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# voltage value??

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#### tariq7868

##### New Member
hello... i wanted to know that what would happen if for example, a 5 v supply and a separate 12 v supply are connected in parallel i.e positive term with positive terminal and negative with negative...what should the overall voltage should be and WHY ?? i tested it by using a simple power supply adjusted at 12v and a computer's power supply giving 5V,, when i checked with my multimeter it was showing 12v but when i varied the voltage from 12 to 1.5 the voltage readings were just the readings of the first supply ,,just as if 5v supply wasnt connected...

#### ericgibbs

##### Well-Known Member
hello... i wanted to know that what would happen if for example, a 5 v supply and a separate 12 v supply are connected in parallel i.e positive term with positive terminal and negative with negative...what should the overall voltage should be and WHY ?? i tested it by using a simple power supply adjusted at 12v and a computer's power supply giving 5V,, when i checked with my multimeter it was showing 12v but when i varied the voltage from 12 to 1.5 the voltage readings were just the readings of the first supply ,,just as if 5v supply wasnt connected...

hi,
For starters, this not a wise thing to do.!

If the 12V has sufficient current rating it could damage the 5V supply.

DONT do it.!

#### marcbarker

##### New Member
It's not a very good idea to connect 12 volts to a 5 V power supply output. It probably wouldn't break it, but then again it just might. In the interests of science I would try as you have too, but I would certainly never do it a PSU that I wanted, or to someone else's PSU !

Why not examine closely the schematic diagram for the 5 V supply and see why the 5V supply behaved as it did when you applied the voltage?

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#### Hero999

##### Banned
It depends on the output of the 5V supply.

If the output of the 5V supply is push-pull and the 12V supply is current limited, the output will be 5V.

If the output of the 5V supply can only source and is reverse feed protected, the output will be 12V.

If the 5V supply isn't push-pull and isn't reverse feed protected, then it might oscillate or be destroyed.

The answer to a school assignment question:
If both the 12V and 5V power supplies are perfect voltage sources, the current will be infinite.

If each supply has a specific output resistance the current and output voltages can be determined using Ohm's law. Hint: the 12V supply will be <12V and the 5V supply will be >5V.

#### crutschow

##### Well-Known Member
While you are still in your random experimental mood you might also try plugging a 110V output into a 220V output. Should be even more interesting than connecting the two power supplies. (Make sure your health, life, and liability insurance is paid up before attempting this however).

#### marcbarker

##### New Member
While you are still in your random experimental mood you might also try plugging a 110V output into a 220V output. .

Not such a dumb concept.... In the USA there is the largest and most powerful man-made machine in the universe, it's trillions of watts of power, and that's the USA Power Grid. Almost all the power is made as it's used and little is stored.

They have what amounts to individual power stations coming online to meet demand and plugging themselves directly into the USA Power Grid.

#### Hero999

##### Banned
I didn't know there is one big US national grid. I thought there was lots of grids in the US, each totally out of phase.

Is the US connected to Canada? Are you both in phase?

#### crutschow

##### Well-Known Member
According to this Distributed Energy Program: U.S. Power Grids there are three main grids in the U.S.: the Eastern, the Western, and (of course), Texas. The Eastern has several subgrids, and both the Eastern and Western are strongly interconnected with Canada. All the interconnects are in phase.

The huge Eastern U.S. blackout in 1965 also darkened a significant portion of Eastern Canada, principally Ontario.

#### Mr RB

##### Well-Known Member
Isn't phase a local phenomenon?

#### tariq7868

##### New Member
the 12v supply i was using is a transformer based(220-30v) and is regulated by LM317.. the other one.. 5v supply is pc's PSU..(can deliver upto 20A ) which i think is transformerless..

#### Hero999

##### Banned
Isn't phase a local phenomenon?
I hadn'e thought of that, yes it is local especially at long distances where delays can be significant.

At 60Hz the wavelength id 4,997km, so at 1/3 of the wavelength the delay will be equal to one phase on a three phase system. This means that if you live 1,666km (just over 1000 miles) away from someone you on a different phase.

Obviously this isn't a problem with a small country such as the UK , like it must be for Australia and the US.

the 12v supply i was using is a transformer based(220-30v) and is regulated by LM317.. the other one.. 5v supply is pc's PSU..(can deliver upto 20A ) which i think is transformerless..
The 5V supply won't be transformerless otherwise you probably would have been electrocuted by now. It'll have a tiny iron powder high frequency transformer.

Was anything connected to the 5V supply?

If not the computer PSU would have probably shut down due to over-voltage. You might also have damaged or shortened the life of some of the components inside the PSU.

For future reference, shorting two supplies of different voltages together is not a very sensible idea and can cause irreparable damage.

#### marcbarker

##### New Member
Isn't phase a local phenomenon?

Well actually, for the US it's going to be GPS-based.

I've been talking with a US company this week who are developing new technology to measure phase differences at multiple points all over the Grid, referencing it to GPS datum.

By analysing phase deltas around the Grid in virtual real time, precursors to problems can be identified and dealt with, saving enormous amounts of energy.

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#### tcmtech

##### Banned
As I see it the 5 volt supply being referenced to being transformers would indicate its a switching type of some sort. So when the 12 volts was applied to the 5 volts outputs the switcher voltage reference circuit would have shut it off fully.

Being most 5 volt power supply's use 10 volt rated capacitors for filtering I suspect after a while the 12 volts will burn through one and pop it!

Technically in standard US domestic service to homes the power system is a 120/240 split. That is there are not independent 120 and 240 volt feeds from the main transformer but rather a center tapped 240 volt transformer with the center tap connected as the common and ground line.
The maximum voltage from common is 120 volts. So to get 240 volts both 120 volt lines are used being they are 180 degrees out of phase from each other.

#### tariq7868

##### New Member
As I see it the 5 volt supply being referenced to being transformers would indicate its a switching type of some sort. So when the 12 volts was applied to the 5 volts outputs the switcher voltage reference circuit would have shut it off fully.

Being most 5 volt power supply's use 10 volt rated capacitors for filtering I suspect after a while the 12 volts will burn through one and pop it!

Technically in standard US domestic service to homes the power system is a 120/240 split. That is there are not independent 120 and 240 volt feeds from the main transformer but rather a center tapped 240 volt transformer with the center tap connected as the common and ground line.
The maximum voltage from common is 120 volts. So to get 240 volts both 120 volt lines are used being they are 180 degrees out of phase from each other.

yes,,, you are quite right.. this must have been the reason...

but .if theoritically speaking,, what should happen,,i.e when two diff voltage source of diff values are connected in parallel ? and why?

#### marcbarker

##### New Member
but .if theoritically speaking,, what should happen,,i.e when two diff voltage source of diff values are connected in parallel ? and why?

Well I did a practical experiment earlier:

I have a car battery, it's on the car and it works fine.

I also have a few AA NiCads (or are they NiMH?)

The AA cells don't read anythingV at all when I measure them, in fact they don't take a charge, no matter how long you have them in the charger for. On ohms each AA cell battery measures about an ohm from + to - (I wondered what the internal resistance was, now I know )

So each AA cell, one at a time, I connected flashed directly accross the car battery, for about 1/2 a second each. As I put each one down, they were quite hot.

When I measured each one, they each (3 of them) read near 1.2 V. A few minutes later, I took them out the freezer where I put them to cool down, and I put them in the charger, with another partly flat AA cell I found.

I now have 4 working AA cells

So when you connect something lower voltage to a higher voltage..., it can: blow it up, or do nothing, or it could even mend it!

The clever (or dangerous) bit is knowing (or not knowing) which it is going to be.

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#### Mr RB

##### Well-Known Member
Cool experiment! Shame you didn't have an ammeter attached...

I've got some iffy NiCds on some test gear here that always need recharging, i might try it.

#### tariq7868

##### New Member
Well I did a practical experiment earlier:

I have a car battery, it's on the car and it works fine.

I also have a few AA NiCads (or are they NiMH?)

The AA cells don't read anythingV at all when I measure them, in fact they don't take a charge, no matter how long you have them in the charger for. On ohms each AA cell battery measures about an ohm from + to - (I wondered what the internal resistance was, now I know )

So each AA cell, one at a time, I connected flashed directly accross the car battery, for about 1/2 a second each. As I put each one down, they were quite hot.

When I measured each one, they each (3 of them) read near 1.2 V. A few minutes later, I took them out the freezer where I put them to cool down, and I put them in the charger, with another partly flat AA cell I found.

I now have 4 working AA cells

So when you connect something lower voltage to a higher voltage..., it can: blow it up, or do nothing, or it could even mend it!

The clever (or dangerous) bit is knowing (or not knowing) which it is going to be.

nicely done . i had also often done the same thing when i need some fast charging for my battery ,even knowing that it can blow my battery or atleast shorten the life of battery ...but i have stopped since i have already blown-up a battery (forgot to disconnect !)..

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#### audioguru

##### Well-Known Member
I mended many old shorted Ni-Cad cells with the ZAP from a 4700uF capacitor charged to 20V. But the mended cells developed the same problem over and over.

I think the problem with shorted cells was due to over-charging them in a product that keeps charging them.

#### marcbarker

##### New Member
i had also often done the same thing when i need some fast charging for my battery.

Yeah sometimes when you've forgotten your cellphone charger, it's the only thing you can do. I once fast charged a cellphone battery from the car, by pulling the headlamp fuse and inserting the battery in place, worked a treat, didn't seem to harm the battery. It's knowing when to disconnect, while it was on and the headlamp was glowing dimmer than usual I did some sums and knew how long I'd need to half-charge it.

I mended many old shorted Ni-Cad cells with the ZAP from a 4700uF capacitor charged to 20V. But the mended cells developed the same problem over and over.

I think the problem with shorted cells was due to over-charging them in a product that keeps charging them.

That's a safer way of zapping them. I think maybe it's 'whiskers' or something inside it that's to blame?

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