Stripline resistor question

jimlg

Member
Premium Subscriber
Joined
Mar 2, 2014
Messages
31
I would like to build a legal limit dummy load using two 800 watt 100 ohm stripline resistors in parallel.
I would like it to cover from hf through 70 cm.
I have attached files showing what I will be mounting the resistors on.
The heatsink is 10x16 with 2.5 inch fins an a .25 inch copper plate.
Can I cut the heatsink in half and still have adequate heat dissipation?
Is this feasible or should I look elsewhere?
 

Attachments

  • 1.jpg
    1.jpg
    96.9 KB · Views: 17
  • 2.jpg
    2.jpg
    98 KB · Views: 18
  • 3.jpg
    3.jpg
    141.1 KB · Views: 18

prcguy

Member
Joined
Jun 30, 2006
Messages
15,233
Location
So Cal - Richardson, TX - Tewksbury, MA
It should work fine and I would use the entire heatsink if you want any extended transmit time.

I have a chunk of tunnel type heatsink with very thick base material and a 4" fan that blows through the middle and I used two 100 ohm 1600 watt load resistors. At 1kW the resistors heat up extremely fast and my aluminum heatsink can't pull the heat away fast enough. I get maybe 30 seconds of transmit at 1kW then you can boil water off the resistors but the heatsink is still fairly cool. Mine needs a copper spreader at least 1/4" thick by maybe 4" square or larger to spread out the heat to the aluminum heatsink. You have a good looking copper spreader, so you should be fine with that heatsink. Or you could sell me that heatsink and start over.
 

jimlg

Member
Premium Subscriber
Joined
Mar 2, 2014
Messages
31
It should work fine and I would use the entire heatsink if you want any extended transmit time.

I have a chunk of tunnel type heatsink with very thick base material and a 4" fan that blows through the middle and I used two 100 ohm 1600 watt load resistors. At 1kW the resistors heat up extremely fast and my aluminum heatsink can't pull the heat away fast enough. I get maybe 30 seconds of transmit at 1kW then you can boil water off the resistors but the heatsink is still fairly cool. Mine needs a copper spreader at least 1/4" thick by maybe 4" square or larger to spread out the heat to the aluminum heatsink. You have a good looking copper spreader, so you should be fine with that heatsink. Or you could sell me that heatsink and start over.
Thanks for the info, I'm curious what 1600 watt resistor did you use, I may consider 1600 watt units, I assume they are physically
larger than the 800 watt units so they should dissipate the heat a bit better. What thermal material would you recommend I use
between the copper spreader and heatsink?
 

prcguy

Member
Joined
Jun 30, 2006
Messages
15,233
Location
So Cal - Richardson, TX - Tewksbury, MA
I looked up my resistors and they are only 800w each and made by Florida RF. Probably similar to yours. I must have been thinking of another high power load I have around here. Anyway since the resistors are fairly small they will not dissipate much power on their own, maybe 30w or so continuous and they need a lot of heatsink for long term key down. I used some white thermal grease I had laying around used for RF transistors, not sure of the brand or type.

Thanks for the info, I'm curious what 1600 watt resistor did you use, I may consider 1600 watt units, I assume they are physically
larger than the 800 watt units so they should dissipate the heat a bit better. What thermal material would you recommend I use
between the copper spreader and heatsink?
 

jimlg

Member
Premium Subscriber
Joined
Mar 2, 2014
Messages
31
I looked up my resistors and they are only 800w each and made by Florida RF. Probably similar to yours. I must have been thinking of another high power load I have around here. Anyway since the resistors are fairly small they will not dissipate much power on their own, maybe 30w or so continuous and they need a lot of heatsink for long term key down. I used some white thermal grease I had laying around used for RF transistors, not sure of the brand or type.
Thanks again for the advice.
 
Top