WW resistor inductance

[Diabolical Artificer]

I read an article in an old Elektor (April 78 issue # 36) recently which showed a way to reduce the inductance of a WW resistor as used as a dummy load say. See pic - 36r should of coarse be 32r.

The way this is supposed to work is that if we get a WW resistor with a centre tap and connect it as shown in the pic, current flows in opposite directions each half of the resistor, so the magnetic fields produced in each half ( and hence the self inductance) tend to cancel out.

I've tried this but could'nt produce meaningful results as I was using a Marconi TF2700 bridge
and could verify the inductance readings I got. It might be easier with an automatic bridge, or modern L meter.

I wondered if this would work with two separate R's of equal value and make/type.

What do you think, is the method valid? Andy

[pwdrive]

To see how it's done Google Ayrton-Perry

Lawrence.

[Mike Watterson]

It's better to wind with a loop, there are such resistors. The coupling isn't good enough with a centre tap.

Today we have Metal Oxide as a better low inductance high power tech.

[Mark Hennessy]

I like the idea, but suspect both windings need to be on the same component. And also suspect that they need to be wound together in bifilar form, as the former isn't magnetic. I don't think two separate resistors will help.

The inductance is usually quite low. Here's a typical datasheet: http://uk.rs-online.com/webdocs/00b9/090...0b9f2b.pdf

My dummy load is 4 lots of 4 1 ohm Arcol HS50 jobs. So that's 4 times 4 ohms, which can be configured as needed from the front panel - usually two lots of 8 ohms by default. Plus a couple of 15 ohm, 25W jobs for vintage stuff. The heat sinks are suitably massive, but need fan cooling for longer periods of use.

Anyway, the 1 ohm HS50 is 0.73uH. Total for an 8 ohm load is 5.84uH. At 20kHz, that's 0.73 ohms of reactive component. I did think about adding a Zobel network to correct for that, but quickly decided not to bother

Of course, if you're going beyond audio frequencies, then that's different...

       

[pwdrive]

Here's some guff on parallel aiding and parallel opposing inductances:

http://www.electronics-tutorials.ws/indu...ctors.html

Lawrence.

[boater sam]

Right and left hand wound resistors could become audiophool money makers. 


Must massively enhance the sound stage by reducing imbalanced inductance reactions on the output transformers, do you think?

[Nick]

I wrote this up in a post on another thread : http://golbornevintageradio.co.uk/forum/...3#pid46633

Basically, add an "N" in front of the standard Arcol HS part number to get the low inductance variant.

[Diabolical Artificer]

Thanks Lawrence, i'll look those links etc up .

To be honest I don't really care about the small inductance present on my dummy load at the moment, but as I,ve got more interested in testing amplifiers and read up on the subject, quite a lot of bumf has mentioned non inductive R's. I was interested in the method as it looked to be a cheap way to get non-inductive loads, and thought I'd run it past the boffins. The Arcol NHS** R's Nick mentioned not being cheap at £50 for one 100w 8 ohm resistor, the HS versions look cheaper though.

Nice dummy load Mark, why the earth? Safety or something else? Wouldn't have thought of using a Zobel network.Those are huge HS's your using. The first power resistors I bought years ago for use as dummy loads were cheap no datasheet jobbies. Last week I bought some Vishay R's which did. For one 50w R it stipulated a HS 5" x 7" x 2" which surprised me, so your titanic HS's no longer seem big. The temp figure is academic as all my HS's are second hand retrieved.

Don't know about lft/rght handed R's Sam, but you could definately sell a few audio dummy loads, maybe with a combined watt meter and two VU meters. Anything with VU, MM meters on is a winner. : )

So it looks like I'm going to need bigger HS's than I'm using. I have some big HS's out of a welder with fans on that look the job, but that means it's going to be big and take up room on my tiny workbench. I found a thermal switch in the junkbox that may be good to include to switch off am amp in case of temperature overload, it switches off at 170 deg C. I've been relying on touch.

Andy.

[Eddystonepete]

Hi. several years ago I knocked up a simple dummy load using a 12v car heater, the ones with the built in 12v fan that take power from the cig. lighter skt. The resistors looked like solid carbon blocks so were low enough inductance for my use, though I didn't measure them. OK I,m a cheapskate but so what!.

Peter

[Valvebloke]

Right and left hand wound resistors could become audiophool money makers. 


Must massively enhance the sound stage by reducing imbalanced inductance reactions on the output transformers, do you think?

You have to get up pretty early in the morning to be ahead of the audio folks. They've been selling Ayrton-Perry wirewounds to eliminate the inductance problem for years e.g. http://www.hificollective.co.uk/components/mills.html.

VB