dc amps vs ac amps
This is a discussion on dc amps vs ac amps within the General Electronics forums.
- 02-09-2008 #1
what is the differece between ac and dc amps if any?
- 02-09-2008 #2
I was told:
DC Amps and AC amps are the exact same thing, they are the measurement of electrons past a given point, the difference is that the electrons of AC go back and forth (alternating) and DC go only in one direction (direct).
Measurement must however be done with two different meters, if you measure AC with a DC ammeter it will twitch a lot but you'll get no reading, if you measure DC with an AC ammeter it might just let it's magic blue smoke loose and you will have a nice christmas tree ornament but no more ammeter.
The reson for my original question regarding amps was because i am currently selecting the correct wire to use as supply lines to each of my driver cards. To futher explain that, i was already aware that diffrent gauges of wire would also draw current and if that is the case i needed to also ask the original question above.
Hopefully this post will also help somone else in the future, who may be looking for the same answer!
What i would like to know now is, how do we determine when a wire will start to draw current?
The reson i'm asking this question is because when i had my out building wired by an electrishion, he told me that you can't use over rated cable because it would draw, build up, demand current itself. So if thats the case really to do the job right i need to make sure that the wire i use to supply each driver, isnt going to do the above.
Maybe you guys could help me with this, am i going "way overboard" for what my circuit will be doing? or is this a good show of consistency in doing the job right.
What i'm basically getting at is this, my PSU is currently supplying my circuit with 38VDC @ 14a. Each of my driver cards are rated @ 4.2a (RMS 3A) MAX, so we know that the driver cards combined could require 12.6a (4.2 x 3 = 12.6).
The wire i have decided to use is rated @ 15a, so 15 - 12.6 = 2.4. So my wire should handle what is being requested in ampage but will the wire pull a little more current from the PSU as its rated @ 15a and a little over what the peak will be, going back to what i said above about wire and the (draw, build up, demand) ?
Now moving onto fuse's to protect my driver cards, i understand it that fuse selection is done so by adding 25% to the peak rating. So 4.2 + 25% = 5.25, so i would need to use a 5.25a fuse to protect each driver but actually a 5a fuse is the closest i can find so thats what i will be using? and if its true that the wire will (draw, build up, demand) do i need to take this into consideration on my fuse selection.
Last edited by Lee Roberts; 02-09-2008 at 03:33 AM.
- 02-09-2008 #3
So reading back on what i'v posted/asked above, i obviously know that mains current is generally allot more then what we are playing with here but shouldnt we be applying the same procussions/standards regardless, if we are to protect ourselves and/or our equipment.
Any help and/or advise is much appreciated!
A Note to mtompson: I'm using the same drivers and PSU as you are, however i'm using two PSU's wired in parallel to give me the 14a.
Last edited by Lee Roberts; 02-09-2008 at 03:35 AM.
- 02-09-2008 #4
You have two things to consider, resistance and capacitance. Fat wires have lower resistance so you lose less power heating the cable. Try to keep the wires between controller and stepper short otherwise the capacitance can affect the current sensing in a chopper circuit. Fat wires have more capacitance but not so as you will notice.
A 5 Amp fuse will handle more than 5A but may tire if you flow 5A all the time.
The great AC/DC debate was thrashed out in America by Edison and Tesla.
Edison's problem with DC was it meant a power station on every city block, but as he pointed out, you can't run a motor on AC.
Tesla responded by inventing an AC motor that we still use today.
Edison said the higher AC voltage was dangerous and arranged for criminals to be executed electrically to prove his point.
Tesla won but blew all his money trying to transmit power without wires.
Edison lost but left a corporation behind who told everyone that he was cleverer the Tesla.
Tesla left no corporation so he lost by default even though he was actually much smarter.
OTOH Tesla has a fundamental unit named after him but we measure nothing in Edisons :D
- 03-09-2008 #5
Wire doesn't 'draw' current, it resists the flow of current. To use an oft quoted analogy, which isnt perfect but works, think of the wires as a pipe, the power supply as a pump and the 'amps' as the flow rate of the water. The voltage of the supply is what drives the system... a bigger pump ( = more water pressure) can push more flow (amps) through the system, but ultimately the size of the pipes, the wires, is what limits the flow... too much and the pipe bursts (overheats/melts). Which is exactly what a fuse does... to fix the point of overheating at a safe location.
Some thoughts.... if you have 3 motors at 5A do you need wire capable of carrying 15A? No, you need wire capable of carrying 7.5A because each motor should be seperately wired back to the power supply via its own 7.5A fuse, never daisy-chain high current motors. Fuses are rated at the current they will carry without blowing, which should typically be 25% higher than the highest current you expect to encounter. Generally wire should be rated 50% higher than the expected maximum so that the fuse becomes the weak point. It doesnt hurt to use thicker wire but it costs more.
Stranded wire is preferable to solid core wire for most applications where vibration is present as it is less likely to fracture. Also for high frequencies and for systems where there are fast transients (like a stepper driver) stranded wire is better as high frequencies travel in the outer skin only so more strands gives a better result (more surface area).
To wrap up - the difference between DC and AC current is - there is none, except that AC current reverses direction in a (generally) cyclical form, whereas DC flows in the same direction all the time. However, consider the power dissipated (energy lost) in a wire. If you have 10meters of cable it may have, say, a resistance of 0.1ohm so a 10A DC current would would dissipate 10W of energy (and incidentally drop 1 volt, so a 50v PSU would give 49volts at the other end). A 10A RMS AC current would do exactly the same, however the peak current would be 14.14A (10 x square root of 2). Why? Because the RMS (root mean square) only truely applies when the shape of the AC current follows a sine wave - or, as in the case of a microstepping drive, averages out to be the same as a sine wave. A higher peak value is required so that the energy transmitted per second is the same as the DC value. It follows therefore that if there are circuit components sensitive to the actual instantaneous current value, like a fast acting fuse or circuit breaker, then they will trip at a lower AC current than the seemingly equivalent DC value because of this higher peak.
Incidentally a typical low-cost AC volt or ampmeter is only accurate when the AC flow is sinusoidal, as it is calibrated to show the RMS value against the actual peak value measured.
Hope that all makes sense.
- 04-09-2008 #6
ok thanks guys, so Irving where i have used 15a wire here for all my supply wires for my fuse's, i need to upgrade the 1 main wire feeding each fuse to 50% of what my expected maximum could be.
So in that case x3 4.2a drivers could ask for 12.6a down that one "supply" line, so adding 50% i need to upgrade that supply line to a 18.9a wire?
Am i right on this? or do i need to expect my maximum to be what the PSU is giving out, so 14a i would need to use 21a wire, please see the pic to understand more what i mean.
Last edited by Lee Roberts; 04-09-2008 at 01:24 AM.
- 04-09-2008 #7
Do not daisy-chain the fuses back to the PSU, wire them individually back as per the diagram below....
The power supply won't be able to supply more current than its maximum rating. depending on the design of the supply it will either go into current limit or will blow a fuse :)
Last edited by irving2008; 04-09-2008 at 11:21 AM.
- 04-09-2008 #8
Ok mate, my PSU says "Peak" 9a.
Will you explain why its not a good idea to daisy-chain the fuse's ?, ill also pop this thread in the FAQ section as i think its going to be a good one for others to read.
- 04-09-2008 #9
Why do you want fuses anyway? I don't use them, just one more thing to go wrong, or in this case, 3 more things to go wrong :D
- 05-09-2008 #10
Irving, i just found this bit of information can you confirm:
it is not a good idea to "daisy-chain" the supply wiring from one drive to the next, as current drawn by a drive that is nearer to the power supply can effectively "starve" drives of current lower down the chain. In fact, if the wiring from the power supply to the drive is likely to exceed about a foot (300mm), it is advisable to fit local reservoir capacitors of 470 microfarads across the supply terminals of each drive - these must have a voltage rating at least 25% higher than the supply voltage.
- By petejw in forum General ElectronicsReplies: 7Last Post: 11-08-2009, 11:40 PM