11.1 volts to 8.0 volts ?

[HarryA1]

Hello all,

I need to connect an 8 volt electronic speed control to an 11.1 battery.
I have an 8 volt regulator and a power mosfet. I have not worked with
mosfets before so I am puzzled on how to make the connections and
what values of resistors are required. Please take a look at my diagram.
Any advice would be appreciated.

This motor control is for the stern of a model blimp to control the direction
much like a helicopter's directional control.

see:

 

[audioguru2]

The datasheet for the very old IRF510 Mosfet shows that it needs a gate-source voltage of 10V (not 8V) to fully turn on. Maybe you should be using a "logic-level" Mosfet instead.

Your Mosfet does not do anything since it is always turned on like a piece of wire. An ESC uses Pulse-Width-Modulation to control the speed of a motor.

I do not know why you have the Mosfet, R2 and R3.

 

[HarryA1]

My "bright" idea is to use the mosfet to get more than the 1 ampere that the 8 volt regulator is rated at.  I have a large 11.1v li-po battery for the main motor so I don't want to add another battery to get the 8 volts.

I wonder if one can put two or three regulators in parallel for more current?

This current is the power supply for the speed controller; it connects to the
receiver for the PWM.  It is the brushed type used with rc cars so it can reverse
the motor for left and right control. 

 

[audioguru2]

My "bright" idea is to use the mosfet to get more than the 1 ampere that the 8 volt regulator is rated at.

Your idea is completely wrong and will not work.
The datasheet for the LM340 regulator shows its current boosted with a PNP power transistor and a current sensing resistor. But the simple circuit needs another transistor added for current limiting and its minimum input voltage is about 10.5V. But an 11.1V battery voltage drops to about 9.6V during a discharge then the regulator will not work.View attachment 41971

 

[HarryA1]

Thanks. I replaced my diagram with the one from Motorola before seeing yours.
I have power transistors coming from my favorite store - ebay.

 

[HarryA1]

I picked up some LM317's today (1.5 amperes max.). Set the output to 7.0 volts and
connected to a brushed motor. The motor drew 0.68 amperes. The output voltage
dropped to 6.86 volts; the li-po battery output drop to 11.55 from 11.80 volts.

This is with no load on the motor; I need to get propeller on it. Someday I will
receive the ESC and can reverse the motor on the fly.  

View attachment 41974

 

[audioguru2]

The LM317 is supposed to have a 120 ohm resistor from its output to its ADJ pin so that its output voltage does not rise when there is no load. Then the value of the pot should also be reduced to half.
The datasheet shows that the more expensive LM117 can use 240 ohms.

The datasheet explains that the voltage regulation is reduced (yours is horrible) when proper wiring is not used (such as using it on a breadboard).

Your Li-Po battery with 3 cells has a voltage of 12.60V when fully charged. Yours was 11.8V so it was not charged much.

 

[Kevin Weddle]

Doesn't he need a good voltage regulator to charge the battery?

 

[audioguru2]

Doesn't he need a good voltage regulator to charge the battery?

Yes. The voltage must never go higher than 12.60V.
An LM317 that has a 120 ohm resistor from its output to its ADJ pin with proper wiring (not a breadboard) is an excellent voltage regulator. When it is set for an output of 12.60V then it can charge a 3-cells Li-Po battery but its current might be too high for the battery. Additional circuitry is needed to detect very low charging current which happens when the battery is fully charged then it must disconnect the charger from the battery.