View Full Version : homebrew dummy load

KE7MRA

07-04-2007, 08:02 PM

I used five 1/2 watt 10ohm radio shack resistors and these got to hot to touch with about 3 1/2 watts but gave me an swr of 1.2 to 1. Encouraged by this I repeated the process with five 10 watt 10 ohm radio shack resistors (wirewound) and the ohm meter read 49.8 ohms but the swr was 2.5 to 1 at 5 watts and off of the scale at 50 watts. why? and can I buy anything at radio shack for a good homebrew dummy load that can handle 50 to 100 watts? I know I can buy dummy loads that can do this but I wont learn anything from this other than how someone else did it.

WC5CW

07-04-2007, 08:21 PM

KE7RMA, et al...

Short answer:

Your problem as described is because you used wirewound resistors as your load...Their resultant impedance are 50 ohms at DC but not at RF frequencies...You need to use a non-inductive load such as a carbon resistor or resistor(s)...If you need to arrange several resistors in a matrix to accomplish 50 ohms at an adequate power rating, be sure to keep the connecting leads as short as possible to eliminate unwanted inductance at the higher frequencies.

Commercial dummy loads are often made of a large, monolithic carbon resistor (made by Carborundum and others) of an adequate power rating for the dummy load's intended service...They are expensive but they perform well...I paid about $30 for a 50ohm 100w unit a few years ago, obtained from a seller on eBay.

FWIW

Bruce

WC5CW

WB7DMX

07-04-2007, 08:24 PM

first of all, you never use wirewound resistors for a dummyload, all you will end up with is one big inductor.

anly use carbon resistors to make a dummy load.

metal film or carbon film work ok under 30 meg.hertz.

for a 100 watt load you could use 30 1.5k 2 watt resistors, this is actually rated for only 60 watts, but will work ok with 100 watts as long as there is enough space between them for cooling, and a very short period of time.

KC9ECI

07-04-2007, 08:34 PM

Quote[/b] (ke7mra @ July 04 2007,15:02)]can I buy anything at radio shack for a good homebrew dummy load that can handle 50 to 100 watts?

Sure, but you can buy light bulbs at Wal Mart for less, and you can get a coffee can to shield it with as well.

KE7MRA

07-04-2007, 08:52 PM

Light bulbs as a dummy? how? how many? how do you wire it?

N4AUD

07-04-2007, 09:10 PM

Here's a thread on dummy loads (http://www.qrz.com/ib-bin/ikonboard.cgi?act=ST;f=5;t=158165;st=50)

I built one based on an ARRL technical article. If you are a member, look up "Super Dummy", though I deviated from their plan because of the size of resistor I used. The Heathkit Cantenna is another dummy load of the same type, a resistor suspended in mineral oil, transformer oil or in the case of the ARRL plan, turbine oil, inside of a paint can (or two or three screwed and soldered together) with a coax connector on top.

Be careful of old Cantennas, though. They might be filled with transformer oils with PCB's, or were filled in the past with them.

KE7MRA

07-04-2007, 09:14 PM

DMX you wrote 1.5k 2 watt resistors, 1.5x30=45kohms? is that 45 thousand ohm's? I dont understand how resistors are marked or what the ratings mean could someone help explan this simple question to me

K4SAV

07-04-2007, 09:31 PM

Quote[/b] (ke7mra @ July 03 2007,18:14)]DMX you wrote 1.5k 2 watt resistors, 1.5x30=45kohms? is that 45 thousand ohm's? I dont understand how resistors are marked or what the ratings mean could someone help explan this simple question to me

1.5K / 30 = 50 ohms

They are in parallel.

K9STH

07-04-2007, 10:27 PM

Light bulbs were "the" recommended dummy load for decades. However, that was back with tube type transmitters with wide range pi-network outputs.

First of all a light bulb is NOT a non-radiating load. Frankly, it is possible to work stations thousands of miles away when using a light bulb as a dummy load. It is just that your signal level will not be anywhere near as strong as when you are using a "real" antenna.

Then there is the fact that a light bulb does NOT present a fixed 50 ohm load. It can be all over the place and the impedance changes when the power is increased or decreased. That wasn't a "factor" when the transmitter had an output circuit that could match all sorts of impedances. But, with "modern" fixed at 50 ohms outputs using a light bulb can present problems.

There are specially "wound" wire type resistors than can be used for dummy loads (especially for HF). Those resistors were made by Glowbar as well as other companies. However, unless the resistor was specifically made to be used as an r.f. dummy load a "normal" wire wound resistor will definitely NOT work as a dummy load. Frankly, all that you are doing with most wire wound resistors is putting a coil across the load.

Glen, K9STH

For fun you can sweep the dummy load across the H.F. spectrum to see just how the unwanted inductance changes SWR. Many professional high power custom loads have compensating components to cancel the same problem that you have. There are loads made with wirewound resistors but the ceramic form is flat instead of round and they are usually used at LF. They might even do something further with the windings... I have several in the pile.

WA9SVD

07-04-2007, 11:56 PM

Most resistors designed for RF purposes are now composed of a ceramic material that has specific, predictable and reproducible resistance characteristics, and as such, are relatively non-inductive, as well as highly heat tolerant, so are well suited for "dummy load" use.

Due to the paucity of non-inductive carbon composition resistors, it is getting more and more difficult to construct an acceptable RF dummy load with "off the shelf" components. A true dummy load should not only be capably of dissipating a specific amount of power, but should also be able to do so at a specific impedance and at as near to 1:1 SWR as possible over it's specified, useful range.

In short, it's getting difficult to be a decent DIY project.

Rat Shack used to carry a reasonably priced "dummy load" with a ceramic resistor rated 50 Ohms @ 15 Watts continuous, 100 Watts maximum (30 sec. on, 3 min off- BIG deal!) with a claimed SWR maximum of 1.15:1 up to 500 MHz. For low power (certainly, 50 watts or less for brief periods) it's an option, considering the labor involved and and difficulty and expense of finding suitable resistors.

For higher power, the current offering from MFJ, (similar to the Heath Cantenna) is an oil-cooled power resistor rated 100 Watts continuous, and up to 1 kW intermitently, is an option. Construction for similar DIY dummy loads are in the 1991 ARRL Handbook, and probably other year Handbooks as well.

In many cases, this may be a case where it's more expeditious to buy than to build, and the cost differential/advantage may be fairly small.

KE7MRA

07-05-2007, 12:00 AM

Quote[/b] (k4sav @ July 04 2007,14:31)]

Quote[/b] (ke7mra @ July 03 2007,18:14)]DMX you wrote 1.5k 2 watt resistors, 1.5x30=45kohms? is that 45 thousand ohm's? I dont understand how resistors are marked or what the ratings mean could someone help explan this simple question to me

1.5K / 30 = 50 ohms

They are in parallel.

I did not know or think to use resistors in parallel to achieve a 50ohm load (see I have already learned something) so if I used say 1k value resistors in parallel I would need 20, 1000/20=50, or say 2.5k I would need 50 resistors 2500/50=50 right unless I am missing something?. what if I used a combiation of series and parallel resistors of diferent value to achive a 50ohm load would it matter which part of the circuit I placed the series resistors?

K9STH I have an old tube type radio and I want to try using light bulb for an antenna just to experiment any advise to keep my safe and to keep from damaging my radio ie type and wattage of bulb? feed line? distance from people ect.. also thanks for the advise I am very glad to have a place like this to ask my many many questions http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif

KA9KQH

07-05-2007, 01:30 AM

If you want to use different values of resistors you calculate parallel resistance using product over the sum.

n *n *n *n

-----------------------

n + n + n + n

gives you the resistance in parallel.

For series parallel networks you would calculate the parallel values first then add up the series segments next.

73 de KA9KQH

WB2UAQ

07-05-2007, 01:56 AM

Ka9kqh de wb2uaq

I tried the product over the sum because it didn't seem correct.

Ex: with three 300 ohm resistors in parallel we know from inspection that the equivalent resistance is 100 ohms.

Using the product / sum the result is 30,000 Ohms.

(300x300x300) / (300+300+300) = 30K???

It does work for two resistors in parallel: Using two 300 Ohm resistors in parallel we have 150 ohms and this works OK. (300 x 300) / (300 + 300) or 150 Ohms.

Please correct me if I misinterpreted your formula.

Thanks and 73, Pete

KD5SHW

07-05-2007, 02:33 AM

Quote[/b] (wb2uaq @ July 04 2007,20:56)]Ka9kqh de wb2uaq

I tried the product over the sum because it didn't seem correct.

Ex: with three 300 ohm resistors in parallel we know from inspection that the equivalent resistance is 100 ohms.

Using the product / sum the result is 30,000 Ohms.

(300x300x300) / (300+300+300) = 30K???

It does work for two resistors in parallel: Using two 300 Ohm resistors in parallel we have 150 ohms and this works OK. (300 x 300) / (300 + 300) or 150 Ohms.

Please correct me if I misinterpreted your formula.

Thanks and 73, Pete

Sum of products only works for the special case of two elements in parallel. Any more elements and you have to add the conductances. 3 300 ohm resistors would be calculated like this

1/(1/300+1/300+1/300) = 1/(3/300) =1/(1/100) = 100

The reciprocal of the sum of the reciprocals.

K7KBN

07-05-2007, 02:42 AM

For n resistors in parallel, the formula is

1 divided by the sum of (1/R1 + 1/R2 + ... 1/Rn).

K6UEY

07-05-2007, 02:43 AM

First off the 50 ohms you are looking for is Impedance=Z the 50 ohms you measured was resistance=R.

You measure resistance with an Ohmeter not impedance.

Suggest you review the basics of DC circuits and the Basics of AC circuits so you can tell the difference.

WB7DMX

07-05-2007, 02:46 AM

the other night I threw together a 10 watt QRP dummy load using 43 2.2K 1/2 watt resistors.

I used the same circuit that is for the Norcal SMT QRP dummy load, but used standard resistors..

K7KBN

07-05-2007, 02:48 AM

Quote[/b] (wb7dmx @ July 04 2007,18:46)]the other night I threw together a 10 watt QRP dummy load using 43 #2.2K #1/2 #watt resistors.

I used the same circuit that is for the Norcal SMT QRP dummy load, but used standard resistors..

Given the normal tolerance for standard resistors, you should be very close to 50-52 ohms.

WA9SVD

07-05-2007, 02:54 AM

Quote[/b] (wb2uaq @ July 04 2007,18:56)]Ka9kqh de wb2uaq

I tried the product over the sum because it didn't seem correct.

Ex: with three 300 ohm resistors in parallel we know from inspection that the equivalent resistance is 100 ohms.

Using the product / sum the result is 30,000 Ohms.

(300x300x300) / (300+300+300) = 30K???

It does work for two resistors in parallel: Using two 300 Ohm resistors in parallel we have 150 ohms and this works OK. (300 x 300) / (300 + 300) or 150 Ohms.

Please correct me if I misinterpreted your formula.

Thanks and 73, Pete

Wrong formula.

R1* R2/R1+R2 only works for two values in parallel.

the actual equation for resistors in parallel is :

1 divided by { 1/R1 + 1/R2 +1/R3 +1/R4...}

That will work for any number of resistors in parallel, from one to as many as you wish to include.

(In many calculations where an approximate value is all that's needed, a single resistance that's greater than 1000 times the lowest resistance can often be ignored as having an insignificant ecffect. Thus, a 10 kilohm resistor in parallel with two 100 Ohm resistors would not have a significant effect, but if there are multiple resistances in parallel, even of seemingly high value, they should be considered.

WB7DMX

07-05-2007, 03:00 AM

Quote[/b] (k7kbn @ July 04 2007,19:48)]

Quote[/b] (wb7dmx @ July 04 2007,18:46)]the other night I threw together a 10 watt QRP dummy load using 43 #2.2K #1/2 #watt resistors.

I used the same circuit that is for the Norcal SMT QRP dummy load, but used standard resistors..

Given the normal tolerance for standard resistors, you should be very close to 50-52 ohms.

right, I should say that they were 5 % resistors and it measures 50 ohms, the same as the bird.

I just wanted something small and portable to go with the QRP rigs.

W7KKK

07-05-2007, 03:20 AM

Hey the light bulbs takes me back to 1967-8.

We used light bulbs on top of the GRC-19 transmitters (T-195 if I remember right) to transmit CW traffic between stations in the classroom at Fort Huachuca where I was a Radio Operator Instructor.

They had to send and receive specific traffic in a given period of time.

WB7DMX

07-05-2007, 03:30 AM

my first transmitter I fired up was a heathkit dx20 or 30 don't rember for sure, but I used a 25 watt light bulb for the antenna.

also have used the heath 2er with a no. 57 bulb.

WA9SVD

07-05-2007, 05:03 AM

Quote[/b] (W7WV @ July 04 2007,20:20)]Hey the light bulbs takes me back to 1967-8.

We used light bulbs on top of the GRC-19 transmitters (T-195 if I remember right) to transmit CW traffic between stations in the classroom at Fort Huachuca where I was a Radio Operator Instructor.

They had to send and receive specific traffic in a given period of time.

I SEE the light (bulb.)

Shades of the ARC-5 pair for 40 Meters and later a DX-40/SX99 pair. http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif

Yep them was the daze.

But now I wonder... Why does somebody need to "copy a circuit" to make a dummy load out of parallel resistors?

WB7DMX

07-05-2007, 05:09 AM

Quote[/b] (wa9svd @ July 04 2007,22:03)]

Quote[/b] (W7WV @ July 04 2007,20:20)]Hey the light bulbs takes me back to 1967-8.

We used light bulbs on top of the GRC-19 transmitters (T-195 if I remember right) to transmit CW traffic between stations in the classroom at Fort Huachuca where I was a Radio Operator Instructor.

They had to send and receive specific traffic in a given period of time.

I SEE the light (bulb.)

Shades of the ARC-5 pair for 40 Meters and later a DX-40/SX99 pair. http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif

# #Yep them was the daze.

# #But now I wonder... Why does somebody need to "copy a circuit" to make a dummy load out of parallel resistors?

to be sure that I wired all 43 of them correctly I printed the circuit and then built it, heck I almost lost count on how many to put on each leg, its 11 of them and there's 4 legs, almost too complicated for this old man.

KE4FES

07-05-2007, 08:58 AM

http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif tnks Posters; nostalgia for me of the 1940's..........................

To the original poster. An "ARRL" hand book IS recommended reading ! An "older " one; say a 1990 printing ; would be cheaper and ok. Search in "used technical books. If you can locate the Radio Shack book

Cat. No. 62-1085 [PAPERBACK]; buy it .Was $ 4.00 back in 1986. Written by William Barden, titled

"Electronics Data Handbook"

Consider yourself LUCKY if you can get one.

CHARLIE

KA4DPO

07-05-2007, 02:32 PM

Here's a web site that may be of some help to you. Choose the non-inductive 50 ohm resistor that best suits you need. You could also use higher resistances in parallel to get higher dissapation but it will cost more.

Non-inductive Resistors (http://www.surplussales.com/Resistors/NonInductive/Res-NonInd_list.htm)

WA9SVD

07-05-2007, 03:26 PM

Quote[/b] (wb7dmx @ July 04 2007,22:09)]

Quote[/b] (wa9svd @ July 04 2007,22:03)]

Quote[/b] (W7WV @ July 04 2007,20:20)]Hey the light bulbs takes me back to 1967-8.

We used light bulbs on top of the GRC-19 transmitters (T-195 if I remember right) to transmit CW traffic between stations in the classroom at Fort Huachuca where I was a Radio Operator Instructor.

They had to send and receive specific traffic in a given period of time.

I SEE the light (bulb.)

Shades of the ARC-5 pair for 40 Meters and later a DX-40/SX99 pair. http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif

Yep them was the daze.

But now I wonder... Why does somebody need to "copy a circuit" to make a dummy load out of parallel resistors?

to be sure that I wired all 43 of them correctly I printed the circuit and then built it, heck I almost lost count on how many to put on each leg, its 11 of them and there's 4 legs, almost too complicated for this old man.

No offense meant, and I'm not familiar with the NorCal dummy load, but 43 resistors of 2.2 Kilohm seems a weird combination if in "banks" of 11 resistors. 43 all in series WOULD make a very nice load of around 51 Ohms +/- resistor tolerances, and with minimal lead length would give you at least 20 Watts dissipation. (Pushing resistors to their "rated" dissipation is unwise, and CAN lead to some temporary changes in resistance; pushing them beyond their rating can result in permanent resistance changes. And that was with carbon comp resistors, which had almost no internal inductance save for the leads; The current carbon or metal film resistors are not quite as good when used at the higher frequencies, as they DO have more intrinsic inductance than the carbon comp. resistors.)

K7KBN

07-05-2007, 03:33 PM

Quote[/b] (wa9svd @ July 05 2007,07:26)]No offense meant, and I'm not familiar with the NorCal dummy load, but 43 #resistors of 2.2 Kilohm seems a weird combination if in "banks" of 11 resistors. #43 all in series WOULD make a very nice load of around 51 Ohms +/- resistor tolerances, and with minimal lead length would give you at least 20 Watts dissipation. #

I'm sure you meant "43 all in PARALLEL"...

WB7DMX

07-05-2007, 03:44 PM

Quote[/b] (k7kbn @ July 05 2007,08:33)]

Quote[/b] (wa9svd @ July 05 2007,07:26)]No offense meant, and I'm not familiar with the NorCal dummy load, but 43 #resistors of 2.2 Kilohm seems a weird combination if in "banks" of 11 resistors. #43 all in series WOULD make a very nice load of around 51 Ohms +/- resistor tolerances, and with minimal lead length would give you at least 20 Watts dissipation. #

I'm sure you meant "43 all in PARALLEL"...

yes, but not all in one stright line, it has 4 rows of 11 resistors each and the 4 rows are ? heck I can't even discribe how there tied together, you would have to go to there web sight and see the circuit yourself.

it all fits nicely in a hamon diecast 1.5 X 3.5 box.