200w+ dummy load 50ohms

[Dan]

Anyone know how to make a dummy load capable of 200w and over. There's many guides online but they are not very straight forward.

[teckniqs]

Be much less hassle just to buy one ready built, unless you prefer to build your own.

[shorty]

Some times you can find decent used bird dummy loads on ebay at reasonable prices, just a matter of keeping your eyes open for a bargain.

Or if you want to build your own, one of these 250w or 500w 50 ohm resistors mounted to a big heatsink with a fan will do the trick.

http://www.ebay.co.uk/itm/DUMMY-LOAD-RE ... SwQItT6yOX

http://www.ebay.co.uk/itm/DUMMY-LOAD-RE ... SwDk5T6yXR

[thewisepranker]

Here is a previous comment of mine on the same subject:

You'd be better off with two 250 W, 100 Ohm flanged resistors in parallel, such as the Florida RF 31-1076-100-F.

You won't be able to afford an adequate heatsink to dissipate anywhere near 200 Watts from a single one of those 50 Ohm flanged resistors suggested, let alone 500 W as is written on the device. For the latter, you'd need a thermal resistance from junction to ambient of around 0.2°C/W, which is lower than the typical junction to case thermal resistance, let alone the rest. Dissipating 200 Watts only takes the required thermal resistance of the system up to 0.635°C/W, which is still largely unachievable at home.

You can find the Florida RF flanged resistors on eBay, here for example, for about 15 EUR each: http://www.ebay.co.uk/itm/RF-Resistor-R ... 1a02635255

Although forced cooling is effective, I would recommend against it for a dummy load, as you often require it in places where you don't have power available to drive the cooling fans.

The eBay link is still valid.

[shorty]

This dry 500 watt bird dummy load looks ok for the price, never used this type before only the bird wet loads, i've read the dry type are easier to blow if your operating near the maximum rating than the oil filled ones.

He also has an offer button on the listing so maybe able to get it for less than listed price.

http://www.ebay.co.uk/itm/Bird-500-CT-F ... SwNSxVSgsd

[shorty]

I think i've given you a bum steer with the link to the bird 500w dummy load, i've re read the ad and the heatsink seems on the small side, i can remember seeing a bird 500w dry load at a friends place from memory it was a lot bigger and beefier than the one in the photo, that heatsink doesn't seem right as i've a 150w load with a bigger heatsink, ignore my above post.

[thewisepranker]

Looks like someone has bolted a Bird module to a random heatsink and sold it to the MoD, who have probably given it away for free to some bloke that sells random shite on eBay, not knowing what it is he's selling.

[Gum]

For a proper silly home brew dummy load, consider an old oil filled electric heater.

The heating element should pull right out and you can shove in a small heatsink or something bolted to a flange resistor in it's place.

Needless to say though, you won't be carrying it about with you much in your rucksack!

[sinus trouble]

Lol ive seen people use allsorts! Light bulbs, Heaters etc!!
I would think they are too inductive and not a true 50Ohm
I do think oil filled is messy but a great performer though! I built an oil filled one using 20 1k 3w resistors for less than 30 quid!
it pissess all over any 300w pro dry dummy loads ive had in the past!

[Gum]

Yeah, light bulbs are a classic!

If you get yourself a nice 50 ohm, non-inductive, pure resistor though it's just about dissipating the heat.

[thewisepranker]

I built an oil filled one using 20 1k 3w resistors for less than 30 quid!
it pissess all over any 300w pro dry dummy loads ive had in the past!

But that's only 60 Watts...

Designed properly, a "dry" 300 W load should perform the same as a "wet" 300 W load. I don't think I've ever seen a "dry" one that has been designed properly.

[Maximus]

Looks like someone has bolted a Bird module to a random heatsink and sold it to the MoD, who have probably given it away for free to some bloke that sells random shite on eBay, not knowing what it is he's selling.

I think I might know the bloke lol

Ram-scrap springs to mind


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[Gum]

So there shouldn't really be any difference between a dry Bird and a wet Bird.................?

[sinus trouble]

But that's only 60 Watts...

Designed properly, a "dry" 300 W load should perform the same as a "wet" 300 W load. I don't think I've ever seen a "dry" one that has been designed properly.

Indeed it is only 60W
I agree, alot of dummy loads are over rated! I previously had an MFJ DL-300M and according to spec it can handle 300 for around 30 seconds? and a 100w continuous?
It failed on 50w!

[sinus trouble]

So there shouldn't really be any difference between a dry Bird and a wet Bird.................?

LOL if a bird is dry?? you aint doing it right!