If I have two identical AC/DC adapters(Such as laptop adapters) can I achieve this as in the following illustration? Would it be safe?:
There is nothing fundamentally wrong with what you propose. Any purpose built dual output power supply is essentially the same thing. So using this approach can be a workable scheme to achieve a dual output + and - rail output.
One thing that must be considered here is that the two power supplies need to have fully floating outputs.
There are a number of serious disadvantages however. Here are some things to consider:
Can the target device powered by this scheme survive safely if one or the other output becomes 0V because one of the wall warts gets unplugged.
A purpose built dual output power supply will have certain fault detection features that will apply equally to both the + and - outputs which will safely shut things down in case of problems. One example would be a primary side safety fuse that blows and disconnects both the outputs from the mains.
Invariably a dual output supply will share some common components and circuits for supporting both outputs. In particular this would be in the mains conversion and isolation circuit. This can lead to a smaller overall design than using two completely separate power supplies.
- This architecture would work if the wall adapters are isolated (some wall adapters aren't on purpose). No power applied (offline), connect the - terminal of the primary to the - terminal of the secondary, and test the resistance between the remaining + terminals.
- HOWEVER when the Earth is isolated/not present (2 prongs or 3 prongs with the earth prong being made out of plastic for example), you are giving up on a very important safety feature: residual current devices. They cut off the supply when they detect a leak to Earth (a person or a short to case) which can be lethal. You NEED to make sure the secondary voltages are inherently safe for you to accidentally touch, given the environment. The voltage between V+ and V- will be twice V+, in particular. Normally, the isolation would prevent any person from closing the circuit rendering it safe, but if not tied to Earth the voltage can float quite high and a fault would be dangerous.
Edit/note: The safest is 2 terminals plus Earth connected to case, and Line-Neutral of primary isolated from secondary. This way even if there is a fault, the residual current devices of your house will trigger. 2 terminals which are isolated from the primary, Earth absent, is safe in the absence of fault but dangerous if there is one. 2 terminals which are not isolated from the primary, Earth absent, is generally very risky: consider a very simple voltage divider as an example, and imagine the bottom one breaks - the entire line voltage is present at the output...
- AND protect the circuit against overcurrents/short circuits if the wall adapters do not include any, to prevent fire.
- The following is circuit specific: some circuits will behave dangerously if one of the supply rails is missing. Since it's very likely that one adapter may be plugged without the other, this should be taken care of. Say, with some logic that inhibit both if one of them is missing.