Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Simple coil question (I think)

Status
Not open for further replies.

JohnAnonymous

New Member
I'm very new to electronics, but have always been into making things. Recently decided to try a DIY metal detector I found on the net. Fo far, everything from making the PCB to wiring everything together has gone fine.

The plan had two coils - a detector coil and a reference coil. The goal as I understand it is to have these at the same or very nearly the same frequency. I picked up a Radio Shack multimeter (my Fluke wouldn't go beyond 50 kHz) to measure the frequency of each.

I woung the large detector coil - it reads 96.5 kHz. I wound the smaller reference coil using a wood dowel - 156.3 kHz. I removed some wire from the smaller coil and remeasured - 121.0 kHz. Removed a little more wire - 0 kHz.

I hooked the larger coil up to the same circuit, and it reads fine. I have triple checked my connections, re connected the large coil at least 3 more times (always works) and have finally succumbed to scratching my head while staring at the coil.

I can rewind the coil (after a trip to Radio Shack for more 26 g wire), but I can't figure out why a simpe wire wrapped around a dowel would suddenly not have a frequency at all.

Can anyone shed a little light as to why this might happen?
 

JohnAnonymous

New Member
Fluke is a 112. The Radio Shack is a 22-811 (don't see a model #)

Both measure frequency, but the Fluke has a lower limit. Surprised me actually. Can't justify an oscilloscope....yet. I've had a ball with this so far, and already have a few more projects lined up. But first I'm going to educate myself with a couple of books and quite a few small projects. I remember electronics being a very popular hobby when I was young (early 80s). I had no idea how far the hobbiest community, and technology itself, has pushed the envelope on what you could do at home. I'm not really new to electrical work, but small electronics are new to me.

Any ideas on the coil are greatly appreciated.
 
Last edited:

edeca

Active Member
The Fluke is only rated to 50KHz because they are a trustworthy brand who have rigorous quality assurance procedures. Therefore they specify an operating range (say 0-50KHz) where they know it will be reliable.

The Radio Shack one might claim to go higher and might not be restricted to 50Khz max, but I'd be interested what the claimed accuracy is. And then I'd be interested how good their design quality is.
 

strantor

Active Member
check your meter's fuse. I had that meter and it's literally a piece of shart. It blows it's own fuse. I ranted about it already. Edeca is right, I think their QA department is comprised of a 5y/o and a blind deaf mute.

oscopes are good.
 

JohnAnonymous

New Member
I love my Fluke. It has been a great meter for almost ten years. I just needed something cheap to measure frequency for this project.
 

strantor

Active Member
I understand, but still you should check your fuse; I suspect that's your problem. That meter blows it's fuse arbitrarily. It happened to me, half way through checking a pile of resistors, so I have no doubt it could happen to you, while checking frequency.
 

dr pepper

Well-Known Member
Most Helpful Member
The issue might be the threshold volatge on your meter, the input circuit for the frequency counter will have a voltage at which it classes the input as the start of a cycle, if your volatge level drops below this voltage then the meter will not count any cycles and show 0 hz, altering the turns on the coil depending on the oscillator may well alter the amplitude of the output.
A ;scope or dedicated frequency counter would give a better reading, I nkow you cant justify a 'scope, you can get counters for around the same price as a reasonable meter.
 

JohnAnonymous

New Member
I tested the fuse and it is good. I also manually set the meter to the range the coil should be in. Still no reading.

Gave up and rewound the coil. Hooked up the meter and...162 kHz! Yay! Unwound about ten turns of wire, snipped it off and...0.0 again. I think dr pepper is closer to the solution. When I hook the larger detector coil up I get a pretty constant 96.5 kHz. I'll keep unwinding and see if I can get it close enough for the meter to read right. But the meter is going back to the Shack.

I did a quick search for a dedicated frequency reader and found them to be around the same price as my fluke, and not too far below the price of a used oscilloscope. Maybe I'm looking in the wrong place?

Related question: what local businesses might have an oscilloscope I could use - or have them measure the coil for me?
 
Last edited:

strantor

Active Member
Well any industrial business that's big enough to have a maintenance department, could possibly have one. Universities have them. Probably a TV repair place. maybe put an ad in craigslist "wanted, to use your o-scope for 5 min" and you could replies from hobbyists.
 

BobW

Active Member
I wound the smaller reference coil using a wood dowel - 156.3 kHz. I removed some wire from the smaller coil and remeasured - 121.0 kHz. Removed a little more wire - 0 kHz.

This doesn't make any sense. Removing wire from a coil should cause the inductance to decrease, and the frequency to increase. If the frequency was too high, you should have been adding more turns, not removing them.
 

JohnAnonymous

New Member
If this is true, it doesn't currently help me with my meter issues. Last night I managed to get a few reading again, but they were all lower than 100, and never the same even when retesting without changing anything. Some time 60 kHz, sometimes 20 Hz, various reading in between. I think the meter is useless.

I've never tinkered with stuff like this, but if I had a working method for measuring frequency, I would have discovered that on my own pretty quickly. Every reading I got from the RS meter is suspect! I'm going to ask around as to where I can use an oscilloscope.
 

dr pepper

Well-Known Member
Most Helpful Member
See if you can find out if theres a local ham radio meet, they are everywhere, one of the boys will probably help you.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top