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I have a tip122 and discovered my multimeter uses Vce=2,8V DC and Ib=10uA for its hFE test. When I insert the transistor on the terminals for hFE test, the gain on the multimeter alternates between 1 and 2 when by datasheet information it should be 1000. Anyone knows why?

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    \$\begingroup\$ Transistor inserted wrong pin pattern (BCE left to right tab up writing side ) / This is an NPN / Transistor broken / DArlington has Vbe x x usual - meter may not to;erate (as Andy suggests) Meter broken / User broken :-) \$\endgroup\$ – Russell McMahon Mar 28 '15 at 10:48
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    \$\begingroup\$ And don't forget, the gain of a particular BJT is highly dependent on current, temperature, day of the week, and whether it's raining in Moscow. \$\endgroup\$ – Dan Laks Mar 28 '15 at 10:57
  • \$\begingroup\$ I'm surprised that a TIP pins would fit the hFE connector. Also you're unusually well-informed about the test conditions. Is this a homework question? \$\endgroup\$ – Spehro Pefhany Mar 28 '15 at 12:53
  • \$\begingroup\$ Haha, no homework here. I was just checking every function about my new multimeter after I short circuited it some days ago. In its manual there wasn't any information about DArlington transistors. \$\endgroup\$ – GabrielRado Mar 28 '15 at 17:37
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Added to get this image in here.. the nominal resistor values vary significantly between suppliers- the below is from a Motorola model with 8K & 120 ohms.

As Andy's answer suggests the problem is in those danged resistors. With 10uA there's only 70mV across the B-E junction of the input transistor and 0.7mV across the other B-E junction. They're both 'off', the "base" current flows out through the "emitter" lead, so hFE = 1.0.

Just for interest, here's a plot of hFE vs. base current. There is a similar shape to the collector current curve in this datasheet, but extended to low currents.

enter image description here

enter image description here

And, finally, to compare with the datasheet curve, here is a simulation of hFE vs. collector current (comparable to the datasheet plot). As you can see, the apparent straight line (on a log-log graph) decline from the datasheet can't be extrapolated very far.

enter image description here

That said, those "danged" resistors are there for a very important reason. Without them, even small leakage (especially important when the transistor is hot) would be amplified by the product of the two hFEs and could cause problematic leakage in the compound device.

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The transistpr is a darlington configuration and most multimeters are not capable of measuring Hfe for darlingtons. Try a regular transistor and see what you get - just to prove the meter works. TIP122: -

enter image description here

Regular transistor: -

enter image description here

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  • \$\begingroup\$ The meter is probably measuring the Hfe correctly, but at 10uA Base current it's just very low. This low Hfe is caused by the internal resistors in the darlington, which rob current from the Base of each transistor and dramatically reduce the gain at low current. \$\endgroup\$ – Bruce Abbott Mar 28 '15 at 13:11
  • \$\begingroup\$ @BruceAbbott I think Andy omitted the answer on a silver platter because this appears to be a homework question.. \$\endgroup\$ – Spehro Pefhany Mar 28 '15 at 13:21

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