Why Use a Bridge Rect and Opto In This Circuit

[bostonman]

The attached is a schematic for the add on MOSFET upgrade to an Anet A8 3D printer.

The hot bed connector on the main board can melt, so this board bypasses the board by using the signal and powering the hot bed directly from the AC/DC supply.

It seems this board is overly engineered. Although it shows AC on the input, the input voltage is DC (even if it were AC, the circuit doesn't have a filtering capacitor). Also, why use an opto since the entire printer is referenced to the same ground?

I'm guessing the bridge rectifier eliminates wiring the DC input backwards (it also wastes voltage/power), but why an opto?

[ledtester]

Yes, the bridge makes it idiot proof from a polarity point of view.

The opto provides more safety and idiot-proofing.

Since this is going to be connected up by the consumer there's a much larger risk that they will screw something up. And if the MOSFET blows it's going to cost the company in terms of bad reviews, time spent troubleshooting the problem with the customer, returned orders, etc. A string of bad reviews can kill a product on ebay/amazon/aliexpress. It's probably well worth the extra cost to add those components.

[bostonman]

Thanks.

It was relatively straight forward, but wanted to make sure I wasn't missing something.

[ledtester]

why use an opto since the entire printer is referenced to the same ground?

Also consider that these extra components allow the module to be used in more situations. With fewer restrictions the module could potentially be used with other printers.