Dilshan Jayakody has published a new build, a DC millivolt source:
DC millivolt sources are often used for thermocouple simulations, DMM, ADC and analog circuit calibrations and fault findings, etc. Most of the DC millivolt circuits are based on expensive (and sometimes hard to find) operation amplifier like OP7, LT1077, etc. In this article we introduce low cost DC millivolt source based on commonly available LM308 precision operational amplifier, ICL8069 low voltage reference and 78L05 voltage regulator ICs. This millivolt source is design to obtain 200mV – 700mV DC output with 100Ω impedance. At the time of writing we spend Rs.800.00 (LKR) to complete this project.
Project info at Dilshan’s blog. All files are available at elect.wikispaces.com.
Yeah, this is a good one, ha ha. Author knock other circuits about hard to find parts, yet his two items ICL8069 and LM308 do not have Digikey pricing! They are also obsolete parts. Intersil, oh yeah…
In this case, ‘precision’ IMHO is a flawed characterization, since the author is working with _old_ parts… Lemme check: A cheap Microchip opamp such as MCP6401 has a better Vos. Pair that with a cheap LDO like MCP1801, has 100ppm/K temp coeff like the precision ICL8069 *cough*. Widely available and cheap.
Since the user has to set and measure the output anyway, the result won’t wander much if used indoors, and it will depend on your multimeter’s error. Why bother with old ‘precise’ (at that time) parts?
KH: You have a point, but I think you missed something.
It took about a minute of digging, but the “Rs.800.00 (LKR)” listed in the writeup apparently refers to the Sri Lankan Rupee, which currently converts to about $6 in US dollars.
I would assume that the parts listed are considered common in Sri Lanka. It also appears that Digikey charges a set rate of $75 USD (!) for shipping to Sri Lanka, making all of your “cheap” parts listed way out of reach.
Digikey is to very roughly determine general availability. I don’t buy from them either.
It’s a voltage ref then a pot then a opamp buffer with a transistor driver. Okay, Digikey was wrong about the temp coeff, it’s 10ppm/K. But I think 100ppm/K is fine for hobby uses, if you want 10ppm/K then everything else must be checked like scientific instrumentation. In any case, the project doesn’t discuss system-wide temp coeff errors or such things.
IMHO it’s not good practice to pick parts from generations ago. I wonder where they got their supply of ICL8069. Many Alibaba suppliers it seems. But unless I am already in Shenzhen, I would buy a 10ppm/K ref part from a reputable source, plenty of parts at say Farnell within a price bracket of USD1-2, but again, I would just use 100ppm/K.
Heh, “not good practice to pick parts from generations” by which I mean cases where non-generic high-performance parts are obsoleted or deprecated and many generations of newer parts exist.
Anyway on the 10ppm/K thing, if a hobbyist need 5-digit measurements or something then it’s a whole new mess, like annually calibrating measuring instruments etc. Then a project like this becomes just a tiny part in the scheme of things and you have plenty of maintenance and procedural issues to consider. Very few hobbyist will want or need to do that.
ICL8069 is obsolete, but I see it on eBay for less than 1$. Seems like its common in eBay!!
Normally I prefer eBay over Digikey / Farnell for my hobby stuff.
The author of this circuit is seems to be from Sri Lanka. In Sri Lanka, you can find lot of electronic component shops with vast range of components and prices are very low.
I think most of these components are imported from Singapore or Malaysia.