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Old 27th November 2010, 04:15 AM   #11
Enzo is offline Enzo  United States
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Unlike a lot of parts, a JFET is ON until you turn it OFF at its gate. SO a JFET just sitting in front of you will usually measure some low resistance between source and drain - I generally look for something like 200 ohms, but it can be a range of values. it isn't the value so much that I watch, it is more looking to see that it isn't open, nor is it shorted. So 50 ohms or 300 ohms or whatever is probably fine.

Then the gate will act like a diode connected to the S-D resistance.

So, if S-D measures low resistance with no gate voltage, that sounds pretty normal. If the gate to either source or drain checks like a diode, than that sounds OK. The part probably works. it may be noisy or leaky, but we can;t tell that without putting it in an active ccircuit.

There is no consensus on lead order. All Fairchild J174s - or whatever - will be the same, but some equivalent may have opposite pin order. In other words the gate could be the left, the center or the right leg, depending on whose part it is.

In fact, before I install a JFET, I ALWAYS take an ohm meter to it to see that the gate is where I think it is. That ohms test is how I identify leads. You will find most times the source and drain are interchangable. Some data sheets even say that. I am sure there are specific parts and applications where it matters, but many places it does not.

A simple test circuit could be set up in minutes on your bench. Wire it to a piece of DIP socket or even use a transistor socket if you have one.

Here is a data sheet for a JFET, scroll down and there is a test circuit. it is intended for fancier tests, but could easily be a go-nogo test. For N-channel and P channel parts, just reverse all the voltage polarities.
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Old 27th November 2010, 07:46 AM   #12
Calvin is offline Calvin  Germany
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donīt rely only on datasheets. They are more than often wrong...even Fairchilds. Their BF246A for example has a different pinning than the datasheet says. The datasheets pinning equals the datasheet pinning of the NXP BF247A, while in praxis it equals the pinning of the NXP BF246A datasheet (BF246 and BF247 are equal parts apart from the pinning). As this example shows there are also sometimes different pinnings of parts to find between different manufacturers.

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Old 27th November 2010, 08:57 AM   #13
andyr is offline andyr  Australia
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Originally Posted by Calvin View Post

donīt rely only on datasheets. They are more than often wrong...even Fairchilds. Their BF246A for example has a different pinning than the datasheet says.
Aaaah, Calvin - don't say that!!

But that is an interesting possibility. I will do the resistance test that Enzo suggests (to identify the Gate and make sure they are functioning) and then change the pin order from what I have now.


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