home built Current transduser for fluke 5700- help needed  - Page 1
Forum section: electronics / projects

author: eevblogfan
author_rank: Frequent Contributor
timestamp: May 31, 2013, 12:59:53 pm
content: heyso usually when 10A is needed , we use the datron for the task , but there was a day when we had several meters and each one got say 10 of them , that's a lot of time wasted hence money ,I told my boss I might be able to build such a transducer for not that expensive price ( ~7K$ for Fluke 5725A )the needed accuracy is 0.005 at least , and on AC 0.01 or so should do ( 50Hz to 10Khz is good I guess )high current source shouldn't be a problem at all , the problem is the loop stability of the CC loop and the shunt's temp-co ...so I thought to ask you firstly , what's the best way to do that ?I thought to make a use of positive feedback since the stability and fast response ,I thought of 20 transistors , each pass 1.0000A , for that accuracy you must use either low temp-co metal and then adjust for the Inaccuracy ,do you have ant idea for good op amp to use ?chopper amp won't be the best due to the internal clock limiting he's BW for several Khz , or you might know any good chopper amp that'll do , ?I don't know what resistance values to use , I think of 0.1% , I uses 10X 0.105ohm shunts I've made out of nichrome , the drift is like 0.0005~0.001% for 24Hrs run , that'll do the task I think , and the op-amp I've used was 0po7 without even nulling it , so I suspect that's gonna be the bear bones circuit , but of course we need all the protections available ( OCP , OVP , when thes'r blown fuse in the DUT , the voltage will exceeds say 2V and we need to trip the output off for safety reasons , OTP , and such )the meters might be fluke 87V , fluke 45 , and such , but the accuracy should meet at least 5X better their specs , preferably 10X ,the output should go all the way up to 20A , if possible 30A is betterthank you in advance !

author: Paul Price
author_rank: Super Contributor
timestamp: May 31, 2013, 02:26:44 pm
content: Even if you somehow managed to build a circuit to measure DC at .005% and AC at 10's of KHz at .005% accuracy how would you ever, ever, ever calibrate it?

author: eevblogfan
author_rank: Frequent Contributor
timestamp: May 31, 2013, 03:18:14 pm
content: firstly - AC current at 10Khz or so . no more ,then I said 0.01% or so for AC current ,calibration ? , we have got 100A shunt , 0.01% accuracy ( on real world it does 0.007% when passing 1A with the fluke 5700 or the Datron at 10A , or ther's some borrowed 20A cipable one , the accuracy is still 0.007% ( when measuring with HP3458 - the shunt's voltage)but since the boss don't let me touch the 3458 anymore ( mybe for that task he will ) , I will do all the measurements with Keithley 2001 , whitch is actually very good . one count within the 3458

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 31, 2013, 04:40:15 pm
content: Hi,Have a look for the Fluke 5220A. This is the previous generation of transconductance amplifiers. There is one eBay for $600.00Be careful using shunts for AC measurements. The inductance of the shunt will give misleading results.Jay_Diddy_B

author: eevblogfan
author_rank: Frequent Contributor
timestamp: May 31, 2013, 08:28:18 pm
content: hey Jay_Diddy_BShall not I use capacitor parallel with the shunt in order to make that to a "resistor" ?I can't find any Fluke 5220A for that 600$ , only 2K$ and more

author: c4757p
author_rank: Super Contributor
timestamp: May 31, 2013, 08:43:17 pm
content: Quote from: eevblogfan on May 31, 2013, 08:28:18 pmAt a fixed frequency? Assuming 10nH parasitic inductance, at 1kHz you'll need 2.5F. At 60Hz you'll need 704F. Yes, farads. And you'll need to account for ESR and the dreadful temperature coefficient of that capacitor, as well as the 10A ripple. In other words, no, you can't.Also, that 10nH inductance will put you out by 6% with a 1 milliohm shunt at 1kHz.

author: c4757p
author_rank: Super Contributor
timestamp: May 31, 2013, 08:46:51 pm
content: Quote from: eevblogfan on May 31, 2013, 08:28:18 pmNeither can I. Where'd you see it, Jay? Perhaps it's already gone!

author: eevblogfan
author_rank: Frequent Contributor
timestamp: May 31, 2013, 08:49:07 pm
content: heyas you may know - I don't yet know how do they do that , might you reveal their secret ?how does the fluke 5700 does say 1A at 10kHz ? and you may tell him you'd like it to be 200mA at 60Khz .does it implemented some high accuracy cap as the shunt ? , I'd really like to know please , as this sort of thing is not familiar to mehow would you create amplitude rated at 20A and frequency rated at 60Khz ? ( the targeted frequency is 10 ) but simply out of interest

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 31, 2013, 09:55:29 pm
content: Hi,Here is a little simulation to show the effect of shunt inductance.I used a value of 25nH which is very small to illustrate the concept.Here is the LTspice modelIf I look at the voltage across the shunt I get:The voltage rises at a frequency 1 / 2pi (L/R)I can introduce a compensation where Rcomp * Ccomp = L/RIt requires an exact match to minimize the error. There will also be some error from skin effect etc.I can't find the Fluke at $600.00 anymore so the seller must have pulled the auction. Some did sell for reasonable money in completed on eBay.com (US site)Jay_Diddy_B

author: c4757p
author_rank: Super Contributor
timestamp: May 31, 2013, 09:58:33 pm
content: Quote from: Jay_Diddy_B on May 31, 2013, 09:55:29 pmOf course, doing it like that didn't even occur to me.That is of course rather reasonable, though it will still be tricky to get right.