inductive voltage divider purpose? - Page 1
Forum section: electronics / projects

author: sarepairman2
author_rank: Frequent Contributor
timestamp: July 17, 2015, 08:18:50 pm
content: A while back on ebay I saw a SR inductive voltage divider, I was wondering what these are used for?Are these just audio frequency precision dividers? Why not use capacitors instead (less distortion / higher bandwidth /cheaper ?)

author: LukeW
author_rank: Frequent Contributor
timestamp: July 18, 2015, 03:22:52 am
content: What on earth is an inductive voltage divider?Can you provide any other information, a picture or a link?It sounds like maybe it's a filter.That's basically what really simple filters are, voltage dividers where the resistors aren't just resistors, they're R/L/C elements where the L and C impedance is frequency-dependent.

author: abebarker
author_rank: Contributor
timestamp: July 18, 2015, 03:31:17 am
content: It kind of sounds like an autotransformer. If you replaced the schematic symbol of the resistor in a resistor divider with an inductor it would act just like a voltage divider except it could also have the ability to raise the voltage which is not possible with a resistor.https://en.wiktionary.org/wiki/inductive_voltage_divider

author: sarepairman2
author_rank: Frequent Contributor
timestamp: July 18, 2015, 04:34:36 am
content: no, its like a decade resistor, but with inductors, buncha switches on it that flip taps on a bunch of variable inductors, not continously variable. it was specifically called an inductive voltage divder.oh duh, it does have the benefit of raising voltage. and I guess it can do DC with AC on it, unlike a capacitive one.http://www.conimed.com/english/prod/induct.htmdo they have capacitive ones that have similar features? or perhaps a few adjustable plate capacitors wired in series with taps? (should be useable and stable to fairly high frequencies unlike the inductive ones)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 18, 2015, 05:17:03 am
content: Yes, such a circuit can be made with capacitors. Without the gain ratio, of course.I wouldn't expect a capacitive form is any better at high frequencies; you still have to deal with reality, and "a capacitor is not a capacitor is not a capacitor" (and so forth for all components).For truly wideband purposes, an attenuator is the only choice. These are available which will operate consistently, from DC to 10s of GHz.Tim

author: sarepairman2
author_rank: Frequent Contributor
timestamp: July 18, 2015, 05:37:06 am
content: well mica capacitors will be alot better then non air core inductors right\i got a metric shit ton of 11nF mica capacitors hmm

author: Kleinstein
author_rank: Super Contributor
timestamp: July 18, 2015, 09:32:30 am
content: In a limited frequency range (e.g. 30 Hz to a few kHz) transformers (not separate induktors) can be used as very precise dividers. So they can be used as a high precision divider. The advantage is the relatively low impedance, despite of low power dissipation.As far as I know they are the most precise dividers in the limited frequency range.Of cause you can't do DC with a normal transformer. Though there are DC current comperators that use some kind of transformers. But this is a different kind of instrument you might find in high grade calibrationlab and NIST and co.Capacitive dividers are less pricise and at the rather low frequency range the impedance is more like impractical high. Also they are just dividers, not transformers.

author: acbern
author_rank: Frequent Contributor
timestamp: July 18, 2015, 10:03:48 am
content: Inductive voltage dividers are primarily used for AC calibration and for precision AC bridges (e.g. for measuring capcitances). The major advantage is that the division ratio only depends on the number of windings (selected through switches). Therefore, they do not need caibration/adjustment; actually there is nothing to adjust. They are highly precise by design. Therefore metrologists love them, and they are still hard to beat in precision.Popular exapmples are e.g. the Dekatran dividers (or the IET capacitance bridges). Resolution is 0.1ppm, accuracy somewhere around 1ppm iirc (datasheets available on the net). Frequency range is some ten Hz to about 20kHz. So if you have a stable AC generator with one specific voltage you can generate very precise AC voltages derived from it to validate e.g. a mulitimeter calibration.

author: LukeW
author_rank: Frequent Contributor
timestamp: July 18, 2015, 10:39:41 am
content: So it's a transformer or autotransformer with a decade-box-selectable turns ratio, in other words. Not really a "voltage divider", but still useful.

author: acbern
author_rank: Frequent Contributor
timestamp: July 18, 2015, 10:53:15 am
content: Well, it is an AC voltage divider, adjustable by means of decade switches to 0,1ppm resolution.Besides just generating a precisely divided voltage, there are other interestig things you can do with it. E.g., in combination with two precise 90° phase shifted AC voltages and a lock in amplifier, you can set up a so called quadrature bride, which is able to precisely measure a capacitance based on a known resistor. So you do not need to send your reference capacitors to calibration if you have a calibrated resistor...

author: TimFox
author_rank: Super Contributor
timestamp: July 18, 2015, 04:16:36 pm
content: I bought a vintage (<1960) Wayne-Kerr B221 admittance bridge, based on decade-tap-switch transformers for the real and imaginary legs, about 10 years ago, that operates at 1592 Hz (10^4 rad/sec), to measure capacitance (positive or negative) in parallel with admittance (positive or negative). That frequency allows easy calculation of inductance from indicated negative capacitance. The manual includes a table of reciprocals to help the operator. The previous owner had butchered the line cord, so I replaced it with a new one, but no further work was needed past the usual cleaning of rotary switches.Since the turns numbers are integers, the ratios are exact. The hermetically sealed standards are still well within specification after 50 years. The vacuum tubes were in the oscillator (still on frequency) and the amplifiers for the magic-eye tubes (two dual units) that give an incredibly easy null indication over a very wide range.The only shortcoming of the instrument is for measuring low impedances, since it has no easy provision for doing a short-circuit calibration. Open-circuit calibration, at the other end of two coax cables, is reliable, however.I notice that the company has since changed its name to WK, for reasons obvious to some.

author: sarepairman2
author_rank: Frequent Contributor
timestamp: July 18, 2015, 07:08:14 pm
content: Quote from: TimFox on July 18, 2015, 04:16:36 pmwu tang klan?

author: eas
author_rank: Frequent Contributor
timestamp: July 18, 2015, 07:37:58 pm
content: Isthisthe sort of thing you saw? This listing caught my eye on eBay recently because I'd been wondering about how I could get precise AC voltages to check some old Keithley DMMs in multiple AC ranges. I didn't buy it though.

author: acbern
author_rank: Frequent Contributor
timestamp: July 21, 2015, 07:47:03 pm
content: you better buy one with 7 decades, they are pretty reasonable on ebay. gives you much higher accuracy up tp say 20kHz than e.g. a 3458A in sync mode.