PDA

View Full Version : Simple Power Question



Navic
08-31-2011, 03:55 PM
I've read the typical water system analogy when it comes to volts amps and resistance but I can't find how electrical power will affect devices. Voltage is the water pressure, amperage is the flow rate, resistance is the pipe size and these characteristics create a specific amount of water at the end of the pipe in which devices like chips and LEDs will use to function. I try to see devices as jars at the end of the pipe. If a chip says it needs 5V and will draw 50mA of current then that's the size of this jar at the end of the pipe. If too little voltage or amperage is present in the water system, the chip 'jar' won't fill up with water, which is analogous to not powering on an operating - right? If there's too much voltage or amperage coming through the pipe into the jar it will overfill with water and crush, which is analogous to burning up the chip due to high voltage or current - right?

If that is correct then I understand voltage requirements because that information is easy to find in datasheets, they typically always list the min, max and ideal voltage for the device but current gets me stumped. Datasheets will list how much current the device will draw, but I don't see maximum current values very often. Is this because amperage is different than voltage?

This is probably a bad way to design, but when I create projects and then want to hook up the power source I always worry about voltage because that's typically the same for all the components in the project, say 5V. Ok, fine but I never look up current draws of the components. If I look at a project that has say a BS2, two servos, a Ping sensor and a compass module I just think "i'll set the power supply to 1 amp, that'll be fine" and it always is because it's ok to provide more amps than the entire circuit needs and it won't burn up - but provide more volts than the components need they will burn up.... why?

I tried to envision amps as something that's 'as needed' by devices and only a lower than normal amperage will stop the device from operating, but higher than normal amperage will power the device and not burn it up - not true about voltage. Is this the correct way of thinking about amps?

When I check the required volts and amps on a retail device, they're always on the back along with the polarity of the connector, you know what I mean but sometimes the device will say something like 0.5A max.... why? I match device requirements and power supply specs all the time but I've seen people take a device that says 12V 1A on the sticker and plug it into a wall wart that's 12V 2A and the device doesn't burn up. If the wall wart was 24V 1A the device would burn. Too much voltage is always bad, but it seems too much amperage is ok sometimes, but not all times.

Please help me understand how amps affect devices in a circuit and if there is a real danger in supplying more current then listed on the device.

Thanks!

Leon
08-31-2011, 04:26 PM
Datasheets usually specify the maximum total current that a device should take, and the current limit for individual pins, in a section called something like Absolute Maximum Ratings. For instance, the PIC18F1220 has a maximum of 300 mA out of the Vss pin and a maximum of 250 mA into the Vdd pin. Each I/O pin can sink or source a maximum of 25 mA. Exceeding values specified in the Absolute Maximum Ratings could result in damage to the device.

Actual current taken by a device when it used in a real circuit depends on the load(s) connected to it. It is up to the designer to calculate the current in or out of each pin that is used, and ensure that the maximum ratings are not exceeded.

bill190
08-31-2011, 04:26 PM
Let's see...

Let's talk water pressure and relate that to voltage. Take a balloon, place it over a water faucet, turn on the water, then the balloon will fill and then burst. Take the same balloon, attach it to a soda pop plastic bottle filled with water, turn it upside down and poke a hole in the bottom of the plastic bottle so the water can drain into the balloon, then the balloon might fill, but not burst. (Lower water pressure.)

So voltage can be thought of as water pressure.

Then think of amperage as an attachment on a garden water hose. You can get one with a small opening and just a trickle of water flows. Or get one with a large opening and a large amount of water flows.

So amperage can be thought of as how much water will flow based on the "opening".

Bobb Fwed
08-31-2011, 04:34 PM
Responses in red:

I've read the typical water system analogy when it comes to volts amps and resistance but I can't find how electrical power will affect devices. Voltage is the water pressure, amperage is the flow rate, resistance is the pipe size and these characteristics create a specific amount of water at the end of the pipe in which devices like chips and LEDs will use to function. I try to see devices as jars at the end of the pipe. If a chip says it needs 5V and will draw 50mA of current then that's the size of this jar at the end of the pipe. If too little voltage or amperage is present in the water system, the chip 'jar' won't fill up with water, which is analogous to not powering on an operating - right? If there's too much voltage or amperage coming through the pipe into the jar it will overfill with water and crush, which is analogous to burning up the chip due to high voltage or current - right?
I would say a device is more like a hydraulic wheel that sends drains the water away. Because devices don't get "filled" with power (like a jar), they continually draw it.

If that is correct then I understand voltage requirements because that information is easy to find in datasheets, they typically always list the min, max and ideal voltage for the device but current gets me stumped. Datasheets will list how much current the device will draw, but I don't see maximum current values very often. Is this because amperage is different than voltage?
Amperage that is listed is usually maximum (unless stated otherwise -- usually there is at least nominal and maximum current for different operations on the device). If the pump can sustain a maximum water flow of X gallons per minutes (amps), your hydraulic wheel can continue spinning at it's intended speed, because the expected pressure is maintained. If there is "too much amperage available" it just is not used. If there is no amperage flow, there is no electrical movement, but the pressure remains.

This is probably a bad way to design, but when I create projects and then want to hook up the power source I always worry about voltage because that's typically the same for all the components in the project, say 5V. Ok, fine but I never look up current draws of the components. If I look at a project that has say a BS2, two servos, a Ping sensor and a compass module I just think "i'll set the power supply to 1 amp, that'll be fine" and it always is because it's ok to provide more amps than the entire circuit needs and it won't burn up - but provide more volts than the components need they will burn up.... why?
There are a number of reasons, one that you could reproduce in a circuit is designing a simple voltage divider. If you don't expect more than 5V you can design a circuit that draws XmA, and be within the wattage ratings for the resistors. Then is someone (or yourself) unexpectedly applied 10V to the circuit, you wouldn't have doubled your wattage, it quadruples, because of math (and physics). Similar things can happen in an IC, as well as the space between the "traces" on the die being designed for a certain voltage, then if too high of a voltage is supplied, the power can jump traces and essentially short (or at least put power where it isn't supposed to be).

I tried to envision amps as something that's 'as needed' by devices and only a lower than normal amperage will stop the device from operating, but higher than normal amperage will power the device and not burn it up - not true about voltage. Is this the correct way of thinking about amps?
Yes, and as a generally rule, even if you calculate your amperage use for a device, you should double the number, to A) be safely within the bounds of your power supply, and B) keep your power supply in a non-stressed state, in theory, they may last a little longer, if not only because they are staying cooler.

When I check the required volts and amps on a retail device, they're always on the back along with the polarity of the connector, you know what I mean but sometimes the device will say something like 0.5A max.... why? I match device requirements and power supply specs all the time but I've seen people take a device that says 12V 1A on the sticker and plug it into a wall wart that's 12V 2A and the device doesn't burn up. If the wall wart was 24V 1A the device would burn. Too much voltage is always bad, but it seems too much amperage is ok sometimes, but not all times.
Again, too much amperage on the supply is not a problem, unless the supply is expected to have a "short" detector. Some things (like USB) are expected to only have a certain amount of amperage available, and some devices use that information in their design. As for a "maximum" rating, the device (at it's discretion) can draw less current. Think of a TV, when it is off, it draws way less current then when it's on. But the amount of available current is always the same.

Please help me understand how amps affect devices in a circuit and if there is a real danger in supplying more current then listed on the device.

Thanks!

Navic
08-31-2011, 04:38 PM
In response to Leon's post - if the max into Vdd is 250mA and the PIC is the only device in the circuit, will it burn up when 500mA is applied to Vdd no matter what or will only 250mA be used by the PIC and the other 250mA ignored?

bill190 - thanks for the explanation. A device is the balloon in your analogy, correct? If the pressure (voltage) is regulated but the attachment on the garden hose has a large opening so a large amount of water flows into the balloon it won't burst because only too much pressure will burst the balloon and since the pressure is already regulated, filling the balloon quickly with the large opening from the hose is fine?

Navic
08-31-2011, 04:42 PM
Thanks Bobb, that answered my question - too much amperage supplied to a device is ok because the extra just isn't used

Tor
08-31-2011, 04:43 PM
I think it's easier if you use Ohm's law instead of relying too much on the waterpipe analogy.
So:
it's ok to provide more amps than the entire circuit needs and it won't burn up - but provide more volts than the components need they will burn up.... why? This is because the current (amps) that the circuit will draw depends on the voltage and its resistance (it's better to think simplistically about this - there are some other factors when semiconductors are involved, but the basic definition works out anyway): Ohm's law says that current is voltage divided by resistance:
I = U / R
So, if you increase the voltage the current will increase, and the number of watts is U*I. So, increase voltage and current will increase, thus watt, which is heat, increases, and it burns up at some point.

When you look at power supplies it doesn't matter how many amps it can provide, as long as it can provide _enough_. So, if your circuit needs 5V and 1A, then you need to use a power supply which provides 5V (not 4 or 6), and 1A _or more_. So, 5V 20A is OK. The circuit won't draw more than 1A, because that's fixed/limited by (what ends up as) its resistance.

If, however, your power supply can only provide 0.5A at 5V then it will kneel when you connect it, i.e. it will either drop its voltage, or, if protected, shut off, or an internal circuit breaker in the PSU will pop.

-Tor

Bobb Fwed
08-31-2011, 04:43 PM
Keep in mind the current ratings are for both supply and draw. If the supply current exceeds the draw current, there aren't any problems, the voltage is maintained, and no devices are going hungry. If the draw current exceeds the supply current, it's possible the supply will be damaged (due to over-current or heat), and the available voltage (on most modern regulators anyway) will drop, until the amp rating is met (because generally a lower voltage means a lower amp draw -- but keep in mind: not always).

Navic
08-31-2011, 04:52 PM
Thanks guys for clearing this up for me. What got me really confused was finding a device (thrift store junk video game) and the sticker on back said 9V, (0.8A Max). Max just blew my mind because like you all said, if there's extra amps on the circuit it doesn't matter. Perhaps as Tor mentioned that the relationship current has with voltage and resistance means the entire device might generate too much wattage if supplied with more than 0.8 amps and then over heat?

bill190
08-31-2011, 04:56 PM
So far as an over amperage situation or a device using too much amperage, I can't think of a way to relate this to water!

Maybe a landlord (appliance) and a renter (extention cord) and a bank (electric plug in house)...

If the renter makes $1500 a month and can afford $500 a month rent and the landlord takes $500 a month rent, then the renter is ok and has enough money left over for food and other things...

But if the landlord tries to take $2000 a month for rent, the renter will quickly get mad (heated up!) because he does not have that much money coming from the bank and that much money can't "flow through him" from the bank to the landlord.

But take a "larger renter" (larger wire size) who makes $10,000 a month, then $2000 a month could easily "flow" from the bank, to the renter, and then to the landlord. The renter would remain "cool".

So same thing with wire sizes and amperage. If you use too small of a wire to connect something which uses a lot of current (flow), it will heat up - possibly get red hot and cause a fire.

And this is the case with a regular light bulb. It is too small of a wire, so it gets white hot. But the power cord to the lamp for the light bulb is "large enough", so it remains safely cool to the touch.

Then everything electrical has wiring or conductors inside it. That wiring or the conductors are a certain "size". They can only handle so much "flow". Exceed that and the wires or conductors will heat up and possibly cause a fire (or melt - then the device will no longer work).

Mike Green
08-31-2011, 04:59 PM
For a power supply, the current rating is the maximum amount of current that it can supply at the nominal voltage stated. An unregulated AC supply often acts as a voltage source at some higher voltage which is in series with a resistor (the internal resistance of the supply). If you draw very little current, a 12V supply might actually provide 15V or even higher. As you draw more current, the voltage will quickly drop to just above the specified voltage. As you draw more current, the voltage will drop slowly until you reach the specified current where the voltage should be as specified as well. If you attempt to draw more current, the voltage will drop further until something breaks in the power supply or the voltage drops to essentially zero. For a regulated supply, the output voltage should remain very close to that specified for the entire current range. Beyond the maximum, the voltage should rapidly drop to near zero. The specific regulator's datasheet should have a graph showing this (output voltage vs. current in the region around the maximum current). Regulators are usually specified in terms of how much (maximum) current they can supply on a constant basis and how much additional current they can supply for a short time.

Most devices don't give a current draw because it depends on what the device is doing and what it's connected to. The Propeller, for example, draws a baseline current depending on its system clock speed. The faster the clock, the more current, and the current goes up roughly as the square of the clock speed. On top of this, each of the 8 processors (cogs) in the Propeller draws current depending on the clock speed and whether it's waiting for some condition or executing ordinary instructions and the current drain varies slightly depending on what kind of instruction. When you connect some I/O device, like an LED, to an I/O pin, it's the I/O pin that provides the power for the LED and that comes from the processor's power supply.

The idea is that you have to figure out the maximum amount of current needed by all your devices under normal (average) operating conditions and under worse case (peak) conditions and match your power supply to that so that the supply can supply what's needed under all conditions. Keep in mind that motors' current drain can vary over a very wide range depending on the mechanical load. A servo motor may draw 0.25A when it's running lightly loaded and over 1A under heavy load or nearly stalled.

Leon
08-31-2011, 05:15 PM
[QUOTE=Navic;1032066]In response to Leon's post - if the max into Vdd is 250mA and the PIC is the only device in the circuit, will it burn up when 500mA is applied to Vdd no matter what or will only 250mA be used by the PIC and the other 250mA ignored?
/QUOTE]

That's wrong! The current taken depends on the load.

bill190
08-31-2011, 05:21 PM
So far as something using "too much amperage", fuses are designed to allow enough amperage to "flow" for a device to work, but will heat up and melt if too much amperage is drawn (thus it cuts off the flow of electricity and protects the device).

Here is a video of a fuse and they draw more and more amperage through that fuse until it turns red hot, then melts (as shown on the amperage meter)...

http://www.youtube.com/watch?v=QjE1k17MsqM

Tor
08-31-2011, 05:48 PM
Thanks guys for clearing this up for me. What got me really confused was finding a device (thrift store junk video game) and the sticker on back said 9V, (0.8A Max). Max just blew my mind because like you all said, if there's extra amps on the circuit it doesn't matter. Perhaps as Tor mentioned that the relationship current has with voltage and resistance means the entire device might generate too much wattage if supplied with more than 0.8 amps and then over heat?
No, the only possible meaning of '0.8A Max' is that it will draw no more than 0.8A Max. Which is just another way of saying that it requires a power supply of 9V which can provide at least 0.8A.

-Tor

Peter KG6LSE
09-01-2011, 07:20 AM
So far as something using "too much amperage", fuses are designed to allow enough amperage to "flow" for a device to work, but will heat up and melt if too much amperage is drawn (thus it cuts off the flow of electricity and protects the device).

Here is a video of a fuse and they draw more and more amperage through that fuse until it turns red hot, then melts (as shown on the amperage meter)...

Most of the time I see a fuse It is there to not protect the device. But to keep the supply from becoming a fire ball on the wall .


il give a ex here


DVD player has a 1 amp rating . @ 120V 120 W

The guts draw 10 amps at 12V DC 120W
Lets say the main CPU runs at 5 amps . 0.5 Of the total 10 amps ..

Lets say it blows and is now drawing 7.5A

So 7.5 + 5 = 12.5 amps @ 12V
because transformers can translate current draws between the windings we now have the
120V side drawing 1.25 amps VS 1.0 with this kind of over load I doubt the fuse will blow .

And this is why

If the nominal Current is 1A most EEs I know would put a 1.5 A or a 2 A fuse to keep a dead short from fireballing .'
A fuse is a heat ( current ) device in the end and it is bad fung shue to run a fuse right at its limit for long terms .
So now that we know that the fuse is 1.5 A there is no way we will blow it with a goofed up dvd player CPU .



Peter

Loopy Byteloose
09-01-2011, 09:55 AM
This may help - a non-mathematical approach.

Power consumption electrically converts into useful work or wasted heat. Excessive consumption of power converts into destructive heat and needs to be restricted by resistance or impedance.

I am not sure how to clearly state that in the water pipe analogy, but I can try. First, if water power is not properly plumbed, you get leaks and burst pipes at the weakest point. So robust pipes and tight connections are just as important as flow and the force behind the flow. So can alternatively reduce the force behind the flow and/or the flow to prevent failure.

Available power would be the equivalent of a rather large reservoir tank (like more available Amps at a given Voltage). So, as long as we consider BOTH the amount of work that needs to be done and the robustness of the conduit, the design is appropriate and durable.

Beginners in electronics tend to try to build without considering Power and it is like trying to push a large volume of high pressure water through a soda straw - the only difference is that failure usually provides a shower of sparks and heat. Fuses are more to protect humans than to protect equipment as there is a fire hazard involved with powerful electrical equipment, but sometimes the fuse happens to protect the equiment as well.