Are Center-Zero Galvanometers mechanically different than the normal ones? - Page 1
Forum section: electronics / beginners

author: HAL-42b
author_rank: Frequent Contributor
timestamp: February 03, 2016, 07:19:50 am
content: I'd like to build an indicator for a LVDT probe, which is basically a null meter. I have a few indicator movements but none of them are center-zero.So the question is how can I hack one of these into a center zero movement? Is it enough if I just apply DC bias or do I need to modify the spring arrangement inside? Obviously I'd like to maintain linearityHow are the ones inside FLUKE 845A constructed? Its needle stays at the center even when the instrument is off so obviously it is not just DC bias.Any clues appreciated.

author: Gyro
author_rank: Super Contributor
timestamp: February 03, 2016, 09:13:03 am
content: Center zero movements are mechanically the same, the difference is the way the hairsprings are set to adjust the pointer position.To modify the meter you will need to get the movement out of the housing - you need to adjust the position of the back hairspring so that the screw fine adjuster on the front one still works. It's delicate work, but doable. The bottom spring mounting can rotate in exactly the same way as the top one (it's used for factory adjustment to get the top adjuster within range).You need to do it in a completely swarf-free environment - use a big sheet of white paper, also check there's no swarf stuck to screwdriver bits. If you need to remove the scale to get at the mounting screws, then it's a good idea replace it after to protect the needle while you're adjusting. Actually you need this anyway so you can set the center position. make sure the front fine adjuster on the is set mid position so you can adjust the zero when it's back in the housing.You probably only get one chance of not screwing it up so be careful. If it's a cheap movement then it's less of a worry. If you're not confident with fine stuff then the DC offset may be your only option.

author: EEVblog
author_rank: Unknown
timestamp: February 03, 2016, 09:16:58 am
content: No different. Just like you can adjust the physical zero position on an analog meter. If those had a wide range of adjustment you could center it.

author: HAL-42b
author_rank: Frequent Contributor
timestamp: February 03, 2016, 09:45:38 am
content: Thanks for the answers guys. Clear as daylight now.The good Russian movement that I had earmarked for this job has a glass window at the back which is sealed shut with some sort of cement. Not possible to get in without doing much damage.Before ruining that I think I would buy a cheap analog multimeter and experiment on that first. Plus, it already has a case, jacks, battery compartment and even a voltage divider I could scrounge.

author: dom0
author_rank: Super Contributor
timestamp: February 03, 2016, 11:42:36 am
content: Left-zero meters are sometimes better than center-zero meters, even when you want to have it center-zero, since they are easier to drive ; for a center-zero meter you either need bipolar supplies or some kind of bridge (these two options are technically the same), be it two OPs or an op working against a resistive divider / rail splitter or whatever.The only real disadvantage is that you might need an additional meter-zero switch, if the display isn't easily zeroed.

author: HAL-42b
author_rank: Frequent Contributor
timestamp: February 03, 2016, 12:19:28 pm
content: The intention is to drive it from a bridge or an instrumentation opamp. I have no idea what the linearity will be in one case vs. the other.It is desirable that the same movement in the LVDS would produce the same deflection in both positive and negative directions on the meter.

author: Gyro
author_rank: Super Contributor
timestamp: February 03, 2016, 12:41:12 pm
content: Have you tried ebay? I see several center zero meters of various ages and quality listed there.

author: HAL-42b
author_rank: Frequent Contributor
timestamp: February 03, 2016, 01:59:30 pm
content: Quote from: Gyro on February 03, 2016, 12:41:12 pmThe point was to either put to use the ones which I already have or throw them away.I'm a hoarder on my way to recovery

author: BradC
author_rank: Super Contributor
timestamp: February 03, 2016, 02:04:28 pm
content: On the matter of swarf, I managed to get some lodged in a movement. I dismantled it far enough that I could use a good little neodymium magnet to remove the swarf, based on the "strongest magnet has more attraction" theory. It worked well. Not sure how it would fare in an environment with low coercively magnets, but it got me out of a jam. I used a tiny disc Neo magnet glued to a wooden stick to just pick off the pieces a couple at a time.

author: Gyro
author_rank: Super Contributor
timestamp: February 03, 2016, 04:17:47 pm
content: Quote from: HAL-42b on February 03, 2016, 01:59:30 pmDon't do it, you know you'll need it as soon as you throw it away!!!

author: HAL-42b
author_rank: Frequent Contributor
timestamp: February 03, 2016, 04:40:17 pm
content: Quote from: Gyro on February 03, 2016, 04:17:47 pm[Heavy Breathing]

author: uncle_bob
author_rank: Unknown
timestamp: February 03, 2016, 06:05:09 pm
content: HiKeep in mind that the sensitivity of the movement also matters in a real application. A meter intended for use with a 50mv shunt is going to be set up very differently than a 50 ua movement.Bob

author: HAL-42b
author_rank: Frequent Contributor
timestamp: February 03, 2016, 06:16:00 pm
content: Quote from: uncle_bob on February 03, 2016, 06:05:09 pmDoes it still matter if I buffer it with an opamp?

author: uncle_bob
author_rank: Unknown
timestamp: February 03, 2016, 06:21:52 pm
content: Quote from: HAL-42b on February 03, 2016, 06:16:00 pmHiYes, it most certainly does. Meters work based on turns of wire and current through that wire. Few turns and lots of current will do the job at low voltage. Lots of turns and very little current will do it at a higher voltage.A 50 mv shunt (or 25 or 10) meter is likely a high current / low voltage winding. It may take more current to go full scale than your op-amp can supply. A lot depends on the specific meter and how much current your op-amp can sink / source. Obviously lower shunt voltages are worse in this respect.It is at least theoretically possible that a 50 ua meter might take more voltage to deflect than your op-amp has available. This is a less likely thing with normal op-amp supply voltages.Best to check this stuff before you go tearing into a meter.Bob