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Newb Question: 12v-80ma transistor switch


Chris-Wilson
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Ok, I know this is a newb question, but i have tried everything. So, I have a 12 volt, 80 milliamp relay coil connected to the emmiter of a transistor (TIP3055), the base input voltage of .7 volts, and the collector leading to ground. To protect the transistor, I have a diode (Rectifier: diode [2.5 amps 1,000v]) between the resistor coil. Previously, with a different transistor (one I scrapped from an RC Car) it just locked in the on state, where as with the current one mentioned above, it doesn't switch at all. ??? Someone please enlighten me! I altered the schematic, replacing the 5 volt relay with a 12 volt from "http://www.rentron.com/Files/TWSAKIT.pdf"

Thanks.

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Yes, the circuit w/ the 5 volt power supply is the one I'm refering too. Audioguru, the circuit you've posted is exactly what I need, only NPN. I assume I can substitute an NPN w/ the same ratings for it. I have gone through several transistors, as long as the 2n4403 can handle the 80ma and 12v, I will be set. Thnaks!

*To clue anyone in who is interested, this is for automating the openning of a locker. 
*Mechanics, now that's something I can deal with. ;D

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Just to be on the safe side, what NPN would you recommend. Everyone I have chosen in the past doesn't work properly.

The circuit is designed for a PNP transistor. It needs to be changed into the other circuit for an NPN transistor. Then the value of the base resistor must be selected to match the current gain of the transistor.

Oh, and the base input to the transistor is .7 volts.

The input to the base resistor must be 0V and the supply voltage.

If you are refering to the supply voltage, why would 5v keep it latched on? Just curious.

A 12V relay needs about 10V to turn on because the armature is at a distance from the electromagnet. But it needs only about 2V to turn off because the armature is touching the electromagnet.
Since the PNP transistor is an emitter-follower then when the base is at +5V then the 12V relay has 6.3V across it and won't turn off.
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Forgot to ask, how do I match current gain w/ a transistor. There are a series of numbers w/ different ratings for current and voltage. The chart only goes to two volts. When i find the right current gain, how do I match it? Won't a resistor drop the base voltage below the triggering tvoltage for a transistor. I only have .5-.7 going out from the receiver circuit which triggers it.

Thanks

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I don't have the time right now to go into everything you need to know but here's what I can tell you. Transistors are current operated not voltage. A NPN transistor needs a positive current to turn on and the load goes between + Supply and the collector of the transistor. The base is where you put the control signal and the emitter goes to ground. A PNP transistor turns off with a positive voltage and must have the base pin grounded to "Turn On". With NPN and PNP the emitter and collector pins are reversed (Kinda). On a PNP transistor the + supply goes to the emitter and the load goes between the collector and ground.

Edit: This may help you to remember
The middle letter tells you whether the base needs a "N"egative or "P"ositive
nPn= Positive base current turns this on
pNp=Negative (GND) turns this on

Also to know whether the transistor you selected will work for your application you must consult that datasheet for the device and find out it's power ratings.
In your case 12V*.08A=0.96Watts. You will need something more substantial than a 2N4403. Rule one of electronics is don't buy from radioshack (this assumes that they actually have the parts you're looking for - not likely). When looking through a website or catalog they will normally clearly show the transistors polarity and it's power ratings. Choose appropriately.

Note: This is not a Inventive/New Idea. This thread will probably be moved.

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In your case 12V*.08A=0.96Watts. You will need something more substantial than a 2N4403.

No, the 2N4403 is fine. The load dissipates 0.96W, not the transistor. The transistor dissipates a max of only 240mW for a moment when it has switched the relay halfway. It dissipates about only 117mW when driving the relay continuously as an emitter-follower and much less when saturated as a common-emitter device. It is rated for a max continuous dissipation of 625mW with a 25 degrees C ambient.
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Forgot to ask, how do I match current gain w/ a transistor. There are a series of numbers w/ different ratings for current and voltage. The chart only goes to two volts. When i find the right current gain, how do I match it? Won't a resistor drop the base voltage below the triggering tvoltage for a transistor. I only have .5-.7 going out from the receiver circuit which triggers it.

A transistor is turned on with base current through the base resistor. With an 80mA load, a 2N4403 PNP or 2N4401 NPN transistor has a min current gain of 100, but even more base current is needed to saturate it, so 11.3V across the 10k base resistor results in a base current of 1.13mA which is enough.

The circuits shown won't work with an input voltage of only 0.7V. The emitter-follower transistor in the PNP schematic needs the full supply voltage as an input. For the NPN transistor circuit, if the input voltage is 1.7V then the base resistor's value should be reduced to 820 ohms to provide 1.2mA to the transistor's base to turn it on.

The 0.5V to 0.7V output of your receiver seems to be extremely low. Maybe it already is limited by a series resistance or voltage divider.
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Use caution when talking about positive and negative voltages in regards to transistor biasing... use "potentials" so that it applies in all cases.


see: http://www.electronics-lab.com/forum/index.php?topic=7097.msg43806;topicseen#msg43806



hehehe.... here we go again  ;D

MP
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