews on Amazon that specific one is more like 2 Amps.

But I agree with what budm said earlier, 5A to start with may be too much anyway, depending on what component is shorted and if there are any fine traces in the way. Which is why I suggested a 24 Ohm resistor to start at 500mA. Even that might be too much. I don't know, I have never actually used this method.

Agent I sent you a PM let me know what you think. I sent one to you to stj thanks.

I watched it. It makes sense, the same thing as you explained, but his idea that the LEDs will "rip all that current through the short" is not the whole story.

Sure, the short will presumably have less resistance than the LEDs, and thus almost the full current his LEDs want to draw (whatever it is) will also go through it, but you don't need LEDs if you want to send lots of current through a short. You just hook it up across the power supply.

Of course he may have tried that himself, and found that his G4 PSU simply shut off (very likely). In addition to that, without current limiting there is a risk of destruction of the board.

So whether he realised it or not, he was also using the LEDs as a current limiter to stop his PSU shutting down and to prevent (perhaps) the board from being toasted - though we don't know how much current his LEDs were drawing.

So to sum up - his idea is not a bad one, but I think more thought needs to be taken into account when selecting the series load to prevent damage to the board under test.

[budm]

Re: Need for testing short something draw 5,10 or 15 amp

Can I see that video or what ever that link shows?

[SteveNielsen]

Re: Need for testing short something draw 5,10 or 15 amp

[QUOTE=SteveNielsen;495574]

What specifically do you see as being wrong?/QUOTE]

You are trying to get a specific current at a specific voltage, it takes a specific resistance and will dissipate a specific amount of power and it seems like you're skirting around that. You aren't going to get 5 amps flowing through that light rope.

MY bad grammer. What I meant was "He is trying to get..." etc.

[Agent24]

Re: Need for testing short something draw 5,10 or 15 amp

Can I see that video or what ever that link shows?

Sure: https://www.youtube.com/watch?v=p83LLbSx4WE (Warning, contains profanity etc, don't watch if you are easily offended)

[budm]

Re: Need for testing short something draw 5,10 or 15 amp

So yeah, he is just using the LED as current limitter which he did not measure to show how much current is flowing through the shorts.
I would start at 12V 500mA by using 24 Ohms 6W min, if you cannot locate the heated device that failed then increase more current by using 12 Ohms 12W.
It will be much easier to use power supply with adjustable current limiter.
He also indicate that the shorts is measure at 2 Ohms "When measuring PPBUS_G3H to ground, I get 2 ohms. this means there is a short to ground on the PPBUS_G3H 12.6v line on the board, which is why it is not turning on." so worst case without current limiter wilL be 6A.
You use LM317 as current source and adjust one resistor for the amount of current needed to get the shorted device to heat up.
I can see why the OP is asking how to get 5, 10, or 15A because the guy said so on Youtube, the guy does not realize that with 2 Ohms of shorts will not let you get anything near 10 or 15A and that is why he needs basic knowledge of Ohms law before saying anything about the current draw 5, 10, 15A; 6A will be the most you get. Youtube can be good or can be bad.

I already explained about the shorted resistance earlier already too in post 6:
"OK if that you want to do using 12V power supply for locating shorts then to draw 5A from 12V, the resistance that the power supply has to see will be 2.4 Ohms, so if the shorts circuit circuit resistance or what ever the resistance on the board is, which you need to find out then the added load connected between the mother ground and the negative terminal of the 12V so the total resistance cannot be higher than 2.4 Ohms to allow 5A to be drawn from the power supply, Ohms law.
BTW 12V @5A is 60 Watts of power!"
So the bottom line, the current draw as shown in video is less than 6A. Details, details, details.