12v to 8v
 

Hello all,
First let me state that I know very little about electronics and this is probably a very simple question. What is required to convert a 12v DC power source down to about 8v. I have an electric fuel pump that just runs to hard on my 12v battery and I would like to slow it down. Any suggestions?
Thanks

12v to 8v
 

The easiest method to reduce the voltage from 12 to 8 vdc is to use a potentiometer or variable resistor. You can purchase it a your local radio shack store. The only criteria is to make sure that the power rating is sufficient to handle the current load. I do not know the current draw of your electric pump but if you can find it on the pump itself you can use the following equation to determine the power rating in watts the potentiometer would have to be. Power = current (in amps) X voltage (12 Volts). For instance, if your pump has a current draw of 1 amp and operates on your 12 volt battery you would need a potentiometer of 12 Watts to handle the current. The good thing about the potentiometer is that you can mount it in a small enclosure and put a knob on it so you can adjust the resistance and hence, the voltage to the pump. It just makes it adjustable to your desired operation. Scott

12v to 8v
 

Honestly, get a manual fuel pump (hand crank) - they work great. Keep the electric one though, they do come in handy. The manual pump would solve your problem. Many models to choose from and they are pretty inexpensive.

With very little knowledge in electronics, you are likely to waste some time and money on this. If you already own a multimeter and soldering iron, then go for it ... I guess.

I agree with cres24. A resistor (fixed or variable) of the proper resistance (or resistance range) could be used to decrease the voltage at the fuel pump. There are many ways to solve your problem ... the variable resistor seems like a good solution - assuming you can find an appropriate one.

Radio Shack probably has the part you need ... you just have to come up with the right value and make sure the power rating is sufficient.

Without getting too technical, the power rating of the resistor is based on the voltage across the resistor. In this case, you are probably only going to have a few volts across the resistor (the rest of the voltage will be across the pump - like you said perhaps about 8 volts or so) ... so hopefully a 12 watt resistor will not be needed. Radio shack and other electronics stores will certainly carry resistors that handle 5 and 10 watts .. no problem. If it turns out that you need more power dissipation, the parts get a bit more expensive and you might have to mail order them. This is not that big of a deal - hopefully.

My old electronics lab in college had these nice little variable resistor boxes. With access to one of these, you would be in business. Splice that baby into the circuit and run the pump. Start of with a value of 0 ohms, and increase the resistance until the pump slows to the desired speed. Then just read the resistor value off of the dials and - bingo you know the resistor value needed. Then measure the current in the circuit and calculate the power dissipated by the resistor (power = current squared * Resistance).

If I had to take a wild guess, perhaps something in the 25 to 50 ohm range would be good. Remember this is totally off the cuff, I never measured the current draw of an electric fuel pump. Assuming the pump draws 100ma (at the desired speed), the 50 ohm resistor would dissipate 1/2 watt, at 200ma the 25 ohm resistor would dissipate perhaps 1 watt.

Radio Shack has some parts that might work. They even have a 25ohm - 3watt variable resistor ... this might work. Probably could handle the power as well .... but like I said ... need to know the current draw to be sure.

Here is a pic of the circuit - it's about as simple as they get. Also put some pics of some likely parts that would work.

Good luck,
Moses

Works great
 

Thanks for the replies. I couldn't find the amps for the pump on its case and I threw the box away. I went ahead and bought the Rheostat from Radio Shack and it works great! Thanks for the input.

PS I have used the manual pumps for years, just wanted to have a electric this time, thanks for the input.

12v to 8v
 

Yep, the only way to know for sure was to splice something into the circuit and test how it works... glad to hear it worked out.

Moses