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Power supply issue

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Hvacdocgary
(@hvacdocgary)
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Joined: 12 months ago
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Hi, new  to electronics and trying to learn. My first project is converting an 400 watt atx power supply to use for powering a car stereo in my shop using the instructions from dronebot video. I have a 15 ohm resistor 50 watt that I used but the instructions I think said a 10 watt? The power on all volt rails is unstable. Is the resistor causing this or is it the power supply possibly bad?     Thanks for your comments.

Gary 


   
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Inst-Tech
(@inst-tech)
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hI @Hvacdocgary,

As I don't have a schematic of what your doing..I assume it's just supplying power to your car stero..Most probably, the power supplies regulated power supply  may be at fault. Try disconnecting the stero  (load) and see if the supply returns to the value it's suppose to be, ie: 5v, 12v.  Then place resistor loads of high value, like 10k ohms and read the voltage drop across that resistor, if the voltage drop is +/1% of rated voltage,then the regulator may be bad. let us know what you find, and supply a circuit drawing of how your hooking up the stero to the power supply..that will be a big help..

regards,

Louis

 

LouisR


   
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DaveE
(@davee)
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Hi @hvacdocgary,

  The comments from @inst-tech all sound reasonable ... personal experience suggests ATX power supplies do have a habit of failing.... and more information about what you are doing is essential for anyone to even begin to help.

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However, in answer to your specific question, is using a 50W resistor instead of a 10W resistor the cause of your problems. The simple answer is NO. The amount of current a resistor will draw only depends on its resistance ... so to a power supply, a 15 Ohm 10 W resistor will appear identical  15 Ohm 50W resistor - though the 10W component would rapidly fail, probably issuing some smoke, if it was forced to dissipate 50W!!

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The difference is the 50W resistor is designed to dissipate (lose) up to 50 W of heat to its surroundings  without failing, whilst a 10 W resistor is only designed to dissipate 10W. Thus, if you try to dissipate 50W with only a 10W resistor, it will rapidly overheat and fail. By contrast  a 50W resistor will happily dissipate only 10W, and probably stay fairly cool.

Elecrically, a 50W part will do nicely in a situation that requires it to dissipate 10W .. the downsides are usually bigger physical size and higher cost .. but if you already have one and space is not an issue, no problem!

The simple formula to calculate the amount of heat a resistor will dissipate is V * V / R,

where V is the voltage across the resistor and R is the resistance.

So if you put 12V across a 15 Ohm resistor, the calculation is  (12 * 12 /15) = 144/15 = 9.6 W

So a 10 W resistor would do nicely.

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Note, higher power resistors (like 10W and 50W) usually get quite hot when run at their maximum power level ... they should be mounted so that air can freely circulate around them and beware of burning your fingers!

Dave


   
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Inst-Tech
(@inst-tech)
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@davee  & @hvacdocgary,

Indeed, all very good information, especially to a newbee..lol

Also I would make sure that your power supply is capable of delivering the required amperage and power require by the device you are trying to power..in this context, a 12v dc and a 15 ohm resistor is delivering around 0.8 amps ( 800 mA), so you need to know what the power supply is rated for in terms of power ( watts) and current .. not all power supplies are equal you know...lol

At any rate, we look forward to what you are learning from this experience, as experience is always the best teacher..and remember , safety first, we don't want to let the smoke out or any burnt fingers,,lol

Good luck with your project @hvacdocgary

regards,

LouisR

LouisR


   
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Hvacdocgary
(@hvacdocgary)
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Joined: 12 months ago
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Topic starter  

Thank you everyone for your excellent replies. I believe from reading the replies that the power supply is most likely the problem. I have another ATX power supply I am going to try to test later today. This is going to be used as a power supply for a low power car stereo and for some testing of small projects. It is a 400 watt ATX power supply. This is the same build as Bill did on Youtube. 

Thanks again and have a safe and happy holiday season!!!


   
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DaveE
(@davee)
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Joined: 2 years ago
Posts: 681
 

Hi @hvacdocgary and @inst-tech,

  To amplify the wise words ... " not all power supplies are equal", having a power supply of say 400W does not mean you can take it all from one output .. I hope you won't mind if I point out a common mistake.

e.g. knowing V * I = P    or colloquially expressed as   Volts * Amps = Watts

then you might expect 12 x Amps = 400 ........ > Amps = 400/12 = 33.3 Amps

However, this is not how power supplies are designed ... the total of say 400W will be split into a number of outputs, each having their own limit. In particular, a considerable proportion will be allocated to the +5V outputs.

If you are lucky, there will be a label or plate which lists them, but if it is from PC sold by one of the 'big names' then that is unlikely, as they will not expect the user to significantly change the contents of the box.

You say you are looking at a 'low power stereo', so this is probably not a problem to you at present, but at least bear it mind for the future.

A great holiday season to all and take care!! Dave


   
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Hvacdocgary
(@hvacdocgary)
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Joined: 12 months ago
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Topic starter  

Thanks everyone problem solved. Was a bad power supply. Picked one up at the used comp store and it works fine. Took old one apart and on close inspection looks like a bad capacitor. Will set that aside for a later project. Now time to move on to another project!

 


   
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RCC1
 RCC1
(@rcc1)
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Joined: 2 years ago
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It is fun to follow along the path as someone figures out what is wrong with their project. I always enjoy reading the "resolution" that fixes the problem.  

RCC1


   
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Hvacdocgary
(@hvacdocgary)
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@rcc1 It's fun to be able to come to a forum where people actually offer good advice without being judgmental or rude. Makes learning so much easier.

 

Gary 


   
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Inst-Tech
(@inst-tech)
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@hvacdocgary Well done @hvacdocgary!.. Yes, power supplies can be tricky some times, but cap failures are not that uncommon..and so are resistors failures..I suspect this is a switching type power supplies and the regulators should be good as they are mostly "crowbar" type, causing the voltage to go to o volts when a over load or short circuit should happen..

Merry Christmas to you and yours, and a happy holiday season to all that's reading this post..

 

regards,

             LouisR

LouisR


   
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