@HIML: I suggested the 1ohm resistors for those who only have voltmeters, but discart this one. What if you use the amp mode only. I've done it and nothing happened! This also avoid the incorrections on the measurements due to the 1 ohm resistor tolerance!
Well, about this, it depend from what current you need from the driver ..... til is matter of 100 - 150mA, i think no problem ..... but over, is again matter of power dissipation and thermal protection circuitry.
Usual commercial DMM, have inside a low resistance circuit, for current measurement, and more low is the resistance, better is the instrument (and less influence the working of the circuti), so the manufacturers always try to keep it as low as possible.
Not speaking about lab instruments or old precision digital units (i have, as example, an old Beckmann pro one that have 0,1 ohm on 200mA scale, and a lab unit, the ones in wood boxes, that is 10 milliohm on 100mA scale, and 1 milliohm on 1A scale
), actually almost all the commercial DMMs uses 2 different systems, depending from the manufacturers ..... someone uses 2 diodes in antiparallel, usually BY255 or similars, and a correction circuit that derive the reading from the variation of junction voltages at different currents, and some others have usually an internal resistance of 1 ohm on 200mA scale, and 0,1 ohm on 2A scale, and read them as mV (like you usually do with the dummy load
) ..... this mean that, if you use the DMM directly, depending from the internal circuit, electrically is the same as if you connect a diode, or a low value resistor, at the end of the driver.
Now, always from the previous example, let me made some other hypothesis about regulating the current with 12V input, related to the better case, i mean the DMM with the resistance as reading probe, and keeping in mind that the maximum ideal dissipation of your LM317 in TO220 is 1W, or few more ..... if you're speaking about 100mA, connecting directly your DMM at the end of the driver (that have a dropout of 3V), is the same, electrically, as if you connect an 1 ohm load ..... the power dissipated from the LM317, in this case, is 9 (12-3) multiplied for 0.1 (100mA), so 0.9W, the driver work good, and you have no problems at all doing the regulation (LM317 is a robust chip, can hold also a 50% or few more power dissipation of the rated one).
But suppose now that you want to regulate it for 400mA for an 8X, and you don't use any dummy load ..... you connect the DMM directly to the driver, on 2A scale (cause you don't have other way for read 400mA), and the internal resistance at this scale is 0,1 ohm, practically a short circuit ..... the LM317, now, need to dissipate a power of 9*0.4, that means 3.6W, that is much more high of the power that a TO220 container can dissipate efficently without a heatsink ..... the junction temperature rise over 120 C in few seconds, and the regulator turn itself off for thermal protection intervent (in the worse cases, it burn itself, but usually the thermal protections of these chips are very efficent) ..... this mean that your driver is continuously turning itself off each few seconds, and on again in other some seconds (when the junction temp go under 90 C), and so on, again and again (until it give up and die
) ..... not so easy to regulate anything, in these conditions, right ? ..... especially cause also the current regulation change a little bit also, with high temperature variations .....
In this case, a dummy load that cause a considerable dropout and help dissipating some power, is not just better, but needed, otherwise you probably simply don't reach to regulate your driver ..... always the previous examples and values, if your dummy load have a dropout like a BR, say, 6V, the power dissipated from the LM317 is just 3*0.4 (12V, minus 3V dropout of the LM, minus 6V dropout of the dummy load), so 1,2W, and also if the LM317 become a bit more hot, it still work in good conditions, and you can do your regulation without problems, cause the rest of the power is dissipated from the diodes in the dummy load.
About the pot / trimmers, instead ..... this is not in the topic opened from the OP (sorry), but better punctualize also this, probably .....
I always use (and always suggest to use) two fixed resistors and a trimmer, and never just a trimmer (for variable drivers, ofcourse ..... no need of a trimmer if you set up a fixed current unit
), for safety reason and for not cook too much trimmers
..... cause, usually, the most part of the linear drivers users, don't realize, or don't care, about the fact that the current that is given from the regulator, pass also through the limiting resistor, with the obvious power dissipation, and possible cooking effects
Another pair of examples, about that (uhm, do i get paid for all the examples ? ..... LOL) ..... suppose you want to regulate 100mA, using just a trimmer ..... for 100mA, you need a resistance of 12.5 ohm (1.25/12.5) ..... with a 50 ohm trimmer, this is some millimeters of track, with a 100 ohm trimmer, maybe 1 or 2 mm ..... this current cause a power dissipation, inside the trimmer, of 1.25*0.1, then, 125mW, and the trimmer can also hold it (the most part of the trimmers are 250mW, the multiturn ones, usually, 100 or 150 mW) ..... now, regulate 400mA, with the same trimmer ..... you need 3.125 ohm ..... maybe 1 mm of track, or less ..... apart the fact that in this position, for pass from 400 to 800 mA, is enough probably just shift 0,1mm more, or also less, your end-of-the-track and cursor, now, are dissipating 0.5W, so, if you're not using an 1W wire trimmer, you probably are already seeing a nice little smoke and burning smell coming out from the trimmer, and at this point, you can also just throw it away
If instead you use the 2 resistors, with the trimmer, you have a good possibility to regulation, with less risks ..... and a more precise regulation, too ..... as example, you need 400mA, and have a trimmer of 50 ohm ? ..... use a main resistor of 4.7 ohm, and put in parallel of it your 50 ohm trimmer
with another 4.7 ohm resistor in serie to the trimmer ..... this give you a regulation from, approximatively, 300mA to 530mA, with the trimmer that is dissipating only a fraction of the total power, having a more long course in the point where you are regulating the needed current, so more precision, and more stability (also if for vibration the cursor shift a little bit, the variation is less than the one that you have using only the trimmer)
If you want to spare yourself of frustration I got through, just get the right resistor for your diode and it is all good. Although, just be sure (and not go against lasersbee's word) measure the resistor with a DMM , just in case the seller made a mistake. Happened on me once!
