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preloading a power source

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wjyates

Member
I have a 12v power supply I would like to preload the power supply with a 4amp draw befor starting the actual load
will adding a 3ohm 50watt resistor in parallel with the load do the trick or should I use something different
 

Sceadwian

Banned
You'll be wasting 50 watts of power as pure heat, and you'll need to use more than a 50watt resistor unless you have a large heatsink attached to it. Why do you need to load it with 4 amps? And are you going to put a switch in to disable this once the load is connected? A few more details about what you're really doing would be helpfull (that is said so much around here it's not even funny)
 
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wjyates

Member
yes there will be a switch, transistor most likely with some sort of currant sensing device. hopfuly when the load is applyed, say 2 amps the currant sensing device will partly close the transistor reducing the resistance to 2 amps threrfor the power supply will never see the change
as to why ? just to see if I can. plus it may become part of anouther project
 

Sceadwian

Banned
Just because is a pretty poor reason to do something, all your effectively doing is keeping the power supply loaded and generating a bunch of heat, what good will that do you? Keep in mind the transistor when it's partially closed will be dissipating a bit of power itself, If you just buy a beafy power mosfet you can skip the resistor completly.
 

wjyates

Member
I have a car alternator and I'm turning it with a motor from a cordless drill. using a seperate power suorce to get it started. ounce removed I can keep it running. but try to add another load it crashes.

I have another post"problem reading diagram" please read let me know what you think
thanks
now I have to learn how mosfets work
no, really.......... thanks for your imput
 
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dknguyen

Well-Known Member
Most Helpful Member
Why can't you just connect the load to the alternator before you start the alternator with a cordless drill?
 

wjyates

Member
I could do that and it would probly work. but I would like to be able to turn different objects on and off. off is easy enough it's turning them on again thats the hard part. so the problem is to be able to get the motor to compensate for the load on the alternator with adding more load to the alternater. does that make any sence.
 

dknguyen

Well-Known Member
Most Helpful Member
Oh, I see. YOu have multiple devices being powered from the alternator and you want to be able to turn them on and off freely without having to restart the alternator because for some reason it shuts down when the load gets too low.

50W...that's kind of a huge resistor just to burn off power, or you'd need circuitry to monitor the output current of the alternator (everything except the resistor) and switch the resistor in whenever that current gets too low.

Any idea why the alternator shuts off when the load gets too low? Why would it stop? It's fuel powered...it's the fuel that keeps it spinning not the load.
 
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MikeMl

Well-Known Member
Most Helpful Member
I have a car alternator and I'm turning it with a motor from a cordless drill. using a seperate power suorce to get it started. ounce removed I can keep it running. but try to add another load it crashes...
Sounds like you are exciting the field of the alternator off its own output. Probably one of the old crummy Motorola-style alternators that had the extra diode-trio.

If you use a battery or other DC source to feed the field terminal, the alternator will always produce output. It only takes a volt or so at about 80mA to bootstrap an alternator. Write back if you dont know what bootstrap means.

A standard automotive alternator will produce about 50A at 14V, or 700W. Since there are 746W per horsepower (HP), it takes a 1HP electric motor to turn a fully-loaded alternator. Your cordless drill motor is likely no more than 1/10 HP.

Why dont you post a diagram of how you have the alternator connected...
 
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wjyates

Member
Oh, I see. YOu have multiple devices being powered from the alternator and you want to be able to turn them on and off freely without having to restart the alternator because for some reason it shuts down when the load gets too low.

50W...that's kind of a huge resistor just to burn off power, or you'd need circuitry to monitor the output current of the alternator (everything except the resistor) and switch the resistor in whenever that current gets too low.

Any idea why the alternator shuts off when the load gets too low? Why would it stop? It's fuel powered...it's the fuel that keeps it spinning not the load.
It's not the fuel that keeps it going,it's not even in my car, it's on my bench.
using 18v electric motor to turn altrenator,once up to speed the altrenators output is 27.5v. using ajustable voltage regulator cuts 27.5v down to 18 more or less witch powers electric motor,witch powers altrenator,witch powers electric motor. at this point I can diconnect power supply and it will run untill something breaks or I turn it off
 

dknguyen

Well-Known Member
Most Helpful Member
Sounds like you're trying to build an overunity generator.
Yeah. Well then, of course it's going to turn off. You got no more energy entering in the system, and nothing is 100% efficient. So every time the energy cycles through the system you lose a bit.

And if you connect a load, it's definately going to turn off whehter it's 100% efficient or not, since now you are sucking energy out of the system with no more new energy coming in.
 
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Sceadwian

Banned
Don't they teach conservation of energy in high school science class anymore? Why do people still build these kinds of devices?
 
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Mr RB

Well-Known Member
I've got a little 12v light bulb inside a cube box made totally from miniature solar cells. There's nowhere for the light to leak out. It's been keeping itself lit up for years!!!
You just cant see it. ;)
 

Sceadwian

Banned
If you open the box does is there a dead cat or a live cat?
 

wjyates

Member
Don't they teach conservation of energy in high school science class anymore? Why do people still build these kinds of devices?
you know, you shouldn't be so negitive. I'm just trying to learn something new
and yes, asking a few dum question along the way. do you remember when you first started. thought so.
do you know how many things we use today that people thought could not be done or thought were stupid. yea me either
so just because it hasn't been done dosen't meen it can't
besides I'm having fun!!!! and what better reason is there for doing anything
oh yea they did teach something abought science in high school but that was abought 10 no 20 well almost 30 years ago. have forgotten most of that
hahahahahaha.
anyway take it easy and have fun. just ordered more parts hope they get here soon hopfuly I won't burn up so many this time. ah well thats half the fun

thanks for your imput and that means all of you
wjyates
 

wjyates

Member
I've got a little 12v light bulb inside a cube box made totally from miniature solar cells. There's nowhere for the light to leak out. It's been keeping itself lit up for years!!!
You just cant see it. ;)
question????? where dose the light go when you turn it off.
 

wjyates

Member
ok enough bashing for now. give me a week or so to recieve my parts and i'll let you all know what happens
later......
 

Sceadwian

Banned
wjyates, the important part is you keep learning. Practice all you'd like doing anything you'd like, but don't replace that with complete disregard of theory. It is upon yourself to educate yourself in what other people have learned over the years, and even more important that you brush up on those forgotten skills from highschool and the scientific method. Before you try to build something look up what other peoples' experiances have been, before you undertake a project, research the facts the fictions and the possibilities, and if you don't like them don't let that stop you, but don't try to be Baron Munchausen and hoist yourself up to the moon on your own bootstraps =)

You have a week to learn in your free time, whatever that may be.
 
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blueroomelectronics

Well-Known Member
Well don't keep up guessing, are you trying to build a motor that generates surplus power to self run?

Yep guess that's what he's trying to do.
 
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