AC calibration source - Page 1
Forum section: electronics / testgear

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: August 27, 2015, 03:52:59 pm
content: I have some old DMMs I would like to calibrate to a reasonable degree. They don't have to by NIST traceable or anything. They just need to be trustworthy for general hobby use. The DC voltage sources as called for in the calibration procedures are typically not a problem. The AC sources are the difficulty. Most want higher voltages (often as high as 1kilovolt), and typically want a particular frequency like 20kHz, or 100kHz.Any pointers on inexpensive ways to generate these signals at higher voltages (my siggen does 20v p2p)? Given these meters were cheap eBay scores that I repaired I don't really want to send them off for calibration. I guess I should check the local St. Louis, MO maker space and see if they have any calibration source..Thoughts? Thanks.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 27, 2015, 06:04:15 pm
content: Usually a very high voltage is not required for a resonable function test. So even to test the 1000 V range something like 100 V should allready give a reasonable test for the dividers.With AC its often more having a uncalibrated source (function generator) and a calibrated meter to do the tests.A crude test can be done with rectangular signals, going through a CMOS driver. The peak to peak amplitude is dirived from the supply voltage of the driver chip.For the frequency dependency a scope could be good enough - though absolute amplitude is often not so good, frequency dependence is usually quite good well below the specified bandwidth.

author: eas
author_rank: Frequent Contributor
timestamp: August 27, 2015, 06:09:19 pm
content: I'm in a similar spot with some 5.5 and 6.5 digit DMMs. To start, I'd just like a stable reference that I can use when comparing them to one at various points in various ranges.I've looked through past posts in this forum and nothing stood out as an obvious solution. Audio amps have come up as a suggest from time to time, but I don't think I've seen anyone say, "I tried this..., this was the result..., and it was good."I've considered the function generator route + some sort of amp. I've seen fixed gain amps for cal go for $~100 or so in "working" but not calibrated condition, but the shipping is another $100. There are also various bipolar amp PSUs from Kepco and HP. But I haven't taken a close enough look at the specs.There are also AC calibrators. A working but not calibrated Fluke 5200a just sold for ~$230 + shipping, but it needs an external amp to source more than 100v. Other units have gone for a similar amount.There are also some olderFluke andHP portable meter calibrators that seem to go for $~350-500, but sometimes less. They are scarce enough that it could be a long wait before you could pick one up for less. One option that I'm considering it is to buy one use it for a month, and then sell it again.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: August 28, 2015, 03:37:38 am
content: I almost bought that Fluke on eBay...Trying to find some easy way to take a siggen and amplify it to the required voltage. I would then use a new meter to set the source to then compare it to the meter being calibrated.I guess I could like use some higher voltage BJT like an MPSA92 and my 120v DC supply, then drive that transistor with my siggen at the required frequency. That ought to work, no?

author: Kleinstein
author_rank: Super Contributor
timestamp: August 28, 2015, 01:04:44 pm
content: At moderate frequencies (e.g. 30-1000 Hz) one can use a normal small mains transformer (e.g. 12 V to 230V, 5 VA) to get something like 100 V from normal 50 Ohm output of a function generator. Just the output impedance is high (e.g. 400 times 50 Ohms = 20 K) , but good enough for 2 meters and a scope.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: August 28, 2015, 04:42:15 pm
content: Quote from: Kleinstein on August 28, 2015, 01:04:44 pmThis is exactly what I realized in a DOH! moment last night. I haven't tried it, but was going to tonight.Thanks

author: HighVoltage
author_rank: Super Contributor
timestamp: August 28, 2015, 05:12:29 pm
content: I also have done the same thing in the past with a mains transformer.Using a toroidal 230/12 V transformer with two secondary 12V outputs and then coupling them in series, you can easily generate 100 VAC from an good function generator. And the output is pretty stable to 100.05V +/- 0.025V in my case.