Practical pulse duration limit for flanged 50 ohm load resistor in dummy load? - Page 1
Forum section: electronics / rf-microwave

author: wb0gaz
author_rank: Frequent Contributor
timestamp: December 28, 2020, 02:13:26 am
content: I'm using as a ham radio dummy load a "CELWAVE" cell base station dummy load; it uses a 250W flanged load resistor about 1cm square, soldered to a small (~25mm x 20mm x 3mm) copper spreader which in turn is attached to a heatsink via thermal compound. These specific dummy loads, and this sort of dummy load (flanged resistor and heatsink) appear on auction site from time to time.During some repair work on a RF power amplifier, I fed it CW 500-600 watts for maybe 15 seconds; at some point during this the load resistor failed open circuit. I've replaced the load resistor without difficulty, and the load again works normally, however, I could not find any practical duty cycle limit for this sort of load resistor in ("ICAS" - intermittent commercial and amateur service) dummy load application.As I think this sort of dummy load design (flanged resistor and heatsink) is pretty common surplus and often ends up in ham shack, I'd like to know if there is any practical limit I should observe (in terms of duty cycle and/or pulse duration) in the future?Thanks!

author: cdev
author_rank: Super Contributor
timestamp: December 28, 2020, 02:33:48 am
content: Do you have a datasheet for your load resistor? I have one too, manufacturer or specifications unknown - (Its marked "250NB") unused, and I am wondering how it would be mounted in order to heat sink it properly (or if the rating is perhaps quite overoptimistic)My guess is the design of the heatsink would seem to me a lot like the design of a beefy CPU heatsink (except a CPU is much larger.. so its thermal interface is larger, so less likely to fail perhaps) to be able to conduct that heat away fast enough. The dynamics of it with DC are likely the same, it shouldn't be that different as far as heat than any other resistor. (My guess is we need more copper spreader close to the resistor - bigger heatsinks attached to a larger chunk of copper, and likely also fans, and we shouldn't go over the rating or we should expect rapid failure)

author: TheMG
author_rank: Frequent Contributor
timestamp: December 28, 2020, 07:16:49 pm
content: Such thick-film resistors have thermal characteristics similar to that of power semiconductors.When adequately heatsinked with a low case-to-sink thermal resistance, the junction-to-case thermal resistance prevails in short duty cycle use. Similar to a semiconductor, the "junction" (in this case the thin resistive element), has a very small thermal mass so it heats up very quickly due to the junction-to-case thermal resistance.As such, pulses much in excess of the resistor's rating, although the manufacturers don't specify this in the datasheets, should be limited to no more than a handful of milliseconds (in most practical implementations this would be pulses in the uS range).So for "ICAS" use, thick film resistor dummy loads the ICAS power rating would be equal to the continuous CW power rating, not more.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: December 28, 2020, 07:21:01 pm
content: Thanks TheMG - this is very helpful indeed. I was wondering if the thermal properties were something like those big LDMOS RF power MOSFETs in terms of thermal behavior - this makes sense that the acceptable pulse duration would be very short (too short to be useful for ham application).Thanks again!!!