Electronics

[darkstar07]

How can i increase my current from microamps to milliamps ? Can i use a transistor ? If so , how much voltage will i require to power it and what are the devices that can be powered with microamps (like a sensor)?
Grateful for your help,
Darkstar07

[rmarz]

darkstar07 - There are many ways and many devices that can be used to achieve current (and voltage) gain. Inexpensive operational amplifiers (Op Amps), bipolar and mosfet transistors will all function in the ranges of gain you are looking for. A current gain of 1,000 (microamps to milliamps) is achievable with just a few components. I suggest you do some simple research on current amplification and transistors to provide a lot of basic background.

Transistors can be biased to operate at very low input voltages and currents. If you can share what your environment is (input current or voltage) and what you are trying to do with the amplified signal, (drive an LED, power a sounding device, a motor etc.,) perhaps we can help a little more.

A dollars worth of components (transistor plus a few resistors) and a small battery is all you may need. Access to a sensitive multimeter will help a lot. Good luck.

Rick Marz

[darkstar07]

rmarz - thanks for your help. My aim here is to recharge a battery and then use the battery to power an l.e.d.. For eg: my source can supply 1.67 - 2.3 volts and my AA rechargeable battery has a voltage of 0.02 ( drained one). However the battery produces much more current than my source. When i hooked up the battery to my source , at the end of 4 hrs. , my battery read 0.21 v. My battery has a capacity of 1.2 v. However , if i am to charge it fully , it would take at least 19 hours. I hope you can devise a method where i can boost my current and so that the rate at which the battery charges becomes faster. Then , I can charge a battery and use it for other purposes. However , without charging a battery , my source can make the l.e.d glow faintly. Can i read the current in a circuit directly with a multimeter or should i connect it to resistor to find the current produced ?

Grateful for more of your valuable and appreciable advice,
darkstar07

[rmarz]

darkstar07 - It sounds like your power source is producing a low output current. It would be good to know what the internal resistance of the source is, and it's ability to supply power to any load. To do that, connect a 1000 ohm resistor to the source and using your multimeter, read the voltage across the resistor and voltage across the output of your source. You can then either insert the multimeter in series with the resistor (so you are not making a 'short circuit reading} and measure the current directly, or calculate the current by using the formula, [current (in amperes) squared, times the resistance in ohms]. That current multiplied times the voltage across the resistor will give you the power dissipated in the resistor. That will give you and idea of the power, in milliwatts, that your source is capable of supporting.

From your description you are using power from the source to try to recharge a rechargeable (nickel cadmium or nickel-metal hydride AA cell) and use that as a power source for the LED. Originally, you were asking about using a transistor to provide current gain. This won't work the way you envisioned because the amplification requires an external source of power. You are simply trying to transform the power to be suitable to drive the LED.

LED's will require a few milliamps of current to be barely visible. Most require about 15-20 milliamps (mA) to be brightly visible. They also require sufficient biasing voltage to conduct and produce any visible light. This forward voltage (Vf) could be in the range of 0.8 to 1.8 volts. I think what is happening is that your source is incapable of supplying the power, both voltage and current, required to light the LED. Your experiment appears to be charging the battery, but at a slow rate. You will never get to a point where you can light the LED continuously with this method. You can spend many hours charging the battery, but then it might only be capable of lighting the LED for far less than an hour.

To get a faster method of charging/discharging, you might consider the use of a small 'super capacitor' in the range of 1 farad or so. These are often used as short term battery back-ups in many systems. I don't know if these are available at a local Radio Shack or not.

This could be an interesting experiment demonstrating the need to convert or transform power from one source to power a load requiring a different voltage/current level using an intermediate storage source, either an electro-chemical battery or a capacitor as a storage device. Good luck.

Rick Marz