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Old 2nd Oct 2022, 12:32 pm   #1
crtgamer
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Default LCR meter for measuring mains and output transformers recommendations?

Hi,
I am wanting to have a go winding some mains and audio output transformers for my self. So obviously it goes with out saying that I need a decent LCR meter to make accurate measurements. When searching for what there is, there is lots to chose from and all with different features and prices points, usually made by companies I have never heard of.

Could somebody recommend me a suitable good quality one able to make accurate measurements at different frequencies(as required for audio output transformers) at a moderate price(£100-£300)?

Thanks.
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Old 2nd Oct 2022, 12:56 pm   #2
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Default Re: LCR meter for measuring mains and output transformers recommendations?

What are you trying to measure?

Turns ratio can be done with voltmeters and a Variac.

Resistances can be measured with an Avo 8. (DMM's can sometimes be confused by the inductance, and cycle through autoranging randomly).

Inductance is like trying to measure spring constant of a piece of chewing gum. I usually use voltmeter and milliammeter, at 50Hz (applicable for both mains and output transformers) to get an idea of large-signal inductance. As long as it's high enough, that's all that matters - any extra is a bonus, and mains transformers (and output transformers!) are generally designed from a no-saturation point of view, not an inductance point of view.

Which just leaves leakage inductance. So here, an audio power amplifier plus signal generator, a voltmeter and milliammeter which are OK to 10kHz or more, will suit your needs - or you can use a 'scope with a current-sensing resistor.
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Old 2nd Oct 2022, 12:58 pm   #3
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Default Re: LCR meter for measuring mains and output transformers recommendations?

The Peak Electronics ones are good value for money, but as they are designed for general use, the built-in frequencies are limited as the range covered is from LF to HF.

You could of course just use an audio generator & a DMM, that way you can select any frequency required.

David.
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Old 2nd Oct 2022, 2:41 pm   #4
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Default Re: LCR meter for measuring mains and output transformers recommendations?

Hi, The measurements I am interested in making are primary inductance, leakage inductance and parasitic capacitance,

Primary inductance at different frequencies so I can measure the difference between mid 1000hz point and low frequency. so I can estimate -3db low frequency when used in various designs(As this will vary depending on the ratio of Primary inductance at different frequencies as well as ratio of valve impedance to the impedance reflected on primary by load)

As well as leakage and capacitance as to whether I have picked a good design choice,
more sectioning reduces leakage and increases efficiency, but increases capacitance and lowers high frequency bandwidth, and these figures will give me a good idea on quality of my construction

I am open to using other methods to make these measurements and using a audio signal generator to measure at any frequency I desire sounds good, if someone could elaborate on the best way of doing that?
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Old 2nd Oct 2022, 3:27 pm   #5
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Default Re: LCR meter for measuring mains and output transformers recommendations?

Primary inductance is frequency and voltage dependent. It shouldn't be, but iron unfortunately doesn't always do what elementary theory predicts...

Just connect the primary to the output of a Variac, in series with a milliammeter, and a voltmeter across the whole lot. Or to the output of your audio amplifier, fed by a signal generator, if you want to measure at frequencies other than 50Hz.

Set the voltage you want, read the current, and calculate the impedance. As long as this is significantly higher than the primary DC resistance (say 5x) the error will be small in assuming it's all inductance. So knowing frequency, you can calculate inductance.

Leakage inductance you do the same way, but with the secondary solidly short-circuited. Or, you can just do a frequency-response sweep, with the representative load across the secondary, the audio amp feeding the primary (at a constant level!) and sweep upwards till the secondary voltage falls to 71% of the mid-frequency value.
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Old 2nd Oct 2022, 5:25 pm   #6
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Default Re: LCR meter for measuring mains and output transformers recommendations?

Thanks for the your answer, I will use the signal generator and power amp, as I own these, but not a variac, only variable dc power supply which of course would be useless for this,
and it will allow me to test at any frequency(and voltage) I choice which was the main thing I was after.
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Old 2nd Oct 2022, 7:36 pm   #7
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Default Re: LCR meter for measuring mains and output transformers recommendations?

A good LCR bridge with external oscillator and possibly the ability to inject DC into the windings, as well as adjustable drive levels would be the top of the range for measurement of inductance.
This is a complex beast so you also need to be able to drive it and understand the results.

Getting into standards lab areas. A variac or tapped transformer is a much better bet

Ed
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Old 2nd Oct 2022, 8:01 pm   #8
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Default Re: LCR meter for measuring mains and output transformers recommendations?

It's quite instructive to plot volts against current for a mains transformer. At very low voltages, the current rises rapidly with voltage (though both are small), then the current levels off, finally rising disproportionately with voltage as saturation is approached.

A simple inductance, of course, would be linear throughout.
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Old 2nd Oct 2022, 10:05 pm   #9
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Default Re: LCR meter for measuring mains and output transformers recommendations?

My experience is with RF transformers rather than AF, but I think if I had to measure an AF transformer here, I would use my Analog Discovery 2 with the external impedance measuring jig. To drive it I use the impedance SW written by 'thestuffmade'.

It takes maybe 20-30 minutes to make up an external impedance test fixture using a sense resistor and then the system is good to go. The AD2 can only do smallish signal testing (up to a few volts) but the SW is very powerful.

A typical AF transformer is going to show quite a change in inductance and resistance across (say) 20Hz to 20kHz and the AD2 can sweep and measure/plot all this in a few seconds. I suppose it would be possible to put a booster amp inline and calibrate it for higher drive levels, but I think this would risk damage to the AD2.

To do the equivalent small signal measurements by hand with a sig gen and scope would take a long time if you wanted to plot 20Hz to 20kHz with 50 steps. Some LCR meters are going to be a bit more accurate than the AD2 but I'm not sure high accuracy matters for something as crude as a transformer.

Sadly, the AD2 is quite expensive now. At one time it cost about £200. I expected the AD2 to get cheaper over the years, but instead, the price has rocketed. It's more like £450 now. This is just out of your budget range but I'm not aware of any low cost LCR meters that are as powerful as the AD2.
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Old 3rd Oct 2022, 1:01 am   #10
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Default Re: LCR meter for measuring mains and output transformers recommendations?

I've been down this road - as others have mentioned it is possible to make measurements and calculate primary inductance of an output Tx and this may be ok for you, however if you feel you need a LCR meter then go for one that has the ranges you will need, output Tx primary inductance can be up over 100Hy so when choosing a LCR meter make sure it has the right ranges to do the job you want. If you are planning on winding SE output transformers it is handy to be able to run a DC bias through the transformer while doing the inductance check, not all LCR meters cater for this.
I picked up a Marconi TF2700 - this does up to 100Hy and allows a DC bias (if you have the Marconi adapter) for approx $AU120 is was in good order and just needed the pots and switch contacts cleaned, be aware if you're one who likes the convenience of instant digital measurements then you'll need some patience when using a typical inductance bridge sometimes a sharp null is found quickly and other times it seems to take forever and you're left wondering "did I find the null point", for me it has been 'practice makes perfect' the more you use it the more adept you become.
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Old 3rd Oct 2022, 11:27 am   #11
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Default Re: LCR meter for measuring mains and output transformers recommendations?

The Marconi TF2700 does look useful.
I do own a vintage taylor bridge already, but it only does resistance and capacitance(not any where close to accurate capacitance) not inductance.
Looked at what is available none of them seem to have any adaptors with them.
Could you give me more info on adaptor what it is? how it works? what it looks like?
Thanks
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Old 3rd Oct 2022, 12:35 pm   #12
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Default Re: LCR meter for measuring mains and output transformers recommendations?

Sometimes you may come across some very serious bridges of the lab grade sort, which use transformers with tappings rather than a potentiometer to set the balance. Wayne-Kerr was the maker. Don't be put off by the complicated panels, they are easier to use and to find the null and can balance resistive and reactive components at the same time. However, the apparent complexity puts people off, and I bought one for well under your budget. I'm not sure nowadays, I think fashionistas would want them just to rob for the nixie tubes.

Some calculation is needed to convert results into Henries for inductors.

These things are versatile and there are adaptors to extend them to more extreme impedances. You can also use external audio sources to work at other frequencies. There are also facilities for DC injection to the thing under test.

Wayne-Kerr also made RF bridges.

Once you know what these things are keep an eye open, and pounce when you see one.

A valve-based simplified one was made for the military in fair numbers and sometimes crops up at radio rallies. But as they contain two round magic eyes, you can guess their fate nowadays.

Oh, if the later W-K stuff looks familiar, they were part of the same empire as Ferrograph.

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Old 3rd Oct 2022, 12:39 pm   #13
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Default Re: LCR meter for measuring mains and output transformers recommendations?

Before having the AD2 I used to measure ESR and reactance using a function generator, a sense resistor and a decent Vrms DVM to measure the voltages across and either side of the sense resistor. This could give very good results if the correct sense resistor was used for a particular task. It is a clunky and slow method, but I'd expect it to be about as accurate as a decent LCR meter and probably much better than an LCR meter for some tasks.

To try and reduce the clunkiness I partially automated it using remote logging of the DVM and a PC calculated the complex impedance. Prior to this I used to do it all manually using an excel spreadsheet. I'd expect this system to be able to measure huge inductances down at AF, but I've never tried it. The system is slow and clunky, but it does measures complex impedance and I've used it at 100Hz through to about 100kHz with very good results.

I had a rummage over lunch today and managed to find my old (Eagle LT700?) AF transformer test jig. This is a tiny thing that I used to use with a 20k/V analogue multimeter meter (similar to AVO 8) to let me measure the (S+N)/N of CB/HAM radios back in the 1980s.

This little LT700 transformer steps up from a few ohms to something like 2k ohm and this pushes the voltage up to the region where the ACV meter scaling is more linear. The old analogue multimeter can't measure true Vrms, but I found the measurement error was predictable and only about 1dB at 10dB (S+N)/N.

This evening, I could have a go at measuring the LT700 transformer using the AD2 and also the old school way with a sense resistor and a Vrms meter.
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Old 3rd Oct 2022, 4:12 pm   #14
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Default Re: LCR meter for measuring mains and output transformers recommendations?

The Marconi TF2700 is a very good bridge for not a lot of money.
Not being digital it needs the correct technique in use.
The manual is on line and it details the adaptors that can be used with the bridge; circuits and component valuse are given

Ed
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Old 3rd Oct 2022, 7:32 pm   #15
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Default Re: LCR meter for measuring mains and output transformers recommendations?

Quote:
Originally Posted by Ed_Dinning View Post
The Marconi TF2700 is a very good bridge for not a lot of money.
Yes I wouldn't be without mine! I rescued it from the tip.....all it wanted was a battery and it's been faultless for the last 15 years. You need a few attempts to learn the controls but it really doesn't take long. Try a few known good capacitors or inductors first so you know how to interpret the readings. You'll wonder how you managed without it...!
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Old 3rd Oct 2022, 11:38 pm   #16
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Default Re: LCR meter for measuring mains and output transformers recommendations?

Here's a couple of screenshots showing an AD2 measurement of the LT700 AF transformer across 10Hz to 100kHz. I've looked into the primary with the (low impedance) secondary open for the first plot and then plotted with the (low impedance) secondary shorted.

It's the first time I've tried to measure an AF transformer with it, and I may not have chosen the ideal sense resistor value.

The scaling legends on the display are a bit strange and I find it best to scroll the vertical orange marker across the range and read off the data from the column on the right. In both plots below I've left the marker at 1kHz. It takes a few seconds to sweep and produce this plot with 50 data points. I'm afraid the forum image file limitations mean the plot is fairly illegible, but the user can choose what to plot and the green trace is mag impedance. The red trace is parallel resistance Rp and the grey trace is parallel inductance Lp. The blue trace is parallel capacitance Cp.

When viewed on a monitor everything is very clear and easy to read. The links below show a slightly clearer image.

https://www.qsl.net/g/g0hzu//RF%20Tr...00_AD2_Pri.gif

https://www.qsl.net/g/g0hzu//RF%20Tr...00_AD2_Lkg.gif
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Last edited by G0HZU_JMR; 3rd Oct 2022 at 11:46 pm.
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Old 4th Oct 2022, 12:46 pm   #17
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Default Re: LCR meter for measuring mains and output transformers recommendations?

I had a look for the latest price of the AD2 on Farnell and it has gone up even more. By the time the essential accessories are included, and VAT is added the price is now well over £500. I think this is way over-priced now although I guess they must still be finding customers at this price. The original AD1 was designed as a learning tool for students and cost just $99 for students to buy. I think it cost $199 for non-students.

There isn't much difference between AD1 and AD2. The AD2 is housed in a clear (and brittle) plastic enclosure when compared to the solid black enclosure used for AD1 and the AD2 feels cheap and flimsy. It can be used as a 14bit (25MHz) scope, network analyser, sig gen, spectrum analyser and logic analyser but all of these features have a fairly clunky and restrictive interface.

I'll dig out my function generator and DVM this evening and try measuring the LT700 transformer the old school way. This old school sense resistor method is essentially the same method the AD2 uses but it all has to be done manually. The AD2 probably measures two voltages and relative phase but it can also be done by measuring three voltages around the sense resistor (so no need to measure phase). With the DVM method, there are three AC voltages to measure, and the results are used to compute the complex impedance of the DUT. This can be done with an excel spreadsheet or it can be semi-automated if the AC voltages are measured remotely using the DVM and a PC.
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Old 4th Oct 2022, 12:46 pm   #18
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Default Re: LCR meter for measuring mains and output transformers recommendations?

I must say That first graph shows the effect of parasitic capacitance very well, as the capacitance increases the more signal is alternated and the inductance falls off sharply as a result, however the impedance does not fall to quite the same extent.

one thing you do not mention is What voltage level was this done at and unfortunately output transformers have lower band width at higher levels.

Testing transformers at all different possible frequencies and voltage would certainly be very time consuming and interpreting and making decisions on all of even more so. focusing on measuring bandwidth at the highest voltage(power level) intended for the equipment to be used at would the most efficient.

using a sweep across the whole frequency is certain very helpful in finding the point with the highest inductance and the -3db points, which would be the far more time consuming making lots of individual measurements.
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Old 4th Oct 2022, 9:01 pm   #19
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Default Re: LCR meter for measuring mains and output transformers recommendations?

The data column on the right of the AD2 plots suggests voltages as high as 3.2V pkpk were used at the source but this is still quite weedy. The AD2 can only test at small drivel levels.

I just had a go with the function generator and my old sense resistor jig and the automated DVM. I tested at 1.095 kHz and tried various drive levels. As expected, the inductance and resistance changes with drive level. I tried various drive levels from 400mV rms to 3V rms. The inductance and resistance at 1 kHz change by about +/- 10% when I do this. However, the general agreement with the AD2 result was quite good.

A few years ago, I made a fully automated version using an AVR MCU, a programmable source and a nice ADC chip that was configured to measure all three voltages around the sense resistor. This worked fairly well and was very fast/fluid in operation when measuring capacitor ESR at various test frequencies. It wasn't as accurate as it could be though as my MCU program code was very simple. I think it would have been better with some averaging for example. Something like this would be the lowest cost option for an initial low-level sweep of a transformer to try and replicate the AD2 plots.

After this, I guess the aim would be to try some high drive level tests using a power amp at several fixed test frequencies to see how the bandwidth changes.
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