Repair

Repair index.

Transistor replacement
Finding short circuit in the domiciliary net

Transistor replacement.

Whenever the original transistor can get it suits to use this, but some transistors are not available sometimes in the commerce or simply there is not information of them or they have the number erased.

The most complicated cases of replacing are the special transistors: high-power, high frequency, of another type like FET MOSFET, IGBT, digital, etc.

If we are lucky and it is not one of these and the transistor is a common bipolar, that is the most probable thing, we can make the following:

Serch for information in manuals as NTE about the characteristics of the transistor: polarity (PNP or NPN), maximum collector voltage Vceo, of base Vbeo, collector current Ic, Power Pd, current gain hfe and maximum frequency Ft.
Choose a transistor that has the same polarity and the mentioned parameters of same or bigger value.

If we look for in the NTE manual, formerly ECG, will we know the characteristics and the NTE number of the transistor, but. How do we know for which we can replace it?

Good, I several years ago have been writing down the commercial number and the NTE number of each transistor that has arrived to my hands of some apparatus or board to repair. So already I have several replacements for transistors of several different sizes:

No NTE	REEMPLAZO
36	2SC2580, C2580, D718
37	B688
51	MJE13005
85	C1959Y, C372, C1815, C945
123A	2N2222A
123AP	BC549
128	2N3053
129	2N4037
130	2N3055
152	2SD234, TIP31Y, C1173, 2SC1061
153	TIP32, B512, A473
159	BC557, BC558
159M	2N2907
162	C1106
175	2N6261
181	2SC1629A, 2N5886, 2N3771
184	D882
198	C1507L
199	C945A, C732
229	9016
236	2SC1945, C1945
261	TIP-120
264	BDX54
280	2SD323
283	2SC1617
291	TIP31C, C790-0
292	TIP32C
327	2N5039
331	2N6100, MJE3055T, BD907
332	BD908
374	BD140
375	BD139
392	TIP3055

This chart is also good to choose some transistor for some project, looking for in the NTE for its characteristics some that serves us and then in the chart the commercial number to buy it.

If there is not information about the transistor or it has the number erased, the only thing left is:

If it's not completely shorted, we still can measure it outside the circuit with a multitester to know if it is a bipolar one and wich polarity has.
If not, it is only to analyze the configuration wher is, so we can determine if it could be a bipolar one and their polarity. If it is a small transistor of the most common and it is a audio frequency circuit serve us general purpose small signal transistors like the 2N2222, BC548, BC107 for NPN or their complementary ones for PNP. If they are medium power and the working current is known it is necessary to see if it serves us someone like the BD139, TIP31 or of more powerful like the popular one 2N3055.

Someone asked me recently for the transistors C3840 and for the A1486. Then an arduous search in internet only found the first one, but later I remembered that to Japanese they are prefixed 2S, so I looked for the 2SC3840 and the 2SA1486 and I found the data sheets quickly. According to what I have seen in several cases: those that begin with "C" are NPN and with "A" they are PNP.

Finding short circuit in the domiciliary net.

It is a very laborious and frustrating task, because in most of the occasions it is not possible connect the circuit breaker again, because it acts immediately. With the circuit deenergized we could only measure resistance, which can confirm us that a short circuit exists, but we won't know wich place, because all the plugs and lamp sockets of the installation are in parallel.

Before anything else we should disconnect all the apparatuses and lamps connected to the circuit and to check if the problem persists.

To avoid the trip of the breaker we can put provisorily in series with this a high-power pure resistive load . Ideally it can be an electric heater, an iron or an stove without electronic control. It do can have thermostat, the one that we have to adjust to make sure that it closes the circuit of our load.

The load should connected to the output of the breaker so that we can make the connection of this with the circuit deenergizad. You can use a female plug to be connected with the male plug of the apparatus that we use like load.

Now we connect the breaker and we should be very attentive to overheating indications in some point of the circuit.

With a clamp ammeter we measure the current that we have through the breaker or the load and that it will be limited to the current that consumes the load if it is a direct short circuit, that is to say there is not significant resistance for bad contact.

Then we track this current measuring with the clamp ammeter the wires in different derivation boxes starting with the nearest to the breaker. You can uncover the points around the derivation box and jerk the cables wich have current mensuration to know where they go to.
The short circuit can be in some plug, lamp socket or inside a duct.

Top