Need to build circuit that shuts off relay when voltage reaches 75 volts

Discussion in 'The Projects Forum' started by rudyauction8, Aug 10, 2015.

  1. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    I'm building a battery charger for my 60 volt ebike. I need a circuit that will shut off a relay when the voltage reaches just under 75 volts (around 73 if I remember correctly). I'd prefer some adjustment if possible. I'd prefer to avoid zener diodes if possible and use what I have available (assorted transistors capacitors resistors rectifier diodes etc.). The circuit can be powered by the battery bank or a separate 12v power supply (or even 120v rectified from a power outlet), the charger will spike to 160 volts when disconnected so using it to power the shutoff circuit isn't a good idea. My relay has a 12v coil
     
  2. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    I'm thinking a voltage divider and a couple transistors to switch the relay is a good starting point. I'm just not sure how to put it all together. The relay can be hooked up so power to the coil turns it on or off.

    Maybe just a voltage divider hooked directly to the relay and set up so the coil has just enough power to switch the relay at 73 volts? How consistent would that be? I'm guessing the voltage the relay shuts off at would likely vary quite a bit.
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    To charge a battery correctly and safely you need to use the proper charging profile, which depends on the battery chemistry. I suggest you visit www.batteryuniversity.com .
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Depends if you want something that is reliable or slapped together with existing components.
    A zener a LM311 etc will give you precise control.
    Max.
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    While rectifier diodes can be stacked up to form a voltage reference for the comparator, it will not have enough accuracy to discern between 73 V and 75 V over much of a temperature range. At some point in your circuit you will need a stable, known voltage that does not vary to be the reference for the comparison with the battery voltage. All of the circuit's accuracy and stability depend on the reference.

    ak
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If the relay shuts off at 75V, what voltage should it turn on at? (What is the hysteresis?)
     
  7. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    I have several 7805 regulators. I can also pick up a comparator at radioshack down the road. They don't have much for zener diodes but I guess a 5 volt zener and voltage divider would be easy enough.

    It won't. Once the battery pack reaches 73 volts the charger will be power cycled before it turns back on. if its easier to build one that does I'd say around 65 volts.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    A 7805 regulator is spec'd to drift -0.8mV/°C which probably is stable enough for your requirements if the 7805 is not dissipating any appreciable power.
    Use that 5V as a reference to one input of a comparator such as the LM339. The other input would be from a resistive voltage divider that generates 5V when the battery voltage is 73V.
    The resistive divider can include a pot so you can tweak the trip voltage to your preference.
     
  9. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    The 7805 will only dissipate the power needed to run the comparator. I think adding a capacitor to filter the output would help as well. does the 7805 require a minimum load to function properly? I know some regulators need to be under at least some load or their voltage can spike. Like an LED with current limiting resistor? I'll get to work tonight and post back if I run into any trouble. Thanks!
     
  10. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    I think I'm going change the charger design. I think I'm going to use 7812 regulators with a couple diodes to get voltage to 13.8 or so and have a separate charger for each of the 5 packs. That way they will all be balanced and likely last longer. I'll put a switch between each pack so they won't short into each other through the chargers. I am planning on running each charger off a laptop power supply and using an external load transistor to allow more current while charging. I have a very large heatsink that will keep all the components cool.
     
    Last edited: Aug 11, 2015
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  11. rudyauction8

    Thread Starter Member

    Jan 27, 2012
    252
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    Is it possible to make a balancing circuit that will switch on a load (a car tail light, power resistor, etc.) when the battery reaches 14 volts? It needs to run off the battery its balancing and consume a relatively low amount of power when idle (no more than 100ma, preferably 50ma or less). Again prefer to stick to what I have available.
     
  12. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    I'm thinking 5v zener to comparator, voltage divider from battery, output drives power mosfet to switch load.
     
  13. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If your charger output voltage is 13.8V the battery voltage will never reach 14V :confused:.
     
  14. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    I know the voltage the balancing circuit turns on at is higher so the load will only switch on if the battery is about to be overcharged.
     
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