# Electrical Quiz, Series Circuits



## atzi (Jun 26, 2008)

I always learn something every day I work on the EV. I will try to explain the problem with figuring the amp draw on a multiple in series electrical circuit.
First, I know or have refreshed myself on a few basic principles. 

Ohm's Law. V= Volts, I= Amps R (resistance Ohms) W= Watts


V=I*R (if you know the amps and resistance you can figure the volts)
I=V/R
R=V/I
W=V*I
Moving to series resistances, you total all the resistances together like 1 ohm, 3 ohm and 4 ohm and you get an total ohm reading of 8 ohms. You can use that figure to predict how much amperage will flow a circuit with 150 volts would be 150/8 or 18.75 amps.

*Application
*​ I am building a thermoelectric air conditioning for the electric vehicle.(I know it is not practical but, better ones will be out someday...) I am using *12 TEC chips* that are rated at MAX 6.0 amps. Recommended efficiency is 2/3 maximum amps so 4 amps is preferred. 
You cannot check resistance of a TEC Peltier module because as soon as you connect a ohm meter to one the voltage that the meter uses to test the TEC will create heat and Seebeck Effect happens and voltage-ohm reading move all about. But, I can send a voltage through the chip and see how much amperage it uses and figure the resistance. (The amperage does drop some as the TEC hot and cold side temperature differences increase.) The resistance is 2.21ohms per TEC and 12 TEC's wired in series makes 26.5 ohms. 
*159 volts* tested in the pack wired to the modules showed over 6 amps at the moment of connection but drops to 5.1 amps when the temperature difference is 27°C. I am joining the cooling fan (for the hot side heat sink) into the series wiring so to drop the amperage to a safer level. I know how to predict the total amp draw by just adding that resistance to the circuit math. But how can I predict the *watts* thru the fan motor? Large resistance in the motor will drop the total amp draw to a lower level and smaller resistance (more powerful motor) will cause less amperage change. 
*Quiz*
​ What will happen if


If I add a fan a cooling fan that is rated at .5 amps at 12 volt?
If I add a cooling fan that is rated at 3.5 amps at 12 volts?
How will the fans do in this series circuit?


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## DavidDymaxion (Dec 1, 2008)

Great project, please keep posting about what you are doing! I'm thinking of doing something similar. Have you thought about reversing the current and also using it as a heater? I had the thought with a heat exchanger it might be more efficient a heater than a ceramic heater.

For your question, are you asking if you can hook up a 12V fan to the full battery pack voltage, hoping the Peltiers have enough resistance to save it? That sounds risky to me, but I'm not an expert on Peltier behavior. Why not run more Peltiers, and run the 12V fan off your 12V system?


atzi said:


> I always learn something every day I work on the EV. I will try to explain the problem with figuring the amp draw on a multiple in series electrical circuit.
> First, I know or have refreshed myself on a few basic principles.
> 
> Ohm's Law. V= Volts, I= Amps R (resistance Ohms) W= Watts
> ...


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## Roy Von Rogers (Mar 21, 2009)

Peltier elements will never make an efficient cooloing or heating system, its in its nature ineffcient.

If you want to spend that kind of amperage you are better off with an electric compressor, and a reversing valve with resistive heating as back up heat. Or in heating, plain resistive is better especially in case of ceramic types.

Now I have never tried a heatpump in an automotive application, but do install and repair heatpump residential systems, so I know a bit about that technology. I know that residential heatpumps can provide heat even at outdoor temps close to freezing, but applying it to automotive situations would take some thinking. 

But I can assure you that peltiers are NOT the answer.

Roy


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## atzi (Jun 26, 2008)

Roy Von Rogers said:


> Peltier elements will never make an efficient cooloing or heating system, its in its nature ineffcient.
> 
> If you want to spend that kind of amperage you are better off with an electric compressor, and a reversing valve with resistive heating as back up heat. Or in heating, plain resistive is better especially in case of ceramic types......But I can assure you that peltiers are NOT the answer.
> Roy


I hereby make a disclaimer
I did not claim this thermoelectric Peltier device is the way or a way. It is just my way, for this period in my life. It will probably go into the pile of my other experimental failures but, I love a country were I can still try & fail. It's OK.

As far as the series wiring. I installed a blower in series with the 12 chips. The extra resistance did drop the amperage (at a similar temperature difference)from 5.1 amps to 4.8 amps. 
All circuits in series will run the same amps but not the same watts.-|
How many watts will run through the small fan verses the big fan?







​


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## atzi (Jun 26, 2008)

DavidDymaxion said:


> Great project, please keep posting about what you are doing! I'm thinking of doing something similar. Have you thought about reversing the current and also using it as a heater? I had the thought with a heat exchanger it might be more efficient a heater than a ceramic heater.
> 
> For your question, are you asking if you can hook up a 12V fan to the full battery pack voltage, hoping the Peltiers have enough resistance to save it? That sounds risky to me, but I'm not an expert on Peltier behavior. Why not run more Peltiers, and run the 12V fan off your 12V system?


Yes, I may do that, but I had only so much room (12) for the Peltiers with the heat sinks. I may change the arrangement. What struck me was the dynamic of different reisistors inseries act so different than when in parallel.


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## TigerNut (Dec 18, 2009)

atzi said:


> Yes, I may do that, but I had only so much room (12) for the Peltiers with the heat sinks. I may change the arrangement. What struck me was the dynamic of different reisistors inseries act so different than when in parallel.


Neither the fan motor, nor the Peltier devices are resistors, and trying to analyze the circuit as though it consists of resistors is going to fail. Still, you can look at the power ratings of the various devices and see how the power ratings compare to what you're trying to draw from them.

For your fan motors:
The little one is rated at 500mA at 12V. So if you assume it's a 24 ohm resistor, you should also assume it's a resistor rated at 6 watts. 4.8 amps through a 24 ohm resistor is about (4.8 x 4.8 x 24) = close to 600 watts. 
Or did it draw 4.8 amps when you connected the other fan? It was rated at 3.5A at 12V, or 42W, and "equivalent resistance" of about 3.5 ohms. So your power dissipation in that fan would have been 3.5 x 4.8 x 4.8 = about 80 watts.

In either case, your motor is not going to last long. It may be that at higher voltages, the current doesn't increase as rapidly; in that case it is a higher effective resistance, but using a much higher than rated voltage will fry the brushes.

The proper way to control the Peltier devices is with a current-limiting circuit. There are lots of those on the market, but you may have to do a bit of searching to find one that will operate at 160 volts. Look into LED lighting solutions.


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## atzi (Jun 26, 2008)

Each Peltier is a resistor (resistance will drop as the Temp difference on its sides rises) and the fan is a resistor. 
When the 12 resistors are in-series with the 159 battery pack each had a potential divider (pd) or voltage drop of 13.25 volts. So at 5.1 amps that would be 67.5 watts each. 
When the 13th resistor (the larger fan or call it the smaller resistor) is added two things happen. The added resistance dropped the amperage to 4.8 amps and changed each voltage drop down to 12.1 volts. So now the original resistors are running 58 watts each.
The 13th resistor showed a voltage drop of 13.9 volts running the same (Of course) amperage of 4.8 giving it a wattage of 67.
*I did not try the following...*
If the smaller fan (higher resistance, 24 ohms) was used instead, the voltage drop across it would have been higher. *Before it melted* the small fan would have, lowered the amperage to 3.2 amps. But It would have a voltage drop of 76 volts. 76 times 3.2 amps would be 243 watts. The 12 resistors would have a combined voltage drop of 83 or 6.9 volts each at 3.2 amps is 22 watts each.
So,
Series wiring... very different. Can't use Peltiers or other devices in series of different sizes without planning and understanding.


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