How to test LM338K and LM309K transistors...

[jdurango]

Okay, relatively new to electronics, restoring an MCI JH110 tape machine that's stuck in FF. Power supply voltage rails test good and everything else has been completely gone through, new IC sockets, IC's, new molex solder/cleaning, etc. Pretty sure I've traced the problem down to some bad transistors in/near the power supply. I just ordered some new LM338K and LM309K transistors (along with 3055's which haven't shown up yet). I'm testing them to make sure I've got the correct pinout and they work properly.

So here's my question. For these transistors, brand new from Jameco, do these #'s look right? I've labelled E=emitter, B=base and C= collector. For the LM309K I'm not sure which is which, the data sheets only shows input (emitter?), output (collector?) and ground (base??). The + and - signs denote which probes are attached to the pin for that reading:

LM338K
E+ / B- = 21 megaOhms
B+ / E- = 19.2 megaOhms
C+ / E- = 3 megaOhms
E+ / C- = 5.2 megaOhms
C+ / B- = 5.7 megaOhms
B+ / C- = OL

Okay, the way I understand it, transistors  are supposed to act like diodes and have VERY high or OL resistance in several directions, which this one definitely doesn't seem to be doing. Is this right for this transistor??? Honestly I'm not sure how to tell from the data sheet.

Similar results with the LM309K.

LM309K
Output+ / Input- = 3.19 megaOhms rising by about .001 Mohm per second (sorry, don't have a scope)
Input+ / Output- = 6.1 megaOhms slowly rising about .001 Mohm per 2 or 3 seconds
Ground+ / Output- = 3.3 kiloOhms slowly dropping about .001 Kohm per 2 or 3 seconds
Output+ / Ground- = 3.3 kiloOhms steady
Ground+ / Input- = 3.16 megaOhms rising about .001 Mohm/ second
Input+ / Ground- = 6.1 megaOhms slow rise about .001 Mohm/2-3 seconds

So once again, we've got well below OL resistance in every direction?!?! Again, I'm new to all this, but it doesn't seem right. I ordered several transistors for each model and they all behave the same way, which leads me to believe they are "right"....but you know what happens when you ASSume??? Haha

So are these readings what you'd expect to see from these components? Thanks so much for your help!!!!

 

[jdurango]

Due diligence:

LM338K data sheet: http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00000548.pdf?s_searchtype=keyword

LM309K data sheet:  http://pdf1.alldatasheet.com/datasheet-pdf/view/318052/COMSET/LM309K.html

Thanks so much fellas!!!!

 

[Tekay]

LM309 / LM338 are NOT transistors, these are voltage regulators and can't be tested with a diode/transistor tester.

 

[jdurango]

Haha, okay, well that explains the weird readings then! lol....they look exactly like power transistors and I thought transistors could be used as voltage regulators, which I what I assumed these were supposed to be. Okay, well thanks very much for clarifying that! I'm going to research what the hell voltage regulators are and how they work so I don't make a similar mistake down the road. Thanks so much!!!

 

[PRR]

> they look exactly like power transistors and I thought transistors could be used as voltage regulators

A transistor is a simple part, not much good alone.

A voltage regulator is several transistors working together. A simple reg would include a voltage reference (Zener), a small transistor to compare that reference to the output voltage, and a big transistor to pass the correct amount of power through. The regs you buy may have 40 transistors to improve action and reduce abuse (little transistor are incredibly cheap).

The first regs had 14 or 8 pins and may have needed an external transistor (such as your 2N3055) to handle big loads. Today they have simplified down to 3 pins, which means they can use the same "frame" as power transistors. But a toaster, a radio, and a PC are all "boxes with wall-plugs", yet not the same thing. While PCs usually don't have bread-slots, with transistors and regulators you gotta go by the part number.

> data sheets only shows input (emitter?), output (collector?) and ground (base??)

That was a Clue. Transitors are simple incomplete functions and the pins have abstract names. Regulators are a specific complete function and the pin names make some sense.

> transistors  are supposed to act like diodes and have VERY high or OL resistance

Also depending what voltage you measure at.

The Silicon diode hardly turns-on until you get 0.6 Volts across it.

In old days we used Needle Meters which worked from a 1.5V battery. If forward across a diode they read about half-scale on all ranges. (0.6V is about half of 1.5V.)

Today we use digital meters which mostly only use 0.2V to measure resistance. As a first approximation, the diode does not turn on at all, should read OL (infinity). In fact the conductance at 0.2V is about a million times less than the conductance at 0.6V (0.060V per 10:1 change of current). This is "too small for real work" but can be detected by many meters. Making some random assumptions about the side of diodes in a '338, I get a number like a few million Ohms. Your 3Meg to 20Meg numbers are close enough.

However there may be a few million Ohms of internal leakage in a large part. Your 0.2V meter puts you way below normal operating point and you can hardly tell forward from backward on the diodes.

All you really proved is that the legs are not broken-off. (They could still be a break inside the plastic-- a barely-not-rejected batch of plastic might have millions-Ohm leakage between pin stubs.) (If you really have metal/glass 309 you can still have Megs of leakage in seals and finger-prints.)

Your ground/output path shows a consistent 3K. When you study regulators you will find a couple of resistors out-to-ground. This divides the output to the same voltage as the internal reference. On some regs this is plain simple resistance, no diodes. The published plan for '309 is not so simple, and I can't identify your 3K at a glance. But you buy pre-made regulators so you do not have to think about these things.

If you must test: wire a not-fresh 9V battery to '309's In and Gnd. Get the polarity right!! Now measure 5V from Out to Gnd. Note that the battery has to hold at least 7V into the part, because the minimum In-Out voltage is 1.5V at teeny current. And this still does not prove that your '309 can carry the BIG current that you paid the big bucks for.

I understand wanting to test. However you bought from a very reputable source, reputable chips have defect rates below 1 in a million, you can just assume the parts are good until you wire them up and check ALL other possible errors. (Wiring errors always exceed parts problems.)