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Author Topic: Need help using NPN transistor with Relay  (Read 4760 times)

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Offline jsmokerTopic starter

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Need help using NPN transistor with Relay
« on: April 05, 2007, 12:25:44 PM »
I found a couple articles online about using transistors to run relays off the unregulated power source.  But, when I try it out by placing an 7.4V battery across the collector to the emitter and trying to "activate" it through a resistor with a DC voltage supply on the base, the voltage supply reads 4V even with the voltage knob turned down to 0V.  Trying to adjust the voltage higher increases the voltage, but doesn't activate the relay.  If I don't use a resistor, the relay activates no matter what, even with the voltage knob turned all the way down.   Anyone have experience with this?

-Jason

Offline Afroman

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Re: Need help using NPN transistor with Relay
« Reply #1 on: April 05, 2007, 01:24:21 PM »


Make sure you have the diode in there?

Offline jsmokerTopic starter

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Re: Need help using NPN transistor with Relay
« Reply #2 on: April 05, 2007, 02:29:03 PM »
Yeah, diode's in there.  The voltage (which I dont' think belongs) is actually at the Base.  So it's not like the current is reversing in the collector, just forcing the base high, which I worry about if I plug it into a PIC chip or something.

Offline jsmokerTopic starter

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Re: Need help using NPN transistor with Relay
« Reply #3 on: April 06, 2007, 12:31:32 PM »
I don't what the deal was, but I used a 2N4401 before and it didn't work.  I just tried using a 2N3904 and it worked perfectly fine.  I hate when things like this happen, what a waste of time! >:(

Offline Admin

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Re: Need help using NPN transistor with Relay
« Reply #4 on: April 06, 2007, 04:15:03 PM »
any reason you are using a basic transistor instead of a mosfet?

mosfets are infinitely better for this kind of situation . . . :P

not sure what power/voltage you are dealing with, but in many cases you can just use the mosfet without the relay, too.

id give you some mosfets if i wasnt in the rep of czech right now . . . :P

Offline jsmokerTopic starter

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Re: Need help using NPN transistor with Relay
« Reply #5 on: April 09, 2007, 08:30:54 AM »
1)I've never used mosfets before in my life, and I'm still not sure if I have a handle on them.
2)I had 12 transistors ready in my posession, 4 sets of 3(now 9 cause I burnt out one set trying to get it two work, then the first of the next set worked right away without a glitch.

Offline sotu

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Re: Need help using NPN transistor with Relay
« Reply #6 on: April 09, 2007, 01:14:27 PM »
« Last Edit: April 09, 2007, 01:15:33 PM by sotu »
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Offline sotu

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Re: Need help using NPN transistor with Relay
« Reply #7 on: April 09, 2007, 01:16:28 PM »
How do u post an image? If i press the image botton i cant upload my picture.. ???
How to build a biped bot:

Offline ed1380

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Re: Need help using NPN transistor with Relay
« Reply #8 on: April 09, 2007, 03:10:28 PM »
link to imageshack, photobucket, etc 
click additional options under where you type up your posts  ;D
                                                                           ^
                                                                      anoying
« Last Edit: April 09, 2007, 03:12:12 PM by ed1380 »
Problems making the $50 robot circuit board?
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Offline sotu

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Re: Need help using NPN transistor with Relay
« Reply #9 on: April 10, 2007, 01:07:44 AM »
Ahh i found it..thanks ed ;)
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