Where to buy carbon power resistors? (2 - 10 watt)

[K1JJ]

I'm finishing up my quad  6LF6 X 6146B PDM rig and need some carbon power resistors to be used for RF.  I checked both Mouser and Digi-Key and they have only wimpy 1/4 and 1/2 watt carbons, etc.

Here's a list of what I need:  (These values are not critical - even 20% off is fine)

(4)  20K  2 watt Carbon

(1) 1K 10 watt carbon

(4) 50 ohm 2 or 5 watt carbon

(4)  5K  2 watt  carbon


Also:  
(4)  6 to 10  ohm, 5 watt wirewound
(1) 30 to 40V,  50 watt zener diode


I understand that carbon film are OK for RF, but metal film are not and have inductance. Is this true?

Anyone know of a source for these components?


** If anyone has these parts in their junkbox and wishes to send a Care package, I will gladly pay shipping and retail parts prices.

Thanks.

Tom, K1JJ

[KA0HCP]

http://www.ohmite.com/

Ohmite makes power resistors, wirewound, non-inductive types.

A number of hams use their non-inductive resistors for ground shunts in Beverage antennas.

[KA2DZT]

I'm finishing up my quad  6LF6 X 6146 PDM rig and need some carbon power resistors to be used for RF.  I checked both Mouser and Digi-Key and they have only wimpy 1/4 and 1/2 watt carbons, etc.

Here's a list of what I need:  (These values are not critical - even 20% off is fine)

(4)  20K  2 watt Carbon

(1) 1K 10 watt carbon

(4) 50 ohm 2 or 5 watt carbon

(4)  5K  2 watt  carbon


Also:  
(4)  6 to 10  ohm, 5 watt wirewound
(1) 30 to 40V,  50 watt zener diode


I understand that carbon film are OK for RF, but metal film are not and have inductance. Is this true?

Anyone know of a source for these components?


** If anyone has these parts in their junkbox and wishes to send a Care package, I will gladly pay shipping and retail parts prices.

Thanks.

Tom, K1JJ

Tom,  I have about 200K resistors in stock.  I may be able to fill some of what you need.  I have a lot of metal oxide resistors, not sure if they are inductive.  The carbon resistors are getting very hard to come by.  My supply of those is very limited.

I know I have 8.2ohm 10W WW Dale resistors.

I have 20K 3W, I think they're metal oxide. I think 2000 of them, so you just can't take one.

The 50ohm 2W, I have some close 56ohm or 62ohm plus I think 47ohm and 51ohm.

The 1K at 10W carbon is a tough one, I'm very low on any 1Ks,  I do have 1K 10W WW, may be sliders. I also think I have 1K 50W non-inductive.

The 5K 2W is a maybe, I have to look.

The 50 watt zener is available, NTE makes them.

Fred

[W0BTU]

For 2W non-inductive resistors, Ohmite type OY will work nicely. I think both Newark and Mouser have them.

[steve_qix]

I understand that carbon film are OK for RF, but metal film are not and have inductance. Is this true?

Hi Tom, I am not aware of metal film resistors being unusually inductive, at least not at frequencies we would normally encounter below 30mHz.

Some of the very low value resistors (1 ohm, for instance) show a Z/R (inductive) value approaching 2 at 30mHz, and the opposite (capacitive) at very high resistance values.  For normal values (and certainly the values you are seeking), I don't believe the device impedance is going to be a factor.  The package and lead inductance/capacitance is the biggest factor you're looking at, and this will exist to a greater or lesser degree with all components.

Some resistor manufacturers show impedance curves for their parts.  I've seen impedance curves for some Vishay parts in the past.

I'm guessing it is similar for carbon resistors, although I have not seen the actual data to prove this out.

I use metal film resistors in critical, RF circuits all the time with absolutely no problems whatsoever.

Hope this helps!

Regards,  Steve

[W1ITT]

Tom..
If you do happen to find some new-old-stock carbon resistors, be aware that they age over time, even in the drawer.  The actual resistance, after a few decades of sitting around, may not be very close to what the color bands tell you, so check them with an ohmmeter.
Come to think of it, my  own resistance to some things has gone up over the years too. But Violet (still) goes willingly for silver and gold.
Norm

[W3GMS]

I understand that carbon film are OK for RF, but metal film are not and have inductance. Is this true?

Hi Tom, I am not aware of metal film resistors being unusually inductive, at least not at frequencies we would normally encounter below 30mHz.

Some of the very low value resistors (1 ohm, for instance) show a Z/R (inductive) value approaching 2 at 30mHz, and the opposite (capacitive) at very high resistance values.  For normal values (and certainly the values you are seeking), I don't believe the device impedance is going to be a factor.  The package and lead inductance/capacitance is the biggest factor you're looking at, and this will exist to a greater or lesser degree with all components.

Some resistor manufacturers show impedance curves for their parts.  I've seen impedance curves for some Vishay parts in the past.

I'm guessing it is similar for carbon resistors, although I have not seen the actual data to prove this out.

I use metal film resistors in critical, RF circuits all the time with absolutely no problems whatsoever.

Hope this helps!

Regards,  Steve

I agree Steve.  The attribute of the Carbon is to create loss at higher frequencies for parasitic's suppression.  In fact the Heathkit dummy load uses film in its construction.   

Joe, GMS 

[K1JJ]

Thanks much for the info guys!

Well, it looks like the consensus is metal film power resistors are OK for RF. That will make it easier to find. I read some stuff on the web that said it was questionable, but maybe they meant for VHF, etc.

Below is from the master, Frank / GFZ who has added more info on power resistor availability.


Fred, please let me know what you have to spare and I will fill in the missing stuff from the suppliers.  The only wirewounds I need are the 5-10 ohm 3-5 watts. The rest can be carbon or film.

"I have 20K 3W, I think they're metal oxide. I think 2000 of them, so you just can't take one."   Is metal oxide inductive?  

Thanks for the effort, Fred.


Steve, most of these resistors are being used as plate suppressors, grid to cathode loading or input loading... all RF positions.

T


More info from Frank:


Here's a list of what I need:  (These values are not critical - even 20% off is fine)

(4)  20K  2 watt Carbon

I found 3 watt metal film at Mouser that will work fine. I just bought a bunch but 1k for my ss linear.


(1) 1K 10 watt carbon

Nobody ever made a 10 watt carbon. Caddock makes some nice 15 watt resistors noninductive resistors
Mouser sells them. I just bought a bunch of 3 ohm 15 watters. Smaller than a TO220 you need to drill the mounting screw hole for a 4-40 screw.


(4) 50 ohm 2 or 5 watt carbon

Metal film will work mouser sells 3 watters and maybe some 5 watters


(4)  5K  2 watt  carbon

Try a 3 watt metal film

 
(4)  6 to 10  ohm, 5 watt wirewound
 A 3 watt wire wound will be fine. I think I used 3 watters in the Viking 2 Mouser

(1) 30 to 40V,  50 watt zener diode

Check RF parts they used to sell big zeners
 

Vishey sells a noninductive metal film. Carbon resistors are rare these days.


 

[KA2DZT]

Tom,

The 8.2ohm 10W WW are 3/8 dia. 1.75" long axial lead, let me know if the size is an issue.  I also have 7.5ohm 3W WW they're about the size of a 1W carbon.

The only 2W carbons 50ohms I have are 51ohm 2W but they all seem to measure closer to 58 ohms, let me know.

The only thing close to 5K 2W carbons I have are 5600ohm 2W, but they all seem to measure closer to 5800ohms, let me know.

All the rest I have (what you asked for) are metal oxide film resistors.

Frank is right, I never saw a 10 watt carbon, they did make 5 watters, but those were seldom seen.

I can just add in some 20Ks as I'll never use 2000 of them.  They measure on the dot.

I can spare all of the above.

Fred

[AB2EZ]

Adding to Steve's comments:

An issue with film resistors is that (because the film is thin), the film's temperature will increase very quickly when the current passing through the film increases. Therefore, short duration current surges (when the power being dissipated by the film briefly rises far above the rated power dissipation value) can destroy the resistor or significantly change its resistance value.

Similar, brief surges would not harm carbon composition or thick film or wirewound resistors because it takes more energy, stored in the resistor in the form of heat, to increase the temperature of the larger volume of resistive material.

Stu

I understand that carbon film are OK for RF, but metal film are not and have inductance. Is this true?

Hi Tom, I am not aware of metal film resistors being unusually inductive, at least not at frequencies we would normally encounter below 30mHz.

Some of the very low value resistors (1 ohm, for instance) show a Z/R (inductive) value approaching 2 at 30mHz, and the opposite (capacitive) at very high resistance values.  For normal values (and certainly the values you are seeking), I don't believe the device impedance is going to be a factor.  The package and lead inductance/capacitance is the biggest factor you're looking at, and this will exist to a greater or lesser degree with all components.

Some resistor manufacturers show impedance curves for their parts.  I've seen impedance curves for some Vishay parts in the past.

I'm guessing it is similar for carbon resistors, although I have not seen the actual data to prove this out.

I use metal film resistors in critical, RF circuits all the time with absolutely no problems whatsoever.

Hope this helps!

Regards,  Steve

[K1JJ]

Vely, bery good Fred!


The smaller 7.5 ohm, 3 watt WW would fit in better.

The 51 ohm  2w sounds FB.

The 5.6K is also FB.

With your additional metal films, looks like I won't need to order any resistors.  Just the Zener and we'll be able to finish up the rig.  Hope to post some pics soon. It's starting to look like a very nice, compact rig. Even the PDM filter is finished and mounted on the back.

I appreciate this nice offer, Fred.  These are probably better suited to the job. Better than I could get from the retail outlets. Let me know your PayPal adr and I'll send you some $$.

Thanks again.

T

Tom,

The 8.2ohm 10W WW are 3/8 dia. 1.75" long axial lead, let me know if the size is an issue.  I also have 7.5ohm 3W WW they're about the size of a 1W carbon.

The only 2W carbons 50ohms I have are 51ohm 2W but they all seem to measure closer to 58 ohms, let me know.

The only thing close to 5K 2W carbons I have are 5600ohm 2W, but they all seem to measure closer to 5800ohms, let me know.

All the rest I have (what you asked for) are metal oxide film resistors.

Frank is right, I never saw a 10 watt carbon, they did make 5 watters, but those were seldom seen.

I can just add in some 20Ks as I'll never use 2000 of them.  They measure on the dot.

I can spare all of the above.

Fred

[w1vtp]

http://www.ohmite.com/

Ohmite makes power resistors, wirewound, non-inductive types.

A number of hams use their non-inductive resistors for ground shunts in Beverage antennas.

WOW!  Why didn't I think of that? 

[K1JJ]

http://www.ohmite.com/

Ohmite makes power resistors, wirewound, non-inductive types.

A number of hams use their non-inductive resistors for ground shunts in Beverage antennas.

I'm curious... are these non-inductive wirewound resistors considered "non-inductive" because they wind the wire in opposite directions to cancel out?  If so, I have tried 50 ohm versions of them for a dummyload and found the swr soared above 1-2 Mhz.   Not sure if this is the same type or not...

T

[Jim, W5JO]

I don't know but I have been told that Ceramic resistors make good replacements for carbon.  I do know the new 3 watt resistors are not as big as the old 1 watt carbon.  Makes one wonder.

[W0BTU]

Some do. These non-inductive ceramic composition resistors will handle quite large surges. They are popular for Beverage termination resistors for that reason. It almost takes a direct lightning hit to damage them.

http://www.ohmite.com/cat/res_ox_oy.pdf

[KA2DZT]

Tom,

OK FB, I'll begin to gather the resistors, I'll measure all and send matched pieces.  I'll send the 8.2s and the 7.5s.

I don't have the 1K 10W carbon (never made), I think I have 1K 50W non-inductive, I'll check.  They are, I believe, WW.  They may be too large and not suited for your need.

You could put 5 or 6 of the 5600ohm 2W in parallel to make about 1K at 10 watts, let me know if that is doable.

Let me know if you can think of anything else.

Don't worry about the $ as I don't have PayPal.  Anyway, the parts are free, the postage you can credit my account

Fred

[K1JJ]

Tom,

OK FB, I'll begin to gather the resistors, I'll measure all and send matched pieces.  I'll send the 8.2s and the 7.5s.

I don't have the 1K 10W carbon (never made), I think I have 1K 50W non-inductive, I'll check.  They are, I believe, WW.  They may be too large and not suited for your need.

You could put 5 or 6 of the 5600ohm 2W in parallel to make about 1K at 10 watts, let me know if that is doable.

Let me know if you can think of anything else.

Don't worry about the $ as I don't have PayPal.  Anyway, the parts are free, the postage you can credit my account

Fred

Sounds good, Fred.

I can put 5 or 6 of the 5600 ohm 2W in parallel to make about 1K at 10 watts. There is plenty of room in that area.

You can use my QRZ.com address.

Thanks for the extra effort, OM!   The Dual Quad rig will have a touch of 51 Watt Fred in it.... :-)   Give me a list of any parts you need and I'll look around too.

T

[John K5PRO]

Allen Bradley was one of the defacto sources of carbon comp resistors, for decades. I still have drawers and jars full of 1/4 and 1/2 watt, but far fewer of the 1 and 2 watt variety. Somewhere in the past 2 years I heard that someone was reintroducing carbon comp. resistors, but perhaps it was only the smaller sizes. I can't find the link now.

Here is one man's story about the drifting values:
http://www.cliftonlaboratories.com/carbon_composition_resistors.htm

In college we had a class where we made a homemade transmission line (twisted pair and parallel line) and measured the Z up to 100 MHz using the venerable HP 4815A Z meter. I remember that 100 ohms was as high as could be done with a termination at the end of carbon comp. resistor. Any value above that started to have too much C or L in it.

I use carbon resistors in work but they are larger than 2 Watt:
Carborundum/Kanthal/Globar, whatever their name is now, still makes bulk carbon power resistors. Some are rated for HV and others for peak energy. All are noninductive for RF loads. I'd use them for dummy loads but their tempco isn't good enough, so they drift when heating. They make small leaded ones for watts or 12 inch long ones for watts and kW. Sometimes you can find them in surplus places like Nebraska Surplus Sales. They make them to order, so there is sticker shock there.

More recently I needed some 'damper' resistors that would function up to 4th harmonic (800 MHz) in my cavity amplifier, in the safety enclosure around the tetrode. I found HVR Power Components in Cheektowaga NY to make some special 35 ohm resistors that are 2 inch OD, about 8 inches long. Bulk carbon in tubular form, sprayed metal ends. They are really working well, but I ran one up to above 180 deg C and it permanently changed value.

There is Altronics in Arkansas, a small family run company, that makes sputtered carbon film on ceramic cores. These are sold to companies like Bird for use in their dummy loads. Altronics makes their own line of loads that work up to 250 kW, water cooled. You can also buy cores from them for hundreds of watts and higher. I have some working at 2.8 MHz in the grid of a cathode follower (500 kW triodes) that they made, with water cooling inside the cores. They can run up to 1 kW and are only about 8 inches long.