Automatic Voltmeter/Ohmmeter selection (Voltmeter mode selected when 0.8V or more is detected across the leads)
Continuity test
Non-contact AC voltage detection
Backlit display (actually a nice feature)
LED for helping to guide the probe
Less than $10 USD in quantity (they were less than $4 USD this past Thanksgiving/Black Friday/Cyber Monday sales cycle)
I thought it was a nice tool until I tried it with setting the current output of a DRV8825 using the built in pot. For at least the past five years, I’ve only worked with TMC2208 and later drivers that have the serial connection in which the current output can be set so this was something of a new experience for me.
My daughter has a school project in which she needed a continually rotating motor. I thought that since I have a ton of old 3D printer parts, I’d throw something together with an old 12V power supply, a RAMPS/Arduino Mega and DRV8825.
With the 0.1Ω sense resistor on the stepper driver module, the current output is defined as:
Ic = Vpot x 2
As noted above, the switching between the Voltmeter and Ohmmeter functions takes place at 0.8V. This is is 1.6A of driver current (or 19.2W) which is too much for the chip in this application as it goes into thermal shutdown after a couple of minutes of operation.
I found that the maximum current that could be run through the chip in this application is 0.8 to 1.0A (it’s hard to very precisely set the pot regardless of the Voltmeter you’re using). This basically matches what you would see in a 3D printer that was running this stepper motor driver module.
This means that while the cheap pen digital multimeter is well suited to my needs, it’s not going to be the only electrical tool somebody who manually sets their output current requires - they’ll also need a separate Voltmeter that doesn’t have a threshold value.
This is why I’m wondering how many people still set their driver currents with a pot and a Voltmeter?
Apologies for the long post, it kind of got away from me as I thought the context and example application were important.
I’m not sure that using a digital meter will give you any meaningful readings other than ballpark. The steppers are essentially driven by square waves‡ and in operation the frequency could well be in the same range as the sampling frequency of the meter (especially older ones as shown in the photo.) This leaves you with 2 problems
The RMS of a square wave is higher than a sine wave.
The actual readings could well fluctuate wildly if sample points drift though a series of highs then though a series of lows.
You would be much better off using a scope.
‡ Sometimes upmarket drivers provide “pseudo-sine” waves made by stacking a series of square waves with varying mark-space but even then you may find the meter reading every 2nd step, then every 3rd step and so on.
@JassMan23, you seem to be too young to discuss about this
This is like listening to music on cassettes and CDs (added Wikipedia links for your convenience) SCNR
In times lost in the mists of history, stepper drivers had their current set via tuning a voltage value that could be set via a potentiometer. Only very few today even know about this black art and even less active practitioners exists.
Regardless of my age, I can read and also have a little knowledge about electronics. I know a little knowledge is a dangerous thing, but that pen style multimeter looks indentical to one I had back in the 80s. Probably why it is cheap enough to give away with a packet of cornflakes - they recouped their R&D years ago.
And yes we had stepper motors even then. The only thing which has changed (and even then only slightly) is the driver designs, as engineers try to eak out every last percentage point in extra efficiency. Yes, I hadn’t checked the spec sheet on the TI DRV8824 but the fact remains that using any sort of a meter instead of a scope to measure the current (derived for volts across the sense resistors is doomed to failure.) The specs say that that response times to current requirements are in the nS range. This is much faster than any meter will respond and produce a reliable reading.
I was trying to point out that @mykepredko 's meter reading of 0.8V is probably very much on the low side and thus current is probably higher than his calculated 1.6A.
The reason @mykepredko is finding the chip getting warm is that even if he has “measured” 1.6A, he is running the at its design limit, and he doesn’t need a meter to tell him that. A better purchase would be a fan to blow some air over that diddy heatsink.
There are many helpful methods on the web showing that one way to set the pot on the driver is to set it so that that the motor doesn’t drop steps and then add a touch more to cover unexpected load demands. Especially in the case of the project for his daughter where she just needs it to be continuously rotating. No unexpected accelerations,
From the TI spec:
Number of full bridges 2
Vs (min) (V) 8.2
Vs ABS (max) (V) 47
Full-scale current (A) 1.6
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@Sineos Yes I too stream my music and haven’t taken a CD out of its box since longer than I can remember. Although I hear that vinyl is now the thing. Maybe I need to use this magic thing called the internet to find a new rubber band for the turntable I have in the attic somewhere.
The point that @Sineos was making isn’t that this is the way stepper motor driver current were set in general “back in the day” but this is the way that 3D printer stepper motor driver current was set using DRV8825/A4988 based driver modules.
To see how the legacy 3D printer stepper motor module current was specified I suggest you do a bit of research and look at pages like:
