Transistor how much current is needed to activate them?

supak111

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Apr 29, 2012
340
I'm trying to build something using a transistor but can't seem to find how much current or voltage is required at BASE or GATE to activate each transistor or mosfet?

Can anyone explain to me what I should be looking for when looking at the data sheet of a mosfet or a transistor?

duke37

Jan 9, 2011
5,364
Junction transistor
A junction transistor needs an input current to operate with a voltage of 0.6V relative to the emitter. The transistor will have a current gain (Hfe) of perhaps 100 so you can get 100 times the input current through the collector.

Mosfet
A mosfet needs a voltage on its gate (but negligible current) to operate. A threshold voltage is specified where the fet begins to conduct. For a power fet this is typically 3 or 4V. If you raise the gate to 12V or so, the transistor will be hard on and will pass lots of current so is ideal to act as a switch.

The data sheets will give the limitations in terms of maximum voltage, current and power dissipation, keep well below these figures.

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supak111

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Apr 29, 2012
340
Ok yea I was able to figure out the transistors base current. If I have less then .6v how can I still use a transistor as a switch?

(I'm trying to use 2 sound wires coming out of a small mp3 player to activate the transistor)

Harald Kapp

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Nov 17, 2011
12,720
Less than 0.6 V is not enough neither for a junction FET nor for a MOSFET. You will need an additional power source (battery). With that you can build an amplifier to bring the signal up to a suitable level.
Alternatively you can use a small transformer to bring up the output voltage of teh MP3 player to a voltage sufficient enough to drive a MOSFET (you will most probably not have enough current for a junction or bipolar transistor). The voltage at the output of the transfomer must be at leat as high as the treshold voltage of teh MOSFET. Using a logic level FET this is approximately 3 V. You need to rectify the AC voltage from the transformer which adds another 0.6 V (single diode, no bridge rectifier). A capacitor to steady the gate voltage plus a resistor to shut down the MOSFET if no signal is available complete the circuit.
Pins 2 and 3 of the transformer go to the MP3 player.
You'd have to find out the component values depending on your application.

Harald

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supak111

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Apr 29, 2012
340
Awesome. I mainly needed to know if the voltage is less the .6v if it was possible without external power.

My mp3 player already uses I think 3.7v battery, couldn't I just use that?

What would be the simplest way to make the tiny amp? Nothing fancy just needs to turn the switch ON with sound, OFF without sound.

Sound is AC, transistor BASE needs DC right? So a diode would fix that

BobK

Jan 5, 2010
7,682
Why are you ignoring Harald's solution that does not need power when that is what you originally wanted?

Bob

supak111

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Apr 29, 2012
340
Why are you ignoring Harald's solution that does not need power when that is what you originally wanted?

Bob

One using the transformer? Because seems to me the transformers are pricey and this needs to be a cheap easy device (newbie here btw so I could be wrong)??

duke37

Jan 9, 2011
5,364
Comment on Harald's circuit.

I think that the fet is shown as P channel, I think it should be N channel.
A Schottky rectifier (e.g. BAT43) will have a lower voltage drop than the standard 1N4148.

A transformer from an early transistor radio set would do the job. Find a repairer or someone at your local radio club and beg one!

supak111

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Apr 29, 2012
340
Yea I just found some cheap ones, for some reason I just though they weren't cheap.

Are transformers always same ratio? Say I get one that has 12vac on primary, secondary is 120vac so 1to10, so if I hook up 1vac I will get 10vac on the other side?

BobK

Jan 5, 2010
7,682
The one I linked to is smaller and cheaper than a power transformer would be. It is the type of transformer used in old transistor radios whcih duke suggested. Are you in the U.S.? If so, there must be a radio shack nearby.

Bob

supak111

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Apr 29, 2012
340
Yes I'm in USA and I will pick it up today. It has 5 wires tho, I get where 4 go but what about the 5th one?

BobK

Jan 5, 2010
7,682
The 5'th is the center tap on the primary (which will be the secondary for you). You want to connect the side with 2 wires to the audio and use the other side as the output. The 3 wires gives you two choices for the output. Choose the one that gives you enough output from Harald's circuit but not too much.

Bob

supak111

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Apr 29, 2012
340
Oh ok I understand the 5th wire now. I just measured the power coming out of the mp3 player. I only get .08volt tops DC (I used a diode to turn ac sound into DC) wondering if even this transformer will be able to bump that up to 3+ volts needed here

supak111

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Apr 29, 2012
340
Just an update I was able to make it work by accident and super cheap and simple.

Not sure if its going to mess up my mp3 player tho. AC sound was only giving me .07v max so not enough for a transistor switch, I accidentally measured Volts between devices main ground (like the metal shell) and one of the speaker wires and it turns out when the speaker wires have sound there is 1 Volt of power between both of them and the main ground, plenty to turn on a transistor and use the transistor as a switch.

Now you guys the experts, WHAT DID I DO? And will shorting out one of the speaker wires to main ground hurt my mp3 player?

I don't care if its distorting the sound, like I said I only need the sound to turn the switch on, I'm not using the sound for anything else.

Harald Kapp

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Moderator
Nov 17, 2011
12,720
@duke: of course you're right. An N-channel type is required. Some sloppiness on my side.

Harald

Harald Kapp

Moderator
Moderator
Nov 17, 2011
12,720
Remove the diode at the mp3 player's output. You need to power the transformer by AC. The diode has a voltage drop of 0.6 V - 0.7 V. You need that power, don't dissipate it.

Whether shorting the mp3's output will do any harm to your player depends on the circuit in the player. If you're lucky the mp3's output is protected, but nobody will be able to tell without knowledge if the amplifier's specificatins.

A good idea would be to limit the current through the transistor by a series resistor in the wire leading to the base of the transistor.

Harald

supak111

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Apr 29, 2012
340
Not using the transformer, just transistor,

Collector to player main ground and to whatever I wanna switch ON
Base connected to one of the sound wires
Emitter just connected to whatever I was trying to switch ON

Works. Yea I was thinking about adding a Base resistor but the current there is already very low, Transistor actually takes a little while to turn ON, as soon as the sound is off switch actually turns right OFF.

QUESTION: I am using a c9013 transistor I had sitting around, does this sound like a good choice or do you guys recommend I use something different that could perhaps have a faster ON?

PS I measure about 6ohms between the C and E when the transistor at its strongest ON.

Harald Kapp

Moderator
Moderator
Nov 17, 2011
12,720
No, you should wire the transistor in a different configuration:
Emitter = GND
Base = audio out of the MP3 player
Collector = to switched item, switch operates between emitter and collector.

The C9013 is an NPN transistor. The way you wired it (collector to GND) you use it in reverse operation. In that mode of operation the gain is rather small which may be a reason it turns on so slowly. If another transistor is more suitable depends on what exactly you are going to switch. I don't know this exact type of transistor but it seems to be a reasonable general purpose switching transistor.

Harald

supak111

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Apr 29, 2012
340
Oh I see. I just need to turn the transistor 180 degrees and re-solder it

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