Dummy Load

[sinus trouble]

Hello All!
Just thought id share this with the forum!
After a catastrophic failure of my dummy load i needed something new to test my rigs!
Instead of buying a new one? i decided to build one!
Many of you have probably seen the oil cooled type of dummy many times on the net and this is my version!
Firstly i found a small tin can to house the oil
Then i cut out two brass discs and pre drilled it to accept a bank of resistors
20 1kohm 3watt resistors were used and mounted to the lid with an SO239 connector!
it can handle 60watt dry and possibly up to 100watt immersed in oil!

20141023_225326.jpg

[teckniqs]

If it can handle 60w continuous mate it should be able to take a bit more than 100w in the oil I'd expect. Be interesting to see lol.

[pjeva]

If you have a proper heatsink, you can buy real, NON-INDUCTIVE dummy load for about 10£... this things are good for low-power (<10w) in-development testing if you work on VHF (which include fm). If you use it for less than 100w, heatsink can be old massive cpu cooler with fan

And here's one link... it is even cheaper!
http://dutchrfshop.nl/componenten/rf-du ... -watt.html

[thewisepranker]

Enigma stock a 250 W 50 Ω resistor for £5 and an 800 W 50 Ω resistor for £20:

http://enigma-shop.com/index.php?option ... &Itemid=51

I've made a load on a nice big heatsink using two of their 250 W 100 Ω resistors in parallel and it works brilliantly for anything up to 300 W. You might ask why you'd need two 250 W resistors as opposed to just one - each can handle 250 W only in ideal conditions. The thermal junction is usually a lot higher in resistance than anticipated, which de-rates the power dissipating ability.

[sinus trouble]

If you have a proper heatsink, you can buy real, NON-INDUCTIVE dummy load for about 10£... this things are good for low-power (<10w) in-development testing if you work on VHF (which include fm). If you use it for less than 100w, heatsink can be old massive cpu cooler with fan

Ahhh i saw them on dutch rf shop! They are compact, powerful and defo a consideration for the future!
Do they use that horrible beryllium oxide stuff in them tho?

[sinus trouble]

I've made a load on a nice big heatsink using two of their 250 W 100 Ω resistors in parallel and it works brilliantly for anything up to 300 W. You might ask why you'd need two 250 W resistors as opposed to just one - each can handle 250 W only in ideal conditions. The thermal junction is usually a lot higher in resistance than anticipated, which de-rates the power dissipating ability.

Sounds good Mr Pranker!!
Are they the large carbon type? And how would they be mounted to a heatsink?
I had one of these previously!

DL-300MN.jpg

It had a large carbon resistor inside and was rated at 300w for 20 seconds?
I only fed it 50 to 60w and it failed? although it was for like an hour or so! LOL

[sinus trouble]

If it can handle 60w continuous mate it should be able to take a bit more than 100w in the oil I'd expect. Be interesting to see lol.

Mr teck! Im really suprised with its performance! This maybe unconventional for RF engineers? but i like a dummy load to stay reliable over a wide length of time? so i can monitor the rig in detail! the metal film resistors and messy oil used in this dummy aint ideal? but it handles 60watt 24 hours a day no problems!!

[thewisepranker]

They are like this:



Not necessarily "Johanson" but that package all the same. You just screw them into the heatsink much like you'd mount a BLF278 or similar, either with self-tappers or better with M2.5 or M3 screws and a bit of your favourite thermal compound, but not too much.

I've also got a Bird coaxial resistor which will handle 600 Watts continuously - I got it on eBay for about £80-90 delivered. It's bloody massive.

[sinus trouble]

Not necessarily "Johanson" but that package all the same. You just screw them into the heatsink much like you'd mount a BLF278 or similar, either with self-tappers or better with M2.5 or M3 screws and a bit of your favourite thermal compound, but not too much.

I've also got a Bird coaxial resistor which will handle 600 Watts continuously - I got it on eBay for about £80-90 delivered. It's bloody massive.

Oh I see! They look similar to the ones Mr pjeva was talking about!
Love the bird Range!! although they look like something out of a world war two submarine lol quality and reliable test equipment!! cant go wrong!!

[pjeva]

They are like this:



Not necessarily "Johanson" but that package all the same. You just screw them into the heatsink much like you'd mount a BLF278 or similar, either with self-tappers or better with M2.5 or M3 screws and a bit of your favourite thermal compound, but not too much.

I've also got a Bird coaxial resistor which will handle 600 Watts continuously - I got it on eBay for about £80-90 delivered. It's bloody massive.

This is high power resistor but not for dummy load. It is 100 ohm resistor which should be used in power dividers or power combiners. That is why its nominal power is rated 250w, but it can handle it for short period of time. The one you want is same as this, but with only one pin.

Here's the difference:

20141028_110548-1.jpg

[thewisepranker]

Well, it depends what you do with the other leg, doesn't it. I used resistors rather than terminations because it's easier to solder some coax to the leg, rather than bolting it to the heatsink some distance away from the termination.

I agree with you that you should use a termination as opposed to a resistor(s), however since we're only at about 100 MHz, the VSWR isn't going to be bad enough to worry about. It will certainly be better than most other things you can brew at home, regardless of whether you do it with RF resistors or terminations, and if you choose some nice terminations/resistors with a beryllium oxide substrate, such as those made by Bourns, you can get it below 1.1:1 quite comfortably. If you need anything better it'd be silly to not buy the real thing in the shape of a Bird load or similar.

As for the power handling, the Bourns datasheet suggests that it will handle 250 W continuously up to 100 °C, and I have two in parallel.

[pjeva]

Well, it depends what you do with the other leg, doesn't it. I used resistors rather than terminations because it's easier to solder some coax to the leg, rather than bolting it to the heatsink some distance away from the termination.

I agree with you that you should use a termination as opposed to a resistor(s), however since we're only at about 100 MHz, the VSWR isn't going to be bad enough to worry about. It will certainly be better than most other things you can brew at home, regardless of whether you do it with RF resistors or terminations, and if you choose some nice terminations/resistors with a beryllium oxide substrate, such as those made by Bourns, you can get it below 1.1:1 quite comfortably. If you need anything better it'd be silly to not buy the real thing in the shape of a Bird load or similar.

As for the power handling, the Bourns datasheet suggests that it will handle 250 W continuously up to 100 °C, and I have two in parallel.

I agree it both can be used, but... These resistors are supposed to take over phase difference between two amps connected to same antenna or in case that one amp fails it takes some portion of rf power and protect other amp from burning. Usually, it handles high power for a short period of time, and then smaller amount of rf power until you repair failed device. On the other hand, dummy load resistor is made to take nominal rf power for long time (with appropriate cooling). And yes, there are some resistors that can be used as both, but usually they are more expensive...

Regarding dangerous chemicals, you are putting your hands in high rf field while making amplifiers, which is bigger risk to your health than a bit of beryllium oxide which will come out only if you heat it over the edge...