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jfet strangness NEED HELP Answered

I just got some npn jfet transistors in the mail and I'm having problems. My setup is a +5.5 volt, gnd, -5.5 volt powersupply. I hooked up the LED to a resistor and going through the drain and source to the +5.5 volt and gnd so the LED lights up. Whenever I apply the negative voltage to the gate the transistor should turn off, right? well whenever I do nothing happends. When I touch the gate with my finger the LED turns on (no current) so the LED turns off. Sometimes when I touch it again it turns off (LED on). sometimes it just turns off when I touch it. When I touch a grounded (real ground, for like anti static stuff) aligator clip it does the same as my body. When I touch the -5.5 volt with one hand and touch the gate with the other nothing happends. Sometimes by how far away my finger to the gate the LED goes up and down in brightness. My questions is how can I turn the transistor on (so that the LED turns off) when I apply a voltage to the gate?

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T3h_Muffinator (author)2007-11-10

If you don't mind me asking, what is a jfet? (how's it different from mosfet?)

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a jfet is a junction field effect transistor. A mosfet is a metal-oxide field effect transistor. They are almost exactly the same but I think mosfets are more prone to static shock. mosfets are actually better though if you ignore that static fact because they have larger gate impedence, 10 to the 14 power ohms compared to the 10 to the 9th power for jfets

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westfw (author)guyfrom7up2007-11-10

a "Field effect transistor" uses the electric field from a signal on the gate to control a (bigger) signal from the source to drain. In a MOSFET, there's a layer of silicon dioxide (or other insulator) between the gate electrode and the reset of the device, so the gate behaves like a capacitor (and conducts zero "static" current), and you have a pretty "pure" electric field. In a JFET, the gate is essentially a reverse-biased semiconductor junction. A practical difference is that JFETs only operate in "depletion mode" - a higher voltage on the gate REDUCES the current through the source. A MOSFET can operate in either depletion mode, or "enhancement" mode, which a higher voltage results in a higher current. FETs are simpler than bipolar transistors from a solid state physics point of view, and they behave a lot like vacuum tubes, but bipolar transistors tend to be easier to actually use (and less fragile.) On the other hand, almost all the transistors in most modern integrated circuits are MOSFETs...

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VIRON (author)2007-11-09

Suggestions: Connect a 10Kohm or more rheostat (pot) to +5, -5, and the slider to the gate. Make sure your GND is a center tap (0 volts) and not the same as -5.5V or worse not connected to anything at all. Try putting another LED on the negative side of the transistor. Try turning the pot to see if it will turn both the LEDs on and off at the same time as it should.

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guyfrom7up (author)VIRON2007-11-10

are you suggesting i make a voltage divider?

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VIRON (author)guyfrom7up2007-11-10

Technically the pot is a variable-voltage-divider, but I didn't think of it in those terms. 2 LEDs on either side of the transistor (one drain,one source) will have voltage drops. JFET's usually switch at a voltage outside of the voltages between drain and source, so the pot may help you find the voltage between JFET on and off, and it's Probably going to be within the voltage drop of either LED. I'm not sure which one. If it doesn't switch on/off at any pot position between + and - 5 volts with 2 LEDs then it's probably bad, BUT the "magic touch switch effect" means it's good.

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guyfrom7up (author)VIRON2007-11-10

it works perectly now with 2 LEDs now, and I need no resistor since the transistor is one. thanks!

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guyfrom7up (author)2007-11-10

all of a sudden it works normal now! How wierd, nothings different than before. Thanks everyone!

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Goodhart (author)2007-11-10

Using an N-channel jfet as a current regulator: Current regulator link

As a Source follower much like Viron's suggestion, or This link to some info on use with an LED array (btw: I like Elelctronic Design News as a source of info)

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westfw (author)2007-11-10

While I can't immediately say why you're having problems, you do realize that JFET style transistors aren't usually used in "switching" applications like this; they're aimed more at signal amplification and such... (it LOOKS like the specs are ok to handle an LED, so it OUGHT to work; it's just an uncommon situation.)

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guyfrom7up (author)2007-11-09

oh, and if anyone wants to know I'm using the 1n4859, it has a metal can. I can't figure out which lead is the source and which is the drain, but that shouldn't matter should it?

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Goodhart (author)guyfrom7up2007-11-09

Having a little trouble sleeping....just thought of something...and as far as I remember, it shouldn't matter (source/drain) but here is a pinout for it anyways:

2N4859 pinout in TO-18 metal can

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guyfrom7up (author)Goodhart2007-11-09

yeah, that what I thought, but these jfets were made crappy (they should still work though) and the tabs arn't correctly aligned so you have to manually test to find out which one is a gate. I hate it when I can't fall asleep cause I was just on instructables for a while and i can't stop thinking, lol.

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guyfrom7up (author)2007-11-09
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guyfrom7up (author)2007-11-09

comone goodhart, I know you know this stuff, and we both live in PA, maybe even in the same town!

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Goodhart (author)guyfrom7up2007-11-09

I do have to get some shut eye tonight, but I do want to look into it further... I will try to see if I can come up with anything, in case no one else does. Where is Las Vegas when you need him LOL

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Goodhart (author)guyfrom7up2007-11-09

Er, I am not AS familiar with FET's as you might think. I only was a hobbiest with electronics....although I learned a lot in a short period of time, I didn't really fool around with these guys much (some of my questions were kind of delay tactics as I looked things up, sorry) and it has been a long time since I did any of this stuff. I am doing the best I can at the moment. If anyone else can jump in, while I am bustling around, feel free...

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Goodhart (author)Goodhart2007-11-09

I may have to leave off looking for the night. I am sorry, I didn't mean to get your hopes up and than find out I was not as familiar as I though with these devices.

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guyfrom7up (author)Goodhart2007-11-09

it's alright, thanks for your input though. it's just that I'm frusterated that I can't figure out what I'm doing wrong because when I was ordering online it was either spending $1.50 on transistors or $10.00 on relays and now I"m having trouble, I'm going to be doing more resaeach... help anyone else?

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guyfrom7up (author)2007-11-09

well, I was never the one for reading transistor data sheets really well, but here's the data sheet from the transistor that I'm using (I got it surplus, so it's older)

http://pdf1.alldatasheet.com/datasheet-pdf/view/78479/MOTOROLA/2N4859A.html

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Goodhart (author)2007-11-09
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Goodhart (author)2007-11-09

Is it a p-Channel or n-Channel J-FET ?

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guyfrom7up (author)Goodhart2007-11-09

I'm sorry, when I said npn I meant n channel.

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