AC square wave constant voltage

hi friends,

Do you have an idea or circuit for "AC square wave constant voltage generator" with short circuit protection ?
The spec:
1. The square wave output is 24v 200mA.
2. Max load's resistance is 8000 ohm
3. When the load's resistance is decreased,the current will drop to a value but the voltage always keep 24v.
I have no idea how to keep 24v.

The frequency must be fixed between 100hZ and 100khZ.

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