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Using Arduino => General Electronics => Topic started by: shiznatix on Sep 15, 2014, 06:11 pm

Title: 0 Voltage from power supply when connected to peltier
Post by: shiznatix on Sep 15, 2014, 06:11 pm
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?
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: jackrae on Sep 15, 2014, 07:03 pm
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.
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: Grumpy_Mike on Sep 15, 2014, 07:22 pm
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.
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: Boardburner2 on Sep 15, 2014, 07:32 pm
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.
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: shiznatix on Sep 15, 2014, 09:21 pm

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 (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.
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: polymorph on Sep 15, 2014, 10:08 pm
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.
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: mauried on Sep 16, 2014, 01:13 am
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.
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: Boardburner2 on Sep 16, 2014, 01:28 am
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.
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: polymorph on Sep 16, 2014, 01:54 am
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.
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: shiznatix on Sep 16, 2014, 12:00 pm
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.
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: jackrae on Sep 16, 2014, 12:25 pm
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.
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: shiznatix on Sep 16, 2014, 12:56 pm

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.
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: Peter_I on Sep 16, 2014, 01:08 pm

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.
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: shiznatix on Sep 16, 2014, 01:28 pm
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?
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: magnetman on Sep 16, 2014, 01:53 pm
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.
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: shiznatix on Sep 16, 2014, 02:10 pm

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.


I want to run more volts through the fan to increase the speed a bit but this is another thing. First I want to just make sure that I understand everything correctly  :).
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: jackrae on Sep 16, 2014, 02:16 pm
If the fan uses a brushless motor (and many of them do) then extra volts will not materially increase speed.  The 3phase module that converts the DC to an AC generally operates at fixed frequency ior at a frequency determined by an external temperature sensor.
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: magnetman on Sep 16, 2014, 02:28 pm
You would be better off by using a more powerful 12v fan and variating speed by using a transistor & pwm.
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: shiznatix on Sep 16, 2014, 03:18 pm
I thank you all for the advice with the fans and everything but I would like to know if my understanding of basic electricity is correct in the previous reply I had here:
http://forum.arduino.cc/index.php?topic=266999.msg1883753#msg1883753 (http://forum.arduino.cc/index.php?topic=266999.msg1883753#msg1883753)
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: Peter_I on Sep 16, 2014, 03:39 pm

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


It is absolutely getting better.
Ohm's and Kirchoff's  laws are your friends (but a pair of merciless friends!)

One for playing with the basics:
http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: polymorph on Sep 16, 2014, 04:46 pm
Yes.

Although for a device like a fan, we don't really assign a resistance value to them, because they are nonlinear. That means that the current draw is not based linearly on voltage. Loading the fan increases current, startup current is more, and as someone else pointed out, the circuits in a brushless computer fan may limit the power drawn even when more voltage is applied.

Although my experience is that I've applied up to 18 or 19V to 12V computer fans and they've run faster. However, you are taking the risk of overheating the internals of the fan.


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?


Generally, when you overload a power supply, the voltage either drops as the load resistance goes down, or suddenly cuts off. It depends on how the power supply was designed.

Rather than use a resistor to limit current to the peltier, which will cause the generation of a lot of heat, I'd get a desktop computer power supply and modify it to work without a computer. The 12V outputs will generally put out a lot more than 10A.

Basically, you need a minimum load on the 5V line, and a way to turn on the power supply. This one does it as an external adapter:
http://makezine.com/projects/computer-power-supply-to-bench-power-supply-adapter/

I repaired a peltier junction cooler, replacing the rusted out fan with two 12V computer fans, and found a 12V 12A open frame power supply surplus, and put it into a case with a fan cooling the power supply so I could run it on AC.
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: MarkT on Sep 17, 2014, 03:40 am

If the fan uses a brushless motor (and many of them do) then extra volts will not materially increase speed.  The 3phase module that converts the DC to an AC generally operates at fixed frequency ior at a frequency determined by an external temperature sensor.


Brushless fans run at a speed proportional to the supply.  They are 2-phase, not 3-phase,
normally, and have hall sensors so the frequency is locked to the fan rotation.  The back-EMF
of the windings roughly balances the supply voltage at equilibrium so that the speed is
nearly linearly related to the supply voltage.  (Most 12V fans work from below 6V up to
15+V if you don't mind taking the risk of damaging them).
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: shiznatix on Sep 17, 2014, 09:29 am
Thanks everyone. I have enough information now to continue learning more :) but at least I am moving in the correct direction.

With all the remarks about using a computer PSU, I have one already set to go that I have been using to charge LiPo batteries so that isn't really a problem. The problem is that the thing is a computer PSU so it is big and heavy and has a tangled mess of additional power lines that I just don't need.

I would like this peltier thing to be somewhat-portable so using a computer PSU certainly isn't optimum.
Title: Re: 0 Voltage from power supply when connected to peltier
Post by: Peter_I on Sep 17, 2014, 01:29 pm
That is a classical problem.

You have something that needs a lot of power, and you want it to be portable.

Peltier based coolers can drain a car's battery if you are not careful.