# power supply repair

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#### waqar ahmed

##### New Member
sir i want to repair my computer power supply how can i test all the components in the circuits like diode,transistors capacitors etc.

plz help me..

with regards,
waqar ahmed.

#### microtexan

##### New Member
how can i test all the components in the circuits like diode,transistors capacitors etc.

Very carefully!

#### tcmtech

##### Banned

Do you know what quit?
Who makes it?
What wattage?
are any of the power outputs working at all?
Stuff like that.

#### BestMimo

##### New Member
you need a test equipement, have you got one?

#### Ayazhaider

##### New Member
Dear Waqar..

I think the best way is that you change power supply of your computer..

and if you wanna learn then first get testing tools (like Volt Meter Etc and soldering Kit )
Then read some circiut diagrams and try on small circiuts first, like Mobile charger, redio Etc... Bcos starting with PC PSU will cuase your Computer burn..

#### spyWarez

##### New Member
u need a mutimeter and test the component u want. and if u not sure how to test them just google them and u will definately will find some solution. gud luck

#### tekmanmike

##### New Member
No one can tell you what do or not to do. But if you have zero experience troubleshooting, you should stay away trying to fix your compuer powersupply yourself. Even though the outputs are low voltage, I believe the power supply itself is a switching power supply, capable of giving you a severe jolt if you touch the wrong spot. Dont troubleshoot it live if you dont have some experience-their cheap, might be better to buy one.

#### eggplant_casserole

##### New Member
The outputs may be low voltage, but at VERY high current!

It isn't worth repairing a PC PSU. You can pick replacements up for very little money - around £15 / $20 US / 15 Euros. I have worked in IT for nearly 10 years and that's what we always did. #### haitham ##### New Member hi ineed circuit power supply pioneer magnatic model no pm3275a-6-5 #### carmusic ##### New Member The outputs may be low voltage, but at VERY high current! It isn't worth repairing a PC PSU. You can pick replacements up for very little money - around £15 /$20 US / 15 Euros. I have worked in IT for nearly 10 years and that's what we always did.
12v isnt dangerous even if it can deliver 100 Amp (you wont get hurt touching the 2 poles of a car battery!), it is the high voltage at input of power supply that is dangerous it can be over 300Vdc, which is enough to kill.

##### Banned
carmusic, that's not correct. Specific example, depending on the person. You have a damp sweaty hand touching the car body (grounded), you drop a screw driver and it penetrates your hand and bumps into the +12 volt battery terminal. Depending on the perosn and the exact conditions enough current could flow across their chest to stop their heart. Simple saying 12 volts is perfectly safe is inviting mischief =)

#### SPDCHK

##### Member
Normal human skin resistance is high enough to limit the current to "safe" levels. but the higher the voltage, the higher the current will be for the same resistance.

24VDC, you can start feeling a tingle
48VDC, you seriously start feeling it
Higher than that, I don't know. Too me that too shocking

Seriously though. 12V is "generally safe", but don't go overboard now. Sceadwian's scenario can be lethal.

#### shokjok

##### Member
What affects you more - voltage or current? I've had a 12V flash when I dropped a wrench while charging a battery, and a slight tingle when adjusting a distributor.
Medical staff will tell you what high current will do to your body and organs.

#### Hero999

##### Banned
It's current that's the killer and mains frequency AC is generally much more deadly than DC.

The voltage you start feeling it depends on the resistance of the skin which can vary hugely: if you have really moist and sweaty hands don't be surprised if you feel a slight tingle at voltages as low as 30VDC, if you've got very dry hands then you might not feel anything even if the voltage exceeds 100VDC.

Note that these are DC voltages, as I said above AC is much more dangerous so the shocking effect can easily be doubled. For example someone who's just been swimming in the sea might get a shock from 30VDC will feel a similar effect at 15VAC or someone with very dry skin might only start to feel a tingle at 100VDC will feel a similat shock at 50VAC.

The regulations in my country state that under dry conditions, all voltages under 60VDC or 25VAC don't need to be insulated and in a moist environment the voltages drop to 30VDC or 12VAC.

#### unclejed613

##### Well-Known Member
actually the way muacles react to AC and DC is something that makes DC more dangerous. during the "War of the Currents" between Edison (DC) and Westinghouse (AC), a lot of "press" was developed by Edison claiming that AC was far more dangerous than DC. it simply wasn't true. it was found that AC has an effect on muscles that for instance makes you pull your arm away. DC however causes muscles to lock up, so if you have grabbed a live busbar for instance, rather than pulling your arm away, you just can't let go, your hand clamps down tight. AC has 2 zero crossings every cycle, and so there is opportunity for neural instructions to get to the muscles and pull away.

#### Hayato

##### Member
I agree with you, Unclejed.

But all deppends on the frequency.

Our muscles respond very well to 50/60 Hz mains, so, for example, if I get electrocuted by touching a 220V 60Hz energized wire metallic rod with the palm of my hand, my muscles will respond very well and my hand will hold so tight that I'll be "glued" to the piece.

So, our muscles will behave like they would behave with DC, locking up.

But, for example, if it was a 220V 6kHz, then my muscles would not respond.

#### unclejed613

##### Well-Known Member
most of the time, the first response of muscles with 50 or 60 hz is to pull away. first response to DC is to grip tighter. higher frequencies tend to travel closer to the skin as the frequency gets higher. a human body can actually conduct enough power (without any ill effects at all) at frequencies of 30khz and above to set a 2x4 on fire. lightning strikes have so much high frequency energy in them that the conduction path is usually on the skin. 2khz would probably not have any direct muscular effects, but might disrupt neural activity enough to kill you by blocking the nerves to your heart muscle.

#### Hero999

##### Banned
actually the way muacles react to AC and DC is something that makes DC more dangerous.
That's simply not true.

Low frequency AC current is much more dangerous than DC current.

Health24 - First Aid, Shock
220v at 60Hz AC travelling through the chest for a fraction of a second may induce ventricular fibrillation at currents as low as 60-100mA (300-500mA if DC).
during the "War of the Currents" between Edison (DC) and Westinghouse (AC), a lot of "press" was developed by Edison claiming that AC was far more dangerous than DC.
They were right, AC is more dangerous than DC. AC only won because it's easier to generate, transform from one voltage to another and can power highly reliable induction motors.

it simply wasn't true. it was found that AC has an effect on muscles that for instance makes you pull your arm away. DC however causes muscles to lock up, so if you have grabbed a live busbar for instance, rather than pulling your arm away, you just can't let go, your hand clamps down tight.
It certainly was/is true.

The DC "let go current" is much higher than the AC "let go current". In other words it takes much more DC current to make you freeze and unable to let go of the busbar than AC current.

ESD Journal - Electrical Hazard
"Let-go" current

The maximum current that can cause contraction of the flexor musculature of the arm but still permit the subject to release his hand from the current source.

DC - this value is about 75 mA

AC - this value is about 15 mA

and varies with muscle mass.
AC has 2 zero crossings every cycle, and so there is opportunity for neural instructions to get to the muscles and pull away.
That makes no practical difference, since the zero crossing time only lasts for a fraction of a second.

higher frequencies tend to travel closer to the skin as the frequency gets higher. a human body can actually conduct enough power (without any ill effects at all) at frequencies of 30khz and above to set a 2x4 on fire.
That's not true, the reason why higher frequencies are less damaging is because the nerves don't respond so well to the current.

The skin effect offers no protection because human flesh is a poor conductor compared to copper so the skin effect is much less pronounced.

The Skin Effect Myth
It is a very common belief that Tesla Currents won't harm you if you touch a Tesla Coil, because of the so called Skin Effect. This is not true! The Skin Effect is connected with the specific restistance of a electric conductor. The electric charges penetrate the deeper into a conductor the lower the frequency and the higher the specific resistance. At Tesla frequencies the charges penetrate several cm into a human body (see here for details). The reason why you don't get an electrical shock is that the nerves can't react at such high frequencies. But the electrical power is nevertheless absorbed by your body. This probably won't hurt at 100-200 Watt output, but touching a large coil will "fry" your nerves internally.
If it were true then you wouldn't be able to heat meat through to the core in a microwave because the skin effect would confine the radiation to the surface.

lightning strikes have so much high frequency energy in them that the conduction path is usually on the skin.
I think that as more to do with tracking and the fact that your body is covered in water than the skin effect.

2khz would probably not have any direct muscular effects, but might disrupt neural activity enough to kill you by blocking the nerves to your heart muscle.
That's probably partly true but I think the burns are more likely to kill you.

PAT Information - Electric Shock, Dangers, Effects, First Aid
Handbook of psychophysiology - Google Book Search

#### Techyboy

##### New Member
Hero is absolutely right! Even extremely low current in the mA range can cause an irregular heart beat and it may not necessarily be evident until hours after the initial shock. No matter how small the electric shock is, you are advised to seek medical assistance immediately!

Please use extreme caution when dealing with AC voltage, your life can depend on it.

#### Martel

##### New Member
It is a very common belief that Tesla Currents won't harm you if you touch a Tesla Coil, because of the so called Skin Effect. This is not true! The Skin Effect is connected with the specific restistance of a electric conductor. The electric charges penetrate the deeper into a conductor the lower the frequency and the higher the specific resistance. At Tesla frequencies the charges penetrate several cm into a human body (see here for details). The reason why you don't get an electrical shock is that the nerves can't react at such high frequencies. But the electrical power is nevertheless absorbed by your body. This probably won't hurt at 100-200 Watt output, but touching a large coil will "fry" your nerves internally.
RF voltages will burn your skin ! Believe me by experience. Having worked with two-way radios, i occasionally touched a component past the RF final transistor while transmitting. I didn't feel any "shock" but felt an acute burn pain at the point of contact between my finger and the RF component. Looking at my burnt finger, i could easily see a burn mark just as if i touched a hot soldering iron tip. Ouutch !

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