TO-247 resistor as RF load - Page 1
Forum section: electronics / beginners

author: AlexRG
author_rank: Regular Contributor
timestamp: June 26, 2025, 12:28:02 am
content: Hello everyone. Has anyone tried making a load for a HF transceiver from TO-247 resistors, like these?https://lcsc.com/product-detail/Through-Hole-Resistors_Milliohm-To-247-100W-25R-1_C5209795.htmlHow do these resistors perform at frequencies up to 30 MHz?I plan to take 8 pcs 25 Ohm resistors. Connect 4 in series, it will be 100 Ohm, and then the second group in parallel. It will be 50 Ohm. All resistors will be installed on a copper radiator. What do you think?

author: thm_w
author_rank: Super Contributor
timestamp: June 26, 2025, 12:57:01 am
content: Its claiming non-inductive design, and 30MHz isnt too high, should be OK?https://passive-components.eu/resistor-voltage-and-frequency-dependence/Another option is to buy RF rated resistor, they are rated to ~2GHz, some are salvaged parts, if you care:https://www.aliexpress.us/item/3256806813889882.htmlhttps://www.aliexpress.us/item/3256806080041256.htmlhttps://www.aliexpress.us/item/2255801082585038.htmlhttps://www.aliexpress.us/item/3256808955200908.html

author: AlexRG
author_rank: Regular Contributor
timestamp: June 26, 2025, 01:40:23 am
content: Quote from: thm_w on June 26, 2025, 12:57:01 amYea, I know about these resistors, they are used by everyone. But, they have a very thin pin, which often breaks.I'm just wondering why I've never seen an RF load made of TO-247 resistors? They are not expensive, their pins are very rigid and reliable. Why are they not widely used?

author: daisizhou
author_rank: Super Contributor
timestamp: June 26, 2025, 02:08:27 am
content: Quote from: AlexRG on June 26, 2025, 01:40:23 amUsually I recommend using a non-inductive winding inductor,Especially for those occasions with special requirements for power load.TO-247 Need a good heat sink,In other words, its redundancy is not high

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: June 26, 2025, 02:44:52 am
content: The exposed tab may be connected to one of the terminals. And this same tab must be in intimate contact with the heatsink. To prevent a short between one resistor and the next, you must add an insulated thermal pad. Meaning you have created a capacitor between the heat sink and a terminal.Which for DC or low frequency it is of no concern. But at 30 Mhz, the capacitive reactance may generate a shunt impedance to the heat sink. I would assemble a device and measure that capacitance and calculate the reactance.

author: AlexRG
author_rank: Regular Contributor
timestamp: June 26, 2025, 04:19:26 am
content: Quote from: UnijunctionTransistor on June 26, 2025, 02:44:52 amNo, the resistor body has no electrical contact with the resistor crystal itself. The substrate here is not copper, but a dielectric made of aluminum oxide. These resistors can be directly screwed to the radiator without dielectric spacers.

author: fourfathom
author_rank: Super Contributor
timestamp: June 26, 2025, 05:28:20 am
content: You might consider using an aluminum substrate, which JLCPCB can provide at very low cost. I used this and a bunch of 1210 resistors to make a multi-channel dummy load / -40 dB monitor board. The return loss was decent (for a non-precision dummy load) up past 100 MHz. and the heat spreading was very good. If I needed more power I could have attached a large heatsink to the back of the board. The aluminum board is single-sided only, and only useful for surface-mount -- not through-hole parts. I used edge-mounted SMA connectors and only soldered the top-side contacts.Here's my ham blog post about it:http://wb6cxc.com/?p=440

author: MariuszD
author_rank: Regular Contributor
timestamp: June 26, 2025, 06:32:29 am
content: TO-247 resistor has the same construction as SMD resistors. It is large so it will have significant capacitance to the heatsink. One TO-247 resistor can be better than bunch of 1210 resistors on MCPCB.

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 26, 2025, 12:39:49 pm
content: Depending on what sort of VSWR you need, using through hole resistors like this is fine. I know of some companies that use TO-247 dummy loads above 100MHz.

author: AlexRG
author_rank: Regular Contributor
timestamp: August 16, 2025, 10:28:21 am
content: All goodup to 150-200MHz load