Is there any quick method to measure if a jfet is dead?

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Check for a short circuit between gate and drain...

I have the same problem as the OP about whether or not some JFETs are "dead", so your answer is very much of interest. :)

OK, so I check for a short between gate and drain - so if there is a short, it's blown, right?

Or does a short mean that it's OK?

If there isn't a short, does that mean the JFET is OK? Or are there other easy tests that can be done?

I have heard that JFETs are very susceptible to being zapped from static. If this happens, what sysmptoms would a JFET display?

Thanks,

Andy
 
I have heard that JFETs are very susceptible to being zapped from static.

Generally, I don't believe JFETs are static sensitive to anywhere near the degree of MOSFETs - they are pn junction devices with no thin gate oxide to worry about. You'd need an impressive amount of static to kill a JFET.
 
Generally, I don't believe JFETs are static sensitive to anywhere near the degree of MOSFETs - they are pn junction devices with no thin gate oxide to worry about. You'd need an impressive amount of static to kill a JFET.

Ah, thank you bigun. That is good to know.

My problem is that I have soldered in a couple of JFETs and am getting a 0v reading from Drain to Source. So I am wondering why - it seemed to me that:
a) I might have got the pins wrong?
b) it might have been blown before I soldered it in place?
c) I might've zapped it with static?

So, as we can eliminate c), it seems likely I might have got the pins wrong? :eek:

The JFETs are MPF102s. There is a cursive 'f' on the flat side - so I have assumed it must be a Fairchild, hence the pins are (from L to R, with pins down and looking at the flat side) D S G.

Do you know whether this is correct? :confused:

Regards,

Andy
 
Andy,

You might have the D and the S interchanged?

No, I'm wrong, you are right, here's the pinout from the Fairchild pdf, all is correct!

Hugh

MPF102pinout.gif
 
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Andy,

You might have the D and the S interchanged?

No, I'm wrong, you are right, here's the pinout from the Fairchild pdf, all is correct!

Hugh

View attachment 198163

I thought I had used the correct pin connections! :) So why am I getting a 0v reading? :confused: :confused:

I will replace the ones I have soldered in with some fresh ones ... as it looks like they are kaput! :( Trouble is, perhaps the whole batch I was sent (16) is all kaput? I need to be able to test them before I go to the trouble of soldering them in.

Would measuring the Idss first, "prove" they were OK?

Regards,

Andy
 
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.
 
Hi,

don´t 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.

jauu
Calvin
 
Hi,

don´t rely only on datasheets. They are more than often wrong...even Fairchilds. Their BF246A for example has a different pinning than the datasheet says.
jauu
Calvin

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. :)

Regards,

Andy
 
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