help with power draw

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help with power draw

Postby Jiminsav » Sun Aug 21, 2005 8:24 pm

Well, let me display a little ignorence here..I want to use a deep cycle battery to run some peltier chips, the chips use about 5 amps of power..the problem as i see it is i need to limit the amount of amps they can draw from the battery, and don't know how to do it..can anyone clue me in?
Jim in Savannah
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Postby Woody » Sun Aug 21, 2005 9:30 pm

limiting the power requirement would affect cooling/heating operation design you intend to use the Peltier coolers for wouldn't? I saw on one of them sites a temp regulator that might help with your power consumption problem by automatically turning them ON and OFF. You might be better with shore power and convert to 120 vac to the required 12 - 16 vdc voltage for operation. I told you they were battery killers
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Postby Jiminsav » Sun Aug 21, 2005 9:41 pm

Woody, i'm not trying to limit the power..i'm trying to limit the amout of current the chips can draw..the chips are rated at around 5 amps max current @ 12 volts..but just hooking them to a battery lets the chips draw way to many amps and they start heating on both sides..i tore apart a small plug in cooler, and they use a small circuit to limit the amperage.
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Postby Woody » Sun Aug 21, 2005 10:06 pm

Jim
You your right I have seen that piece installed on the one I took apart also. It said on it what the rating it allowed, I'll have to find on see exactly what it is though. The only way to limit might be a large resistor, or something of that nature, to limit power. Let me see if I can find that part and get back to you Okay
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Postby Steve_Cox » Tue Aug 23, 2005 6:07 am

Jim,

Woody was in the right direction in his first post about turning the power on and off. Resitors would use the same amount of energy as running "wide open" just limiting how much get to the peltier chips and turning the rest into heat at the resistor. What it seems is needed here is Pulse Width Modulation that cycles the power on and off to prevent overload. The technology is pretty cheap. Controllers like that are used in small battery trickle chargers, rechargable spotlights, small motor controllers, etc. Might try to google "12vdc PWM" or something close. You should be able to find small modules with the right controls for current and voltage sensing pretty cheap. Of course this early in the morning I might just be talking out my a$$. :lol: Good luck

Steve in St Augustine 8)
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Postby Woody » Tue Aug 23, 2005 8:42 am

The unit had a thermistor installed on it to limit power consumption( from manufacture so your battery wouldn't melt after a 1/2 hour of use, it now takes 4 hours) and disapated the difference in heat. From the articles I read, they all state in one form or another of diminished or decreased efficiency by limiting input amps along with voltage. Now granted it also is a function of how and what the chips were originally designed for and there usage and or application. It also said of the use of either ON and OFF switches to control power consumption or the use of tempeture control modules (Automatic ON/OFF thermostatic switch device) for when temp control functions are required. Since temp regulation can not be simply controlled or maintained by varing the input voltages and amps due to the efficiency problem associated by the way these chips are designed to operate. I would still suggest using shore power and 120 vac to 12 - 16 vdc converter and some form of thermostat switch device to save rampant battery power consumption. Also by using the hot side of the chip to heat water might affect to the cooling side efficency, the same way ambient outside temps have on the chips cooling ability but to a larger extent. By holding the heat in the heat sink ( heating water) instead of disapating it into the surrounding enviroment rapidly would have a limiting factor on the effective cooling. At first it probally would aid in heat dissapation, but as the water warms up the heat exchange would be greatly diminished, which in turn would inhibit the cooling function you desire
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Postby TonyCooper » Tue Aug 23, 2005 1:40 pm

Jiminsav wrote:Woody, i'm not trying to limit the power..i'm trying to limit the amout of current the chips can draw..the chips are rated at around 5 amps max current @ 12 volts..but just hooking them to a battery lets the chips draw way to many amps and they start heating on both sides..i tore apart a small plug in cooler, and they use a small circuit to limit the amperage.


Is this with the heatsinks installed? With a large enough heatsink on the hot side you should not have a problem with the cold side heating up. You may need a small fan to help cool the hot side heatsinks as well.

BTW, I'd use 120VAC shore power to 12-16VDC to drive them.
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Postby Jiminsav » Tue Aug 23, 2005 7:53 pm

Yes Tony, it's with the heatsink installed, and some "cold shoes" on the cold side to move the cold away from the hot side..it's just a never ending battle..i could have bought a small A/C by now..
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