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Electronics => Beginners => Topic started by: yosmanor on June 21, 2016, 11:28:16 am

Title: Newbie - Switching base on bias
Post by: yosmanor on June 21, 2016, 11:28:16 am

I need a switch that will turn on a 5V circuit when the input is zero while the circuit will remain off while the input is 5V.
I have minimal education in electronics. Any suggestion is welcome.
I experimented on the attached circuit without success (naturally). 

Title: Re: Newbie - Switching base on bias
Post by: bitshift on June 21, 2016, 12:25:44 pm

You need to use a PNP transistor like this:

(http://www.rason.org/Projects/transwit/image1.gif)

R2 is there to tie the base of the transistor high (off) when the base is floating.

Title: Re: Newbie - Switching base on bias
Post by: yosmanor on June 21, 2016, 12:50:09 pm

Thanks

Title: Re: Newbie - Switching base on bias
Post by: rstofer on June 21, 2016, 01:02:45 pm

It would be interesting to know how much current is being switched.  If it's tens of milliamps, no problem.  If it's tens of amps, that poor 2N3906 will go up in smoke.

Title: Re: Newbie - Switching base on bias
Post by: bitshift on June 21, 2016, 01:04:57 pm

Absolutely. The 2N3906 is just what was in the first image a quick google search found. Replace with a suitable transistor.

Title: Re: Newbie - Switching base on bias
Post by: yosmanor on June 21, 2016, 01:16:02 pm

Thanks guys,
The current will be 2A or less.

Title: Re: Newbie - Switching base on bias
Post by: Helen95 on June 23, 2016, 12:36:49 pm

Forgive me please for butting in but where is the difference. Or rather when should I switch from 2N3906 to something more powerful?

Title: Re: Newbie - Switching base on bias
Post by: bitshift on June 23, 2016, 02:02:32 pm

Each transistor will have a maximum collector current which will be listed as "Ic" in the transistors datasheet. The most basic reason to get a "larger" transistor would be when the amount of current you need to pass through the transistor approaches the maximum specified in the datasheet.

Title: Re: Newbie - Switching base on bias
Post by: StillTrying on June 26, 2016, 12:05:53 am

 2N3906 is 200mA Max. and that will only be if it's properly turned on/saturated, - so well below 2A.

A 3A is probably needed, and the correct base current calculated.

Random silly question: Are yosmanor and Helen95 both the same person.

Title: Re: Newbie - Switching base on bias
Post by: yosmanor on June 26, 2016, 01:52:36 pm

yosmanor and Helen95 are not the same person

Title: Re: Newbie - Switching base on bias
Post by: Audioguru on June 26, 2016, 03:38:50 pm

In the first post the load is an LED that will go up in smoke if a current-limiting resistor is missing.