Voltage Reduction

Discussion in 'General Electronics Chat' started by Ericvic, Aug 22, 2013.

  1. Ericvic

    Thread Starter New Member

    Aug 22, 2013
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    0
    I have a problem that I can't seem to get my head around. I am designing a solar charger that uses the MCP73871 to charge a single cell Li-Ion battery and provide power to the load. The loads that are connected to the charger board are existing devices that have a MCP1700 3.0v LDO regulator on them and they can't be changed. They have a sleep mode where they draw about 5uA and an active mode where they draw about 100mA

    The problem that I am running into is that the MCP1700 regulator has a max input voltage of 6v and the 6v solar panel I am using actually puts out more like 7v in bright sun. The MCP73871 has voltage regulation on the battery output but not on the load output. I need to come up with a way to keep the output voltage below 6v but still be able to use all of the available capacity of the battery at night or when the sun is not shining. I thought about using a zener diode but I have never used them before so I don't really know where to start, also I would rather not have the series resistor in the circuit when the voltage is below 6v to keep everything as efficient as possible.

    Thanks,
    Eric
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,033
    Have you actually observed the MCP73871 output voltage while everything is "live"? I would suspect the panel voltage to be reduced by any load on it, and combined with any dropout in the regulator, it seems it would be rare to actually exceed 6V on the output.

    You might consider a blocking diode for the panel, which will drop ~0.7V. This is an efficiency loss but adds a bit of protection against reverse current into the panel, if that is relevant.

    An excess current dump circuit might be a candidate. I've seen such a thing used in a solar circuit before. I can supply a link it that's interesting.
     
  3. Ericvic

    Thread Starter New Member

    Aug 22, 2013
    3
    0
    I already have a blocking diode on the panel input and I have observed over 7v at the output with the battery in place. I didn't have one of the devices hooked up because I didn't want to damage it with an over voltage condition. I will try that tomorrow if we have sun around here.

    A link to a current dump circuit would be nice.

    Thanks,
    Eric
     
  4. Ericvic

    Thread Starter New Member

    Aug 22, 2013
    3
    0
    OK, I've had some time to test the charger with one of my actual devices connected. Even with the device connected it will still output over 6.5v in bright sun.

    My device seems to work fine for a while and then at some point it quits talking (wireless) to my master device and will not start talking again until I remove power and apply it again. I don't think it is some bug in my firmware because if I supply 5v to the device it will run indefinitely but hooked up to my solar charger it will quit "talking" after a while. It is not a set amount of time which is why I'm thinking that it is having an over voltage event so I would like to somehow reduce the output voltage of the solar panel when it gets over 6 volts. Thanks for any help anyone can provide.

    Eric
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,033
    Take a look at the lower right of the schematic here. BY tweaking the zener value and the resistor divider, you can adjust the voltage at which current begins to dump thru the transistor.
     
  6. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,034
    1,631
    You might want to look at making a shunt regulator circuit based on a LM431 device.

    They are a simple three pin shunt regulator IC that is very handy for making reasonably precise shunt type voltage limiters using only two resistors and a optional potentiometer, the LM431 IC and a properly sized power transistor.

    Very simple circuits to make and tune.
     
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