In a quick look at the JTech site, I did not find a 7.8w laser. I did find a 7w laser. I found this page for upgrading the MPCNC (Ramps board). On that page, they used the fan pin which is 12v. The one trick with the lasers is to make sure the driver will handle the voltage of the signal line. If this really is a JTech laser, they say about their driver, “It also can be remotely controlled with an digital isolated input with a super huge range of acceptable voltages for turning your driver on and off.” So you should be golden using either of the two fan pins on your board. On the JTech referenced page there are also instructions for using the fan g-codes to drive the laser.
After you have it working, you may want to consider enabling the laser support on Marlin for your MiniRambo. Once enabled, you can drive the laser using M3/M4/G0/G1 g-code commands (though you won’t be able to use the fan commands anymore). I posted a “theory” about how to enable the laser for the MiniRambo here. One MiniRambo user followed my outline and was able to get the support working.