- Component idenification - 3 Updates
| Cursitor Doom <cd@notformail.com>: Nov 10 05:57PM On Sun, 06 Nov 2022 20:48:15 +0000, Cursitor Doom <cd@notformail.com> wrote: >except for two tiny blobs of paint. >Anyone know? >https://disk.yandex.com/i/jmR716xUNBt8OQ Got 'er! I'm not at all familiar with the topology they've used in this PSU, but I believe it's the chopper transistor (I should have guessed) that's gone low-resistance: Q1050. It was releasing quite a bit of heat even at only 18VAC supply with the current limited via 4x40 bulbs to boot. I removed it from circuit to check and it was dead. So I'm hoping that's the only part that's been damaged. It's an IRF820 and I have a bunch of IRF840s so will press one of those into service in place of the burnt-out one. Pin-outs just the same, just a lower Rdson figure, which is no bad thing. One question, though: they've mounted these transistors without using a screw through the mounting tab of this TO-220 package. The devices are just 'leaning' against some white pad mounted on the heat sink (see photo). Is there some special grease required to mate the transistor to the white pad or can I just use the regular silicon grease for this? It doesn't seem like a very good way of ensuring thermal conductivity. :-/ https://disk.yandex.com/i/cJDolujNfYucYw |
| whit3rd <whit3rd@gmail.com>: Nov 11 12:35AM -0800 On Thursday, November 10, 2022 at 9:57:16 AM UTC-8, Cursitor Doom wrote: > (see photo). Is there some special grease required to mate the > transistor to the white pad or can I just use the regular silicon > grease for this? That'd be 'silicone' grease; the picture showing studs over the power package indicates that an L-shaped or Z-shaped pressure arm (maybe three separate ones) was designed into the system. Why else would there be a threaded rod there? <https://e2e.ti.com/resized-image/__size/1230x0/__key/communityserver-discussions-components-files/196/7888.Mounting-a-TO220.png> <https://media.rs-online.com/t_large/RD504059-01.jpg> |
| Cursitor Doom <cd@notformail.com>: Nov 11 03:31PM On Fri, 11 Nov 2022 00:35:03 -0800 (PST), whit3rd <whit3rd@gmail.com> wrote: >into the system. Why else would there be a threaded rod there? ><https://e2e.ti.com/resized-image/__size/1230x0/__key/communityserver-discussions-components-files/196/7888.Mounting-a-TO220.png> ><https://media.rs-online.com/t_large/RD504059-01.jpg> If that's the case, maybe it was removed at some time, shortening the life of the chopper. I'll do something about that regardless. BTW, the threaded rods are there to retain plastic covers which encase the 3 devices (don't ask me why as I haven't a clue what purpose they serve but certainly not to press bear the transistors to the heatsink, that much is certain). |
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