User avatar
EmAtChapterV
Posts: 244
Joined: Sun Jan 06, 2013 6:49 pm
Location: Vancouver, BC

AC Ammeters - you get what you pay for

Post: # 27350Unread post EmAtChapterV
Thu Oct 03, 2013 7:43 pm

Last week I was pleased to finally find a 0-1 AC ammeter for sale at a local electronics shop. "Huzzah," I thought to myself, "combined with a spool of AWG34 wire and a meat thermometer, I can finally do some proper research into coil heat buildup, as well as safeguard my cutters."

Alas, I quickly found out why it was inexpensive: it has a DC impedence of 1.8 ohms (cf 0.213 ohms at 60 Hz for a Simpson meter of the same scale), and must have a huge inductance - power in any circuit it's placed in falls off a cliff above 800 Hz. The loss at 16 kHz is anywhere from 12 dB in series with a 9.5 ohm coil, to 26 dB in series with a 2.2 ohm resistor. Yikes. Before I figured out what was happening, while running tests my poor beleaguered amplifier (rated 40W at 8 ohms) was trying to plow 70W into the circuit just to get some usable readings. No wonder the coil was heating up more quickly at higher frequencies, it was probably dealing with square waves.

I guess I'll rewind the coil to be much, much longer and use an impedance-matching transformer, and restart the experiments with that in mind. And leave this lemon of a meter out of any cutting circuits.

User avatar
boogievan
Posts: 142
Joined: Thu Feb 21, 2013 3:43 am
Location: Dutchess County, NY

Re: AC Ammeters - you get what you pay for

Post: # 27724Unread post boogievan
Fri Nov 15, 2013 12:08 am

Hi Em, Maybe you can try an electrical mouse-trap which is humane (like FloKa)? A meat thermometer will usually heat up faster than what it's measuring and if it shows you something really hot, it might be too late for a winding? Thankfully, resistance rises with temperature rather reliably with most coil metals, so Ohm-logic is a safe way to automate fast shut-off. The Fonofilm Industri stereo cutters constantly measure the DC resistance of the drive coil. Because of the ability to reach over 200 degrees Centigrade before melt-down, they wait for a target DC resistance of about 15 Ohms to build up before the automatic opening of the mercury-wetted reed relay in each amplifier chassis. When the coil is cold, it measures about 9 Ohms or less. But once it's 200 degrees Centigrade, that same coil measures at almost double its cold measurement in Ohms. The circuit is calibrated to open the relay using no mercury-wetted thermometers. (: I suspect you can devise a similar circuit to use with your coils.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/restmp.html

-boogie

Post Reply