Peltier & Amps

[Cliff Rohrabacher]

I can drive a Peltier with a switching power supply like that from a PC.
An ATX power supply Volt output matches the demand by the peltier unit.
But some the Peltier only wants 12 VDC at 6.5 amps
The ATX I have in my hand has the following listing for Max output:
+5 VDC at 5 amps 200 watts
+3.3 VDC at 28 amps 200 watts
+12 VDC at 45 amps 540 Watts

Then there are some other call out I don't understand:
-12 VDC at 0.6Amps 7.2 Watts
+5VSB at 2.5 amps 12.5 Watts

Will the excess amps available 28A harm the peltier or will it simply not draw that?

[Chuck Wintle]

the peltier will only draw the current it needs and nothing more.

[Cliff Rohrabacher]

I found an answer . It matters not about the excess amps available it'll only draw what it needs.

[Cliff Rohrabacher]

the peltier will only draw the current it needs and nothing more.

Thanks Charles

[Ken Fitzgerald]

Amperage rating on a power supply is just the normal operating capability. If the load the power supply is supporting doesn't demand that amount of current it won't exist or be used.

[Cliff Rohrabacher]

So then to be safe, I can experiment with Amp limit Fusing trying to get the Amps that can flow down to the lowest possible?

Last I checked a lousy milliamp can kill

[Rob Young]

Cliff -

I wonder if the +12V @ 45A is a typo and that should read 4.5A. I've yet to see an ATX supply that was rated for a kWatt...

You got your answer but here is more detail...

PC Power supplies are designed for CONSTANT VOLTAGE. That is to say they regulate their output voltage to be in spec (let's say +12V +/-10%) so long as their load is drawing less than maximum current.

In some switching supplies, you also need a minimum current draw for the supply to start and enter regulation. On "cheap" PC supplies this is often the +5V or +3.3V rail. But not in all cases. A well designed supply will start and regulate with any load between 0 and maximum current.

And one more comment. Depending on how the internals of the supply are configured, it may "crowbar" if the current draw exceeds maximum. That is to say it will start folding back the output voltage such that the product of output voltage and output current stays constant. Again, a well designed supply can do this all day long. A cheap, poorly designed supply can do this once and for a very short time.

[Cliff Rohrabacher]

Cliff -

I wonder if the +12V @ 45A is a typo and that should read 4.5A. I've yet to see an ATX supply that was rated for a kWatt...

That came from the side panel of an LSP Ultra 750 watt PS.

I haven't figured out what the negative symbol means:
-12, 0.6A, 7.2W

I did learn that the +5VSB is the 5 Volt Standby voltage that wakes up sleepy devices from standby mode. There is always 5 volts available on that lead if the PS is plugged in and the switch in on.



PC Power supplies are designed for CONSTANT VOLTAGE. That is to say they regulate their output voltage to be in spec (let's say +12V +/-10%) so long as their load is drawing less than maximum current.

[John Coloccia]

-12, 0.6A, 7.2W

It just means -12V with respect to the return/common, that's all. If you go back far enough, you'll find supplies with -5V on them too.

re: crowbar
Just a small point. It normally won't limit the current so much as put a dead short across the output, hence the name (it's like dropping a crowbar across the output). This removes power from the output (goal number one). Goal number two is to keep the supply from blowing up. That's left to the breakers, fuses etc. You were describing more of a clamp, which I don't normally see outside of lab power supplies. No biggie...I just noticed the terminology went funny for a second

[Cliff Rohrabacher]

It just means -12V with respect to the return/common, that's all.

Ok I haven't a clue what that means.
My Electronic skills are more in line with wiring dual pole light dimmers installing panels and subs etc.

Ask me what a MOSSFET does or why a Zenier only lets power flow one way and my eyes roll into the back of my head.

I think it took me like three days to figure out how to make a bridge rectifier.

[Dan Hintz]

Cliff,

A few milliAmps can kill you if it enters directly into your soft tissue (like someone jamming a wire through your skin next to your heart) where resistance of bodily fluids is very low, but your skin resistance is so high (10s to 100s of kohms) it's not an issue in everyday use. This is why you can touch a 9V battery across your tongue and not die... it tingles, and you probably have a few hundred mA flowing over that short distance, but it's not going to kill you.

Supplies offer positive and negative voltages (the direction in which the electrons flow). Most people are used to specifying a voltage without a polarity, and in such cases the polarity is assumed positive (+) with respect to ground... under the right circumstances (floating supplies, which I won't get into here), a +12V supply could be used as a -12V supply, and vice versa. In most cases, however, the two polarities are NOT interchangeable.

And from a 10,000 foot view, MOSFETs aren't difficult to understand. They essentially act like switches for power control. Three legs: an input, and output, and a control. Power is attached to one leg (the input), what you want to control power to is attached to a second (the output), and your control signal is attached to the third (like some guy with a light switch). FETs offer very little resistance to power flow, which is why they are used in high-power circuits, like power supplies. Transistors operate with a similar function, just with different pros/cons.



Rob,

kW supplies are becoming quite popular these days... biggest I've seen is 2kW. With some graphics cards requiring two and sometimes three +12V supply plugs, this isn't surprising. One of Invidia's latest graphics cards sucks down a cool 800W all on its own.