I am studying various ATX power supplies schematics and don't understand the need of the voltage selector as switching power supplies work at range of voltages. In most schematics i see the 110V line connected after the bridge rectifier between the resistors and filtering capacitors. Why?

Schematics example


2 Answers 2


In these schematics, 110VAC is connected through a voltage doubler schematic in order to get the same 310V DC after the rectifier.

These solutions are pretty outdated now. The typical modern PSU is universal and can work from 90..250VAC on the input without voltage selector.

  • \$\begingroup\$ Thanks. I simulated similar circuit and results were really double voltage (a bit more). \$\endgroup\$
    – user19978
    Nov 7, 2013 at 20:03

"Switching power supplies work at range of voltages." They work at a range of voltages they are designed to work at. The larger the range you want to support, the more it will cost in the design (in $, size, reliability, etc). It's one thing to make an 80W laptop supply work across 100-240V, but it's quite another to do the same for a 450W ATX supply, and the benefit to the customer is much lower.

[Update: this paragraph is speculative and wrong:]

The switch likely rewires the primary in the transformer so that the regulation circuit doesn't have to work over such a large range.

[Update - comment added after downvotes:]

OK, I've been voted down. The 2nd paragraph is speculative and wrong , so I won't protest. I am well aware than SMPS transformers don't run at line frequency, but I wasn't aware that you couldn't still have a split primary with a switch on that transformer. And the schematic was not up at the time.

My ignorance of that doesn't invalidate the main point - the switched voltage doubler still serves to reduce the range over which the supply needs to adapt automatically. So that brings me to a question: is it not harder, in terms of cost/efficiency/reliability, to make a PSU that can adapt over a 1:2.6 range as opposed to 1:1.3? And, does not the tradeoff become more significant with higher power? As long as a PSU with a switch is more efficient & reliable than one without, I'll take the one with a switch for a desktop. @johnfound answer says that modern PSU are univeral without a switch - that's absolutely true for wall warts and laptop supplies, but most ATX supplies have switches.

So it's a good question, and I believe my first paragraph is a good answer. But I'm prepared to learn more.

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    \$\begingroup\$ It is quite unlikely that there is a transformer on the input to a ATX power supply. More likely, some capacitors and diodes are switched around so that the peak to peak voltage is rectified for the 110 V setting, and 0 to peak is rectified for the 240 V setting. \$\endgroup\$ Nov 7, 2013 at 19:17
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    \$\begingroup\$ +1 Olin, -1 greggo: the de facto ATX solution is indeed via a voltage doubler, not by changing transformer turns ratios. I have never seen an ATX unit with a mains-frequency transformer. \$\endgroup\$ Nov 7, 2013 at 21:02
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    \$\begingroup\$ Actually, I don't think any vendor has ever built an ATX power supply with a conventional, 50/60Hz transformer, all are exclusively SMPS. And in the SMPS world, it is not possible to "rewire the primary". \$\endgroup\$ Nov 8, 2013 at 8:13

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