PS-1 centipede trying to lower voltage to smoke units

So I have this non powered ps-1 centipede. All components removed, except for 2 smoke units and the lighting board. If I want to pull it around the layout with the smoke units turned on, I would like to know how to limit the voltage to the smoke units. I would normally be pulling it with 18 volts on the track as I use DCS and TMCC/Legacy. At 18 volts smoke is just too much. When I lower to 9 volts I  get just the right amount. So what components do I need to limit the voltage going to the smoke units. I am thinking I may have to insert the component between the smoke unit switch and the positive wire going to the smoke board. Here is a few pics. One pic shows way too much smoke and the other shows 9 volts applied on the track. 9 -10 volts is what I am looking for. I am guessing I need some kind of voltage regulator. The 2 smoke units together draw about 2 amps (displayed on my z4000. If someone knows what I need, can you please be specific with item #'s and where to get the item.  Thanks in advance