Faulty transistor?

Chivvyp · 24th June 2011, 02:51 PM

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

Andy5112405 · 24th June 2011, 03:02 PM

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.

Chivvyp · 24th June 2011, 03:26 PM

The measurements above are taken out of circuit.

Andy5112405 · 24th June 2011, 03:30 PM

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.

lineup · 24th June 2011, 03:39 PM

BF245 is one JFET
but maybe the same goes for JFETs as for testing MOSFETs

Andy5112405 · 24th June 2011, 03:45 PM

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.

Andy5112405 · 24th June 2011, 03:47 PM

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.

Chivvyp · 24th June 2011, 03:53 PM

G-S = overload
G-D = overload
D-S = 290 ohm

Andy5112405 · 24th June 2011, 03:58 PM

Yep. I would say that they are goosed.

Especially any reading from G-D or G-S. Those two should be virtually open circuit.

Andy5112405 · 24th June 2011, 04:01 PM

Readilly available. BF245A are 30p each at http://www.cricklewoodelectronics.co...=search&page=1

24th June 2011, 02:51 PM	#1
Chivvyp diyAudio Member Join Date: Apr 2006 Location: London	Faulty transistor? 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

24th June 2011, 03:39 PM	#5
lineup diyAudio Member Join Date: Dec 2005 Location: the north	BF245 is one JFET but maybe the same goes for JFETs as for testing MOSFETs __________________ lineup

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24th June 2011, 03:02 PM	#2
Andy5112405 Banned Join Date: Nov 2006 Location: Torpoint	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.

24th June 2011, 03:26 PM	#3
Chivvyp diyAudio Member Join Date: Apr 2006 Location: London	The measurements above are taken out of circuit.

24th June 2011, 03:30 PM	#4
Andy5112405 Banned Join Date: Nov 2006 Location: Torpoint	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.

24th June 2011, 03:45 PM	#6
Andy5112405 Banned Join Date: Nov 2006 Location: Torpoint	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.

24th June 2011, 03:47 PM	#7
Andy5112405 Banned Join Date: Nov 2006 Location: Torpoint	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.

24th June 2011, 03:53 PM	#8
Chivvyp diyAudio Member Join Date: Apr 2006 Location: London	G-S = overload G-D = overload D-S = 290 ohm

24th June 2011, 03:58 PM	#9
Andy5112405 Banned Join Date: Nov 2006 Location: Torpoint	Yep. I would say that they are goosed. Especially any reading from G-D or G-S. Those two should be virtually open circuit.

24th June 2011, 04:01 PM	#10
Andy5112405 Banned Join Date: Nov 2006 Location: Torpoint	Readilly available. BF245A are 30p each at http://www.cricklewoodelectronics.co...=search&page=1

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