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Vintage threads from the first ten years

inductance tester

5/4/1999 9:03 AM
Kyle	inductance tester Hey guys, does anybody have a schematic for a DIY inductance meter? I'd like to build one so I can start getting into this coil stuff. Thanks.

5/6/1999 10:35 AM
Doc	There may be a hobbyist kit available, but I'm not certain, to allow you to read inductance with your hand held digital multimeter. I know they have them for temperature, and for capacitance. Check: MCM, Jameco, All Electronics, places like that. We're talking $20-30 range, and they give you all the parts in one package. Ranges for some of these add-on circuits can be limited, but may be useful for the area you plan to work in. Next step would be a hand held L-C-R meter, $150 and up, which will be more accurate and have wider range capability.

5/6/1999 10:35 AM

Doc

There may be a hobbyist kit available, but I'm not certain, to allow you to read inductance with your hand held digital multimeter. I know they have them for temperature, and for capacitance. Check: MCM, Jameco, All Electronics, places like that. We're talking $20-30 range, and they give you all the parts in one package. Ranges for some of these add-on circuits can be limited, but may be useful for the area you plan to work in. Next step would be a hand held L-C-R meter, $150 and up, which will be more accurate and have wider range capability.

5/6/1999 4:14 PM
Terry	I've seen the little kits, possibly already assembled (can't remember) at Ham Fests for about $12-$15. I was tempted to try one but didn't get around to it. Also I'm not sure about the reliability and range. I believe I saw one in a catalog not too long ago. If I can locate, I'll let you know.

5/7/1999 10:02 AM
Kyle	Thanks guys. I'd just get a LCR meter if I had the money, but being a student and all... I figure it'll be good practice to build one anyway!

5/7/1999 11:00 AM
Carl	I remember that I had to caclculate inductance of a coil in one of my physics classes years ago. The apparatus was pretty simple so you might want to scrounge around some electronics or physics lab manuals to see if you can find something useful there. One more reason to never throw anything away! Regards; Carl

5/8/1999 9:41 AM
Dave Stork	Impedance bridges Carl wrote: I remember that I had to caclculate inductance of a coil in one of my physics classes years ago. The apparatus was pretty simple so you might want to scrounge around some electronics or physics lab manuals to see if you can find something useful there. You can make decent "ballpark" measurements of inductance and capacitance using very simple apparatus if you don't mind doing a little math. The method used in your class may have been the "voltage divider" method, which is the simplest method. You build a voltage divider using a known resistance or inductance on top, and the unknown inductance on the bottom. You feed an AC signal (usually 1kHz)of known amplitude into the top of this voltage divider and read the voltage at the junction of the two devices in the divider. The ratio of input to output voltage is the ratio of the reactance of the upper device (at 1kHz) to the reactance of the lower device. From this, you can calculate the reactance in ohms of the unknown inductor at 1kHz and from that, its value in Henries. This method does not take things like distributed capacitance (for coils), ESR (for caps) and so on into account, so the result is an approximation, albeit a useful one in noncritical cases. A somewhat more sophisticated and much more accurate method is the impedance bridge. Most of these have switchable arrangements for different bridge types depending on if you're measuring resistance, capacitance, high-Q inductors, or low-Q inductors. I won't get into the particulars of the different bridge circuits, but suffice to say that even a "lab-grade" bridge is pretty simple compared to modern electronic L-C-R meters, even though you can achieve the same level of accuracy in a well-designed and carefully-built bridge. Usually, all the components are passive except for the signal source, unless you use an active detector for greater sensitivity. A bridge is built mostly from resistors, switches, pots and a couple of caps. You can find a very good discussion of impedance bridges, complete with circuits you can build yourself, in the book Instrumentation and Test Gear Circuits Manual by R. M. Marston, currently in print from Newnes/Butterworth-Heinemann. Keep an eye out for deals on used impedance bridges and older bridge-based inductance-capacitance testers. I own an Eico bridge that works great and has many features, and I think I paid $5 for it. One of the showpieces of my antique test equipment collection is my old General Radio impedance bridge, which works great (after I made a minor repair to the internal 1kHz oscillator), is beautifully built, and is very precise. I paid $12 for that one.

5/8/1999 9:41 AM

Dave Stork

Impedance bridges

Carl wrote:

I remember that I had to caclculate inductance of a coil in one of my physics
classes years ago. The apparatus was pretty simple so you might want to
scrounge around some electronics or physics lab manuals to see if you can find
something useful there.

You can make decent "ballpark" measurements of inductance and capacitance using very simple apparatus if you don't mind doing a little math. The method used in your class may have been the "voltage divider" method, which is the simplest method. You build a voltage divider using a known resistance or inductance on top, and the unknown inductance on the bottom. You feed an AC signal (usually 1kHz)of known amplitude into the top of this voltage divider and read the voltage at the junction of the two devices in the divider. The ratio of input to output voltage is the ratio of the reactance of the upper device (at 1kHz) to the reactance of the lower device. From this, you can calculate the reactance in ohms of the unknown inductor at 1kHz and from that, its value in Henries. This method does not take things like distributed capacitance (for coils), ESR (for caps) and so on into account, so the result is an approximation, albeit a useful one in noncritical cases.

A somewhat more sophisticated and much more accurate method is the impedance bridge. Most of these have switchable arrangements for different bridge types depending on if you're measuring resistance, capacitance, high-Q inductors, or low-Q inductors. I won't get into the particulars of the different bridge circuits, but suffice to say that even a "lab-grade" bridge is pretty simple compared to modern electronic L-C-R meters, even though you can achieve the same level of accuracy in a well-designed and carefully-built bridge. Usually, all the components are passive except for the signal source, unless you use an active detector for greater sensitivity. A bridge is built mostly from resistors, switches, pots and a couple of caps. You can find a very good discussion of impedance bridges, complete with circuits you can build yourself, in the book Instrumentation and Test Gear Circuits Manual by R. M. Marston, currently in print from Newnes/Butterworth-Heinemann.

Keep an eye out for deals on used impedance bridges and older bridge-based inductance-capacitance testers. I own an Eico bridge that works great and has many features, and I think I paid $5 for it. One of the showpieces of my antique test equipment collection is my old General Radio impedance bridge, which works great (after I made a minor repair to the internal 1kHz oscillator), is beautifully built, and is very precise. I paid $12 for that one.

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