DC-DC converter with some limitations

[MrMoore]

I need a way to boost the voltage in a circuit in which I have a limiting resistor that cannot be removed, and a limited max current. The input is 5vdc coming from a 9v battery put through a 7805 ic regulator, and the limiting resistor is 120ohms. The purpose of all of this is to make an 18ohm motor (with a start up current described to be around 40mA) run as quickly as possible within the provided parameters. An additional power source can be used to power another circuit, such as a PWM or something of that sort, so long as none of that power makes it to the motor.

Here is an image of the circuit as it stands with no attempt to boost anything:

I have run many tests, and discovered the following:
- Before the start up requirements are met for the motor (before it's running), the current is at its maximum, around 37mA
- After start up requirements are met (when it is running), the current is kicked down to somewhere between 14mA and 16mA. This leads me to believe that the equivalent resistance of the motor as it is running is somewhere in the 270ohm area.

With this information in mind, I scoured the internet looking for different ways to boost my voltage using what was left of my current; keeping in mind that whatever current I lost would be irrelevant, so long as I left enough for the motor to run on, and that I could always use programing to start the boost after the motor was already running. I've tried nearly everything I could find, including but not limited to home made boost converters, switch mode power supply ICs that I had purchased online, charge pumps, different types of transistors, and a few other more abstract ideas that don't have names yet. All I've managed to do so far is heavily frustrate myself. I'm willing to try just about anything that someone else has made work before.

 

[(*steve*)]

DC to DC converters are (at best) constant power devices -- the power (V * I) going in is slightly more than the power (V * I) coming out.

To make the motor go faster, you need more power, so a DC to DC converter isn't going to help.

Placing a large capacitor across the motor may help because it will store power while the motor is not running and release it when you start the motor. In this case it is important to have the capacitor connected between the 120R resistor and ground all the time, and the motor switched separately.

You would want the largest capacitor you can get, with a voltage rating at least at least a little greater than that of the battery.

You would also need to ensure that the motor would not be damaged by attaching it to a low impedance power source of power (that resistor must be there for a reason)