Simpson Analog Wattmeter - Page 1
Forum section: electronics / projects

author: Crossphased
author_rank: Regular Contributor
timestamp: January 30, 2019, 06:41:06 am
content: Hey guys,I'm doing some work with a power measurement IC, and I'd like to compare readings from the chip vs readings from this vintage panel wattmeter I have. Meter is in great shape and needle movement is free and seems good. Its a Simpson, scale 0-10 watts. Fine print on the meter face reads typical current is 100 mA (max 175) and typical voltage is 100 (max 300). So that tells me it was intended for single phase usage. I know nothing about the internal construction of these types of meters, would it also be suitable for measuring DC? Also, theres four connections on the back of the meter, I'm guessing the LOAD connections are intended to be in series w/ the current? Or are these intended to be used with a shunt? Thanks for any help, couldnt find much info on this vintage meter.

author: wilfred
author_rank: Super Contributor
timestamp: January 30, 2019, 07:26:56 am
content: Youtube channel "The Signal Path" did a teardown of an instrument that sounds similar to what you have. I have no real idea though.Why not give it a look. "TSP #140 - How Do Purely Passive Watt-Meters Work?"It is a very high quality channel and well worth a look.

author: capt bullshot
author_rank: Super Contributor
timestamp: January 30, 2019, 07:48:07 am
content: Wow, that's a really nice meter.Internally, they "do the math" by means of magnetic fields. It's a "moving coil" meter like a DC current/voltage meter, but using an electromagnet to provide the field instead of the permanent magnet,The load current is routed through a large gauge wire that is wound around the stator, providing the field. The voltage is fed to the coil moving the pointer. Pointer movement is proportional to coil voltage multiplied by field current, smoothed in the time domain due to the mechanical mass. These meters work "wideband" from DC to some hundred hertz, the upper limit is given by the inductive characteristics.The binding posts are labelled "line" and "load", so I'd suggest to connect the source to "line" and the load to "load", routing the load current through the meter. It says "compensated for potential current loss", so I'd guess the voltage sense (the moving coil) is connected across the "line" bolts, while the current sense (the field) is connected from one "line" bolt to one "load" bold, while the others are connected directly. So the meter doesn't display its own consumption. It would show positive power flowing from "line" to "load".Edit: As this meter is designed to be wired directly "in-line", it would be rather difficult to modify its voltage and current ranges. A shunt wouldn't work at all, rather a current transformer (limiting its usage to AC). Depending on the internal construction, it might also be difficult to modify the voltage range without opening and modifying the meter itself.Edit: "TSP #140 - How Do Purely Passive Watt-Meters Work?" shows excactly this kind of meter

author: Crossphased
author_rank: Regular Contributor
timestamp: January 31, 2019, 03:41:10 am
content: Thanks for much for the info. I watched the video, it was very instructive. As I understood the video, the two different coils should be electrically isolated, if there are no connections to the binding posts. I did a quick resistance check between different terminals on the back. Here is what I measured:Between the line and load terminal on the left side, the measurement was quite jumpy, dancing around from 60 Ohms to 300 Ohms to 120 to 500, very unstable. Between line and load on the right side, the reading was much more stable around 50 Ohms. Jumping around between maybe 47 Ohms and 55 Ohms.Between the two line terminals the measurement was a steady 18K, and the same for between the two load terminals.I tried wiring up in the following configuration, as suggested above, but no dice. No meter movement. Understandable with 18K series resistance! I tried swapping terminals a few times and at one point was able to get the needle to move backwards, but never forwards.I should also state I was doing this at low voltage- adjusting PS from 0-10 vdc with 15R load. Wanted to start at low voltage to avoid damaging the meter. I realize meter expects 100V-300V so perhaps this is my error.What are your guys thoughts? Damaged meter? Try different wiring connections?Thanks

author: floobydust
author_rank: Super Contributor
timestamp: January 31, 2019, 05:36:01 am
content: The high resistance wandering around between the left line-load terminals is a problem I think.I'm not sure if this is an AC or DC meter, if it contains rectifier diodes. Try take readings on diode test.The cramped 0-1W scale portion "compensated for potential current loss" looks like around 0.5W worth. I think that is the coil resistance as a current shunt or the rectifier diode drop.The meter has Line and Load terminals, I would try the meter connected that way. Incoming power to the Line terminals only, and the Load at those two terminals only.If the line-load resistance is too high on one side, the coil might be damaged, or the meter needs an external shunt, or there's a bad connection warranting disassembly of the meter.The bakelight is very brittle and it's hard to take these apart without patience. Usually you can see most of the guts taking the face off.

author: capt bullshot
author_rank: Super Contributor
timestamp: January 31, 2019, 06:21:49 am
content: Quote from: Crossphased on January 31, 2019, 03:41:10 amThe wiring should be like this: two wires from the battery to the two line terminals, and two wired from the load terminals to the load.When testing, bring at least one of the both (current or voltage) to the nominal level to get a significant movement. (e.g. 100mA at 10V is 1 Watt)