Hi,
I've have a circuit with BF245A transistors and it's not working properly, I think the transistors may be faulty.
With a diode test I get
G/S 0.73v, S/G 0v Could be other way round
G/D 0.73v, D/G 0v Could be other way round
S/D 0.15v, D/S 0.15v My understanding is that these should be 0v both ways.
Are these faulty?
Regards
Pete
I've have a circuit with BF245A transistors and it's not working properly, I think the transistors may be faulty.
With a diode test I get
G/S 0.73v, S/G 0v Could be other way round
G/D 0.73v, D/G 0v Could be other way round
S/D 0.15v, D/S 0.15v My understanding is that these should be 0v both ways.
Are these faulty?
Regards
Pete
Can you measure them out of circuit as diode volt drops across MOS-FETs are meaningless.
Out of circuit you should get Infinite (or close to it) between Gate and Source and High Resistance between Drain and Source.
MOS-FETs usually fail with low Gate to Source resistance (Gate breakdown) or Low Drain to Source resistance.
Out of circuit you should get Infinite (or close to it) between Gate and Source and High Resistance between Drain and Source.
MOS-FETs usually fail with low Gate to Source resistance (Gate breakdown) or Low Drain to Source resistance.
Diode drops across MOS-FETs are meaningless.
They are not constructed like normal BJTs and cannot be measured as such.
Just measure the resitances between G-S and D-S.
They are not constructed like normal BJTs and cannot be measured as such.
Just measure the resitances between G-S and D-S.
A JFET should measure in a similar way to a MOSFET.
With a JFET you might see some leakage from D-S but very little.
With a JFET you might see some leakage from D-S but very little.
As with ANY device. Best test is to look at the datasheet and to build a little test circuit to see if it behaves iaw its datsheet curves.
Nothing special. At Vgs = ** should it be conducting or not.
Check to see that Ids changes with Vgs.
Its not brain science.
Nothing special. At Vgs = ** should it be conducting or not.
Check to see that Ids changes with Vgs.
Its not brain science.
Yep. I would say that they are goosed.
Especially any reading from G-D or G-S. Those two should be virtually open circuit.
Especially any reading from G-D or G-S. Those two should be virtually open circuit.
Readilly available. BF245A are 30p each at http://www.cricklewoodelectronics.com/Cricklewood/search.php?mode=search&page=1
Hi,
I don't know how your DMM indicates infinite resistance (or greater then max of the range).
I suspect "Overload" means greater than max of scale selected.
BF245 can be in a variety of pin outs depending on manufacturer.
You need to identify D, G & S.
Then connection G to S and test Idss.
Then connect G to D and test Idss. It should be almost identical to the first measurement.
Remember that for a Nchannel jFET that -ve of the test supply goes to the gate pin along with that pin you have chosen to connect it to.
I don't know how your DMM indicates infinite resistance (or greater then max of the range).
I suspect "Overload" means greater than max of scale selected.
BF245 can be in a variety of pin outs depending on manufacturer.
You need to identify D, G & S.
Then connection G to S and test Idss.
Then connect G to D and test Idss. It should be almost identical to the first measurement.
Remember that for a Nchannel jFET that -ve of the test supply goes to the gate pin along with that pin you have chosen to connect it to.
BF245, being a J-fet is a depletion mode device and should measure fairly low resistance from D to S. The gate-source junction should be reversed bias during operation. So a positive voltage on the gate vs source should measure a forward junction. You could build a simple J-fet Vgs tester rig using a simple 741 op-amp. This circuit is common and can be found by searching.
I bought 10pcs BF245B from 100% feedback seller on ebay UK. 6 of 10 were faulty. I gave seller curve tracer proof and a friendly advice that he should be more careful about the items he is selling and nothing. He said that my circuit isn't functioning properly etc.
So, nowadays, you can't buy a 'simple' JFET without being too careful. Fakes of faulty everywhere.
So, nowadays, you can't buy a 'simple' JFET without being too careful. Fakes of faulty everywhere.
The measurements on the fet seems rather normal.
G-S and G-D are both diodes , conducting when G is + .
S-D gives a low resistance with the gate at allmost the same voltage as the other electrodes.Rubbing the plastic envelope sometimes charges the floating gate and the resistance S-D goes up.Dropping again if you make contact G-S or G-D.
Mona
G-S and G-D are both diodes , conducting when G is + .
S-D gives a low resistance with the gate at allmost the same voltage as the other electrodes.Rubbing the plastic envelope sometimes charges the floating gate and the resistance S-D goes up.Dropping again if you make contact G-S or G-D.
Mona
I know it's an ages old post but for some reason misinformation seems to persist longer than real information!
Given that 99.9% of all JFETs are depletion mode and 99.9% of all MOSFETs are enhancement mode, as well as that J in JFET means JUNCTION, in fact diode junction, whereas a MOSFET has an insulated gate, then you are at least 99.9% wrong and please do not put wrong information that you did not check on this forum as real information.
It only takes 10 seconds to type BF245 into google, and simply knowing what the transistor types are should give you an idea of some things you should measure. For one thing it's easy to identify insulation (as open circuit) vs a PN junction, also a channel on a depletion device which is not merely 'leaking' but very low resistance - at the very least.
The JFET measurements look OK. It does not, however, mean that the part is OK.
Because it's a depletion mode device, you will get a small value resistance between D and S, which is how you can identify these pins. This is the JFET channel, and the G pin is one end of a PN junction towards it, in this case it's a N-channel device, so placing a multimeter set to test diodes, should show a forward biased diode between G(+) and either S or D (-).
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I tried to inform on the misinformation in post 13 but with little obvious success. - Status
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