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Non-inductive resistors for dummy load




 
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Author Topic: Non-inductive resistors for dummy load  (Read 6483 times)
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k4kyv
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Don
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« on: February 23, 2012, 07:39:44 PM »

I have finally started experimenting with the revised antenna tuner to match a nominal flat 450Ω balanced feedline (actual Zo=438Ω) to tuned feeders running up the tower.  To-day I began with the 40m tuner, and was able to get a good null with the MicroMatch impedance bridge.

The problem is to determine that the load to the balanced line really is 438Ω + j0 (or 450Ω +j0, close enough for government work). I have a couple of dummy  loads made up of Glo-bar composition resistors.  My small one uses nine 50Ω 5 or 10 watt Glo-Bars wired in series.  The larger one uses three banks each of four paralleled 600Ω Glo-bars, wired in series.  In each case, with all the resistors wired in series, there appears to be enough residual reactance to skew the readings from the predicted, at least on 40m.

I have considered using a bank of something on the order of 5K 2w carbon composition resistors wired in parallel, to make up a 20-watt or so 450Ω resistor, and then submerging the whole thing in a jar of mineral oil, la Cantenna, to give enough wattage rating to make appropriate measurements, without a string of anything in series to accumulate inductance. I could dig through my collection of hundreds of new and used resistors, but that would be a real PITA, and no guarantee I would find enough of the correct value to complete the project, so I considered purchasing new ones.  I checked out Mouser and Digikey.  Looks like the  largest carbon composition resistors now available is 1-watt.  Neither company lists anything at 2 watts.

Film resistors are  listed at up to 2 watts, and their tolerances are much closer than anything you can find in carbon comp, but film resistors are made up of a layer of carbon or metallic film deposited on some kind of ceramic or composition form, with a spiral groove cut along the length. IOW, they are similar to wirewound resistors, except that the conductor is a flat film of carbon or metal, instead of wire.  I don't believe that would be satisfactory for what I want to do, since it would have a small amount of inductance instead of a pure resistance. Also, my experience with film resistors is that they are very fragile, unlike  carbon comps; just a little bit of overload and they flame out in a puff and flash, and become useless.

More and more common ordinary components are becoming unobtanium.  The way it looks, in a few years we won't be able to buy any kind of newly manufactured electronic component except pre-packaged dipped-in-epoxy digital widgets.  It appears that useful rf type analogue components are rapidly disappearing off the face of the earth.   Angry Angry
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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Dave K6XYZ
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« Reply #1 on: February 23, 2012, 07:56:50 PM »

Hi Don.....have you seen the Ohmite OY series of resistors? I've been using them for parasitic and other applications.

http://www.mouser.com/catalog/catalogUSD/644/741.pdf
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Jim, W5JO
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« Reply #2 on: February 23, 2012, 08:36:32 PM »

Don, MFJ sells non-inductive resistors  that are 22 ohm 35 watt cont. up to 100 ohm 35 watt cont for 22.50 each in several values,  there is a 50 ohm 200 watt  for 32.50 and a couple of other values, none are over the 22.50 price except the 200 Watt 50 Ohm.  If that will work for what you want, that may be a somewhat reasonable path for you.

Other than that there is a place to buy carbon comp reistors but I don't know the values they have.

http://www.tedss.com/Resistors/

TEDSS will probably be high but it is a source.
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aa5wg
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« Reply #3 on: February 23, 2012, 08:53:31 PM »

Don:

Years ago I aquired some noninductive resistors from Omite.  I think they were 600 ohm at 50 watts.  They were large, maybe 8 or so inches in length.  They may have 450 ohm that you need.

Surplus Sales of Nebraska has some type "C"  noninductive 450 ohm resistors for sale.  Here is their link: http://www.surplussales.com/Resistors/NonInductive/Res-NonInd_list.htm

These are expensive!

Chuck
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KM1H
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« Reply #4 on: February 23, 2012, 08:54:17 PM »

The 2W Ohmite OY resistors, at least 50 and 100 Ohms, show virtually no parasitic inductance to at least 6M, where they are used as part of the parasitic suppressors for the 6M conversion biz, as well as replacement for tired carbon comps in HF amp repairs.

Before that Id been using 5W Mouser MOX that required some XC to cancel the XL which was enough to be a problem even at 10M.

Carl

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WA1GFZ
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« Reply #5 on: February 23, 2012, 08:59:29 PM »

Check out Caddak. SP?
I just bought some 50 watters in TO220 cases, they have higher power devices in TO247 cases but need a good heat sink. I suppose you could dip the heat sink into oil.
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k4kyv
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Don
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« Reply #6 on: February 23, 2012, 09:47:32 PM »

Hi Don.....have you seen the Ohmite OY series of resistors? I've been using them for parasitic and other applications.

http://www.mouser.com/catalog/catalogUSD/644/741.pdf

Thanks.  I had never heard of them.  They may be just what I am looking for.

The 7W Ohmite OY resistors, at least 50 and 100 Ohms, show virtually no parasitic inductance to at least 6M, where they are used as part of the parasitic suppressors for the 6M conversion biz, as well as replacement for tired carbon comps in HF amp repairs.

Where do you find the 7-watt OY series?  Mouser lists only 1 and 2 watt versions.

Anything at Ripoff Sales of Nebraska is bound to be expensive!
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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This message was typed using the DVORAK keyboard layout.
http://www.mwbrooks.com/dvorak
WB3JOK
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« Reply #7 on: February 23, 2012, 09:52:24 PM »

Caddock Wink
Mouser has them... http://www.mouser.com/catalog/catalogUSD/644/734.pdf
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KM1H
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« Reply #8 on: February 23, 2012, 09:54:22 PM »

I let too much smoke out of 100W Caddock samples I tried about 3 years ago. The aluminum or copper sink plus air required was a show stopper. Perhaps as a load for a grid driven tube in the forced air stream under the chassis they would be OK with a smaller sink.
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Detroit47
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« Reply #9 on: February 24, 2012, 08:05:27 AM »

Don
I will look thru my junk. I have a lot of large no inductive stuff. How many watts total do you want and the finished restiance.

John
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WA1GFZ
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« Reply #10 on: February 24, 2012, 08:26:44 AM »

I use caddock resistors and they are fine as long as you heat sink them properly and derate for temperature rise. Simple let the heat out or let the smoke out.
I did take a 20 watt part out when I tried to dissipate 38 watts.
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W3GMS
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« Reply #11 on: February 24, 2012, 08:49:03 AM »

I use caddock resistors and they are fine as long as you heat sink them properly and derate for temperature rise. Simple let the heat out or let the smoke out.
I did take a 20 watt part out when I tried to dissipate 38 watts.


Here is an interesting application note on using the low inductive TO-220 & TO-247 resistors.  I have used them successfully as long as the heat sinking is done properly.  Other manufactures make them as has been stated.   

http://www.token.com.tw/pdf/power-resistors-to220.pdf

Joe, W3GMS
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Rob K2CU
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« Reply #12 on: February 24, 2012, 08:53:24 AM »

Why not just use low power for testing?
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KL7OF
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« Reply #13 on: February 24, 2012, 10:34:19 AM »

Why not just use low power for testing?

How much fun is that?
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WD5JKO
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WD5JKO


« Reply #14 on: February 24, 2012, 10:57:27 AM »


   I see these are good for 2.5 to 3.5W dissipation without a heat sink. Should be good for a big tube plate side parasitic suppressor where we used to use 1 or 2 carbon comp 2W resistors in parallel across a 2-3 turn VHF coil.

Jim
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KM1H
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« Reply #15 on: February 24, 2012, 03:29:34 PM »

Quote
Where do you find the 7-watt OY series?

Fixed the typo. I use a pair of 100 Ohm in parallel for 3-500 size tubes.
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