Digital Voltmeter  - Page 1
Forum section: electronics / projects

author: Tioleco
author_rank: Contributor
timestamp: June 20, 2011, 12:50:19 am
content: Which IC would be better to build a voltmeter module for power supply?Intersil 7107 / 7135 , PIC, other?

author: Unknown
author_rank: Unknown
timestamp: June 20, 2011, 01:02:22 am
content: There are very cheap ready modules available, are you sure you want to build it? I would go for the ICL7107 or its brother and find a schematic online or in the datasheet as it is a standard application. There are also kits you can buy to build a meter with around this IC. You can use an MCU to also indicate various states (CC, CV, output ON/OFF etc)Alex

author: slburris
author_rank: Frequent Contributor
timestamp: June 20, 2011, 01:59:27 am
content: Note that the 7107 can be kind of drifty, because its internalvoltage reference is affected by the heat generated by theLED segment drivers. You can throw another reference chipin to get around this, but something to be aware of if you aretrying for a minimum chip solution but still want reasonableaccuracy.Scott

author: saturation
author_rank: Super Contributor
timestamp: June 20, 2011, 10:48:29 am
content: The 7107s brother is 7106, which has an LCD driver. If you want to build a voltmeter around these chips its easier to modify the popular pre-built LCD or LED panel meters then modify their ranges and reference voltage:http://www.mpja.com/products.asp?dept=52The PM128 is very popular, and its far cheaper to buy the panel than building to the same level yourself; the trick is improving its accuracy. If you build from scratch you'd like to have more than 3.5 digits.

author: PStevenson
author_rank: Regular Contributor
timestamp: June 20, 2011, 06:07:00 pm
content: I did make a stripboard Layout for the 7107 a while ago, someone mentioned the drift problems it has with regards to heat from the displays, I had heard about this and separated the 7Seg LED's from the main board using a piece of aluminium sheet but honestly I've never had any issues with drift at all and I do use the 7107 in anything that needs a voltage/amp readout. but that is because I had a load of the IC's given to me so it worked out cheaper for me that way.Also I just love making things. but for a good range and cheapness get the pre built ones, if you're a glutton for punishment build it with 7107

author: slburris
author_rank: Frequent Contributor
timestamp: June 21, 2011, 12:38:28 am
content: Here's a link to an Intersil app note that talks about building digital panel metersout of 7106's and 7107's.www.intersil.com/data/an/an023.pdfThe discussion about drift with LED heating is on page 4.I'm not sure if all 7107's are created equal though. They are made at leastby Intersil, Maxim, and Harris.Scott

author: PStevenson
author_rank: Regular Contributor
timestamp: June 21, 2011, 12:05:59 pm
content: aye yeah, I see - it does state that the plastic ones are poorer than the ceramic ones for thermal drift which is a bit shit for those who got the plastic ones hahaI guess it all depends on how compact the application is. I've only used it in big fan cooled bench supplies really so compact design wasn't an issue. but yeah definitely something to watch for with the 7107's

author: saturation
author_rank: Super Contributor
timestamp: June 21, 2011, 12:47:32 pm
content: Even if the 7107 is sensitive to heat, its to 100uV tops, so unless your range resolves 0.1mV it may not be a consideration.If you don't used matched resistors for tempco and other features, then even if they are precision they will drift at different rates.If designs internal temperature varies at maximum warmup, it will still drift the 7106, so what matters is to carefully consider operating temperature against the components thermal stability and drift. As the number of digits in your panel meter increase, the more all these small factors will matter; if you are just doing 1% for a 3.5 digit DVM, most resistors and precision in the 1% or less will do, including non-matched pairs, and they use of either 7107 or 7106 is more the visual appeal of LED vs LCD than a question of accuracy.

author: PStevenson
author_rank: Regular Contributor
timestamp: June 22, 2011, 11:38:04 am
content: picking up on what you just said... I have to say my preference is for LED displays, I don't know why, I have always found them so appealing ever since I was little, I remember begging my dad to get me the 200 in 1 kit cause it had a "little red 8" on it. I know that is completely off topic but what the hell.but I was careful with my component selection because I knew that some of the externals were part of the internal clock so I think the ones I had were 0.4% only because they were the most precise ones I had on hand at the time.I also got a load of 7135's direct from TI but I have not got a clue what to do with them yet. all the data for those seems to be based on micros and I'm not much of a fan of programming my attempts at that are slap dash at bestsorry I went on abit there.

author: saturation
author_rank: Super Contributor
timestamp: June 24, 2011, 07:57:46 pm
content: My preference too, the key thing is its very easy to see, it has a fairly large 'viewing angle' and indoors its very well lit. if I buy test gear, I look for LED versions over LCD if they are not battery powered.Quote from: PStevenson on June 22, 2011, 11:38:04 am