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

**budm** · 10-24-2014, 01:49 PM

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

"I need a idea for something that will draw at least 5 10 or 15 amps that will be powered by a ATX power supply's 12v rail." Are you saying you want the load that will draw 5A, 10A, 15A from 12V power supply?
The load or what ever you connect to the power supply output will dictate how much current it will want to draw from the power supply.

http://www.edn.com/design/test-and-m...acer-technique

http://www.edn.com/design/pc-board/4...gineering-PCBs

http://www.edn.com/design/pc-board/4...uses-low-power

What are you working on? Pictures, make and model.

**macattack600** · 10-24-2014, 02:07 PM

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

That info is good but it does'nt help

**budm** · 10-24-2014, 02:10 PM

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

I thought you wan to be able to locate shorts circuit. So what is the 5A, 10A, 15A question is about? Info and pictures on what you are working on will us helping you.

**macattack600** · 10-24-2014, 02:13 PM

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

I am wanting to put a demanding load on the output of the ground on motherboard, using the 12v PSU rail connected to a mosfet
but the led strip I have isn't enough . So in this order, PSU-motherboard-led strip or equivalant, -back to PSU in that order. The board is the middle man to maybe a light source that is drawing the amps but I don't know what I could use maybe lamp type LED light bulb?

Since there is a short to ground on the mobo I should be able to feel it get hot is the end result i'm looking for.

**budm** · 10-24-2014, 02:31 PM

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

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!

**macattack600** · 10-24-2014, 02:47 PM

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

I don't think i'm getting this across right but thank for the information.

I've seen this done before where the board was the middle man and the output was a whole bunch of led lights putting demanding load on the in between motherboard and the gpu was the one getting hot I just don't have while bunch of led rope. Trying to find something equivalent

**budm** · 10-24-2014, 02:53 PM

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

Using LED, RESISTORS or what ever, the power supply has to see 2.4 Ohms to get 5A draw. Yes the board is the middle man and it is connected in series with the LED/RESISTOR or what ever. The current draw from power supply is dictated by the total load resistance.

**macattack600** · 10-24-2014, 02:58 PM

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

so actually I would want to put like a halogen bulb before the motherboard back to the psu

**Agent24** · 10-24-2014, 03:05 PM

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

If the motherboard is shorted the bad part will heat up when you apply power. The trick is having enough juice from your power supply, and not have it shut down when the PSU sees the short.

It sounds to me like the LED strip is simply for current limiting so the PSU doesn't shut down when you are testing. The LED strip is not for "pulling more current through the short" - A short circuit by definition will try to pull as much current as is being supplied!

**budm** · 10-24-2014, 03:06 PM

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

Check the resistance of the bulb, the cold resistance will be 10 times lower than hot resistance, the lamp has positive resistance, as it heats up the resistance goes up, so as you start drawing current, the resistance will go up so the current draw will go down.
Agent; I hope you can explain it better than I can about the current draw, I think he needs to understand the relationship between Voltage, resistance, and current. Seeing how it is done and understand why it is done that way, and what the expected result is will be a big help for him to understand.

**macattack600** · 10-24-2014, 03:08 PM

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

So it would be enough for a lamp like led light bulb before or after the motherboard for current limiting? What size should I use.

**budm** · 10-24-2014, 03:13 PM

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

Agent: please do the explanation, I cannot do any more.

**Agent24** · 10-24-2014, 03:38 PM

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

Originally posted by budm

Agent: please do the explanation, I cannot do any more.

OK, I'll give it a go.

macattack600: You need to understand Ohm's law for this to make sense. The basic idea is that Volts (V) = Current (I in Amps) x Resistance (R in Ohms). Very basic maths here.

What we have is the equation V = I x R.
This can be rearranged to find current or resistance also.
To find current it's I = V divided by R (or I = V/R) and to find resistance it's R = V/I.

So for example: If you connect the shorted motherboard to the 12v rail as is, and want to find out how much current it will draw.

The shorted motherboard will have a very low resistance. Let's say it's 0.1 Ohms.
You can even measure this resistance from the 12v rail input to ground with your DMM, but 0.1 is a good start, it will be something close.

So, we would say I = 12v / 0.1 Ohms, which gives us 120 Amps! Yes, the shorted motherboard wants to draw 120A from your 12v rail. This is a crazy amount of power and the PSU can't even supply it, so it shuts down to prevent itself from being damaged from this overload. This means that we can't do any testing. It also means that if we gave the shorted motherboard all 120A that it wanted, it would go up in flames. That doesn't help either.

We need a way to limit the current flowing through the board, so that 1) Board doesn't catch fire and 2) PSU doesn't shut down.
This is where the LED string etc in series with the motherboard comes in.

**macattack600** · 10-24-2014, 03:49 PM

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

Got it very good detailed answer. I will overnight the led rope. Thank you for such a detailed answer you've both been great help.

**Agent24** · 10-24-2014, 03:53 PM

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

There are some more things to know here. 1) The LED string has its own resistance, 2) When resistances are in series, they add up. and 3) The current through series-connected resistances is the same.

Let us say for example that we have an LED string that is 2.4 Ohms (note I am oversimplifying it, but the point is valid)
Again, if we check Ohm's law we get I = 12v/2.4 Ohms which equals 5 Amps.

If we connect the LED string to the PSU, it will draw 5 Amps. The PSU will not shut down because 5 Amps is a pretty small load for a PC power supply.

Now think about the LED string and shorted motherboard in series, eg:
PSU 12v ------>[Shorted Motherboard]------>[LED String]-----|
PSU GND <---------------------------------------------------|

Power will go through the motherboard AND LED string, and return.
Remember the motherboard is 0.1 Ohms, the LED string is 2.4 Ohms. We add them and the total is 2.5 Ohms. This works out at 12v, for a current of 4.8 Amps.

This means 4.8 Amps will be drawn through BOTH the LED string and motherboard. This also means the PSU won't shut down, and the motherboard (will hopefully) not catch on fire.

**macattack600** · 10-24-2014, 04:09 PM

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

How much rope light do you think I need 12ft 24ft ? The more the better

**stj** · 10-24-2014, 04:15 PM

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

halogen lamps make a good resistive load.

**Agent24** · 10-24-2014, 04:18 PM

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

Originally posted by macattack600

How much rope light do you think I need 12ft 24ft ? The more the better

I don't know. It all depends on what its specifications are. Do you have a link to some more information about these lights?
The easy way would be to find an LED string that works on 12v and draws whatever current you wish to test the motherboard with.

But as budm pointed out, 5A @ 12v is pretty high for short testing, you may still roast the shorted part (and this can result in damage to the PCB which is beyond repair)
With equation Watts (P) = V x I, your 12v x 5A is 60 watts!

You may wish to start with something small like 500mA, and increase from there if nothing gets noticeably hot or starts smoking.
You could get by with a simple 10-watt, 24 ohms resistor for 500mA (which would be dissipating 6 watts)

**macattack600** · 10-24-2014, 04:21 PM

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

Originally posted by stj

halogen lamps make a good resistive load.

What size type you recommend.

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

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

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