Dropping Vdc

[cels]

I am a novice in the electronics field and am full time sailing with my wife. I know just about enough to keep all the onboard systems functioning. With that out of the way, I wish to modify a small rechargable vacuum cleaner so that it runs directly of the boats 12vdc system. This of course varies from 12v - 13.8v. The vacuum cleaner motor runs at 9.6v and according to my multimeter draws about 7 amps. None of my scavenged collection of regulators can handle that current draw. Is there an easy (and cheap) solution to this?
Thanks, Chris

 

[colin55]

Just put two "one ohm" 5-watt resistors in parallel and connect them in series with one of the wires to the motor.
Or you can try a single 1R 5-watt wire wound resistor.

 

[MikeMl]

Have you just tried running the vacuum directly from the 12 battery? I'll bet it only runs like 10% faster than it would on 9.6V, while not drawing much more current due to the back emf...

 

[Papabravo]

I am a novice in the electronics field and am full time sailing with my wife. I know just about enough to keep all the onboard systems functioning. With that out of the way, I wish to modify a small rechargable vacuum cleaner so that it runs directly of the boats 12vdc system. This of course varies from 12v - 13.8v. The vacuum cleaner motor runs at 9.6v and according to my multimeter draws about 7 amps. None of my scavenged collection of regulators can handle that current draw. Is there an easy (and cheap) solution to this?
Thanks, Chris

Many regulators can be configured to work with a series pass transistor. Look for that term in the application notes for three terminal regulators. I'd also get myself a lab power supply with voltage and current controls to do the experiment where you run the vacuum cleaner from the ship's power supply. That's just me, I'm naturally cautious. If you got your wife to agree to go sailing, you probably don't have any hobby dollar constraints.

 

[cels]

Thanks for the replies.
I have run the vacuum cleaner directly from the vessel's 12v supply, but only very briefly. As you suggest, it did run quicker and I was a little afraid of burning out the motor, although the windings look very heavy.
Also, we are very tight-fisted with our power usage since we rely on solar and wind generation - am I right in assuming that dropping the voltage to 9v - 9.6v will use a little less of our precious electricity?

 

[audioguru]

A 9.6V Ni-Cad or Ni-MH battery is fully charged at 11.2V to 12.0V. It needs a resistor to limit its charging current. When it is fully charged then the charger should be turned off.

 

[mneary]

I like to use diodes (bridge rectifiers like these) for dropping voltage. Although they don't take care of the regulation problem (12V to 13.8V input) their voltage drop is a fairly consistent 0.8 to 1V per diode.

I would use the 25A or 35A 200V units (FWB-252 or FWB-352), because of the solderless terminals and easy mounting. Those terminals are standard 0.25" spade lugs. Voltage rating doesn't matter so cheap is good. I used big cheap bridges like these when the 'low-speed' resistor in my 1964 Chevy heater fan died.

By dropping with diodes, resistors, etc. you're drawling 7A from 12V (84 watts) and delivering 7A at 9V (63 watts) to the motor. If you hooked your motor directly to 12-13.8V it would draw a lot more than 84W, and you can only vacuum so fast.

 

[cels]

Thank you - that's great. I wondered about using a bunch of diodes with the understanding that each one would provide a forward voltage drop of 0.8v. I was unsure as to whether they would handle the current. If I use sufficient diodes to get 10v from 13v, then I'm sure I will be O.K with my on board 12.2v - 13.8v and the vacuum cleaner's motor requirement of 9.6v.

 

[mneary]

Just remember they'll dissipate just as much power as a resistor would.

Larger parts have to be mounted properly to achieve their full rating. That's why I suggest 35A diodes for your 7A motor. The advantages of the bridges I showed are consistent voltage drop, nice terminals, and easy mounting.

 

[Sceadwian]

I would personally just keep using the vacuum on the native 12 volt system. Yes, if you run it for a long period of time you do risk burning out the motor, but as long as you keep the filter clean, don't run over anything that completely stalls the motor and make a good faith effort to make sure it doesn't overheat you're wasting more time than you're going to gain anything from atemping to attach a regulator.

If you're serious, do the research to figure out how to rewind the motor to use 12 volts nativly or replace it with a 12 volt motor rather than the 9.6 volt one. Honestly on many devices they may be the same motor but with better cooling on the 12 volt rated one. Basically if you pay attention and avoid the smell of burning plastic you should be fine.

 

[kchriste]

Also, we are very tight-fisted with our power usage since we rely on solar and wind generation - am I right in assuming that dropping the voltage to 9v - 9.6v will use a little less of our precious electricity?

Too bad you don't have a couple of 10000mah D cells that need charging.
If you are not as tight-fisted with money as with wattage, then purchasing a 12V car/RV vac may be the better solution if the 9.6V one burns out on 12V.
If you want to be a real power miser, look into a manual roller broom. Basically a housing/chamber with two roller brushes which spin when the broom is pushed back and forth. The brushes throw the dirt into a central holding area which can be emptied later.

 

[colin55]

The fact is we have absolutely no idea how much current the motor will take on 12v and no idea of the RPM at this voltage. You have to test the device before you can make any predictions.
We are all just running around in circles.