Choosing a replacement 1W TO-92 NPN for a 2N3405 in this vintage circuit - Page 1
Forum section: electronics / beginners

author: cincin
author_rank: Regular Contributor
timestamp: March 24, 2025, 10:10:08 pm
content: I'm working on an old Petereson Model 400 strobe tuner. The motor is having a hard time. I found a blown power transistor on the motor board and decided to change all 4 since they are old and have probably been stressed by the death of their co-worker anyway.One of them is a long discontinued2N3405(NPN, 50Vcb, 50mA Ic, 1W, ECB) in a TO-92 with a metal clip as a heatsink, and I'm struggling to find an adequate substitute. There seem to be very few TO-92 that can handle 1W, but does this circuit really push 1W onto it? I can't tell from the schematics alone, do I need to measure the actual current or can it be inferred from the schematics? The clip-on heatsink makes me think it is getting pushed a bit.The other transistor I still need to find a replacement for is the2N3414(25Vcb, 500mA Ic, 560mW, ECB).2N4424? 2N5172?Thanks!

author: TimFox
author_rank: Super Contributor
timestamp: March 24, 2025, 10:33:32 pm
content: Each of those two NPNs will be limited by their 4.7k collector resistors and +50 V supply to about 10 mA collector current. The worst-case instantaneous power dissipation would be (50 V / 2) x (0.01 A) = 0.25 W, so the average (with switching) will be much less than that.(That worst case is the maximum power that the resistor can deliver to a load.)However, neither of your two suggested replacements is rated for the 50 V that the transistor sees when off (from the 50 V source).You wantVceo> 50 V.

author: cincin
author_rank: Regular Contributor
timestamp: March 25, 2025, 12:00:07 am
content: Thanks! That makes the choice much simpler. I'll review the numbers to understand how you got to that conclusion.The two replacements I linked at the end were for the 2N3414, which is a 25V rated tranny.Cheers!

author: TimFox
author_rank: Super Contributor
timestamp: March 25, 2025, 12:03:47 am
content: To get that conclusion, consider the transistor turned "ON", which means thatVce= 0 and calculate the current through the collector resistor.

author: JoanBS
author_rank: Contributor
timestamp: March 25, 2025, 12:23:55 am
content: BC637 -> (NPN, 60Vceo, 1A Ic, 0.83W, BCE, TO-92)

author: David Hess
author_rank: Super Contributor
timestamp: March 25, 2025, 12:46:56 am
content: I wonder why they used two different driver transistors since they are both NPN and operating under similar conditions.The 2N5551 is the common higher voltage TO-92 transistor and should work fine even though it is only 625 milliwatts. Larger TO-92 parts are a lot more expensive. For a more powerful part I might use a ZTX653 or similar e-line type transistor from Diodes Inc., previously from Zetex.

author: David Hess
author_rank: Super Contributor
timestamp: March 25, 2025, 12:47:48 am
content: Quote from: JoanBS on March 25, 2025, 12:23:55 amThe BC637 was on my list but apparently is discontinued?

author: JoanBS
author_rank: Contributor
timestamp: March 25, 2025, 12:58:38 am
content: Quote from: David Hess on March 25, 2025, 12:47:48 amIt seems to be discontinued, at least in TO92 format, but it is available at some distributorshttps://www.rocelec.com/part/01t4w00000POnjrAAD-BC637

author: Kleinstein
author_rank: Super Contributor
timestamp: March 25, 2025, 09:09:53 am
content: The 60 V BC637 may be a bit difficult to get, but the 80 V brother BC639 is still easy to get.If one really needs a bit more power there is the BD137/139 in a slighly larger SOT32/TO126 case that allow heat sink mounting. AFAIK it's the same silicon. One can usually twist the pins enough to get in a TO92 footprint, though the holes may be too small.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 25, 2025, 10:53:50 am
content: Hello cincin,To replace the 2N3405, I would suggest theZTX651(NPN 60V 2000 mA1Whfe > 150 up to 1A) custom package similar to TO-92, pinout does not match (EBC). So you'll have totwistthe pins.The ZTX651 is immediately available in large quantities fromMouser.

author: David Hess
author_rank: Super Contributor
timestamp: March 25, 2025, 11:01:07 am
content: Quote from: Kleinstein on March 25, 2025, 09:09:53 amI really like SOT32/TO126/TO225 parts but when replacing TO92 parts, the leads are inconvenient, and higher power TO92 parts like the ZTX653 actually cost less.Update: Although in this case, the SOT32/TO126/TO225 pinout with the collector as the center pin actually matches better.