Using a 5v input with a 500mah draw which resistir wattage should I be looking for? The input will be 3-40 volts and will output 5 volts and sometimes 3.3v would a 1/4 watt resistor handle this on the converter? I am powering my project through a vehicle batteey terminal. I plan on using the LTM8021
Watts is volts times amps (last time I checked).
Multiply by a factor of two or more if you don't want it to get very hot.
(deleted)
Fungus got it. I need to find a 10 watt 19.1kohm resistor apparently. Is there such a thing as a current regulator?
We REALLY need to know what you are trying to do...
The LTM8021 is a voltage regulator. It puts-out constant voltage as long as the input voltage and output current are within allowable ranges.
got it. I need to find a 10 watt 19.1kohm resistor apparently.
No! With the LTM8021, the current to the load does NOT flow through the adjustment resistor. A 1/4 or 1/8th watt resistor should be fine.
Is there such a thing as a current regulator?
Yes. there are "constant current" power supplies. These are somewhat rare, but often used with high-power LEDs. As with voltage regulators, the voltage, current, and load resistance/impedance must remain the allowable range.
The relationship between voltage, current, and resistance (or impedance) is determined by [u]Ohm's Law[/u] . This is a law of nature* and it's always true. If you try to get more current out of a power supply than is available, the voltage will drop (and the power supply might burn-up). If you disconnect the load from a constant-current power supply, current-flow stops completely and the voltage will rise to the maximum available as it "tries" to supply the current.
Generally, most electronic circuits are "constant voltage", which means the voltage doesn't change (or doesn't change much) with the load impedance, but the current does. For example, an output pin on the Arduino is either 0V (digital low) or 5V (digital high) as long as you don't "overload it" and try to get too much current out. This is a generalization, so it's not always true.
- Of course, the units of measure (Volts, Ohms, Amps) are defined by humans.
ViLeGlyph:
Is there such a thing as a current regulator?
Yes.
Using a 5v input with a 500mah draw which resistor wattage should I be looking for? The input will be 3-40 volts...
On the LTM8021 datasheet there is a graph labeled "Load Current vs Input Voltage". It shows you need at least 12V in to get 500mA out.
OK. You want to know the wattage rating required for the resistor on the ADJ terminal of the LTM8021.
Understanding is required. That resistor does NOT have half an amp flowing through it. Looking at the datasheet, that is merely a very low current meant to set the output voltage. I'm not quite sure what current will be there, but if we say 0.8V/100k (page 6, resistor in the block diagram between Vout and ADJ) = 8uA, we can guess that we have a current somewhere in that range through that external resistor.
Power is also equal to Current Squared times Resistance.
19.1k x (8uA^2) = 1.222uW
So... I don't think there is a resistor made with a low enough power rating to be a problem here.
You see, it is important to understand the question before giving an answer, and before accepting an answer.
This is all really useful information thank you all! I am headed in the right direction now. I will be powering my board of my car battery. I know that its rated for 700 amps cca. But I want variable amperage tolerance on the device so that when I swap my battery I wont have to worry about matching amps. Or does the amperage not matter as long as its greatee than the requirement?
A 5V device connected to a 5V power source will draw the current it is designed to draw. It doesn't matter the current available as long as it is as much or greater than the current the device will draw at its design voltage.
The regulator, for instance, is rated to supply a maximum of 500mA when the output is set for 5V. But if you connect a 5V rated device that draws 100mA, that is all the current that will be used. Not 500mA.
http://cds.linear.com/docs/en/datasheet/8021fd.pdf
It is really important for you to study the basics before you start throwing projects together and hoping for the best. Understanding Ohm's Law, for instance, is an absolute requirement. Same for Kirchoff's current and voltage laws. And understanding of at least the 2nd approximation of semiconductors, as well.
DVDdoug:
The relationship between voltage, current, and resistance (or impedance) is determined by [u]Ohm's Law[/u] . This is a law of nature* and it's always true.
Its not a fundamental law of nature though, its an emergent property of various materials due to electron/lattice
and electron/phonon interactions.
Ohms law breaks down at high current densities (very high), and only applies to metals(*) and semiconductors of uniform
composition. Gases don't obey it, a vacuum doesn't obey it (insulators in general don't obey it), supercoductors don't
obey it. Plasmas don't obey it. Quantum tunnelling devices don't obey it.
(*) above absolute zero by a sufficient margin
ViLeGlyph:
This is all really useful information thank you all! I am headed in the right direction now. I will be powering my board of my car battery. I know that its rated for 700 amps cca. But I want variable amperage tolerance on the device so that when I swap my battery I wont have to worry about matching amps. Or does the amperage not matter as long as its greatee than the requirement?
Your car's batter amperage rating is about what the batter can supply, not what it must supply. What matters is how much your particular circuit is drawing. If you are pushing your regulator to provide its maxium current rating of 500ma, then your regulator will be drawing something less than 1A from your car battery, but more than 500ma. The reason is that the regulator is not 100% efficient. That means it consumes some amount of power just to provide power to your circuit.
So short, answer any battery that will start your car, will work fine with powering your circuit.
Most linear regulators are very current efficient(*), but power efficiency depends on the ratio
of output voltage to input voltage.
(*) they draw something in the order of 1mA to power themselves, so a 500mA regulator providing 500mA
will be drawing 501mA or so, but wasting 3.5W due to the 7V difference between input and output.
Read the datasheet.
http://cds.linear.com/docs/en/datasheet/8021fd.pdf
It is a switchmode power supply. It does -not- require 12V in to get 5V out. However, with 12V in and 5V out (page 5 of the datasheet, top center), with 500mA out, it requires 250mA in.
MarkT:
Most linear regulators are very current efficient(*), but power efficiency depends on the ratio
of output voltage to input voltage.(*) they draw something in the order of 1mA to power themselves, so a 500mA regulator providing 500mA
will be drawing 501mA or so, but wasting 3.5W due to the 7V difference between input and output.
polymorph:
Read the datasheet.http://cds.linear.com/docs/en/datasheet/8021fd.pdf
It is a switchmode power supply. It does -not- require 12V in to get 5V out. However, with 12V in and 5V out (page 5 of the datasheet, top center), with 500mA out, it requires 250mA in.
I appologize for my imprecision in specifics, I didn't bother to look at the data sheet, nor does it really affect my point. The capacity/specifications of the OP's car battery will never cause him/her to have any issues with his projects power supply-If it can start a car it can provide whatever current a 0.5 A power supply is capable of demanding.
Using a 5v input with a 500mah draw
What do you mean by this ? Input to the arduino, or input to something else ? An arduino input is not going to draw 500 mA. It is not going to draw 500 mah either.
Electronics does demand precision.
mah = ??
ma = ??
mA = milliamperes or thousandths of an amp
mAh = Rated capacity of a battery as a function of current times time. IE, 500mAh would mean that you can draw 500mA for one hour and then the battery is effectively empty.
MA = Megaamperes or millions of amperes
ViLeGlyph---STOP STOP STOP---
Do you know the difference between voltage, current, power and resistance.
If not then you need to learn about these before building your regulated supply.
Do you have a DMM, Digital Multimeter, if not -----STOP STOP STOP---
Do you know the difference between voltage, current, power and resistance.
I hate to repeat myself, but it looks to me that your understanding of electrical current needs improving.
Tom......... 