Quantsuff says he’s getting one of these DC to DC buck converters combined with a meter, from a vendor on eBay named Kuuco (this link may go dead after a time, so do a search for “4.5-24V to 1-20V 2A Voltage Regulator DC Buck Converter Voltmeter Ammeter Blue”. What I think is a stinking shame is that the meter has two digits after the decimal point, but they waste the rightmost digit with a letter U or V depending on your country leaving a single digit. My ancient Weston analog ‘wiggle stick’ meter that I blogged several days ago has better accuracy than this.
A typical digital multimeter costing only a few dollars has a 200 millivolt range, and the higher voltage ranges are created by putting resistors on the input to make a voltage divider. So if the meter displays 0 to 2 volts, then it has a 10 to 1 voltage divider on the input. If this converter / meter is displaying 0.1 to 20 volts in 1/10 volt steps, then it most likely has a 100 to 1 divider on its input. What I would try to do is put another ‘shunt’ resistor in parallel with the divider resistor to make the range 10 to 1, so it would display 0.01 to 2 volts in .01 V steps. When the output voltage exceeds 2 volts, it would display blinking 1s, or overrange. I suppose a switch could be put in series with the shunt resistor so both 2V and 20V ranges may be selected.
The only thing that would have to be jiggered is the decimal point, which would be in the wrong place when the range switch is switched. The decimal point could be blacked out with a tiny piece of black electrical tape. Or if there is circuitry on the board for moving the decimal point, then it might be possible to add that to the switch I discussed above. But even if this range is added to the meter, it still can only resolve down to 1/100 of a volt, and that cheap DMM can do at least ten times better. This leads me to seriously consider taking a cheap DMM and gutting it, removing the case, and just use the board and the two sets of resistors needed to give the 2V and 20V ranges. The only problem is that I would have to come up with an 9 volt supply that is isolated from the common of the converter.
I got the Harbor Freight flyer the other day, and it had a coupon for a FREE Centech DMM with any purchase. I could walk in and buy a fifty cent key ring and walk out with it and the meter. Really.
I reread the info at the bottom of the eBay auction I linked to in the first sentence. The buttons select V or A etc, which seems to me that there may be a microcontroller on the PC board. If so, then my guess is that it and its software determine how the meter works. If that’s the case, then why does there seem to be a limitation on reading only a single digit after the decimal point? Is it because of the way that the microcontroller handles the analog input? In any case, the point I’d like to make is that the software might be modified to take advantage of the second digit after the decimal point.
One thing I’d like to delve into is window comparators. If a supply is typically mostly set at one or a few voltage points, then a window comparator could be used to do the setting. The idea here is to add to the power supply a circuit that is compact and small, simple, and draws little power. This circuit will then eliminate the need for using a DMM to accurately set the voltage most of the time.
Using a dual opamp or comparator chip, the window comparator can tell when an input is between two voltage points, and if those points are within a narrow range, it makes it capable of setting a power supply to a certain voltage. The circuit needs a stable reference voltage, The window comparator’s output is a simple Go-No-go indicator such as a LED. A switch can be used to change between 2 or 3 set points, such a 1 volt, 1.25 volt, and 1.5 volt.