| Electronics > Beginners |
| Checking Darlington Transistor |
| << < (2/2) |
| 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:
--- Quote from: AngraMelo on June 18, 2018, 11:08:04 pm ---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 --- End quote --- 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. ----------------------------------------------------------------------------------------------------- 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. |
| Navigation |
| Message Index |
| Previous page |