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what resistor to use for transistor base? Doing a touch circuit with 2 transistors Answered

Greetings all,

I am doing a touch switch which will be activated when the touch plate is touch, however i do not know why is the resistor used is 100k ohms. As i will be using another surface mounted transistor, how do i know what value of resistor to use?

i have found a transistor base resistor calculator online: http://kaizerpowerelectronics.dk/calculators/transistor-base-resistor-calculator/

i will be using the FZT651TA, NPN Bipolar Transistor for the circuit.

Datasheet:  http://docs-asia.electrocomponents.com/webdocs/0026/0900766b800261a4.pdf

Can somebody tell me what resistor to use for this transistor?

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9 Replies

iceng (author)2013-09-08

Ok, I like a single TO-92 MOSFET...
The zener leaks a micro current keeping the gate low and
zener voltage can be anything from 8Vz to 15Vz ..

verence (author)2013-09-07

The 100k protect the transistors in case there is a shortage across the touch plate. The resistance of a finger will be higher than 100k, so the value is not critical at all.

ashbite1 (author)2013-09-07

Thanks! Will it be possible to use less than 6V too? I'm considering using a surface mounted LED too.

verence (author)2013-09-07

The form factor of the LED (SMT, through-hole, 3mm, 5mm, round, square etc.) doesn't matter at all. The colour does as the forward voltage depends on the colour (red about 1.6V, green/yellow about 2.2V, blue/white 3V and up - read the data sheet). You will need at least 1.4V for the two base-emitter diodes in the transistors plus the forward voltage of the LED plus a bit to drop on the current limiting resistor. So with a red LED you need at least 3V. All the numbers from top of my head, they may not be 100% exact but good enough as a rule of thumb.

ashbite1 (author)2013-09-07

so its actually okay to use around 4-5V batteries if im using the red LED with 1.7V? as i will be using button batteries

verence (author)2013-09-08

Hmm, let's do the math...

You'll need about 1.4V for the two B-E diodes: 4V - 1.4V = 2.6V. (worst case, batteries empty). The Uf (forward voltage) of a red LED is about 1.6V (give or take some ten millivolts), so about 2.6V - 1.6V = 1V has to be dropped at the current limiting resistor. Let's say you want to have a current of at least 20mA through the diode, you would need a resistor of 1V / 20mA = 1V / 0.02A =50Ohm (try 47Ohm).

You could try a low current LED, they need only about 3mA, so the resistor should be around  1V / 0.003A = 330Ohm.

Josehf Murchison (author)2013-09-07

It depends on your base current and your voltage I would go with 10K.

I think the datasheets are better here

http://www.maxim4u.com/

steveastrouk (author)2013-09-07

100K should work fine.

ashbite1 (author)2013-09-07

Thanks!