Hamfest Ammeter - Page 1
Forum section: electronics / projects

author: neo
author_rank: Super Contributor
timestamp: August 31, 2016, 05:57:57 am
content: So i recently went to hamfest i got a bunch of stuff including a matching set of gauges a volt meter with 4 ranges the bottom one is 0-1 in .1 increments next one up 1 to 10 in increments of 1 then next up 0-20 in increments of 2 and finally the mode i have it on 0-50 in increments of 5 and then theres the matching ammeter gauge 4 ranges minus3 to plus 10 increments of 1, next one up minus 15 to plus 500 increments of 5, next one up minus 30 to plus 100 increments of 10 and finally minus 150 to plus 500 increments of 50.Now believe it or not i dont post this just to talk about them, i want to use the 100 amp scale of the ammeter for positive measurement is it as simple as getting the right shunt? fs=15-0-50mvdc.I ask because these were expensive and i dont want the ammeter to goand ruin a nice set.

author: ivaylo
author_rank: Frequent Contributor
timestamp: August 31, 2016, 06:44:51 am
content: Edit: Somehow YouTube copied the url of their stupid ad. Apologies... What I wanted to post is this:https://youtu.be/wbRx5cQZ8Ts

author: calexanian
author_rank: Super Contributor
timestamp: September 01, 2016, 04:39:42 am
content: Yes. It is as simple as that. Finding accurate meter shunts is a little on the difficult side these days.50mv is a common movement and eBay may be of use. generally they stay calibrated over the decades.http://www.rammeter.com/media/specsheets/Ram_Meter_Inc_F_Series_Specs.pdfHere is a link to Rams shunts. 100 amp for a 50 millivolt movement would be a .0005 ohm shunt. They have them for a little under $40 retail. Some people will use a calibration meter in series and use a length of wire coiled up as the shunt and adjust the length of wire until the meter reading tracks with the calibration meter so a whole shop may only have one fancy shunt. You could also cobble something together with a bunch of low ohm resistors as a crude shunt and a low ohm pot across that to fed the meter but the temp/co would probably be all over the place at 100 amps.

author: neo
author_rank: Super Contributor
timestamp: September 02, 2016, 01:24:19 am
content: Thanks to both of your im the paranoid type so i wanted to make sure, just to clarify i don't want to go up to 100 amps necessarily i just want it in that range i dont have anything that can get that high.

author: calexanian
author_rank: Super Contributor
timestamp: September 03, 2016, 04:28:08 am
content: Just remember the 50 millivolt movement and adjust your current shunts accordingly. there are a number of meter protection methods out there of shunts and steering diodes. a germanium or shottkey diode across the input going both ways will shunt any stray signal over 100 millivolts or so away from the meter.

author: neo
author_rank: Super Contributor
timestamp: September 10, 2016, 05:55:54 am
content: As i said in that range meaning that the meter itself has multiple ranges at 100 amps full scale each line is 1 amp but thanks for the useful information, in my opinion they really are beautiful meters.

author: jh15
author_rank: Frequent Contributor
timestamp: September 10, 2016, 06:16:08 am
content: As mentioned, look into protecting them, and basic electricity books of old. The back to back diode scheme worked for me. just use Ohm's law to make sure diode's knee won't be conducting till after full scale.

author: neo
author_rank: Super Contributor
timestamp: September 10, 2016, 07:01:21 am
content: Quote from: jh15 on September 10, 2016, 06:16:08 amQuoteBut wouldn't 100 mv destroy the meter?

author: JimRemington
author_rank: Regular Contributor
timestamp: September 11, 2016, 12:59:21 am
content: QuoteNo, not by a long shot. But with a very sudden turn-on, 100 mV will bang the needle on a stop pin and might possibly bend one or the other.

author: w2aew
author_rank: Super Contributor
timestamp: September 11, 2016, 01:18:47 am
content: Quote from: neo on September 01, 2016, 04:00:28 amMy follow up video discusses using shunts to change ammeter scales:

author: jh15
author_rank: Frequent Contributor
timestamp: September 12, 2016, 03:01:11 am
content: "100 mV will bang the needle on a stop pin."Also called the bend-against or wrap-aroundhttps://www.eevblog.com/forum/Smileys/default/evil.gif

author: neo
author_rank: Super Contributor
timestamp: September 12, 2016, 07:34:00 am
content: Thanks all, on a side note to all who im sure will find it funny a bought a shunt from china and then found a ge branded one in my stuff i didn't even know i had, just a little cosmic joke and im the subject of it.