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Topic: 0 Voltage from power supply when connected to peltier (Read 5505 times) previous topic - next topic

shiznatix

I am trying to power 2 fans and a Peltier module from a laptop power supply with On/Off switches for each being controlled from an Arduino. Sadly, I am having issues.

The power supply is a laptop power supply with an output of 19V and 5.27A. When I connect a fan to it, everything goes well, the fan spins and no problems. But, when I connect the Peltier, nothing happens. If I measure the Volts with the Peltier connected, it reads 0V, but any other time it fluctuates around 20V.

Also, when I measure Amps directly from the power supply, I get 0 amps regardless of anything being connected. If I have the fan connected and spinning and try to measure the Amps, the fan dies until I stop trying to measure (but I measure around ~3A).

So, I am guessing that I am not understanding a basic electricity concept here. Can someone explain what is happening here so I can better understand?

jackrae

What is the power rating of the peltier.  If it draws in excess of the power supply rating, the power supply's protection circuit will be cutting off the output (crowbar protection) and won't reset until you remove the load.   
Typically a 50mm square peltier will pull 5 amps at 12 volts, so at 19 volts you could be talking about 7.5A which exceeds the PSU rating.

Grumpy_Mike

Quote
when I measure Amps directly from the power supply, I get 0 amps regardless of anything being connected. If I have the fan connected and spinning and try to measure the Amps, the fan dies until I stop trying to measure (but I measure around ~3A).

You measure current by inserting your meter in the load. So +ve supply to meter, other wire of meter to what used to go to the +ve supply.
If you try and measure across a load like you would for voltage, you meter puts a dead short across the load and you blow up the meter's input fuse ( if you are lucky ). Sounds like that is what you have done.

Boardburner2

Laptop supplies do not like inductive loads.
If i try to run a 4 a motor off. Mine it shuts down.

The peltier should work provided it has its own current regulator.

shiznatix


What is the power rating of the peltier.  If it draws in excess of the power supply rating, the power supply's protection circuit will be cutting off the output (crowbar protection) and won't reset until you remove the load.   
Typically a 50mm square peltier will pull 5 amps at 12 volts, so at 19 volts you could be talking about 7.5A which exceeds the PSU rating.


This is probably the case and I should have put the Peltier datasheet up in the original post but here it is now:
http://www.hebeiltd.com.cn/peltier.datasheet/TEC1-12710.pdf

I seam to have some trouble understanding the concept of how something can draw more current than what the PSU can give. Shouldn't the PSU push the current out and any device will take what it gets? So, if I have 12V with 10 ohms resistance then the current will be 1.2Amps, how can the device demand more current from the PSU?

@Grumpy_Mike
Yep, seams that is what I stupidly did. Will have to change that tomorrow now.

polymorph

No. The power supply is a constant voltage (as long as you don't draw more current than it can supply) and the load draws the current it is designed to take.

So you can put a 12V 0.25A computer fan on a car battery that is capable of putting out 800A starting current, the fan will draw 0.25A.

That peltier junction is rated to draw 10A at about 15V when the hot side is at 25C. Your power supply is 19V but at a max current of 5.27A. So the peltier would try and draw even more than 10A, but the power supply is shutting down as Jackrae suggested.

You are shorting out your power supply with your meter. You do NOT measure current by placing the leads across the power supply. The meter needs to go in series.

I strongly suggest that you do some studying on things like Ohm's Law, series and parallel circuits, and the proper use of a meter.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

mauried

What is the specs of the peltier, ie power rating at what supply volts.
Laptop power supplies are protected from overcurrent and will shutdown if overloaded.
Most common peltiers run off 12V , so 19V is far too high.

Boardburner2

From the spec , the peltier cooler is capable of drawing more current than your psu can supply, this will cause the supply protection circuitry to shut dowm.

polymorph

mauried, he has a link to the datasheet here:
http://forum.arduino.cc/index.php?topic=266999.msg1882840#msg1882840

It is characterized for more than 12V, but it is 10A at only 15V, and is still 8A at 12V.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

shiznatix

Ok, I understand that I need a power supply that can push out more amps.

What I have trouble understanding is how a device can pull in more current than a PSU can supply. I can understand that a device can _want_ more amps than supplied, but I have trouble understanding how it could demand those amps instead of just taking what it can get.

jackrae

It's a bit like beer drinking.

If you have a barrel of beer and you know that you go bonkers after only 2 pints, you'll still try and get four or five down your gullet.

shiznatix


It's a bit like beer drinking.

If you have a barrel of beer and you know that you go bonkers after only 2 pints, you'll still try and get four or five down your gullet.

Ha, ok, should I just accept this as fact then? Is the real explanation quite a bit more complicated? What factors dictate the amp consumption of a device and how can demanding more amps than are available damage the PSU?

I am fine with knowing that $thing draws $amps with supplied $voltage. Still, is there an easily sorcable and not ultra bulky power supply that can push 8 - 10A? The only thing I can think of is a power supply unit from a computer but that isn't something that can be carried around.

Peter_I

#12
Sep 16, 2014, 01:08 pm Last Edit: Sep 16, 2014, 01:11 pm by Peter_I Reason: 1

It's a bit like beer drinking.

If you have a barrel of beer and you know that you go bonkers after only 2 pints, you'll still try and get four or five down your gullet.



Or economy:

Let's say you earn 1000 "currency units" a day.
As long as you use less than that, you have no problem.
But when you get close to spending 1000 CU per day, things start getting critical. You strain your relations to your girlfriend because there is no buffer to pay for a trip to town, and small fluctuations will build debt.

If you start spending a bit more than 1000, you will start wearing things down. You will have to sell your canary and the unicycle to pay, and the girlfriend will leave because you never take her out.
(Your power supply will overheat and the parts will wear down)

If you really overspend (draw too much current or short circuit), you will pop the fuze, trigger the over-current protection or set the supply on fire.

You should not "just accept it".
You should realize, that your supply is designed to deliver a given amount of power, and that drawing more power takes it beyond what it is designed for, in which case it will either shut down to protect itself or be ruined.
"Nothing is foolproof to a sufficiently talented fool"

shiznatix

Ok, I think I am getting it. The PSU values of 19V and 5.27A are just that it will push 19V out and the amp is a rating, not a constant value. Meaning that it can push out max 5.27A or less, just not more. No matter what the amp draw is (as long as it is below its rating), the voltage will always be 19.

If that is correct, then the resistance will vary across each device connected to it. So I have a fan that is taking 19V and drawing 0.25A meaning the resistace of the fan is 76 Ohms. The peltier would have to have a resistor set up infront of it to limit the voltage but at the same time it would limit futher the current available and pull even harder for that current, but would still have the same affect on the PSU.

The device itself will pull the amps out instead of the PSU pushing them in. If the device exceeds the PSU limits, then things happen that can be bad.

Is this correct or am I still lost in the woods over here?

magnetman

But is the fan a 19v fan? Most of them are rated for 12v, so that may overheat and burn out too if left connected to 19v for a longer time. For powering a peltier your best bet would be a computer psu. I use a small desktop computer psu for mine, rated at 150w, and i still get enough extra power to use some fans, an arduino or other low power consumption electronics on the same psu.

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