Homebrew Dummy Load...

[acyddrop]

I'm thinking that these Caddock - MP9100-100-1% - Passive Components - Resistors - Allied Electronics would make one helluva dummy load. Put two of those in parallel for a nice solid 200+ watt dummy load. Two ways I could think to use them would be...

1) Mount the resistors to a heat sink, put the heat sink in a paint can and fill the paint can with mineral oil. You need to put some terminals on the top and with some zener diodes you could get a watt reading too.

2) Mount the resistors on two heat sinks, force air through the fins with a fan for a dry dummy load.

I'm thinking with two of those in parallel in mineral oil you could probably hit 300+ watts before they got in any danger of over heating and self destructing. But that is just a guess. With a dry dummy load and forced air you could do the rated power no problem. Total cost of the project wouldn't be very expensive at all either. Those are 1% tolerance resistors I think they have 5% and 10% in 100 watt 100 ohm options for less money in the TO-247 package.

 

[LtDoc]

First I've heard of them, but they sound like they would work okay. 'Pushing' them is certainly possible, but I have no idea how far they can be 'pushed'. I'll let you find that out!
- 'Doc

 

[acyddrop]

Well I've seen a 50watt setup with an array of 1 watt resistors handle 120 watts key down because it was submerged in mineral oil. I do know that the unsinked handling power of those 1000 watt resistors is only about 2 watts, so they absolutely need to be sinked properly. I noticed they had 1k Ohm 100 watt resistors, if my math serves you could use 20 of those in parallel for a 2KW dummy load.. But that might be getting carried away. LOL.

 

[acyddrop]

Here's a basic schematic for a dummy load with two resistors and BAV21 small signal diode for measuring transmitted power. Not in the schematic are the heat sinks the resistors need to attach to, nor the can with mineral oil to dissipate the heat from the resistors.

 

[k3td]

Looks good!

By the way, how do the receivers in your K3 and FTDX-5000mp compare?

 

[acyddrop]

In my opinion the audio produced by the FTDX-5000mp is more pleasing to me, even though the K3 can pull out the weak signals like nobodies business. To answer your question, I like them both but I like them for different reasons. Pulling out the muck with the K3 is easier, and being able to talk in a pile up is done well. But for overall quality of sound produced and access to options I like the FTDX-5000mp. They both have a place in daily use.

Looks good!

By the way, how do the receivers in your K3 and FTDX-5000mp compare?

 

[fineshot1]

Flange Resisters

I could be wrong but I believe most of the better quality dummy loads use flange resisters
so that the thermal transfer to heatsink is efficient. See pic

 

[acyddrop]

well the entire back of the TO-247 package is metal, you'd obviously have to put thermal paste on that to conduct heat. Dissipating heat becomes a function of surface area on the heat sink and the amount of air moving over it. You can get away with no air moving over the surface of the sink if you suspend it in a medium that will absorb that heat and move it away. You obviously can't use water but you can use mineral oil which is not electrically conductive but conducts heat nicely. You then move that heat to a large surface area of the can it's contained in to be dissipated into the heat.

I suspect the style of resistor you're actually referring to is chosen for a reliable and low VSWR across a wide spectrum of frequencies; rather than heat dissipation which is more easily overcome by choosing the proper amount of cooling required for your load (which you can derive from the Datasheet and widely known equations).

I could be wrong but I believe most of the better quality dummy loads use flange resisters
so that the thermal transfer to heatsink is efficient. See pic