Outer Limits 4x4 Board

what size resistor do i need to drop from 12 to 1.5 volts. its for a small air pump on my boat

cheers Brad

The voltage drop across a resistor depends on the current flowing through it - so we can't calculate a resistance value without more information.

If the air pump has a small DC motor the current will probably vary depending on the pressure, so a resistor may not work well.

A voltage regulator is the obvious answer, but I don't think they are available in 1.5V. So you could try a 5V regulator (very common) and then drop the 5V down to 1.5. A zener diode could do this, but without knowing the current draw (power dissipation) we can't size either the diode or the regulator properly either.

AA battery not sufficient?? You can get D cells up around the 7-8 AH mark or parallel up some tag topped AA cells to get the AH capacity you need, then use a resistor and a diode to provide a charging current from the boat battery. The batteries themselves will work as a voltage regulator.

The good thing is your pump will still work if you boat battery or electrics dies in the arse (more common due to corrosion etc, but you already knew that )

As Scott said, more info = more help, especially current needed, and time needed to run.

ok it is an aerator yes it runs on D sized batteries but the switch is stuffed and the batteries are a pain in the arse, so i was thinking i could just run the pump with a swithch off of the boats battery. I dont think there are any markings on the motor but i will have another look.

thanks Brad

Could try something like this. Although you'd probably want to drop the voltage to 5v (8v as a minimum) before cutting it down to 1.5v. This will prevent too much heat being dissipated by the regulator. Nice chunky heat sink would be beneficial too.

http://www.jaycar.com.au/productView.as ... rm=KEYWORD

http://www.dse.com.au/cgi-bin/dse.store ... duct/Z6578

You'll need to add a few other components (and possibly a heatsink), but it will do exactly what you want with a guaranteed O/P voltage.

try it on 12vlot. We ran a 1.5 volt motor on 12 in a scalextric car. Need less to say the thing was the fastest on the track. still going strong

pongo wrote:try it on 12vlot. We ran a 1.5 volt motor on 12 in a scalextric car. Need less to say the thing was the fastest on the track. still going strong

heh.

either way i reckon it would be better to find a 12V solution than to stuff around organising 1.5V for it.

eg: http://www.greatoutdoorsdirect.com.au/p ... volt-.html

probably cheaper than any of the voltage step down options.

thanks guys just bought a 12volt motor for it cost like 5 bucks

thanks Brad

92mav wrote:thanks guys just bought a 12volt motor for it cost like 5 bucks

thanks Brad

Good choice