24V R/C car power supply  - Page 1
Forum section: electronics / beginners

author: mcdesca
author_rank: Contributor
timestamp: January 23, 2016, 11:29:12 am
content: Dear everybody,I am currently working on a r/c car that is 24V battery-powered (28.8V real). It has a main electronics board with converters directly fed from the batteries (they withstand 42V max). The H Bridge motor supply is directly connected to the batteries.- When the motors brake (regenerative), an overvoltage is produced, which is already limited by the motor controller to 35V. In order to improve the braking capability, I am planning to build a small load that activates once the voltage is higher than 34V, so the motor controller has always room for braking and the converters never reach the 42V limit. Can you spot any deficiency in the implementation? I am using just a Darlington transistor activated by a TL431, current and temperature protected. I could even disconnect the motors when the overvoltage protection could not be able to absorb all the energy. I was thinking of using a mosfet that fully turns on and all the energy is burnt in a resistor. Is this a good approach or is a linear approach better?- Another solution could be to avoid any overvoltage correction and use another converter to supply the low-voltage converters with 12V. Options are flyback or SEPIC up/down-converter. My concern here is the overlapping inefficiency of two converters (and worse reliability figures). I guess that is the EV-industry approach (main converter + PoL converters for the low voltage electronics), but since I am running the motors on relative low voltage, I could save a converter and improve the reliability figures. What do you think?- I am using two batteries that discharge in parallel. I have implemented a power supply oring feature that will refrain as well from charging the batteries. Since the vehicle will have little kinetic energy (20kg vehicle at 20km/h hit 70V 300ms peaks maximum), I have no plans to charge the batteries from regenerative braking. I was thinking though, on charging a small capacitor bank up to the 34V limit, which will be consumed by the electronics/motors themselves.I am including varistors/TVS + LC filter on each board, which will suffice for small transients. I am just not confident on the little safety factor from the 35V to the 42V limit of the low power converters.Thanks a lot for your thoughts.

author: ADC-1995
author_rank: Regular Contributor
timestamp: January 24, 2016, 02:31:07 pm
content: You might look into the Sabertooth series of motor controls for what you are trying to achieve or possible design ideas. I have used the Sabertooth for a while and its very versatile. They do also make some smaller versions designed for lower amperage circuits.http://www.robotshop.com/en/sabertooth-dual-motor-driver.html

author: macboy
author_rank: Super Contributor
timestamp: January 25, 2016, 04:48:05 pm
content: I would build what amounts to a ~32 V shunt regulator across the battery. When the voltage begins to exceed 32 V, the regulator shunts (to ground) whatever current is necessary to bring the voltage down. This will provide all the braking current you need* and protect the battery/controller. (* if the regulator is sufficiently large).I've attached a design of a very simple shunt regulator that will do this. The regulation isn't great, but it will shunt more and more current as the voltage increases slightly higher, which is what's important in this case. The Power transistor is one or more big (TO-247 or TO-3P) Darlington NPN power transistor(s) (I modeled it using some discretes built into LTSpice, but I would build it with a Darlington device). The zener provides the working voltage, about 1.3 V higher than the zener voltage (zener breakdown + transistor Vbe). The 10k resistor is necessary to prevent the transistor from conducting due to any leakage through the zener, when the voltage is still below the threshold. The 2 ohm resistor is an arbitrary value for the source resistance of the braking controller, which itself is modeled by the simple voltage source. Try running this in LTSpice and compare the voltage of the voltage source to the battery terminals.Also, remember that your regenerative braking power will be better with partially discharged batteries than 100% charged ones. The reason should be obvious: nearly fully charged batteries don't want to accept much charge current. Using a shunt as described above will overcome this until the batteries drain enough to accept current.

author: mcdesca
author_rank: Contributor
timestamp: March 09, 2016, 08:12:50 am
content: That worked a treat, thanks.