I am an electronics noob, and want to build a lab bench power supply from my atx psu. I watched a bunch of guides and they all say that I need to put a dummy load on the 5v rail or 12v rail, but I don't know where. I attached a picture of my psu, now I need help on what resistor should I use and where?

 

10W 5 ohms resistor across the 5V supply...

 

but why to use load resistor.

 

10W 5 ohms resistor across the 5V supply...


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I have 2x 10ohm 9w ceramic resistors, can I use those in parallel?

 

I have 2x 10ohm 9w ceramic resistors, can I use those in parallel?

Hi welcome to the forum
Yes the two resistors in parallel will be fine. you may find that just one 10 ohm resistor is sufficient, depends a lot on the particular atx supply you are using. some don't need any fixed load at all and work fine without one, try it and see what works best for your atx.
P.s, I've just looked at one I modded some years ago, an old 250 Watt thing. It works fine with a 27 ohm 5 Watt resistor on the 5Volt output.

 

Oh, and the reason you sometimes need a fixed load is, A) some will not turn on without some sort of load, and B) the regulation will be all over the place without it.
Convert ATX PSU to Bench Supply to Power Circuits
www.electronics-tutorials.ws › Miscellaneous Circuits
Go to head power supplies tutorial, then look at sub heading power supplies.
There is tons of helpful info on this site.

 

but why to use load resistor.

Only the 5V & 3.3V outputs are regulated - once they've charged the filter electrolytics; the PWM throttles back so the voltage doesn't go any higher. the 12V output doesn't know that and the voltage collapses if you draw any current from it.

 

Oh, and the reason you sometimes need a fixed load is, A) some will not turn on without some sort of load, and B) the regulation will be all over the place without it.
Convert ATX PSU to Bench Supply to Power Circuits
www.electronics-tutorials.ws › Miscellaneous Circuits
Go to head power supplies tutorial, then look at sub heading power supplies.
There is tons of helpful info on this site.

Thanks for a complete answer and for the welcome, really appreciate it . But I have another probably really stupid question. I want to build a slayer exciter circuit and power it via the atx psu, but I'm scared that the circuit would draw a lot of current and ruin my psu or worse. Everyone on online tutorials usually uses a 9v battery that probably can't deliver much current. How do I know for sure how much current the slayer exciter lets thru?

 

The few slayer exciter circuits I have seen, using this one as an example, draw very little current. My read is they can easily be powered by an ATX form factor PSU 12 Volt rail. Different designs of the circuit will draw different levels of current but for the most part they all seem to be low current devices. Considering they can be powered with one or two 9 volt general purpose batteries.

Ron

 

As one of my hobbies is amateur radio operator (Ham) I have a great dislike for anything that generates R.F.I /E.M.I (radio frequency interference) including poorly designed and shielded switch mode power supplies, Tesla coils, and such like devices when used without consideration of the (illegal) interference they produce.
So sorry, but not something I endorse or wish to see proliferate.

 

I got my first ticket in '63 and when I would work 6 Meters (50 to 54 MHz) every now and then a neighbor would call. There was no cable and all TV was OTA (Over The Air). I was in the greater NY area and Channel 2 was CBS (54 - 60 MHz). For the most part I doubt a slayer exciter is going to generate much RFI over any distance. I doubt that any RFI generated would interfere much with anything. Larger Tesla Coils yes I can see the potential for a problem with some forms of communication but what I linked to I seriously doubt it.

Ron

 

Try 20metres and 40metres, nearly wiped out at times, especially weekends! Mostly by the cheap mobile phone chargers that come from china.

My local club did a test about 5 years ago, they bought replacement chargers from flea markets and local phone shops. Tests were done in real life conditions, over 80% of them caused interference ranging from S1 to S5 at a measured distance of 200 metres from inverted G5RV completely masking weak signals. My Yaesu FT950 with DSP did a good job of filtering a lot out, but my FTone could not cope at all.

 

Try 20metres and 40metres, nearly wiped out at times, especially weekends! Mostly by the cheap mobile phone chargers that come from china.

Maybe a thread should be started on the subject. I would like to see that. With a thread and some good well documented test results which involves defining how the test was conducted and equipment used. Additionally terms like S Units need defined for those who have no idea what a S Unit is or how it relates to a measure of power. The fact that junk pours in from China which does not meet our FCC emission standards is no surprise but getting the government to take any corrective action is about hopeless. The average end user here in the US is going to be clueless as to RFI, EMI or any other emission as well as the signal strength of those emissions.

Anyway, I see it as an interesting topic for discussion. Up in the attic I still have an ancient R392 Receiver from many decades ago. Maybe one day I'll resurrect that beast.

Ron

 

Like I said I know very little about electronics I just started taking interest. I had no idea that a small and simple circuit could have such an impact, especially the fact that it can be powered by a 9v battery.

 

Like I said I know very little about electronics I just started taking interest. I had no idea that a small and simple circuit could have such an impact, especially the fact that it can be powered by a 9v battery.

Behold the amazement of a little inductor (coil). If you continue and interest in electronics in time you will see and maybe feel what a coil of wire is capable of.

Ron