dc amps vs ac amps

[Robin Hewitt]

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

[irving2008]

CheekieMonkies

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.

regards,
Irving...

[Lee Roberts]

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.

Thanks,
CM

[irving2008]

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.

Thanks,
CM

CM,

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 :)