2N3904 transistor . . . is this normal?

[zachtheterrible]

I have a transmitter circuit that specifies using a 2n3904 transistor. the transmitter only transmits about 30 feet though. the funny thing is that on the package of the transistor, it says that it will have an approximate hfe of at least 200 . . . when i plug it in to my multimeter, it says that the hfe is only 20 or so. I've tested about 4 of the 2n3904 transistors, and they all say about the same. I know that I am reading it right because i tried a different transistor in my multimeter, and it had an hfe of 800. am I missing something, or should i just try a new transistor? any suggestions?

Oh yeah, one other thing, I tried putting two transistors in the same exact spot so at to double the gain. should this work? cuz it didnt.

 

[Nigel Goodwin]

I have a transmitter circuit that specifies using a 2n3904 transistor. the transmitter only transmits about 30 feet though. the funny thing is that on the package of the transistor, it says that it will have an approximate hfe of at least 200 . . . when i plug it in to my multimeter, it says that the hfe is only 20 or so. I've tested about 4 of the 2n3904 transistors, and they all say about the same. I know that I am reading it right because i tried a different transistor in my multimeter, and it had an hfe of 800. am I missing something, or should i just try a new transistor? any suggestions?

Try connecting the collector and emitter the other way round to your multimeter, if you've got them the wrong way you will get low gain.

Oh yeah, one other thing, I tried putting two transistors in the same exact spot so at to double the gain. should this work? cuz it didnt.

No it shouldn't, and it won't :lol:

 

[zachtheterrible]

I tried connecting the transistors the other way, and it didnt work. Could there just be something weird going on w/ my multimeter? thanx so much :lol:

 

[John Sorensen]

Could there just be something weird going on w/ my multimeter?

It's always possible. The rule in the 'industry' is, "If you have doubts about your meter, throw it away." For us opinionated basement hobbiests, we don't always have that option :cry: . One of my meters, a Triplett 2202, the dial needs a little "wiggle" sometimes...

Why don't you set up a little experiment and calculate the beta?

j.

Just for kicks, I measures the betas of all my 3904s using my Triplett meter, and I got: 158, 178, 195, 195, 169. I wouldn't hang my hat on them, and I usually don't depend on any given beta when I do my designs (but I'm a digital guy, and can get away with all kinds of analog blunders).

 

[zevon8]

Two things come to mind...

1) Maybe your meter supplies a very small signal to the transistor, perhaps only DC, to be on thhe "safe" side with different transistors.

2) Your batch was made first thing Monday morning, or last thing Friday afternoon ( old factory Quality Control joke )

Motorola data sheets list the 2n3904 as having a gain of between 100 and 400 with a 1000 Hz signal, Collector current of 1mA, and Vce of 10 Volts. DC current gain is listed at minimum 70, with no maximum given. A pretty wide range, but is is a general pupose, low cost , device.

 

[zachtheterrible]

So is 2n3904 a good transistor for RF stuff? or is there a better transistor that i could buy @ radioshack?

 

[zevon8]

Radio Shack doesn't really have a big variety of transistors ( according to their website anyways ,) I'm guessing you bought pn 276-1617 which is a bag of general purpose devices. They also list the 2n4401, not a whole lot different, but maybe worth a try. One thing you may be able to do is find some old battery powered radio and "cannibalize" it for parts. If it was made with individual parts, not an IC you could get all kinds of usefull parts. The 2n3904 should work for your circuit, you may just have a batch of low gain devices.

Many repair shops keep a bin of "parts-donor" equipment around just for these occasions.

 

[mozikluv]

3904??

hi,

it could be a case of fake parts, i won't mention what country it usually comes from. these parts does not last long. have been a victim those fakes. :x

 

[John Sorensen]

I heard that Radio Shack parts are factory floor sweepups, and some places won't let their technicians use them.

j.

 

[lavenatti]

I agree with John, I've had better luck scrounging parts from old radios and TV's than with Radio Shack components. Besides, lots of places will offer a 100 2N3904 transistors for $5 sometimes. And you could always check ebay.

 

[zachtheterrible]

Thanx all, ill try and look 4 some other electronics store