MCC amplifier
Sometimes you want an MCC output to control more than the standard 20mA a MCCdec-output can deliver. This can easily be done by amplifing the output with a MOSFET.
In the given example we want to control a large amount of LED's. The complete amplifier consists of just 2 components: a resistor and a MOSFET The MOSFET we use here is a BSH103. This is a so-called "logic-level MOSFET", which means that the MOSFET is completely opened even when just an few Volts is applied to the gate. The advantage of a MOSFET over a transistor is that a transistor has a forward voltage of about 0.6V in conductive condition. That is no problem in most circumstances, but when only 2.4V is available, it is a large percentage. A MOSFET behaves in conductive condition as a small resistor. The BSH103 has a maximuml resistance of 0.5 Ohm in conductive state. At 500mA there is only a Voltage loss of 0.25V. Keep as maximum current the BSH103 can stand 500mA. If you want to control more loads, or the total load is too large for 1 single BSH103, you may put more BSH103 on 1 MCCdec output. Because the gate of the MOSFET does not use any current, in theory you can use as many as you like. Problem is, the MOSFET behaves as a capacitor. When you put a lot of them parallel, the switching will go a little slower. We talk milliseconds here, so for most applications it is not a real problem. The resistor of 47..180 kOhm is only there to keep the BSH103 off when the MCCdecoder goes into sleepmode. The decoder-output is then isolated. The value of this resistor is not critical at all.
Calculating the series resistors for the LEDs is a little trickier.
Most yellow, red and green LEDs have an operating voltage between 1.8 and 2.1 V. The exact voltage depends on the current you drive through it and the brand and type. Normal LEDs have a maximum current of 20mA. Low-Current LEDs usually have a maximum current of 2mA.
High Efficiency LEDs usually have a maximum current of 20mA, but then they are extremely bright. Fot this type of LEDs 2 or 3 mA is enough in most cases. CAUTION: this figures general estimates.
Every LED can be different. The only way to be sure, is to read the datasheet of the manufacturer!
If you put LEDs in parallel, you can give each LED an individual series resistor (the most stable solution), or you can give multiple LEDs 1 common resistor. CAUTION: This will work in most cases, if all LEDs are of the same type and manufacturer. We calculate the series resistance first when using 1 resistor for all LEDs together.
Imagine your battery voltage is 2.4 V and the operating voltage of your LEDs is 1.9 V. Then the resistor has to bridge: 2.4V - 1.9V = 0.5V Imagine you put 10 LEDs in parallel, and they all use a current of 5mA. Then they use together a current of 10*5mA = 50mA. The required resistance is V/I = 0.5V / 50mA = 0.5V/0.050A = 10 Ohm. The BSH103 has an internal resistance of about 0.5 Ohm, so 9.5 Ohm remains for the resistor. In this case you can round off at 10 Ohm.
When you give each LED its own resistor, then the current for each LED is 10 times smaller, while the resistor has to brigde the same voltage. So the resistors need to be 10 times larger, in this example 100Ohm. CAUTION: When you use a battery as power supply, the voltage from the battery is not constant. When a NiMH is completely charged, the voltage can raise to 1.4V per cel, 2 cells will give 2.8V. Make sure the resistors are calculated so that the maximum current of the LEDs and the maximum current the BSH103 can handle is not exceeded. Always be on the safe side. In most cases 50% of the maximum current is a very good value for normal practise. Click the image below for a larger version.
