# Drawing maximum amount of current from power supply ?

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#### Ivan's Assay

Nov 16, 2018
6
Hi there.

After some research i could not find right answear so i decided to find help here as usual.

Question is pretty simple but can be answear too ??

I got 12v DC PSU wich is able to deliver 24A max.
It has short circuit protection (turns off) so i can't just touch wires and watch them melt.

I tryed with using diodes, inductors and capacitors but could not get it to work.
I am just amateur in electronics so hoping for smart people to jump in and save my day.

My goal is to hook up graphite from pencil with GND and +12v to make it red hot but as i said is not possible because of short circuit protection.
Does solution for this exist ?

Thanks.

#### dave9

Mar 5, 2017
1,181
Ohms law. 12V will put 24A through 0.5 ohms resistance.

Measure the resistance of the graphite. Add a series resistor to increase the resistance above 0.5 ohms. Use ohms law again to calculate how much wattage the resistor of the particular ohm value needs to dissipate and pick a power resistor above that wattage rating.

Granted if you don't need to make the pencil glow long, and it might disintegrate anyway (suppose it has plastic or some other non-heat-resistant binder mixed with the graphite instead of clay) then for very short periods of time, the resistor power rating can be a bit lower because it won't have time to overheat itself.

Even so, that's going to be a big resistor. If all you want is to glow graphite then how about connecting multiple pencils in series to increase resistance. As above, measure their resistance and put enough in series so the total resistance is above 0.5 ohms.

A brief look online indicated a 17.5cm pencil should (or at least could) be around 0.7 ohms which should be enough by itself. Is your pencil too short? Maybe try a different brand?

Last edited:

#### BobK

Jan 5, 2010
7,682
I tryed with using diodes, inductors and capacitors but could not get it to work.
Stop, now, before you hurt yourself. Connecting random things to a power supply capable of 288 Watts is dangerous! You are lucky you have not yet created an explosion, or at least a fire.

Bob

#### Ivan's Assay

Nov 16, 2018
6
Ohms law. 12V will put 24A through 0.5 ohms resistance.

Well.. how could i forget.. Ohms law. yea thanks a lot i did it with my peace graphite with 4Ω.. I have shorten it to 5mm so it had bellow 1Ω. But the graphite was not only thing that was glowing now.. all wires melted and burned due to 15> amps. Just GORGEOUS!

Soulution has ben found!
Thanks !

#### Ivan's Assay

Nov 16, 2018
6
Stop, now, before you hurt yourself. Connecting random things to a power supply capable of 288 Watts is dangerous! You are lucky you have not yet created an explosion, or at least a fire.

Bob

Well, too late. But don't worry it happend under controlled conditions.

#### BobK

Jan 5, 2010
7,682
Well.. how could i forget.. Ohms law. yea thanks a lot i did it with my peace graphite with 4Ω.. I have shorten it to 5mm so it had bellow 1Ω. But the graphite was not only thing that was glowing now.. all wires melted and burned due to 15> amps. Just GORGEOUS!

Bob

#### dave9

Mar 5, 2017
1,181
You can get nichrome wire and ceramic terminal blocks (to mount it) for cheap on ebay if you want something a bit longer lasting and with consistent resistance.

#### dave9

Mar 5, 2017
1,181
I once met a fellow who picked up a trick in jail... he'd take the spiral metal binder from a notebook pad, a piece of pencil graphite, and shove it into 110VC outlets to light contraband cigarettes.

#### davenn

Moderator
Sep 5, 2009
14,150

too much dangerous stuff being spoken about in here

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