Question about analog meter movements - Page 1
Forum section: electronics / beginners

author: Audiofanman
author_rank: Contributor
timestamp: February 19, 2025, 11:59:54 pm
content: I created a 100 to 1 voltage divider / reducer as mentioned in the following posthttps://www.eevblog.com/forum/beginners/ee-required-for-dvm-to-have-1100th-result/with the intent of using a DVM and probe test points added to the chassis...I wanted to measure or have the ability to measure 5 test points but now realize that6 additional holes in the chassis (+1 for ground) will look like poop..Got me thinking, I already have an analog meter across a 10Ω sense resistor measuring the output tubes mA or bias and thought that may be I could use that movement with a 5 or 6 position rotary switch.. That analog meter swings to full deflection with 1mA of current...The voltage divider PCB is intended to see no more than 600v on the high side which puts out 6v at 500nA.My problem is now, that the circuit I designed is intentionally a high impedance circuit. The circuit it attaches to should barley know its thereand that's the way Id like to keep it. That being said, it will not supply 1mA while at 6v (meaning 600V is being measured at B+)So, Im thinking an OpAmp could be used, tie the 6v 500nA to the non inverting side of a LT1001, the output fed back to the inverting input a few resistors here and there and my LTSpice sim is happy giving me exactly 1mA while the opamp is seeing the 6v at the +input...This would be full deflection on the analog meter, with a new scale showing 600vWhat I dont know for sure is if say a 56v input of the 100/1 voltage divider, putting out 0.56 volts to the opamp would track in a linear enough towards the low end...I see the problem as being two fold, one being how linear the opamp can be made to work from say 0.1v to 6vand secondly determining what current the analog meter would need to have flowing though it to display 10v...The only guaranteed workable solution would be to use a 5 position rotary switch and drill only two holesfor test jack tips.The thing is now Id really like to use the analog meter already installed...Anyone have any ideas on how this could be accomplished ?

author: ledtester
author_rank: Super Contributor
timestamp: February 20, 2025, 12:47:40 pm
content: Quote from: Audiofanman on February 19, 2025, 11:59:54 pmOne idea is to have a range switch the operator can use to more closely examine lower voltages. For instance, the HP 6289A has two voltage ranges - 50V and 5V full-scale (see attachments).QuoteSee figure 11 in this Nuts and Volts article:OP-AMP COOKBOOK — Part 4 -- Ray Marstonhttps://www.nutsvolts.com/magazine/article/op-amp-cookbook-part-4

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 20, 2025, 01:23:39 pm
content: Maybe this will help. I have a Fourdee PSM analog meter that uses op amps. It is quite nice. Perhaps the schematic hay help. Hopefully it is attached but if not Google Fourdee PSM 37... Good Luck

author: themadhippy
author_rank: Super Contributor
timestamp: February 20, 2025, 02:09:49 pm
content: QuoteIf the meters set up to show 10v as fsd then 1ma,if its set for 100v fsd then 10v would require a tenth of 1ma

author: David Hess
author_rank: Super Contributor
timestamp: February 20, 2025, 11:21:23 pm
content: Buffering a high impedance signal to drive a low impedance meter movement is a great application for an operational amplifier. In the past this could be done with one or a couple of transistors.An LT1001 is a great choice but overkill, and if you like the LT1001, then an LT1012 or LT1097 is better yet and practically made for this application. Any common inexpensive JFET or CMOS input operational amplifier is more than sufficient.I am sure have seen a meter driver using the circuit shown below, which is commonly used as an ohms converter current source in old multimeter designs. A single operational amplifier and two resistors can operate as a precision current divider or multiplier.The other configuration I have seen places the meter within the feedback loop of the operational amplifier.

author: Audiofanman
author_rank: Contributor
timestamp: February 21, 2025, 01:01:45 am
content: All,Thank you so very much!The Op-Amp cookbook looks like a nugget of gold!As someone who still struggles with the basics, op-amps are very new to me..It will take a little time to get more of a grip but the cook book looks like it is perfect, TY again...Shoulda learned more about this stuff 40 years ago, when i was in my 20's..I think I can defrag and may be find a little free spaceI think a quick order is in order, and bread-boarding the circuit to get a feel for whats happening...Stay tuned as Im sure I will have questionsGlenn