Is it possible to replace panel meters with a new display? - Page 1
Forum section: electronics / testgear

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: February 22, 2014, 12:07:40 am
content: What sort of signaling do panel meters use? I looked at a panel meter datasheet and see mention of d'Arsonval (rectifier type) and iron-vane.Also if it's possible, how difficult is it?There are tons of PS with panel meters and I may just buy one and use it as-is but if it's possible I might retrofit a new display.

author: nowlan
author_rank: Frequent Contributor
timestamp: February 22, 2014, 12:29:05 am
content: I had a similar question, except wanting to know about retrofitting lcd with oled displays. Was considering replacing old lcd on older bench meters. I asked some friends who said they were very noisy, and not good for precision meters. Im told current models have heaps of shielding.Figured you could put a little mcu inline to translate signals if not the Hitachi standard.

author: N2IXK
author_rank: Frequent Contributor
timestamp: February 22, 2014, 12:47:37 am
content: In short, yes.But you will need to determine what the full scale current of the original panel meter is, and its internal resistance. The driving circuitry may depend on working into a certain resistance, which your new digital readout will have to emulate. The "signaling" used is simply an analog voltage or current, depending on the panel meter type being replaced.A step-by-step explanation of how to use a DPM module to replace an analog meter is available here:http://www.arizona-am.net/misc_files/TV7MeterB.htmSome of this is specific to the particular instrument being discussed (an old military tube tester), but the basic principles are the same no matter what you are working on.

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: February 22, 2014, 03:00:35 am
content: Quote from: N2IXK on February 22, 2014, 12:47:37 amYou are saying the panel meter is driven by sampling a portion of the current or voltage that's generated and the sample is sent to the panel meter directly. Is my understanding correct?I will find a schematic and see if I can decipher it.

author: N2IXK
author_rank: Frequent Contributor
timestamp: February 22, 2014, 03:36:06 am
content: It depends on the meter.Most panel meters are just a "raw" movement, with a certain sensitivity (usually written on the lower portion of the scale, in tiny text). Typical wording would be "100 uA FS". That meter needs a 100 uA current through it for full scale deflection. Its DC resistance is usually not marked, but can be measured with an ohmmeter (do not exceed the rated FS current when testing). The FS current on a meter movement may have NO relation to whatever the scale markings are. The scale can be printed to read in whatever units you want. The actual meter in an analog VOM is a microammeter. It is adapted to measure voltage, current, or resistance by the external circuitry wrapped around it.Some panel meters incorporate internal resistors in series or parallel with the basic movement, to create a voltmeter or a higher range ammeter. These usually have a scale that directly corresponds to their input requirement, direct voltage or current input.To adapt something like a DPM module that needs 200 mV full scale input, you need to look at the circuit and figure out what is being applied to the existing meter. Then adapt the circuit as needed. You may need to add a voltage divider to drop voltage down, or a shunt resistor to develop a voltage drop proportional to current (and that shunt should ideally be close to the DC resistance of the original meter movement).Once you have a suitable input signal for the meter module, you can calibrate it to read in whatever engineering units you want. by using a trimpot as a voltage divider.Post some specifics of what you are working on, a schematic would be a good start.

author: pickle9000
author_rank: Super Contributor
timestamp: February 22, 2014, 04:01:24 am
content: This style case used to be everywhere have a look.http://www.electro-meters.com/jewell/jewelldpms/dw_series/

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: February 22, 2014, 04:08:50 am
content: @N2IXK, great info, it's actually making sense to me! I will try to put something together and post here. First I have to decide on my target PS.

author: 6E5
author_rank: Regular Contributor
timestamp: February 22, 2014, 04:28:40 am
content: Make sure to use a modern digital multimeter for measuring resistance of meters. Analog multimeters such as a Simpson 260 will exceed the meter's current rating and might burn them out.

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: February 22, 2014, 08:05:37 pm
content: @6E5, roger wilcoAs a naive participant on the electronics field, reading the PS data sheets is an eye opener. I'll have to find a datasheet for a modern PS to compare but those old datasheets are a surprise. They even have definitions and provide detailed testing schematics. They show how and where to connect an oscilloscope, etc.I found mention of sampling resistors and so I think I'm on the right track, whether or not I can come up with something sensible as a solution is up in the air.