Multi meter 200 mA question and a solar cell

Discussion in 'General Electronics Chat' started by joulian, Nov 21, 2009.

  1. joulian

    Thread Starter Active Member

    Aug 27, 2009
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    Hey guys I am testing a solar cell for the amount of amps and voltage it puts out. In order to run a 24v CPU fan I need of course 24v, 5.5w, and .23 amps.

    The solar cells I have are from the little solar lights you see around walkways. I have been tearing them apart and savaging the solar panels ( I am fixing to post a new thread about these solar panels asking some questions )

    I was testing it yesterday in the sun and it shown +/- 4.43mA it put out about 4volts.

    Am I reading that correctly? Its the mA's that are getting me confused. When I place it on 2mA it shows 1 which of course tells me I have to go to the next over which is 200mA

    If the sun isn't bright enough to turn the fan on will it burn up the motor on the fan?
     
  2. steveb

    Senior Member

    Jul 3, 2008
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    Can you explain how you did the tests exactly? It sounds to me like you tried reading the voltage with a voltmeter, and then tried to read the current with a current meter. However, you haven't told us how you loaded the solar cell. Did you use a load resistor, or did you connect the fan? Or, did you just measure open circuit voltage and short circuit current?

    By the way, it's hard to say if the motor will be damaged when light level is too low to move the fan. Generally, you don't want to power a locked fan, but then again, if the power input is so low, it may do no damage.
     
  3. ELECTRONERD

    Senior Member

    May 26, 2009
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    Greetings Joulian,

    There is no need to post another thread about the solar panels, this thread is for the same application and very similar. Your readings of 4.43mA seems to be accurate as well as the 4V, but your going to need at least 25V output from the solar panels for your application. So using six solar panels in series you can achieve that and then use a Low Dropout (LDO) voltage regulator that will supply you with the current needs.

    Austin
     
  4. steveb

    Senior Member

    Jul 3, 2008
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    Actually, we don't know if this will work until he answers my question about how the cell was loaded when the measurements were taken. You see, if the 4.43 mA is the short circuit current, then 6 cells in series will produce enough voltage, but will not provide enough current. He needs 230 mA which may only be possible if he has cells in parallel as well as in series.
     
  5. ELECTRONERD

    Senior Member

    May 26, 2009
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    It would be simpler to arrange them just in series for the voltage and then use an LDO to regulate the voltage as well as supply 230mA.

    Austin
     
  6. steveb

    Senior Member

    Jul 3, 2008
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    But, the LDO can not supply 230 mA unless 230 mA is available from the solar cells. Linear regulators can't amplify current, as this would violate conservation of energy.

    In order to use a series combination only, you would have to stack many more cells in series to generate over 1000 V, and then use a DC/DC converter to generate 24 V at .23 A.

    It would be much simpler to generate an appropriate series and parallel combination. An LDO would still be useful to help regulate the voltage.

    A more advanced approach is to regulate with a DC/DC converter.
     
  7. joulian

    Thread Starter Active Member

    Aug 27, 2009
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    Steveb

    I placed the red lead into mA terminal on the meter and set to 20mA ( i am sorry I think i said 200mA previously now that I have grabbed the meter and am holding it ).

    I then took the alligator clip leads and placed them on the wires coming out of the solar panel
     
  8. joulian

    Thread Starter Active Member

    Aug 27, 2009
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    Interesting , I am not familiar with how to wire up a series and parallel combo?

    Hmmm? Why is that a more advance approach with DC/DC converter. I am not familiar with this method. Its interesting to say the least. Now would that be a more efficient way to use the electricity being produced?
     
  9. steveb

    Senior Member

    Jul 3, 2008
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    If i'm understanding you correctly, you have basically done a short circuit test. A current meter acts like a short, hence you get no voltage and you measure the maximum current.

    Similarly, if you measure voltage with a voltmeter, this is an open circuit test. You measure the maximum voltage with no current.

    Note that both of these conditions results in no power because Power = Voltage X Current.

    In actual operation, you will have a load resistance, and there will be a voltage and current which implies power. The voltage will be less than the open circuit voltage, and the current will be less than the short circuit current.

    One trick with solar cells is trying to operate near the maximum power point. There is usually a particular resistance that maximizes power for a given light condition.
     
    Last edited: Nov 21, 2009
  10. steveb

    Senior Member

    Jul 3, 2008
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    Parallel is hooking + to + and - to -, while series is hooking + to -. Parallel cells will still give 4 V but will allow the current from each cell to add. Series cells will still give 4.43 mA, but will allow the voltages to add. If you do both, you can multiply the voltage and current to match your fan. However, you basically need over 300 (or maybe over 400) cells to get 5.5 W. Power is what matters here.

    Most likely it would be more efficient, but it really depends on your specific situation.

    For example, let's say that you hook 420 cells in an arrangement of 7 series banks, each with 60 cells in parallel. These 420 cells can provide your 5.5 W power (just a guess mind you), and the arrangement give you just about the correct voltage and current (just a guess). Now you have to ask the question of whether the cells are operating at their maximum power point. If they are, then a DC/DC converter will not help because you will lose 15% power in the conversion process, but will get no benefit, other than regulation.

    However, let's say that the above puts you at a power which is not at the maximum power point for the cells. Then you may want to change the parallel series arrangement to get 50 V at 125 mA (for example) if you believe this is the maximum power point operation. Then a DC/DC converter will be used to give you 24 V at 230 mA. Although you lose 15 % in the conversion, you may gain more than that back because you are now at the maximum power operation for the solar cells.

    Note that I'm highly simplifying this, and probably am not describing it in the most understandable way.
     
  11. steveb

    Senior Member

    Jul 3, 2008
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  12. joulian

    Thread Starter Active Member

    Aug 27, 2009
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    Thank you sir
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    The little solar panels on my solar garden lights have a short-circuit current of 60mA at noon in summer when the panel is outdoors pointing at the sun. It winter with the sun low in the sky at noon outdoors pointing at the sun the short-circuit current is 10mA and the sun is not in the sky for long.

    Your solar panels are worn out or dirty.
    Don't try it behind a window. The window reduces the output current to half or less.
     
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