Low Ohms

[bigrex]

Hello again,

I've been getting some more advice from an Australian fellow who I'm actually able to talk to through something called "freshtel firefly" online. Anyway, just wondering if anyone could recommend a certain meter (brand/model/or type) for measuring low ohms. I have been looking on eBay and am not certain exactly what to look for. Thanks.

[bbsailor]

An LCR meter is usually good for reading low ohms. Some models have a zeroing function others you need to short out the leads, note the reading usually in tenths of ohms , then subtract that reading from what you are measuring.

Some 4.5 digit multimeters also give you good low ohm resolution; two digits after the decimal point.

I hope this helps?

bbsailor

[Unregistered]

Be careful of using the ohms function of an LCR Meter...

as most are AC Ohmeters(Impedance Bridges) not DC Ohmmeters. Read the manual to be sure. The easiest way to tell if you don't have the owners manual is to hook up a coil to it and measure the ohms at both frequencies(Most LCR Meters have two or more frequencies) If it's an AC Ohmmeter the resistance will be different at each frequency as you actually measuring the impedance of the coil. If it has a DC Ohmmeter the resistance will not change if you switch frequencies while measuring resistance.

As far a good meter to measure low ohms. Pretty much any of today's Digital Readout Multimeters that have 200 ohm range will read out in tenths of an ohm and can easily and accurately measure the Tx and Rx windings of metal detector coils.

Hope this helps

[bigrex]

Hello unregistered user,

As I mentioned I've been talking with an Australian fellow. He mentioned that multi-meters would need an adaptor to read low ohms such as the one he recommended found at:
http://transmille.com/metermate_adaptors.htm
The meter is manufactured in England. Just wondering if there is something different about meters in foreign countries that would require an adaptor for multimeters and why the discrepancy between the information I'm receiving at different ends. He also mentioned that LCR meters wouldn't be great for measuring low ohms. Thanks for reviewing this message, anyone can feel free to respond if you're out there.

Regards,

Rex B.

[Unregistered]

Low Ohms Adaptor

For Metal Detector Coils you don't really need something like that. Most of today's meters have the 0-200 ohm range that measures in tenths of an ohm. I am not sure what the resistance is of PI coils but Most VLF's are 1ohm and higher. The Minelab Explorer for instance has a 1.1ohm Tx winding but there are slight variations between different size coils from different manufacturers. I have seen it as low as 0.8 ohms and as high as 1.3 ohms. The key is to get both the resistance and inductance as close to the manufacturer's coil as possible. You will find that for a specific size coil there is only one wire size that will get you to that resistance and inductance.

One thing to keep in mind is to always use the same meter to measure your coils once you settle on the meter you will use. Most meters when used on the 0-200 range do NOT read zero when the test least are shorted together because the meter will actually measure the the resistance of the test leads. Usually between .1 and .3 ohms depending on the guage wire the test leads are made of and how good the mechanical connection is between them and the meter itself. I tested both high and low end meters before buying mine and found that it didn't mstter whether it was a $20 meter or a $200 dollar meter they all measured the resistance of the test leads when used on the Low Ohms range. A friend of mine has a $300+ Fluke Meter that he swears by and it did the same thing and since Digital Meters don't have a Zero Adjust on them like the old Analog meters did it is just something to be aware of.

[bigrex]

Unregistered guest,
Thanks for the added info. The minelab GP series detector coils need to have the ohms right at 0.4 with an inductance of 300 uH (microhenries).

[Unregistered]

Interesting

I don't know which size coil those numbers but we can use an 11" coil as an example. Using the inductance formula from the website at the address as a starting point it comes out to roughly 24 turns of 17.5ga wire for an 11" round coil to come out to the resistance and inductance numbers you gave. Here's a web address with forumlas that come in handy. For round coils they are pretty accurate but for D-shaped coils the number of turns are off a little on the high side.

http://www3.telus.net/chemelec/Calcu...alculators.htm

[bigrex]

Thanks unregistered,

I already have gotten direction when it comes to the calculators, they were quite helpful. With the one I used the 25" coil I hope to construct came out at about 11.7 turns and 76.576' of coil. and about the same AWG you mentioned. You can look at the info. I've received at the thread entitled Instruction monocoil for minelab GP series in the coil forum if you haven't already browsed through it. Let me know if the above stats seem wrong to you.

Regards,

Rex

[Unregistered]

Tryed your values

for the 25" coil and it came out to 16ga wire, 16T, and 105ft. As I said the calculators are just a starting point. The one at the link I posted above is usually quite accurate on the resistance but is usually off by a turn or two on the Inductance...hence the need for the LCR Meter. The inductance can vary by a few turns one way or the depending on how tightly you wind your coil. The PI that you are building the 25" coil must have a very hi power Tx section to need 16-17ga wire. Good Luck with your project!!! But be warned building coils can become addictive ...just like this hobby ) When I started building coils this past Summer I was only planning on building one coil. Now I am making different size and shape coils for both my Explorer and my Sov

[Charles (Upstate NY)]

Hello again,

I've been getting some more advice from an Australian fellow who I'm actually able to talk to through something called "freshtel firefly" online. Anyway, just wondering if anyone could recommend a certain meter (brand/model/or type) for measuring low ohms. I have been looking on eBay and am not certain exactly what to look for. Thanks.

When measuring DC resistence I found a variation between cheaper digital multi-meters. My $35 Radioshack meter measured different than my $80 Extek. Both of the cheaper meters also tend to bounce around e.g. you connect the leads and then see 1.2ohm, 1.1ohm, .9ohm, 1.0ohm, 1.2ohm bouncing on the screen. I purchased this 50,000 count BK meter below and put an end to that BS.

My advice is buy the better quality test equipment from the start versus wasting money on cheap test equipment only to replace it later.

Also...BEWARE the internet test equipment dealers who have nothing in stock. Here's the scam, lets say you live in NY as I do and you place an order say with a company in NJ. But the shipping charges seem high and they say it will be 7 to 10 days before you receive the equipment. UPS ground is normally next day between NY and NJ. Hmmm.

Charles: Do you actually have this in stock?
Scam-o-company: Yes (that's a lie)

Charles: You are located in NJ?
Scam-o-company: Yes

Charles: Where is the item shipping from?
Scam-o-company: CA

Charles: Why is it shipping from CA when you are located in NJ?
Scam-o-company: Our warehouse is there.

Long story short, they are just drop shipping from the manufactures warehouse in CA. In fact they have no warehouse and stock none of the hundreds of items they advertise. You pay more for shipping and get to wait 7 to 10 days for your order to arrive.

The place I buy from in the link below almost always have what I want in stock. The price is reasonable and the equipment arrives within 2 days.

http://www.testequipmentdepot.com/b+...ision/5390.htm

[bigrex]

.

[chemelec]

If you know the gauge of wire, number of turns and the diameter of the coil, you can than calculate the actual length of the wire.

Most wire chart will tell you the resistance of a given wire size in ohms per 1000 feet.

So to Mathmatically calculate the DC Resistance is Quite easy this way.
And probably more accurate than those cheap ohm-meters.

Or pass a Known DC Current through the coil, with a simple constant current source. Than measure the voltage directly across the coil.
"Ohms law" will give you the DC resistance.

[bigrex]

Thanks Chemelec,
I have now calculated the correct length of wire by looking at the ohms per 1000 ft dividing to get ohms per ft., etc.
One final query on this thread,
To double check the ohms when I hook up the coax, connector, etc. Does this multimeter look all right for measuring ohms? Here is a link to take a look:
http://www.multimeterwarehouse.com/vc9806f.htm

And here are the specs if that doesn't work:

DCV:0-200m-2-20-200±0.5%-1kV±1.0%
ACV:0-200m±1.0%-2-20-200V±0.8%-700V±1.2%
DCA:0-2m-20m±0.5%-200m±0.8%-20A±2.0%
ACA:0-200m±1.2%-20A±2.5%
R:0-200-2k-20k-200k-2M±0.2%-20Mohm±0.5%
CAP:0-2n-20n-200n-2u-20uF±2.5%
Audible continuity:<90ohm
Frequency:0-20kHz±1.5%
Transistor and Diode test
Batt: popular 9Vx1

[chemelec]

Any Ohm-Meter will work, But Most Meters are Not very accurate below 1 ohm.

I have a meter that measures accurately down to .01 ohm, but its a specialized piece of eqipment.

For Simple, Accurate low resistance measurements, make yourself an Accurate Constant Current Source using an LM317.
Maybe about 10 mA or 100 mA.
Keep it Relatively Low, so as to Not apply too much current and overheat what your testing.
Than applying this to the coil or other unknown resistance, measure the voltage at the coil terminals.

Knowing the voltage, divide it by the current to get the resistance.

Quite Simple.

Thanks Chemelec,
I have now calculated the correct length of wire by looking at the ohms per 1000 ft dividing to get ohms per ft., etc.
One final query on this thread,
To double check the ohms when I hook up the coax, connector, etc. Does this multimeter look all right for measuring ohms? Here is a link to take a look:
http://www.multimeterwarehouse.com/vc9806f.htm

And here are the specs if that doesn't work:

DCV:0-200m-2-20-200±0.5%-1kV±1.0%
ACV:0-200m±1.0%-2-20-200V±0.8%-700V±1.2%
DCA:0-2m-20m±0.5%-200m±0.8%-20A±2.0%
ACA:0-200m±1.2%-20A±2.5%
R:0-200-2k-20k-200k-2M±0.2%-20Mohm±0.5%
CAP:0-2n-20n-200n-2u-20uF±2.5%
Audible continuity:<90ohm
Frequency:0-20kHz±1.5%
Transistor and Diode test
Batt: popular 9Vx1