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

Thread Starter

rudyauction8

Joined Jan 27, 2012
250
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
 

Thread Starter

rudyauction8

Joined Jan 27, 2012
250
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.
 

MaxHeadRoom

Joined Jul 18, 2013
28,507
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.
 

AnalogKid

Joined Aug 1, 2013
10,943
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
 

Thread Starter

rudyauction8

Joined Jan 27, 2012
250
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
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.

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

crutschow

Joined Mar 14, 2008
34,024
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.
 

Thread Starter

rudyauction8

Joined Jan 27, 2012
250
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.
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!
 

Thread Starter

rudyauction8

Joined Jan 27, 2012
250
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.
 
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Thread Starter

rudyauction8

Joined Jan 27, 2012
250
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.
 

Thread Starter

rudyauction8

Joined Jan 27, 2012
250
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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