If you have the current but not the voltage to drive your dc motor you might be disappointed. You might have the ability (in your battery) to supply 1 amp at 3 volts but, to power a 1 amp motor with 9 volts means your battery will need to supply maybe 3.5 amps into the boost regulator.
Power in equals power out plus losses and to obtain 9 volts at 1 amp requires a power in of 9 watts plus about 15% extra for the boost converter losses. That takes the input current up to about the 3.5 amp level. Can your battery sustain this level I wonder?
My objective is to create a device that will generate a constant
current to a load that varies in resistance from approximately
1Kohm-40Kohm
OK, so far so good.
For the sake of this question, the current I am trying to generate is
5mA.
5 mA through a 1 kohm load means the circuit has to supply 5 volts to the load. 5 mA through 40 kohm means 200 volts applied to the load.
I've done a bit of reading over the last few days and the concept of a
boost converter in its simplest form is rather straightforward.
Here's where the first hurdle is arising because a boost converter can only produce an output voltage that is greater than the input supply voltage (you specified 12 volts). If your supply is 12 volts and you want a current of 5 mA through a load that can vary between 1 kohm and 40 kohm then you need a buck-boost converter; a booster won't do what you want.
You could use a buck regulator to set a voltage at (say) 4 volts then use a booster to deliver 5 volts to 200 volts but that is pushing technology too far. I would strongly consider generating (say) 210 volts using a "booster" then using some form of linear regulation to deliver the required voltage for the correct current to the load.
Any advice or tips would be greatly appreciated. I am looking for
similar ways to achieve a constant current given different loads.
The type of "booster" I would use would be a step-up flyback converter like either of these two: -
So, you get a high voltage regulated DC supply and use a linear current regulator to control the current to the load. If you generate 210 volts and you need to supply 5 mA the worst case power dissipation is going to be just over 1 watt into a short circuit so you will need a small heatsink.
If you can allow your load to be floating then a fairly simple constant current sink will do what you want providing you choose an output transistor that is rated at overs 210 volts (300 volts recommended): -
Applications in which a microprocessor can be used are helpful because
I can choose different output currents with ease (say, 6mA instead of
5mA)
If you want it microprocessor controlled then you could generate a PWM output and use a low pass filter to create an analogue voltage that feeds Vset.
Best Answer
Why not add a low dropout 12 volt voltage regulator instead of loading the output with resistors. It's likely to produce less heat. This will only work for very light loads like the one suggested and might keep spurious switching noise (from the boost converter) from interfering with the RF circuits and get better performance.