# voltage vs. current

#### PackratKing

Joined Jul 13, 2008
847
It is my understanding [ possibly wrong ] that any electrical device, when supplied with its proper voltage, will only draw the current it needs to operate at peak design efficiency.

Conversely, if supplied an over or under voltage, a device [motor] will promptly " release its magic smoke "

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#### studiot

Joined Nov 9, 2007
4,998

#### Audioguru

Joined Dec 20, 2007
11,249
In a car, all loads should be fused to prevent a fire if a wire gets shorted to ground in a crash.
A current-limiter would get extremely hot: 12V x 20A= 240W of heat.

#### SgtWookie

Joined Jul 17, 2007
22,210
Perhaps they want to run a motor at a slower speed?
Or perhaps they want to protect against a motor operating in a stall condition.

A DC motor when running no-load generates back EMF that causes it to stabilize at a particular RPM. As the load on the motor increases, the RPM decreases as does the back EMF. If the motor is stalled, the current will be at maximum because there will be no back EMF.

#### thingmaker3

Joined May 16, 2005
5,084
Depends on the 12 volt device... Many would not need one. Some might occasionally overload themselves.

#### studiot

Joined Nov 9, 2007
4,998
Why would a current limiter be needed for a 12 volt device to be run off a car battery
A car battery can, and does every time you start the engine, supply hundreds of amps.

The starter motor is designed to accept this for short periods, but other additional devices eg a winch motor may well burn out if it pulls against to great a mechanical load.

#### jpanhalt

Joined Jan 18, 2008
10,073
loosely quoting one wanting to place a 20 amp current limiter of sorts, on his auto battery of 70 some-odd amps. ----- What's the point.

Is that " 70 A battery" the rating in amp-hours or cold cranking amps. If it is amp-hours (which I suspect), the the battery can deliver far more than that if shorted. Thus another reason for protection. John

#### yikes

Joined May 19, 2007
11
[
A DC motor when running no-load generates back EMF that causes it to stabilize at a particular RPM. As the load on the motor increases, the RPM decreases as does the back EMF. If the motor is stalled, the current will be at maximum because there will be no back EMF.[/QUOTE]

Is there a system (method, circuit?) to use or eliminate the bemf?

#### thingmaker3

Joined May 16, 2005
5,084
Is there a system (method, circuit?) to use or eliminate the bemf?
One could use a screwdriver to short the terminals, but doing so would be quite dangerous. Metaphorically, bemf is the electronic equivalent of Newton's reactive force. No bemf means no work being done, and all the input energy being wasted as heat.

A current limiter would prevent this, but as noted by Audioguru, it would itself waste energy. Soft-start control would be more efficient, but more complex.

PackRatKing: what is the device your friend wishes to protect?