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Old 14th January 2004, 05:57 PM   #1
Calebay is offline Calebay  Denmark
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Question How to match MJL3281A?

Hello

I need to match 2 pairs of MJL3281A NPN, for my JLH 15 amps.

But i do not know how this is done. or what i need to do it.

I have a multimeter so i can mesure Ampers. and i have a +-22volt power supply. do need more then this?

Hope someone know more about this than me...

tnx

Carsten
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Old 14th January 2004, 06:02 PM   #2
Calebay is offline Calebay  Denmark
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can i use this?

and can i use +22 and -22volt for the supply?

How do i chose the value for Rb

Tnx
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Old 14th January 2004, 06:04 PM   #3
markp is offline markp  United States
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Some multimeters have a hfe setting. It makes life easier than setting up a rig to test for hfe. Look into Tenma for a cheap meter that does hfe, capacitance, and the usual stuff for under $100us.
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Old 14th January 2004, 06:13 PM   #4
Calebay is offline Calebay  Denmark
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Ok.

my multimeter does not have that funktion.
i think i read i another thread that this funktions on multimters is not very accurate.

and will is be posible to mach em whitout i go indvest? since these are the only transistors i need to match.
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Old 14th January 2004, 06:15 PM   #5
markp is offline markp  United States
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Multimeters are not extremely accurate but they are consistent. So, while the actual hfe might not be right on the nose, the unit to unit numbers will be consistent. If two units come up with the same hfe then they match each other.
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Old 14th January 2004, 06:22 PM   #6
azira is offline azira  United States
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From the leach amp construction pages....

http://users.ece.gatech.edu/~mleach/lowtim/part2.html
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Old 14th January 2004, 06:23 PM   #7
Calebay is offline Calebay  Denmark
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ohh i see what you mean.

but do you know how to set up a rig to test for hfe?
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Old 14th January 2004, 06:33 PM   #8
azira is offline azira  United States
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Same rig.

Ic = Ib * Hfe...

So.... you can measure the collector current by the voltage accross the resistor, and you're already measuring the base current.... do some division.

EDIT:

You don't really need to measure Hfe, you just need to match them. Or match Ib/Ic's as Dr Leach suggests. Since the values are so random for given transistors, amp circuits are sort of designed to take advantage of there being an Hfe but not it's actual value.
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Old 14th January 2004, 06:40 PM   #9
Magura is offline Magura  Denmark
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Kig på www.passdiy.com

There youll find anything you need about mosfets / transistors and the matching of them.

Cheers

Magura
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Old 14th January 2004, 08:07 PM   #10
Calebay is offline Calebay  Denmark
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thx for the links.

i just tryed to mesure 10. MJL21194

40.4
40.1
47.1
38.5
43.5
40.4
44.3
43.5
41.0
40.1 mA

Mesured 30 sek.
using 12v DC over E and C.
and 0.59 volt on B? is that not to high?

do this then mean i have 3 matching pairs. 40.4,40.1 and 43.5?
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