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Author Topic: Newbie - Switching base on bias  (Read 3150 times)

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Offline yosmanorTopic starter

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Newbie - Switching base on bias
« 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). 
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Offline bitshift

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Re: Newbie - Switching base on bias
« Reply #1 on: June 21, 2016, 12:25:44 pm »
You need to use a PNP transistor like this:



R2 is there to tie the base of the transistor high (off) when the base is floating.
« Last Edit: June 21, 2016, 12:43:03 pm by bitshift »
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Offline yosmanorTopic starter

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Re: Newbie - Switching base on bias
« Reply #2 on: June 21, 2016, 12:50:09 pm »
Thanks
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Offline rstofer

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Re: Newbie - Switching base on bias
« Reply #3 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.
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Offline bitshift

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Re: Newbie - Switching base on bias
« Reply #4 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.
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Offline yosmanorTopic starter

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Re: Newbie - Switching base on bias
« Reply #5 on: June 21, 2016, 01:16:02 pm »
Thanks guys,
The current will be 2A or less.
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Offline Helen95

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Re: Newbie - Switching base on bias
« Reply #6 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?
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Offline bitshift

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Re: Newbie - Switching base on bias
« Reply #7 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.
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Offline StillTrying

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Re: Newbie - Switching base on bias
« Reply #8 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.
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.  That took much longer than I thought it would.
 

Offline yosmanorTopic starter

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Re: Newbie - Switching base on bias
« Reply #9 on: June 26, 2016, 01:52:36 pm »
yosmanor and Helen95 are not the same person
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Offline Audioguru

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Re: Newbie - Switching base on bias
« Reply #10 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.
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