DSE ESR meter

what would be really handy is something nice and easy that lets me measure 10 milliOhms max down to 10 microOhms.

I'm doing some transformer windings at the moment that have a DC resistance of (calculated) 25uOhms. thats a real pain to measure, even when I stick 100A thru it - which is why I use 1000A, but I have to be fairly quick about it; although the winding only dissipates 25W or so, the gear I use to generate the 1kA gets HOT, fast! I might have to invest in a 6.5 digit keithley voltmeter, but thats a lot of money for one measurement.

Cheers Terry

What uncertainty do you want at 25 microhm? What do you need the measurement for?

MrT.

There's a few micro-ohm meters around. Dunno the price range though. Have a look at:

and other Google results on a search for -> measure micro ohms

Now thats unfair....I stupidly thought I could buy components from the local DSE store last year (wanted them then, not after the WES order arrived)...The young chick behind the counter had no idea what a capacitor was, or even what electronic components were, so she went & rounded up the manager....he fumbled around for a while saying " I think we have them over here somewhere"..yes, they had two values of electrolytics and nothing else. I'd told him what I values I was after, and his reply after he'd found the solitary parts bin...."What size were you after? Big or small?". I had no choice but to piss myself laughing and walk out.

The moral to this story is, yes, some of them do know what capacitors are, they stumbled accross them in the catalogue one day when they were looking for batteries :)

James

Thanks for the laugh. I stand corrected! :)

Bob

Does anyone know if Jaycar ever started marketing in the UK?

I have a 2004 catalogue which IIRC came through the post back then, I never ordered it from anywhere and it has a cover price of $3.95 which no one ever asked me to pay. The only addresses on the back cover are Aus & NZ.

They're currently advertising in Elektor. I can't tell you when they started.

Thanks - I never read the adverts, maybe I should.

It seemed a little odd that the cover price printed on the front was in $ and all the prices inside were in £, all the ordering addresses printed on the back cover were Aus or NZ so it probably had an order slip with a UK address slipped in the pages.

Hi Bob,

thanks for that, some of those are pretty neat gadgets. 5A and 0.01uOhm resolution up to 20uOhm is pretty good - 10 bits, 1LSB = 50nV.

Cheers Terry

I'd be happy with 10% (its better than I get now), but 1% would be fabulous.

a good uVolt meter would do the trick, but they are expensive.

so I know the DC resistance. Dont you measure things after you've designed them, to make sure they (or you) were right?

Cheers Terry

fabulous.

You are not going to get that cheaply I'm afraid.

Well you do realise that the *resolution* of the Keithley 6.5 DVM model 2000 is

100 microhm on the lowest 100 ohm range. OTOH the voltmeter ranges will get you a reading with a lot less than 1000A. What your measurement uncertainty will be is another matter though. But then you have no idea what it is at the moment do you? This is where the problem of calibration comes in.

But you mentioned a coil? I usually find that the resistance will be close enough to the calculated figure not to matter, if the measured inductance is also correct. If you know what the resistance is per 100 metre for your wire, and you know what the length is, you should be able to calculate within 10% IMO. That's why I wondered why you need a better figure, and if so, you ARE going to have to pay for it.

MrT.

Terry Given wrote:

If you can periodically reverse the current through your winding (e.g. with a relay or MOSFET bridge, and not too fast, since the inductance will prevent you from doing it fast and accurately), then you do not need a low-offset voltmeter. For example, turn the current on through the winding forwards for 10 seconds and then reversed for 10 seconds and then forward and then reversed, repeat... Average all readings of the voltage across the winding with the current forward, and average all of the voltage readings with the current reversed. Find the difference between these two average voltages, and then divide by twice the magnitude of your current source (since the delta in current is twice the magnitude of the current). You can thereby remove any fixed offsets in the voltmeter, and any thermal EMFs should also be removed because the heating would be the same for forward and reversed current. If your voltmeter does not have enough resolution then you can build a pre-amplifier, and as long as the drift and offset are within reasonable limits, they will be cancelled out by the measurement technique. You will have to determine the gain of you preamplifier accurately however. The easiest way to do this measurement might be to hook up a PC to the GPIB or RS232 port of your voltmeter, if it has one, and also to the control signal to the big relay that reverses your current source. Take appropriate precautions against back-EMF from the winding lest it blow out your DVM. I bet you could get a reasonable reading with just a few amps this way.

Chris

Having watched the responses emerge to the above, I felt compelled to add my 2c worth.

Firstly, metering connection resistances will no longer be something you can ignore. I found this out decades back using a Gossen geohm bridge device for low (milli-)ohms work.

Secondly, a bridge approach may be better than trying to simply invoke ohms law.

Finally, as one other poster alluded to - if you are winding the transformers and know the length of wire you used, you have the answer to better than 10% already. And you should be able to keep track of the length of wire used.

FWIW

Just out of curiosity should I decide to do so, where can I order a spare micro in the UK and how much would it likely be?

Good question. I think a few people in England still have spares available. Can I contact you directly by e-mail at the address you've given, or do I need to modify it?

Bob

The return address from my usenet account is never downloaded as its always full of spam, rather than repeat my email return address here I've replied to an email you sent me when the LDO mod 27k resistor was mentioned.

... and the customer is willing to pay enough to cover the price and delivery. Tom

Some Farnell/RS prices are actually as low or lower than DSE/Jaycar/ Altronics et.al, and they both have excellent trade counters you can pick the parts up at. But the fact that they have in the order of 50,000 parts that the hobby players don't have, it kinda makes up for any price difference IMHO.

Dave :)

Yeah, ain't that the truth? :-) That's why I avoid them even when I'm ordering for the company I work for. Whatever happened to Radiospares (RS). Are they still around? Farnell seems to have pulled the rug out from under them since they started in Aust.

Bob