well, here is the issue:
I am building a project (a small CNC milling machine), and for the steppers am going to be using some L298N chips.
these will be driven from a Raspberry Pi, possibly with step-up transistors (data sheet seems to imply it is intended to be driven with 5v 30mA signals, but RPi outputs are 3.3v 3mA, with a 1k resistor).
issue (uncertainty after reading lots of random forum posts):
apparently, there is a need to current limit the L298N chips, so that hopefully they don't fry (apparently pretty common with these things).
reading stuff online implies that they need 0.5 ohm to ground on sense (regardless of whether or not current sensing is used? people seemed to debate this, claiming it is safe to connect them directly to ground), and need to be current limited to around 3A so that they don't fry themselves if any inputs get stuck on for too long, as well as them needing a good heat-sink (screwing them onto an old PC heat sink with a fan?).
there is also mention of people using a 10 ohm power-resistor into the Vs pin.
for resistance, I have found a few possible options:
some old clothes line from outside has some metal wire in it, which seems to have around 1 ohm per foot (measured via the voltage drop method), so about 6 inches of this wire should be sufficient for the sense pins. (it is also springy, resistant to bending, yellowish, and ferromagnetic). it also seems to accept solder well.
could possibly wrap one or more pieces of this wire around something, and maybe put some clay on it to make some resistors. (don't really want to order power-resistors, these resistors would cost nearly as much as the motor drivers...).
I have tested the wire, and have noted that it starts warming much past 1A, gets hot and starts blackening at 2A, faintly glows red at 3A, and basically lights up like a toaster at 5A (it did not burn up or break in these tests, but did start to become fairly limp and prone to sag, as well as glowing a bright yellow-orange color, as well as increasing its voltage drop). I have little idea what its properties will be like with prolonged use, or if heating it red-hot adversely effects its metal properties.
so, around several feet of this wire (and a fuse?) should be sufficient to get enough resistance to keep the motor drivers safe.
probably need to limit total power to around 3A, or 4 ohm (3A is within L298N max amps, and roughly sufficient to run 3 steppers at 1A each). for a single strand, this would be 4 feet of wire.
another noted option is that I have some L7812CV voltage regulators, which claim to have built-in current limiting (to 1.5A), which should work. though (untested), with a 12V input, these should only produce around 10V output (they are designed for a 14V-35V input), but would require multiple regulators.
also have some LM317 adjustable regulators. apparently these can also be used as current regulators.
I could also build some proper current-limiting circuits for this, but this option looks like more effort. for this though, I could probably use a wire-tie ("twist tie") as a shunt (sense resistor), noting that wire tie is approx 0.15 ohm, or around 0.075 ohm if folded in half (in my tests, I got similar measurements from several styles of ties). in a test with wire ties, only slight warming was noticed at 3A. though, solder doesn't really stick well to ties IME.
it would then be a matter of fiddling it into allowing the desired current flows.
thoughts?...
I am building a project (a small CNC milling machine), and for the steppers am going to be using some L298N chips.
these will be driven from a Raspberry Pi, possibly with step-up transistors (data sheet seems to imply it is intended to be driven with 5v 30mA signals, but RPi outputs are 3.3v 3mA, with a 1k resistor).
issue (uncertainty after reading lots of random forum posts):
apparently, there is a need to current limit the L298N chips, so that hopefully they don't fry (apparently pretty common with these things).
reading stuff online implies that they need 0.5 ohm to ground on sense (regardless of whether or not current sensing is used? people seemed to debate this, claiming it is safe to connect them directly to ground), and need to be current limited to around 3A so that they don't fry themselves if any inputs get stuck on for too long, as well as them needing a good heat-sink (screwing them onto an old PC heat sink with a fan?).
there is also mention of people using a 10 ohm power-resistor into the Vs pin.
for resistance, I have found a few possible options:
some old clothes line from outside has some metal wire in it, which seems to have around 1 ohm per foot (measured via the voltage drop method), so about 6 inches of this wire should be sufficient for the sense pins. (it is also springy, resistant to bending, yellowish, and ferromagnetic). it also seems to accept solder well.
could possibly wrap one or more pieces of this wire around something, and maybe put some clay on it to make some resistors. (don't really want to order power-resistors, these resistors would cost nearly as much as the motor drivers...).
I have tested the wire, and have noted that it starts warming much past 1A, gets hot and starts blackening at 2A, faintly glows red at 3A, and basically lights up like a toaster at 5A (it did not burn up or break in these tests, but did start to become fairly limp and prone to sag, as well as glowing a bright yellow-orange color, as well as increasing its voltage drop). I have little idea what its properties will be like with prolonged use, or if heating it red-hot adversely effects its metal properties.
so, around several feet of this wire (and a fuse?) should be sufficient to get enough resistance to keep the motor drivers safe.
probably need to limit total power to around 3A, or 4 ohm (3A is within L298N max amps, and roughly sufficient to run 3 steppers at 1A each). for a single strand, this would be 4 feet of wire.
another noted option is that I have some L7812CV voltage regulators, which claim to have built-in current limiting (to 1.5A), which should work. though (untested), with a 12V input, these should only produce around 10V output (they are designed for a 14V-35V input), but would require multiple regulators.
also have some LM317 adjustable regulators. apparently these can also be used as current regulators.
I could also build some proper current-limiting circuits for this, but this option looks like more effort. for this though, I could probably use a wire-tie ("twist tie") as a shunt (sense resistor), noting that wire tie is approx 0.15 ohm, or around 0.075 ohm if folded in half (in my tests, I got similar measurements from several styles of ties). in a test with wire ties, only slight warming was noticed at 3A. though, solder doesn't really stick well to ties IME.
it would then be a matter of fiddling it into allowing the desired current flows.
thoughts?...