Darlington readings

[DaveC]

Testing a 2SD711A. B-E reads 0.25v drop in both directions. Normally I’d
throw it in the trash but looking at the equivalent schematic I see a
resistor paralleling each B-E junction. Would a healthy device give a v-drop
in reverse polarity like I’m seeing?

Thanks.

 

[Phil Allison]

DaveC wrote:

Testing a 2SD711A. B-E reads 0.25v drop in both directions. Normally I'd
throw it in the trash but looking at the equivalent schematic I see a
resistor paralleling each B-E junction. Would a healthy device give a v-drop
in reverse polarity like I'm seeing?

** Yes, if tested with a typical DMM.

Apply a current of say 10mA from B to E and see what you get.



..... Phil

 

On Friday, January 22, 2016 at 5:57:38 PM UTC-5, DaveC wrote:

Apply a current of say 10mA from B to E and see what you get.
.... Phil
What value should I monitor while doing this?

Thanks.

The voltage drop. However I am not so sure 10 mA is enough.

Bottom line, I think it is good.

Maybe the spec sheet gives the value of the shunt resistors. If it is reading what the spec says it is probably good.

 

[Phil Allison]

DaveC wrote:

With 11 ma: forward biased: 1.1v, reverse: 1.8.

Being 2 junctions it looks like the 1.1 is spot on. Spec sheet doesn't say
shunt values.

** 1.8 divided by 0.011 gives 164 ohms for the two.

One will be larger then the other, like 120 ohms for the driver and 40 ohms for the output.


..... Phil

 

[DaveC]

Apply a current of say 10mA from B to E and see what you get.
.... Phil
What value should I monitor while doing this?

Thanks.

 

[DaveC]

The voltage drop. However I am not so sure 10 mA is enough.

Bottom line, I think it is good.

Maybe the spec sheet gives the value of the shunt resistors. If it is reading
what the spec says it is probably good.

With 11 ma: forward biased: 1.1v, reverse: 1.8.

Being 2 junctions it looks like the 1.1 is spot on. Spec sheet doesn’t say
shunt values.

Thanks.

 

On Friday, January 22, 2016 at 8:30:06 PM UTC-5, DaveC wrote:

The voltage drop. However I am not so sure 10 mA is enough.

Bottom line, I think it is good.

Maybe the spec sheet gives the value of the shunt resistors. If it is reading
what the spec says it is probably good.

With 11 ma: forward biased: 1.1v, reverse: 1.8.

Being 2 junctions it looks like the 1.1 is spot on. Spec sheet doesn't say
shunt values.

Thanks.

Those readings sound good. The fact that they differ based on polarity is a good sign. The problem I have is that means the total shunt resistance (which is in series) is low.

The spec sheet says at 6 volts Vebo, Iebo is 50 mA. In a normal transistor this is a leakage spec. but in this case it should tell us the value of those resistors. So 6 / 0.05 = 120.

You got 1.8 / 0.011 = 163.63Ż.

With the 1.1 forward, it is likely good. The EB junction is not open. I assume you checked the BC junction and found it open one way and a diode the other way. The whole thing is not shorted nor open, the odds are 99.9 % this is a good part.

This is a switching transistor. Not the greatest but good enough for some things. Still maintains 50 HFE at 15 amps with Vce 1.5 volts. Back when that thing probably came out is was considered pretty good.

Now, what to do with it ?