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R Value for Base of NPN 2N3904

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Turbo Boss

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I am using a NPN transistor 2N3904 to triguer a small relay

This is what I have: Emiter is conected to ground, Collector is the negative output that will triguer the relay and what I need to find is a value of R that I can use on the Base in order that it will work "FROM" +3V to +12V

At the present time I am using a R of 1K and its work perfectly with +12V but when the voltage decay to +3V it does not work.

I know I have to lower the R but my concern is if I lower it to much in order to work with 3V maybe when ir receive 12V its going to blow the transistor.

What is your advise? What R value you recomend me?

THX,

Malibu
 

Roff

Well-Known Member
Are you trying to get this relay to work when it is connected to 3v-12v, and the base drive is 3v-12v, or is the relay connected to a constant voltage?
 

Turbo Boss

New Member
Ron H said:
Are you trying to get this relay to work when it is connected to 3v-12v, and the base drive is 3v-12v, or is the relay connected to a constant voltage?
The relay has a contant +12V at one end of the coil the other end of the coil is the one I want to triguer with 0V that the Collector will output. (NPN 2N3904 Transistor )

What I need is to triguer the transistor base with a voltage from +3V to +12V so I need to put a R to limit the current to the Base of the Transistor in order to not blow the transistor. What R value I can use????

The factory system that I am interfasing is working with +3.6V and +12V thats why I need to triguer the base of the transistor with those voltages.

THX for your help,
 

Roff

Well-Known Member
Use 3.3k for your base drive resistor. It will work with 3v or 12v drive, and won't damage the 2N3904. If the 12v source can't drive the 3.5ma load, that's another issue.
 

Roff

Well-Known Member
Boss, that page asks me, in Spanish, for a login. I have long forgotten my Spanish.
 

Turbo Boss

New Member
At the present time I am using a 1K 1/4 watt Resistor and Its working OK with 12V but NOT with 3V. So thats why I dont think is going to work with a 3.3K

I call the electronic shop and they guy sugest me to use a R of 100ohm thats why I ask you about the 100 ohm..


What you think???

What is the lowest R I can use there?????

THX
 

Roff

Well-Known Member
You said the relay was 12v, 200mw. 200mw/12v=16.7ma. That should require less than a milliamp of base drive. Either your 3v source impedance is really high, or your transistor is bad. What sort of device does the 3v logic level come from?
I think that 100ma of base drive for a 2N3904 is too much.

You didn't mean 200ma for the coil current, did you? That seems like too much for a mini relay.
 

Turbo Boss

New Member
The relay that I saw inside the module I want to interface is very small and I pretty sure I saw a print on top of the relay that say 200 ohm.
Maybe I am wrong and it was a part numbert. :oops:

Ok, let do the following. Forget about the relay. lets asume I want to drive the base of the 2N3904 with 3 to 12V. and the emitter is to ground.

What is the lower resistance I can use?

Some body recomend me to do this:
 

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nettron1000

New Member
TBoss, what you're trying to design is called a transistor switch, in which the transistor is operated between saturation and cut off. This is usually done using known fixed voltages, but it appears you've thrown a curve ball with a circuit design that requires a variable base drive voltage.

What you have to do is design for the lowest drive voltage which will allow for saturation and at the same time protect the transistor from accessive base current ( when the input rises to 12 volts )which could destroy the transistor. The best way i can see to do this is to connect a zener diode between the base and ground, so the zener will be acting as a voltage regulator.

A suggestion :

Use a 450 ohm resistor in series with the base, connect a 4.7K resistor from the base to ground and put a 5 volt zener in parallel with the 4.7K
 

Roff

Well-Known Member
Turbo Boss, you led me astray with your "200mw" statement. Now you say it's 200 ohms. If you expect good advice, you must be careful to answer questions accurately. A 200 ohm load will draw 60ma, so we should probably have at least 3ma of base drive (conventional wisdom would require 6ma, but we'll fudge a little). So, for 3v drive, we'll have about 2.3v across the resistor (the base-emitter voltage is about 0.7v). The base resistor will then need to be 2.3/3ma=767 ohms. We can use 750 ohms, but now we will get about 15ma of base drive with a 12v input. This may be too much drive for a 2N3904, but worse, it's probably too much current to draw from your 12v driver. If not, use 750 ohms.
Below is a circuit which will provide 3ma of base drive for any input from 3v to 12v. It is complex, but it only draws about 3.3ma from the driver over the input range.

Nettron, your zener idea won't work. The base voltage will never get above around 1v, because the base-emitter is a diode to ground. You must be thinking of a MOSFET. In fact, a MOSFET might be the best solution if there is one available with low on resistance at 3v base drive.
I've been assuming Turbo Boss wants to use the 2N3904.
 

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nettron1000

New Member
Nettron, your zener idea won't work. The base voltage will never get above around 1v, because the base-emitter is a diode to ground. You must be thinking of a MOSFET. In fact, a MOSFET might be the best solution if there is one available with low on resistance at 3v base drive.
I've been assuming Turbo Boss wants to use the 2N3904.
Heh, i guess after seeing your schematic he may decide to go with the MOSFET anyway.

what i was thinking with the zener idea was to have a voltage controlled current source, i guess my explaination was abit lame. I know these are usually used in voltage regulation circuits but i was thinking it might work here.
Heres a schemy to make it alittle clearer, not sure if it will work tho, i dont have the zener to try it out and theres insufficient info about how much input drive current there is.







After some thought i came up with this....




A zener controlled voltage source.
 

Roff

Well-Known Member
That's not a current source. The current from the emitter of Q1 is limited only by the 10k base resistor and the beta of Q1 and the output resistance of the 12v driving source. I ran a sim and got over 60ma of Q2 base current for 12v drive, and 18 ma for 3v drive. This is with zero output resistance for the source, which, of course, may be higher - but it could be less than an ohm. We don't know.
I think you're doing a disservice to the OP when you post a schematic that could destroy his parts.

My circuit is a current source, but, as you pointed out, is relatively complex. If Turbo Boss only needs one, that's probably not a problem. If he needs 100, it probably is. I did some searching and found some SMD MOSFETs that would work. This would be the cleanest solution.
 

nettron1000

New Member
That's not a current source. The current from the emitter of Q1 is limited only by the 10k base resistor and the beta of Q1 and the output resistance of the 12v driving source.
Ack! yes thats a voltage source not a current source, noted and fixed. I knew i should have omitted the value for the base resistor (10K), i thought you might concentrate on that, its was meant as a design example. Heres where i got the idea:

http://niufrm.physics.niu.edu/~labelec/lab/p375_lab07.pdf


I think you're doing a disservice to the OP when you post a schematic that could destroy his parts.
I agree and i think it would be a disservice of OP himself to not consider a simpler design approach by sticking with bjt's. As i said in earlier posts above, the circuits have not bin tested it was just a design idea to consider. But if you think the design ideas wont work, and according your sim it doesnt (BTW thanx), then thats fine.
Ive seen zeners used in transistor current sources before but ive never had the opportunity to use one in a circuit, so i dont have much experience there.

I dont have zeners on-hand with the proper voltage to prototype a circuit, so i did a search to find some examples that explain the idea in more detail. Heres one such find:

http://www.dwelle.de/rtc/infotheque/semiconamps/semiconductor_amps5.html

Maybe theres something im overlooking but would a constant current source designed around that idea work ? This is mainly for my own edifying.


P.S. what zener voltage did you use in your sim ?
 

Turbo Boss

New Member
I did some searching and found some SMD MOSFETs that would work. This would be the cleanest solution.
Sorry if I dont reply earlier but as was bussy with customers. People are buying for christmas season :lol:

I want to thank you for your help. Its there is a web were I can found more info about MOSFETs? Like some projects usijng MOSFET?

Can you post a diagram using MOSFETs for my situation???

Also, I have the Multisim 2001 what program you have???

THX againg for your help.
 
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