Adjustable DC current source with integrated hi-z DVM [130287-I]

With this portable instrument, you can safely test and measure very quickly all diodes, zeners, LEDs, etc... in their actual conditions of use, that is with a known and adjustable current, and up to a significant voltage (opposite to classic DVMs : unknown current, output limited to 3V or less). You can see the light efficiency from a LED, connect LEDs in series to make a visual selection, characterize a zener knee, estimate the leakage current of a diode with temperature, measure the avalanche voltage of a junction, measure very high resistances up to 2 gigaohms, and even capacitors with help of ... a stopwatch !

You can also disconnect the current source and use the instrument as a simple DVM with the particularity of a very high input impedance.

Main characteristics :

20 output current values from 10nA up to 20mA in 1-2-5 steps.

1.4% + 100pA output current precision

20V maximum output voltage.

Adjustable output clamping voltage from 1V up to 20V with LED indicator.

Integrated 3.5 digits two-range high impedance voltmeter.

0.2% voltmeter precision

4 x AA cells power supply.

110 hours battery life @ 1mA output

Project Elements

author's instrument (final version) (JPG, 658 KB)

schema page 1/4 (PNG, 24.86 KB)

schema page 2/4 (PNG, 16.48 KB)

schema page 4/4 (PNG, 27 KB)

csv9812-schema-rev3-page3-0.png (PNG, 17.33 KB)

CSV9812_parts_list_rev1.zip (ZIP)

parts list

CSV9812_photographs.zip (ZIP)

instrument pcb views

csv9812a_schema + description + pcb.zip (ZIP)

schema + description (fr + en) + pcb info + miscellaneous

CSV9812-T_led_sorting_accessory.zip (ZIP)

LEDs visual sorting accessory

Discussion (6 comments)

johanh 1 month ago

Did anyone else ever build this project? Looking at the schematic I see one issue. There is no connection between VSS and ground, so VSS is basically floating in the schematic. IC2 pin 7 VSS is however connected to ground on the PCB (I think). But the VSS side of C14 isn't connected to ground anywhere (at least that I can see) on the PCB (and gerbers), so that includes IC1 pin 37 and VSS side of R48 and R47. I'm not sure if the low battery indicator works as intended due to this. The VDD designator found in one place in the schematic isn't used anywhere else, from what I can see.

johanh 1 month ago

I got the above a bit wrong. The original schematic makes it clearer. So IC1 ICL7136 pin 37 TEST is used as a logic ground. And that's why there is a VSS designation. But still, IC2 4070 pin 7 is NOT connected to VSS in the gerber and resulting PCB, but to common ground. I'm not sure if it makes much difference, but at least it would be better if the schematic and PCB were to agree. Or if there were an explanation as why the 4070 is referenced to GND instead of VSS as per the schematic.

1 Comment(s)

patrickwie 11 years ago

http://youtu.be/p78B4jb7JhU

0 Comment(s)

JanB 11 years ago

Finished!

The finished project complete with fine Elektor-style schematics, a full description of its operation and adjustment appears in glorious color print in the April 2014 editons of Elektor magazine (except France).

Thanks Henri and Luc for engineering this one into publication.

Jan Buiting, Editor

current-source-project-title-page.jpg (235kb)

sigo 10 years ago

@ Luc and Henri Could you provide an bill of material in EXCEL importable format, please? Thanks, Ralf

1 Attachment(s) 1 Comment(s)

Lucky 11 years ago

Finally the project is finished in the lab. We did change most of the component references when we redesigned the schematic and PCB. In the ZIP you'll find all documents, including the BOM, the author's circuit description + adjustment procedure 'translated' to Elektor's version of the circuit. Also some lab notes about assembling the adjustable current source.

Elektor lab files 130287.zip (698kb)

1 Attachment(s) 0 Comment(s)

Lucky 11 years ago

Looks quite good, still some work to do, like wiring the rotary switches.

As expected there's always some tweaking to do on the PCB, some unexpected things like wrong dimensions in the datasheet of the LCD. The picture shows the display mounted on the PCB, after some bending of its pins. On top of it the same LCD in its original shape, this has to be corrected in the layout.

The fourth picture shows a silly mistake: the fixing nut of the potentiometer (almost) touches its pins and there's a trace + via underneath it. I overlooked that one :-(

The good news is that the voltmeter part appears to be working and most voltages on the test point look okay.

Time to finish the rotary switches, but first discuss with our photographer if he wants to takes pictures of this process.

copper-side.JPG (368kb)
130287-component-side.JPG (515kb)
lcd.JPG (101kb)
potmeter.JPG (87kb)

4 Attachment(s) 0 Comment(s)

Lucky 11 years ago

There are some corrections on the PCB of the author, I just finished drawing the schematics in Altium Designer. First thing to decide is if we will use the (rather expensive) rotary switches of the original design or the more affordable (and bigger) switches from Lorlin.

LED visual sorting accessory (1139kb)

henrip4 11 years ago

Mounting concerned resistors vertically around the switch on a 30mm diameter circle (with one terminal soldered on the switch and the other one on the pcb) won't consume much more area than that of my instrument (in which this kind of mounting has been used intentionally for high value resistors, see photos). And if they are available, switches with solder lug terminals instead of pcb pins (CK1029 and CK1031) will help mounting these resistors.

Lucky 11 years ago

The layout of the PCB is quite a challenge. Room for the big rotary switches makes the PCB much larger than the original design. Since we want to make this project with through hole parts only (easier for our readers to build), the only way to keep it small (portable/hand held equipment) is to mount some resistors vertical. Not the most elegant solution...

Lucky 11 years ago

Agreed, if the (larger) Lorlin switches fit on the PCB, there will be enough room for (nearly) every rotary switch available. Can be a nice subject for publication too: comparing performance of this instrument with cheaper or more expensive switches. And making people aware of the fact that even a 'simple' part like a switch can be important.

henrip4 11 years ago

I agree that rotary switches are very expensive, but the choice was governed by the insulation resistance which must be very high in order to avoid leakage currents directly affecting the low range current precision (this is mainly true for S2 switch). This is also for the same reason that 10M and 50M resistors are directy wired on the rotary switch, without passing thru the PCB. The parts list gives a series of manufacturer references in the order of preference. GrayHill parts are a good choice with 50000M initial insulation resistance, and their price is acceptable, but they will require a shaft extension. I do not recommend Lorlin parts because : 1) their insulation resistance is specified at only 1000M 2) from my own experience, their mechanical lifetime is rather short, and for that instrument they will be frequently and quickly operated 3) they are big. Because the rotary switches are not printed circuit versions, the room granted to them allows any part up to 18mm diameter to be mounted, and so leaving the final choice to everyone's budget.