About amps:You must always use a power supply unit(PSU, charger) with more amps, than the device you're charging can draw.We don't have stickers on the back of our device, so we can't say for sure how much the device needs to charge. However, we can see the original PSU has 1500mA output.If you're using a PSU with less output than your device draws, it will struggle to meet the demand of the device.In this case the poor 300mA PSU is working it's ass of to charge your (1000 mA?) device. And so, it heats up.Heat is bad. The PSU is just insulated copper wound around a core.. Copper insulation melts at high temperatures, and can cause short-circuits which in turn can cause a fire in your house. I'd strongly suggest you only use the original power supply, unless you want to wake up to a warm fire, or a smoldering heap of plastic on the floor/desk/wall.Side note, this works both ways. So if you have a 3000mA DC 9V PSU using it with the HSG will only draw 1500mA and leave us with 1500mA that's not used.The mA ratings on devices, are simply indication what they can draw as max(device), and can output as max(PSU).