Hey Dummy; that's a Load . . .

[RatsoW8]

You will have to open it up to add the oil any way.

They are the exact same DL with a different label on the front of the can. When my Cantenna finally developed a slow leak because I had it placed under my house I went down to Home Depot and bought a new empty 1 gallon paint can. Popped top with the load on the new can filled with mineral oil (Walmart brand baby powder scented of course). So for $5 I have like new Cantenna. These things just about last forever.

 

[Robb]

Hasn't anyone built their own dummy load?

The type 'c' carborundum resistor with a 90 watt rating is found in the cantenna IIRC.

Has anyone purchased a 200w rated resistor and put it in a 2gal container with xformer oil? Guessing that should be rated for another kw of dissipation over the Ameritron or the MFJ.

 

[vo1ks]

At work we have a 25kW dummy load that uses a 200 watt resistor. Before power is applied a water pump comes on, and the water run through two radiators with two fans blowing air through. The resistor shatters instantly if it isn't properly cooled. It's shown in the lower right of the first picture.

The discolored 25kW load suffered major meltdown without proper cooling with only 10kW applied. The aluminum melted and dripped...

 

[Robb]

Got the MFJ-250 yesterday from the HRO in Sunnyvale, CA for $69 & tax.

The resistor used in this thing looks like the 100w Goldline dry dummy load resistor that I use for radio alignments. If I had known that before; I might of just bought a paint can and some xformer oil and built my own with that Goldline instead.

But I don't know where to buy xformer oil around here. No way would I use mineral oil as a substitute either. The applied power/derating curve explicitly shows a vast difference between real xformer oil and mineral oil. Xformer oil will rate the loading time for 5-10 minutes @1.5kw's. Instead of 5-10 seconds if mineral oil is used. The oil made and used for xformers smokes mineral oil for heat dissipation. No pun intended . . .

So, if anyone is considering building their own high power dummy load; don't scrimp on the quality of the oil. If you can get genuine xformer oil; then by all means - do so.

 

[Mudfoot]

I like it. It's a fine looking dummy load.

You could use mineral oil in the interim until you score transformer oil. Find an industrial janitor or electric company worker, maybe they'll help you out.

 

[W5LZ]

Your power company has that transformer oil. No idea if they'd sell it to you though. My local company gave me some once, haven't asked in a lot of years, but that'd be my first stop for it.
- 'Doc

 

[Robb]

The MFJ-250 comes with xformer oil; that is what I bought for the $69.

The MFJ-250X does not come with the xformer oil; dunno what it cost.

The reason I bought the MFJ-250, is because I needed an adequate load to handle an amp that has ~1.8kw output and can handle a newbee's hand on tuning the amp off-the-air. The time factor under load was the real issue. The xformer oil is the reason for the better time factor alone.

 

[Robb]

At work we have a 25kW dummy load that uses a 200 watt resistor. Before power is applied a water pump comes on, and the water run through two radiators with two fans blowing air through. The resistor shatters instantly if it isn't properly cooled. It's shown in the lower right of the first picture.

The discolored 25kW load suffered major meltdown without proper cooling with only 10kW applied. The aluminum melted and dripped...

Just a 200w resistor? So; the real factor is in the ability to cool that load. I priced a carborundum/type 'C' 50 ohm/200w resistor online. Think it was about ~$250, IIRC . . .

 

[vo1ks]

Yes, exactly! That's what the derating curve for the MFJ load shows as well. The 25kW rated load literally melted with only 10kW applied when its cooling fans shut down. Find a way to get rid of the heat and a resistor can dissipate immensely higher power.

 

[Captain Kilowatt]

Got the MFJ-250 yesterday from the HRO in Sunnyvale, CA for $69 & tax.

The resistor used in this thing looks like the 100w Goldline dry dummy load resistor that I use for radio alignments. If I had known that before; I might of just bought a paint can and some xformer oil and built my own with that Goldline instead.

But I don't know where to buy xformer oil around here. No way would I use mineral oil as a substitute either. The applied power/derating curve explicitly shows a vast difference between real xformer oil and mineral oil. Xformer oil will rate the loading time for 5-10 minutes @1.5kw's. Instead of 5-10 seconds if mineral oil is used. The oil made and used for xformers smokes mineral oil for heat dissipation. No pun intended . . .

So, if anyone is considering building their own high power dummy load; don't scrimp on the quality of the oil. If you can get genuine xformer oil; then by all means - do so.

Thousands upon thousands of real hams have been using mineral oil in thousands of homebrew dummy loads without an issue. Heathkit recommended mineral oil, the heavy type for their Cantenna. The Cantenna was rated at 1Kw for 2 minutes with mineral oil and about 10 minutes max with transformer oil.The light mineral oil will not handle the heat as well. Transformer oil is indeed better however there is nothing wrong with mineral oil, the heavy type, as long as you monitor the temperature which you should be doing with transformer oil anyway. I used a small quart sized load with 10 500 ohm 2 watt resistors with a 100 watt transmitter using mineral oil and it hardly got warm. If you need more than a few minutes of key down time you either need a better tuning technique or an even bigger load.

http://www.orcadxcc.org/content/cantenna_va7jw.pdf

 

[Mudfoot]

My Cantenna is so old that I'm sure it has the nice PCB goodness. It's starting to rust along the bottom and has a few war wounds. I got a new lined can with bail that's all shiny looking. I'm gonna powder coat it and change everything over. I popped the lid today, the old oil is nice and dark, looks heavy and barely has an odor. I guess it should be okay to use.