# PC PSU max load

#### drjackool

Joined May 23, 2021
34
Hi
I have a old PC PSU, and I want use it to turn on a car lighter to use it as a hot air gun! but problem is when I turn on the PSU by connecting green wire to ground and then connect the lighter, PSU goes OFF!! Why?
I know car lighter uses much amps but on youtube I see a clip that uses 12V 5A SMPS PSU and for him it works fine!

My PSU spec is: 12V 10A, 5V 15A, 3.3V 16A
Thanks

#### ericgibbs

Joined Jan 29, 2010
19,015
Hi dr,
The problem with heater elements, they can draw a very high current when cold, until the heater element reaches its operating temperature.
[the cold low resistance of the element increases with temperature, to its final operating value resistance]

So your PSU goes into current limit.
E

Last edited:

#### drjackool

Joined May 23, 2021
34
Hi dr,
The problem with heater elements, they can draw a very high current when cold, until the heater element reaches its operating temperature.
[the cold low resistance of the element increases with temperature, to its final operating value resistance]

So your PSU goes into current limit.
E
How avoid current limit?
Some guy said me connect a low resistance to psu before connecting the lighter. I not tested it yet.

#### ericgibbs

Joined Jan 29, 2010
19,015
Hi dr
One method would be to have a high power resistor in series with the heater at power up then, when the heater partially heats up, short out the resistor.
This is a similar method used on some motors at start up.

E

#### AlbertHall

Joined Jun 4, 2014
12,385
You probably need a load on the +5V supply - 10 Ohm 5W would do the job.
These supplies monitor the outputs for, among oher things, overvoltage. When you load the 12V supply all the other outputs voltages rise and that can trip the overvoltage protection.

#### Ya’akov

Joined Jan 27, 2019
9,237
One method would be to have a high power resistor in series with the heater
The more sophisticated version of this is to use an inrush current limiter which is an NTC (Negative Temperature Coefficient) thermistor. A thermistor is a resistor the changes values as it heats up. A PTC (Positive Temperature Coefficient) variant increases in resistance as it heats—this makes it suitable to protect against constant over current as it regulates the draw by increasing in resistance.

The NTC type reduces in resistance as it heats making it suitable to limit the sudden inrush of current before things in the circuit stabilize. For example, large capacitors that will look like dead shorts at first, until they charge; and of course heating elements that behave similarly until the heat up.

The heating element is effectively a PTC thermistor and so an NTC inrush current limiter can be used to neutralize the effects of this. This application note from TDK has good information in it.

#### MisterBill2

Joined Jan 23, 2018
19,033
Certainly the comment about needing a load on the 5 volt supply is correct for many versions of PC power supply. So connect the lighter to the five volt output and try it.
What happens is that with no load the 5 volt section can not regulate and so it shuts down.
In adition, a car lighter is for starting fires, not heating air. It will not be simple to use to heat an air stream.

#### Tonyr1084

Joined Sep 24, 2015
7,960
In adition, a car lighter is for starting fires, not heating air. It will not be simple to use to heat an air stream.
That would make for an extremely small heater. Useful if you want to heat a mouses tiny house.

I have an older computer PS. I use it from time to time as a power supply. Fortunately for me I don't need to have a load on the 5V line. But yes, the newer ones probably do. I say "Probably" because I don't know that for certain. But I've certainly heard it here enough times to think it's "Probably" true.