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| Marconi 2380 SMPS capacitor replacement |
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| brian_mk:
Hi, I am attempting to repair the power supply in the display unit of a Marconi 2380/2382 Spectrum Analyser. It's a SMPS using a SG3526 controller. A blocking capacitor (C9) in series with the main transformer primary has failed. The original device was a TRW-61 2.2uF. These are 250V Metallised Polyester high current capacitors rated at 4 Amps. They have been obsolete for some time. A possible candidate for a replacement is a Vishay MKP1839 522 634HQ. These are Polypropylene, rated at 630V and intended for high current, high pulse applications. I am a bit unsure if the Vishay part will handle the current. Can anyone confirm if it will be suitable? I should add that the PSU operates at 125kHz. I have attached some files that should help:- AC1.jpg : The schematic from the service manual. The Failed capacitor C9 is ringed in red. AC1_Current.jpg : Shows the normal pulse current (3A peak) through the capacitor and transformer primary in series (from the service manual). 2380PSU.pdf : The relevant section of the parts list and the spec for the original TRW-61 capacitor. 3651112.pdf : The Vishay replacement capacitor spec. Thanks in advance. Brian. |
| brian_mk:
My calculations for the power dissipation in C9 are as follows:- Curves in the Vishay spec for a 3.3uF capacitor indicates tan delta at 125kHz is around 0.07. We have 2.2uF, so assume it's somewhere between the curves for 3.3uF and 1uF. Say 0.05. ESR = tan delta / (2 * pi * f * C) = 0.05 / (2 * pi * 125000 * 2.2 uF) = 0.086 Ohms. From the current waveform (3A peak pulses), Irms = sqrt ((3^2) / 4) = 1.5A Average Power Disspation in C9 = I^2 * ESR = 1.5^2 * 0.086 = 0.19W. That doesn't seem like much. I would expect the capacitor to handle 200mW without overheating. Have I got the calculations right? |
| brian_mk:
I'm wondering - is 'Test Equipment' the best forum to post this question? It really needs someone with an understanding of SMPS, capacitors and circuit design. |
| Martin72:
Hi, I don´t see a problem not to use the vishay, I´ve looked to wima MKP10 impulse caps, there are also no current ratings in their specs. Maybe asking the manufacturer directly. We got some caps for exactly the same usage, but "unfortunately" I got holidays actually... 8) And yes, maybe the repair section could bring more answers. Martin |
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