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I was taking apart an old game-show buzzer system (like something they might use on Jeopardy) and I was confused by the a particular section of the circuit board:

rectifier connected to power switch

I'm pretty sure the circular component labeled "W005G 840A" (datasheet) is a bridge rectifier. It is connected immediately after the power coming from the switch.

This board runs of a 9V DC wall supply, so why would they add a rectifier? It seems to me that all it would do to a DC voltage is drop it slightly.

I thought it might be used for some sort of circuit-protection, but I don't understand what they are protecting against. Wouldn't the 9V wall adapter prevent any harmful voltages from reaching the board? Also, if they are trying to protect against reverse polarity wouldn't single diode work fine?

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They aren't protecting against reverse polarity, they want the device to be operational, this ensures that. Good guess though. As long as there is no external device connection or need to share a ground this is perfectly fine and safe.

You will notice that there is space for a cap on board too. So one could also put in AC as well.

Semi-universal.

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  • \$\begingroup\$ Given that the device came with a 9V wall wart soldered directly to the board, was the board probably designed by a different person than whoever assembled the final product? If they knew the power supply they would be using, couldn't they have just removed the rectifier? \$\endgroup\$ – SuperPig Jun 8 '16 at 23:35
  • \$\begingroup\$ Indeed. This could operate with 9V, center positive; 9V, center negative; or 9VAC. Depending on the current draw, the 7805 could possibly even handle higher input voltages. \$\endgroup\$ – user2943160 Jun 8 '16 at 23:36
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    \$\begingroup\$ @SuperPig it's easier to manage the supply chain for a complete product if you can source any 9V power supply. Run out of one? Use a different one. \$\endgroup\$ – user2943160 Jun 8 '16 at 23:37
  • \$\begingroup\$ @SuperPig it could be any of a number of things. That bridge is only rated to 50V so it could also be a good anti-ESD device too. \$\endgroup\$ – placeholder Jun 8 '16 at 23:49
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A single diode will protect against reverse polarity, but a bridge rectifier will allow operation with reverse polarity. That way you can use whatever cheap wall wart you can find that week in the market.

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    \$\begingroup\$ beat me by 50 seconds ! \$\endgroup\$ – placeholder Jun 8 '16 at 23:17

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