Checking Darlington Transistor

[AngraMelo]

Hey guys,
I received 3 different orders from 3 different sellers from 3 different countries.
They all had 5 pairs of Darlington Transistors. The MP1620 and the MN2488

The reason why I bought them is to replace some blown up transistor on a old receiver.

Now, after checking those 3 batches I realized that whenever I checked the Emitter to Collector junction, I always get a diode drop reading from my DMM.

All of them. 100% of them show this diode drop which is something that a regular npn or pnp transistor do not show.

Is there something I dont know? or am I the unluckiest customer of all times and got 3 batches of blown up darlingtons?

[drussell]

You do know what a Darlington transistor is, right? 

It is a compound pair, actually composed of two transistors with one feeding the other to get increased current amplification.  This means that you will see some aspects change from a regular "standard" transistor because it is two transistors, so things like voltage drop across the compound pair will be higher than a single transistor, must be biased to two diode drops to begin to conduct in an amplifier situation, etc.

That being said, what exactly are you measuring, and how? ... and compared to what?  You shouldn't be seeing significantly different behavior across only the C-E junction on a Darlington...  Are you sure you're measuring correctly for the pin-out?

[AngraMelo]

You do know what a Darlington transistor is, right? 

It is a compound pair, actually composed of two transistors with one feeding the other to get increased current amplification.  This means that you will see some aspects change from a regular "standard" transistor because it is two transistors, so things like voltage drop across the compound pair will be higher than a single transistor, must be biased to two diode drops to begin to conduct in an amplifier situation, etc.

That being said, what exactly are you measuring, and how? ... and compared to what?  You shouldn't be seeing significantly different behavior across only the C-E junction on a Darlington...  Are you sure you're measuring correctly for the pin-out?

I do know that darlingtons are different from regular BJT transistor. Im using a DMM on the diode mode and checking to see if there is any shorted junctions. That is the point of me measuring them. Im not concerned (yet) about the value of the drop.
When I measure the C-E junction on a regular BJT transistor, I get an OL on my DMM, but when I measured those darlington ones I get a diode drop. When I get a reading on the DMM while measuring the C-E of a BJT I assume the transistor is bad because all my good, brand new transistor do not show any readings on the C-E junction.

And yes, I do know the pinout and Im aware of how to measure pnp/npn transistors

[MK14]

I suspect it might be a parasitic (PN) diode, between the Collector and Emitter, because of the way some Darling transistors are made.

Collector-Emitter Diodes
In the most popular high voltage Darlington transistors,
there is a diode between the collector and emitter terminals.
Most often, this diode is a parasitic element.
The parasitic nature of the C-E diode can cause

Source:
http://www.onsemi.cn/pub/Collateral/AN875-D.PDF

I.e. The darlington transistors are probably fine.

[exe]

Uhm, I thought it's a protection diode... But looking at TIP120 datasheets (e.g. [1]) it's not mentioned it is for protection. So, should be parasitic.

[1] https://www.mouser.com/ds/2/149/TIP120-890130.pdf

[madires]

It's a little bit different. Some power BJTs have a flyback/freewhelling diode between collector and emitter. Those types are usually used for switching applications like driving the flyback transformer for a CRT. Some of those BJTs also have a base-emitter resistor. The flyback diode and B-E resistor aren't always mentioned or depicted in the datasheet. Or a diagram might show the B-E resistor but its value isn't listed anywhere in the datasheet.

In case the BJT has a flyback diode integrated the diode could be on a separate substrate (simply a diode in parallel) or on the same substrate. A diode (p-n junction) on the same substrate becomes part of the BJT and causes parasitic features. The application note linked by MK14 lists a few. For example, it also creates a parasitic second BJT. A NPN would get a parasitic PNP, and vice versa.

[AngraMelo]

So how can I know if the E-C junction is bad? It is not open, but when I place the positive of the DMM on the E and the negative on the C, I get a reading of around 0.4V

[MK14]

So how can I know if the E-C junction is bad? It is not open, but when I place the positive of the DMM on the E and the negative on the C, I get a reading of around 0.4V

A (power) Darlington transistor, is basically similar to a small integrated circuit, with two transistors, sometimes multiple resistors, and diode(s). So a simple multimeter, is not really suitable for directly poking around integrated circuits or similar (darlington transistors), as they are too complicated inside, to test them that way, easily.

You could build a simple circuit to test them, if you want. taking into account any inbuilt resistors, which will complicate its design. The circuit should put enough current into the base to overcome any inbuilt resistors and have a suitable output load (e.g. a resistor).
Then check the voltages with a multimeter, or use a bulb/Led as the output.

[MK14]

You would usually use a circuit, something like this (with a normal transistor):



But Darlingtons are more complicated, because their gains are usually considerably higher, they can have resistors inside (I think yours have resistors), which need to be overcome.

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I think one of yours looks like this:



As already discussed above, in other posts. It probably has a (parasitic) diode between the collector and emitter. The application note (see link in one of my earlier posts), seems to say that the process of adding the resistor(s), to the darling transistor, also creates the parasitic diode. Because of how they are made.

[mzacharias]

The MN2488 and MP1620 were mostly used by Sony - I've not seen them elsewhere, anyhow. The damper diodes across the emitter-collector are normal.