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I'm working on replacing some power transistors in an old power supply. One of the transistors that I need to replace is an old MJ900 pnp power transistor with a newer close to equivalent one since it's hard to find the originals and or expensive. The power supply was made in the early 1970s.

What are some key concepts when choosing equivalent parts? What parameters are important to match, no so important, and not important at all?

fbmdude235
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    That depends on the exact purpose of the transistor and the design criteria its role has to meet. – PlasmaHH Sep 04 '16 at 20:13
  • This summary http://alltransistors.com/transistor.php?transistor=40573 suggests it's a Darlington, which will narrow the choices. –  Sep 04 '16 at 20:16
  • In an emergency you could build up your own pair of transistors into a darlington but this may be more work and can be hard to get right if you have a sensitive or high speed circuit. Have the power transistors failed already or are you trying to do preventative maintenance, if so I would first focus on the electrolytic capacitors. – KalleMP Sep 04 '16 at 21:09
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    A 2n6052 came up based on the parameters in Brian's link. It is a TO3 case so this may be a problem or just what the doctor ordered. You need to make sure you can fit it in and all the ratings are within circuit limits. If you have a 24V circuit the 60V rating is in excess and a 50V replacement may be enough, otherwise pick higher ratings to prevent edge case operation. - http://www.onsemi.com/pub_link/Collateral/2N6052-D.PDF – KalleMP Sep 04 '16 at 21:27
  • http://www.ebay.com/itm/MJ900-MOTOROLA-POWER-TRANSISTOR-PNP-DARLINGTON-8-A-60-V-PNP-/122079603449?hash=item1c6c82eaf9:g:OXkAAOSw9NdXq8D2 – Bruce Abbott Sep 04 '16 at 22:39

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The most important parameters depend on the application, but the main ones generally are:-

  1. Type (Silicon/Germanium, NPN/PNP, Darlington, Audio/Power/RF etc.)

  2. VCEO (Collector-Emitter breakdown voltage) equal or higher than original.

  3. IC (maximum Collector current) equal or higher.

  4. HFE (DC current gain) equal or higher - but not too high.

  5. Case/Package. (Will it fit? Can it be kept cool?)

  6. PT (maximum power dissipation). This rating is often 'optimistic' (actual power handling is mostly determined by the case and heat sinking).

Some circuits are critically dependent on other parameters such as VCE(sat), safe operating area, capacitances etc., while others were designed around popular 'industry standard' components that may not be optimal (and might even work better with more modern parts). Often you can get away with using a generic replacement, but you should always monitor its performance in the circuit to make sure it is doing the job properly.

The MJ900 is described as a PNP Power Darlington in TO-3 case rated for 60V, 8A and 90W max. To match its characteristics You will need another PNP darlington transistor, in a case that can dissipate similar power on the same heatsink. If your circuit runs on low voltage and current then these parameters could be relaxed. Darlington HFE is usually quite variable so the circuit probably isn't very sensitive to it, but avoid transistors with built-in Base resistors since their gain drops off at low current.

Bruce Abbott
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