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rectifier before transformer?
rectifier before transformer?
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Old 13th October 2018, 08:10 PM   #1
zgtc is offline zgtc  Europe
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rectifier before transformer?
Default rectifier before transformer?

Hi,

Please bear with me, I'm a complete newbie on all these theory terms and conventions. Today I opened a Panasonic DVD-S35 that is malfunctioning (doesn't recognise the discs being inserted). I also downloaded the service manual. Just out of curiosity, not that I plan to even touch the mains or modify anything.

...And I found that the rectifiers come before the power transformer, see attach:
panasonic-dvd-s35-power-supply.png

I found this quite shocking, but as I said I'm a complete illiterate on this ground. Is this really a weird design or common use of rectifiers?

Thank you
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Old 13th October 2018, 08:19 PM   #2
thimios is offline thimios  Greece
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This is a block diagram of a SMPS, not a classic liner one.
See here. Switched-mode power supply - Wikipedia

Last edited by thimios; 13th October 2018 at 08:25 PM.
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Old 13th October 2018, 08:53 PM   #3
FauxFrench is offline FauxFrench  France
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Yep, as just said by thimios, this is not a traditional transformer but an SMPS. The overall idea here is to rectify the net-voltage and then "chop" this primary voltage to a high frequency AC, which is then transformed by a smaller transformer to a plurality of secondary voltages.
Because the frequency is higher, the transformer can be smaller.
"Power transformer" is not a well chosen name for that block and you rightfully spotted it appears weird. You are on a good track.

Last edited by FauxFrench; 13th October 2018 at 08:59 PM.
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Old 13th October 2018, 09:01 PM   #4
wg_ski is offline wg_ski  United States
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T001 may in fact be the power transformer, if the “constant voltage control” happens to be a fly back or forward converter circuit. That is exactly how the transformer would be connected - between the HV bus and the collector of the power transistor (or IC). Most larger switchers are a full or half bridge, but stuff under 200W is often implemented in a simpler way with a single ended output.
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Old 13th October 2018, 09:06 PM   #5
zgtc is offline zgtc  Europe
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rectifier before transformer?
hey thank you so much guys. i’m sorry i missed this (key) info, what i understand is the “power transformer” in the scheme would be a small black box in the middle of the board. i can take a pic tomorrow if you’re interested, again out of curiosity. for what you all are suggesting, rectifiers before transformer is not a weird scheme in SMPSs, right? then question solved. thank you so much
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Old 13th October 2018, 09:20 PM   #6
FauxFrench is offline FauxFrench  France
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Rectify net-voltage -> chop rectified net-voltage into AC at high frequency -> transform the high frequency in a small transformer -> rectify the secondary voltage(s) -> eventually use a post regulator.

Fully normal design for an AC -> DC (step-down) converter.
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Old 14th October 2018, 12:17 AM   #7
PRR is offline PRR  United States
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Yes, a switcher, and commonly turning 115V/230VAC into 320V of DC, then chopping that and transforming down to your 5V, 12V, 3V, etc.
Attached Images
File Type: gif DVD-S35-1.gif (56.2 KB, 161 views)
File Type: gif DVD-S35-2.gif (61.8 KB, 162 views)
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Old 14th October 2018, 12:31 AM   #8
nigelwright7557 is offline nigelwright7557  United Kingdom
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Switch mode power supplies are becoming very popular. Mains transformers are getting to be expensive.

Switch mode just rectifies mains to peak mains voltage DC. Its then chopped and put through a high frequency (100KHz?) transformer. The output is rectified and smoothed. The output is sometimes fed back to the input for regulation purposes.

SMPS are very efficient especially if using power factor correction. You can sometimes get residual switching frequency on the output but that isn't insurmountable with filtering.
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Old 14th October 2018, 01:02 AM   #9
TMM is offline TMM  Australia
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In laymans terms a flyback SMPS (that's the isolated type discussed here with the transformer instead of an inductor) effectively converts DC to AC by switching it on an off with a transistor, passes it through a transformer and rectifies the AC on the secondary side back into DC.

Because the transformer in an SMPS usually operates somewhere in the region of 100kHz instead 50Hz, it's inductance can be a lot lower than a typical mains transformer and that means that it can be a lot smaller, lighter and cheaper to manufacture.

The rectifier before the SMPS is purely because a flyback SMPS is a DC-to-DC converting device so the input must be DC. The simplest and cheapest way to achieve that is to straight rectify the 50Hz AC mains input. Since the SMPS can be designed to work at the resulting ~300V DC bus voltage, there is no need for a conventional 'step down' mains transformer in addition to the SMPS transformer.

Last edited by TMM; 14th October 2018 at 01:07 AM.
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Old 14th October 2018, 09:45 AM   #10
zgtc is offline zgtc  Europe
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rectifier before transformer?
OK, thank you everyone, again.

Just for the record, here's a pic of the power supply
panasonic-dvd-s35-power-supply.jpg
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