need help reducing voltage and current

[jclaudii]

I need help reducing my car battery from 12v down to 8v and 1.5v The amperage also needs to come down to I persume since my car battery produces something like 800 amps. I am thinking about using resistors but I'm not sure which ones. I've gotten this far the device I use uses 8 v so 12-8 =4 4\20a=.2ohms Now How big does this have to be to handle at least 10 amps easly?

The other sid is 12-1.5=10.5 accross circuit. 10.5\5=2.1 Ohm This one is the one that is giving me the most trouble because it has to disipate heat fast I am leaning toward a 3w or 5w wirewound encased in concrete. what do you think?

 

[folarinv]

I need help reducing my car battery from 12v down to 8v and 1.5v

You may simply use voltage divider circuit. i.e if you connect 5k and 10k across the 12V, you get 8V just across the 10k.

 

[instruite]

jclaudii:

Now How big does this have to be to handle at least 10 amps easly?

Are you planing to drive some motors or solenoids together??

To get the reduced voltage use some voltage regulator IC
But I doubt any voltage regulator available will be able to provide 10A

I've gotten this far the device I use uses 8 v so 12-8 =4 4\20a=.2ohms

For resistor Wattage; W = V * I (W)
V = 4V, I = 20A (as you have taken in caculation)
W = 4*20 = 80W
I doubt you will get resistor of this wattage

Tell us about your application we may be able to help better

 

[stevez]

It sounds like you have a 12 volt battery and you want to power some things that require 8 volts and some things that require 1.5 volts. It would help to know what it is that you are trying to power and how much current would be required.

Resistors may or may not work - it all depends on your load. If the 8 volt load is a constant 10 amps then a series resistor that will provide a 4 volt drop at 10 amps is what you'll need. You'd need a 0.4 ohm resistor rated for at least 40 watts dissapation.

The problem - if the load isn't a constant 10 amps then the current thru the 0.4 ohm resistor won't be constant and the voltage drop across the resistor won't be constant - so you won't get a constant 8 volts. How much variation there is is quite dependent on the load. If the load were a light bulb the load would likely be constant - once the bulb is fully on. A motor might present a different problem.

If the load isn't somewhat constant and you need a relatively constant 8 volts from a 12 volt source you'll need some kind of regulator. A linear regulator could be fashioned from a 3 terminal regulator with pass transistors to handle the current. A switching regulator might prove useful in this application and would likely produce somewhat less heat. As with the resistor, more needs to be known about what you are working with.

 

[evandude]

First off... you do not need to "reduce" the current of your battery. The battery does not blindly supply 800 amps to anything you plug into it... V=IR... with the voltage fixed at 12v, the current depends on the resistance (the load) You only need to reduce the voltage, and make sure the voltage regulator can handle the current of whatever load you're trying to use.

do yourself a favor... don't use resistors. As said before, they will only work properly with a constant load, and if you're up in the 10 amp range, that seems unlikely.

If you need 10 amps of output current, you're getting into switching regulator (DC-DC converter) territory... building your own is NOT something it sounds like you should be building yourself (they can be very difficult, even for those with a lot of electronics knowledge)

So your best bet is probably to try to find a commercial DC-DC converter to buy.

Why don't you tell us what you intend to power?

 

[jclaudii]

Ok...I put this in another post with little respone. I have a r/c car that runs on Nitro. They are very tempermental when it comes to starting. The factory sends this little box with a 7.2 battery pack and a lil box that turns the starter motor (standard hobby motor) and also supplies power to the glow plug ( not sure what the amperage is on this, it just has to glow bright red. I have a ac-dc converter for tinkering but it only goes up to 3 amps. It won't supply enough current to turn the lil motor over and it will light the glow plug but it pegs the needle out so I'm guessing it is more amperage than it can support. The glow plug is basically a heating element with resistance on it. I decided to go with 8v to the starter and 1.5 to the glow plug.

Now the current will fluctuate on the starter when starting the engine, it is high compression. It starts fine with the 12v applies to it except it is going to greatly reduce the lifespan of my lil engine.

Now you guys know what I'm trying to build mabie you could help me further. I mean, Afterall I'm not building a switching powersupply for a carputer.

Thank you guys for taking the time to read and post.

 

[evandude]

Okay, well despite what I said in my previous post, in your case resistors should be OK. an electric starter motor and glow plug aren't going to be very voltage sensitive. You will just have to experiment with proper resistor values for each one. For the starter motor you are going to need a resistor of less than 1 ohm, and you should probably stick with 10 watt resistors to be on the safe side. I would advise you to buy a couple of 1 ohm 10 watt resistors, and a bunch of 0.1 ohm 1 watt resistors. That way you can experiment with different parallel and series combinations of resistors to find out what value gives you the right amount of current to spin the motor without over-stressing it (start with a high value, like 2 ohms, with the 1-ohm resistors in series) and once you find an appropriate value, you can either permanently solder up your combination of resistors, or try to buy a single resistor with that value (or close to it) and proper power rating.

as for the glow plug... well it's hard to say what current that is going to require, you should probably try to use a multimeter to measure the current it draws and choose appropriate resistors from that.

 

[zevon8]

There are many circuits on the Web that will show you exactly what you will need to do. Basically you could use a PWM driver circuit. This will supply the full 12 volts in small pulses, that when averaged out, do the same work as a fixed lower voltage. Think of it as "blipping" the throttle in short bursts to keep a vehicle moving along at a slower speed.

Some links to get you started:

http://www.fer.nu/rc-electronics-links.htm

Try a search for "rc glow plug circuit" in google and you will get alot of pages. This is a very common R/C project that many others have done.

 

[jclaudii]

dealing with the heat issue. Now on those 10 watt resistors is their any way to mount heatsink to them. The factory resistor would heat up so hot it melted my lil plastic box it was in. I plan to keep these in the open without them being coverd. Also for clarification most 10 watt reistors are those encased in the ceramic housing.

Thanks for the help I'll hit radio shack up later this week and let you know how it turns out.

 

[Nigel Goodwin]

dealing with the heat issue. Now on those 10 watt resistors is their any way to mount heatsink to them. The factory resistor would heat up so hot it melted my lil plastic box it was in. I plan to keep these in the open without them being coverd. Also for clarification most 10 watt reistors are those encased in the ceramic housing.

You can clamp them on to heatsinks, but even better is to buy bigger resistors, which you can get in metal housings with mounting holes to fit directly on to a heatsink.

 

[evandude]

yeah, those metal ones can be expensive but they may be a good solution...

you can clamp the resistors to a metal heatsink of some sort, but as nigel said, using a higher-wattage resistor is generally a better option. Alternately, you can make a larger series-parallel combination to get the wattage you need. (for instance, last year I needed a 200W, 20 ohm resistor, and I had 20 ohm, 50 watt resistors... so I made two pairs of two 20 resistors in series per pair, and then put the pairs in parallel. total resistance of each branch = 40 ohms, then in parallel it goes back down to the desired 20 ohms... total power rating of the combination = 200 watts)

basically you split the power load across multiple resistors so each one is only dealing with a fraction of it, and then none of them should get very hot, especially since your starter shouldn't be running for a very long period of time at once.

 

[Nigel Goodwin]

Yes, I made some dummy loads last year, two lots of eight 1 ohm 17W resistors in series - just soldered across tagstrips, with plenty of spacing for free movement of air.

I can either use them as two 8 ohm loads, or wired in parallel to give one 4 ohm load (or even in series for 16 ohms if required?).

 

[Roff]

<snip> an electric starter motor and glow plug aren't going to be very voltage sensitive. <snip>

My understanding of DC motors is that starting torque is severely hampered by series resistance. The voltage drop across the resistor is highest when the motor is stopped, because the motor is creating zero back EMF. Seems to me this is exactly what you don't need when trying to spin a high compression engine.
What's wrong with the 7.2V battery and a charger? It won't spin the R/C motor fast enough?

 

[evandude]

perhaps it would be simpler to replace the motor in the starter box with one that can be run directly from 12v?

 

[zevon8]

I would do the PWM circuit. Some are simply a decent sized FET driven by a 555 timer, with a pot controlling the duty cycle. You can take out the glow plug and experiment with duty cycle settings that give you a nice glow.

Here is a circuit and layout similar to what I mean:

https://www.webx.dk/rc/glow/glow.htm


Do you have a cordless drill? they make great starters with a rubber nose cone attachment.

 

[evandude]

Do you have a cordless drill? they make great starters with a rubber nose cone attachment.

he's starting a nitro R/C car, not a plane :lol:

 

[zevon8]

/me slaps self

I guess he needs a spline drive for the drill then, LOL

 

[jclaudii]

thought about the drill. but I would have to expose my crankshaft each time I started...and drove it....so it would be open to debris. I am not leaning toward the starter box yet because I don't have the money. The 7.2 battery just dosen't last as long as I would like and the weaker it gets the weaker the glow plug which makes for crappy starts. So I got a sealed lead acid 12v motorola battery out of a bag phone (remarkbly it was still in the box LOL) Plugged it into a ac-dc converter and pumped 14.5v at 3 amps into it and it charged. Now I could use that battery for portable starts and if it goes dead, just have some aligator clips handy and hook it to the battery on my mustang.

Traxxas has a device called the "E-Z Start" It is a great idea for beginners and is a heck of alot betterh than pull starts. I would like to utilize the same device except for the control box. I believe the resistors in paralel with smaller resistors to reduce the glow plug should be the way to go. When I get paid in about a week I'll buy some supplies to test this method out. You know some people have even mounted a small lightweight 7v battery on the r/c and using a thrid channel radio they can start the car remotely! Thats pretty cool.

Thanks alot guys for the posts I really appreciate it. Now when I get the project done are you all going to help me out with my dc power supply for use ina car computer?