1ma meter instead of 50ua - Page 1
Forum section: electronics / repair

author: m3vuv
author_rank: Super Contributor
timestamp: September 22, 2018, 08:56:33 am
content: hi all im looking of driving a 1ma meter in a project that specs a 50ua meter,anyone know of an amplifier that can do this?

author: gbaddeley
author_rank: Regular Contributor
timestamp: September 22, 2018, 11:58:34 am
content: Might be possible without an amplifier. Can you provide more details on the project, how the meter is used, and the schematic.

author: Gyro
author_rank: Super Contributor
timestamp: September 22, 2018, 12:15:37 pm
content: Quote from: m3vuv on September 22, 2018, 08:56:33 amTrying (and failing) to avoid quoting one of your own recent posts back at you...Quotehttps://www.eevblog.com/forum/projects/guesses-on-what-i-am-attempting-here/msg1830503/?topicseen#msg1830503How on earth do you expect anybody to attempt an answer to a question like that with zero context or background information. Schematic at least?

author: m3vuv
author_rank: Super Contributor
timestamp: September 22, 2018, 01:33:02 pm
content: its for an esr meter,find attached schematic.

author: Gyro
author_rank: Super Contributor
timestamp: September 22, 2018, 01:55:49 pm
content: Probably just a matter of lowering the value R17 and the 25k series Zero pot. The impedance of C3 is only 100R at 156kHz, so wouldn't need changing. Worst case, you'll need to re-scale the Collector and Emitter resistors of Q1.

author: m3vuv
author_rank: Super Contributor
timestamp: September 22, 2018, 02:02:19 pm
content: forgot to say im using it without the pot for zeroing the meter,i get about quater scale deflection at the most with the dut leads shorted,would using a darlinton for the transistor help?

author: m3vuv
author_rank: Super Contributor
timestamp: September 22, 2018, 02:05:22 pm
content: forgot to say r17 is just a link ie zero ohms,would bridging r12 help?

author: Gyro
author_rank: Super Contributor
timestamp: September 22, 2018, 02:15:06 pm
content: So what's therealschematic then, have you got R17 in circuit, if so, what value?A darlington might help but has a higher Vce drop,It's a charge pump, all the meter current has to be sourced by R11, Q1 is there to pull down, not up. You're running out of headroom on a 5V supply by the time you include the voltage drop across the 1N4148s. Try tweaking the values of R11 ans R12 to increase the stage gain.P.S. Please find some time to add some capitals and spaces in your text.EDIT: Ok, if you've already removed R17 then try reducing the value of R11, try 1k or 470R

author: m3vuv
author_rank: Super Contributor
timestamp: September 22, 2018, 02:55:48 pm
content: Hi also forgot to mention,im running this on 9 volts not 5,the oscillator was jittery on 5 volts.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 22, 2018, 03:14:39 pm
content: The 74HC14 does not like running on 9 V. So this is not a real option. It might need really good decoupling around the 74HC14.If running at a higher voltage one would also need to adjust R10 to get a reasonable DC level for the transistor amplifier.The normal way would be to reduceR12R17 to a lower value (e.g. 1 K instead of 10 K). For higher gain one could consider a 10 µF cap (maybe with 22 Ohms in series) in parallel to the 100 Ohms resistor at the emitter to increase the gain. Loading it with 1 K or less will reduce the amplifiers gain - so some extra gain would be good.

author: m3vuv
author_rank: Super Contributor
timestamp: September 22, 2018, 03:21:58 pm
content: r12 is 100 ohm not 10k

author: bsfeechannel
author_rank: Super Contributor
timestamp: September 22, 2018, 04:08:27 pm
content: Can you post a picture of the meter, please? The front and the back side.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 22, 2018, 05:05:17 pm
content: Quote from: m3vuv on September 22, 2018, 03:21:58 pmOf cause R12 is the emitter resistor. Reducing R12 is tricky, as this also makes the DC gain higher and thus the working point might get unstable. The suggested cap parallel to R12 is effectively shorting R12 for AC only and still keeps is for DC.

author: m3vuv
author_rank: Super Contributor
timestamp: September 22, 2018, 07:58:37 pm
content: well i cant post the pic but have the manual for the piece of kit i robbed the meter out of its a 1 ma meter.the reason for using it is its nice and big,my intention is to use the case as well.its a bradley ct471 multi meter from the 1960s,did try and atach the manual but its too large,its deffo a 1ma meter,the orig esr meter calles for a 50ua unit.I just need to be modify the circuit to drive it.

author: bsfeechannel
author_rank: Super Contributor
timestamp: September 23, 2018, 03:59:02 am
content: You have a very rare piece of test equipment in your hands. So, please be extremely careful!I have a suggestion.What I would do would be to introduce a buffer after Q1 so as to drive 1mA. Since 1mA is 20 times 50µA, I would reduce the value of R17 and increase the value of C3 by that factor.You won't escape the 1k5 trimpot in parallel with the meter. It's there to adjust the full scale.I would then:1) Start with the trimpot fully shorted out before even turning the circuit on.2) Short-circuit the terminals of the ESR tester (ideal good capacitor).3) Then gradually adjust the trimpot until I get 1mA.IMPORTANT!I would test this circuit with a multimeter set to mA first instead of the CT471. When sure that it drives around 1mA then I would proceed to connect that rare meter in place repeating the three steps above.Since I haven't tested the circuit, if you decide to implement it, do it at your own risk.Hope that helps.

author: m3vuv
author_rank: Super Contributor
timestamp: September 23, 2018, 05:27:46 am
content: just for your info the ct471 is dead,i got it from birkets in lincoln,,he has dozens of them all ex raf at £5 each im just going to use the case and meter movement,im going to buy a couple of more next week,worth £5 a go just for parts,hopefully be able to get a good one from 2 or 3,i will try your schematic later,im still in bed,its just 6.30am here,cheers Paul m3vuv.

author: m3vuv
author_rank: Super Contributor
timestamp: September 23, 2018, 08:29:58 pm
content: well just an update,I put in the buffer transistor and changer cap and resistor values,still had next to bugger all meter movent,looking at the scope,i was getting zero amplification,looked like q1 was saturating so guesed it was a biasing issue,i ended up hooking up two variable pots in parelell with r9 and r 10 ,after a bit of fidling it seemed better,i then measured the resistance removed the pots and fitted fixed resistors,Then did the same with q2,result was plenty of drive to the meter,well over enough tp push it well over full scale!,just out of interest r9 came out at 1.8k r10 at 5.7k and finaly r12 at 1.3k,as an aside it seems to be working on 5volts as well!,just in the process of fitting led illumination to the meter(dimable) a 9v battery check a 5volt out put check from the regulator,next job is to make the frequency switchable over say 4 ranges,sort of guilding the lilly but it males use of the original wafer switches,and means i wont have holes in the meter + most of the controls will be put to use,im pretty pleased with it so far,putting a defunct busted piece of test gear to another use,not bad,total cost a fiver and a few hours work,will post some pics when its finished,cheers for your help chaps,73 Paul m3vuv.

author: ebastler
author_rank: Super Contributor
timestamp: September 23, 2018, 09:04:04 pm
content: Quote from: bsfeechannel on September 23, 2018, 03:59:02 amSeems to late if he has already cannibalized the Bradley for the meter?

author: m3vuv
author_rank: Super Contributor
timestamp: September 23, 2018, 09:52:51 pm
content: it doesent bother me birketts have loads of them for a fiver each,i have one non working and the one ive turned into an esr meter,im in lincoln next wed so will buy another and try and get a good working one out of 2,last time i was there i picked up an original pair of rf probes for one for 50 pence each!.