Dear all,
I read <<Intrinsic safety circuit design>> from Paul S.Babiarz. Somewhere make me confuse, may you give any advice?
Like you see in the documentation they apply 120V on low voltage circuit.
The whole idea of intrinsic safety is to limit the energy which can flow from the safe area to the hazardous area (explosive gasses etc).
To acheive this energy limitation devices known as "IS Barriers" are used.
The power to a circuit which uses a barrier is usually 24v DC.
Anything higher that about 26volts (from memory) will "blow the barrier", isolating the hazardous area from the supply.
120volts is NEVER used in an IS circuit.
I'm wondering what type of power supply they may have? how to transform 120Vac to DC voltage?
Usually regulated 120 or 240 volts input power supplies with a 24volt output.
Something like these shown here. (Yes, there are lots of different types).
The power supply may be linear or switchmode.
JimB
On edit:
It is not the circuit that is intrinsic, there is no such thing as an "intrinsic circuit".
It is the safety which is intrinsic.
The idea is that the circuit from the safe area (for example a plant control room) out to the instruments in the hazardous area (a petro-chemical process plant), is intrinsically safe.
That is the instrument circuits cannot pass dangerous voltages and currents out to the field.