RF Dummy load with damaged resistor - Page 1
Forum section: electronics / beginners

author: kelemvor
author_rank: Regular Contributor
timestamp: July 16, 2021, 03:44:33 am
content: Greetings,I just bought an MFJ-250 dummy load. It's just a big 50 ohm resistor installed in gallon sized paint can that gets filled with transformer oil to cool the resistor. When I opened the can to fill it, I discovered the resistor had a chip broken off the corner - probably in shipping. Resistance measures very close to the targeted 50 ohms at 50.002 with my trusty 121gw.Should I send this thing back and get a replacement? Can I safely disregard the chip in the resistor? I'll probably never put the 2kw that it's rated for into the load but I will definitely run 500w into it and maybe someday 1kw.Opinions?

author: bob91343
author_rank: Super Contributor
timestamp: July 16, 2021, 05:31:01 am
content: I wouldn't worry, but if you bought it new it should be returned for replacement.

author: radiolistener
author_rank: Super Contributor
timestamp: July 16, 2021, 06:15:32 pm
content: It can still work, but the power rating and lifetime will be significantly reduced, so there is a reason to request a new resistor element for replacement from the seller.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 16, 2021, 06:28:58 pm
content: Quote from: radiolistener on July 16, 2021, 06:15:32 pmWhy do you think the effects will be significant? It looks like the undamaged end is only clipped in place and the clip is the contact. The damaged end looks like there's a bolt and the clip but the damage barely touches the clip. Also, since the unit will be submerged in oil, the power will be dissipated through the oil rather than the mounting so the power rating should be unaffected.Based on how little of the damage relates to the connection of the resistor, I was thinking that the effect will be negligible. But, it's annoying to see in a new unit so I'd contact the company and see what they say.Ed

author: james_s
author_rank: Super Contributor
timestamp: July 16, 2021, 06:35:26 pm
content: It will probably be ok, but personally I would ask them for a replacement resistor, it's brand new, there should be no damage to even wonder about. It looks like replacement resistors are under 40 bucks so it isn't going to be a major expenditure to send you one.

author: dcbrown73
author_rank: Regular Contributor
timestamp: July 16, 2021, 08:15:33 pm
content: I agree it will likely be okay, but if it was new. I would return it for a replacement.

author: kelemvor
author_rank: Regular Contributor
timestamp: July 16, 2021, 10:56:50 pm
content: Thanks guys, I asked the seller as well and they volunteered to send a replacement

author: radiolistener
author_rank: Super Contributor
timestamp: August 25, 2021, 01:43:23 am
content: Quote from: edpalmer42 on July 16, 2021, 06:28:58 pmRF current flows primary at surface (due to skin effect) and part of surface is damaged, so it means that resistance will be a little higher and it will be more hot at broken area which leads to a less power rating. When you dealing with high power, such damages reduce conductivity and as result it will be more hot and a high current will destruct it further. We don't know how much power rating will be affected, but it will be affected and it's definitely is not acceptable for a new device.If you got a used device with such defect, you can use it, just note that it's power rating is less than for a good one. But if you bought a new item with such defecti it's better to replace it.