By Steve Bush 26th May 2021
I need an adjustable power resistor that can be switched with a resolution of 1Ω.
After some thought and web surfing, I bought some 10W wirewounds, which are available in E24 values so that 1, 3, 12 and 24Ω are available.
This meant that a binary progression incrementing in 3Ω steps is easily possible, with 2x 3Ω in series to get 6Ω, and a couple of 1Ω resistors to achieve the required resolution.
These are 10% resistors. 2W metal film resistors are available in 1%, but I thought the construction would be so fiddly that it would take too long to make.
Only after the bits arrived did I realise that the whole thing could be constructed by drilling a few holes in a sheet of plain pcb (I was expecting 3d printing plus faffing), so making the wirewould version only took an hour or so and the more accurate version would not have taken much longer.
After mounting the resistors to the plain pcb, I carved the required isolation pattern into the copper with an electric rotary tool fitted with a little cutting disc.
The resistors are wired in a long series chain, each one with a shorting switch (this arrangement is thanks to friend of this parish Wise Mr Kurt who saved me from wiring them weirdly with change-over switches – I was having a bad day, is my only excuse…. 🙂
In cutting the isolation free-hand, what an untidy mess I made – no wonder the poor art teachers despaired. Luckily, this is hardly noticeable compared with the untidy free-space wiring….
Still, it works, and is only a little bit let down by the switches I bought off ebay, some of which seem to have quite a lot of resistance, that fluctuates from actuation to actuation. Update: contact resistance per switch has settled to ~40mΩ after operating them all lots of times.
BTW, I am a generally a great fan of ‘miniature’ toggle switches. Many of the quality brands are specced at 10mΩ (initial).
If I have to change the switches, I might also go for 1% resistors as well, but for now, I have what I set out to have, with a maximum error under 0.5Ω.
Tagged with: EinW maker passives resistor
the copper-clad board arrangement looks fine! I’m not as confident in the wiring… it looks like there is no insulation on the wires?? Or is the insulation silver? As far as lack of precision in the resistors… as long as you measure the resistor values, you should be able to get fairly precise results. There may be some temperature coefficient concerns, though, so you might need to measure resistance at different temperatures.
Hi Mr Kurt. Well spotted, there is indeed no insulation on the wires. My reasoning was that thee is acres of bare copper around, so insulation on the wires would not add much 🙂 My plan is to space a sheet of insulator underneath the whole thing. On the precision front, I am going to try to measure the resistors and switches this evening. Hopefully the switches are mostly settled now.
Your email address will not be published. Required fields are marked *
Download the Elektra Awards 2022 media pack and book your sponsorship package to be part of the most prestigious awards event celebrating its 20th anniversary this year!
Get our news, blogs and comments straight to your inbox! Sign up for the Electronics Weekly newsletters: Mannerisms, Gadget Master and the Daily and Weekly roundups.
Read our special supplement celebrating 60 years of Electronics Weekly and looking ahead to the future of the industry.
Read the Electronics Weekly @ 60 supplement »
Read the first ever Electronics Weekly online: 7th September 1960. We've scanned the very first edition so you can enjoy it.
Read the very first edition »
Electronics Weekly teams up with RS Grass Roots to highlight the brightest young electronic engineers in the UK today.
Read our special supplement celebrating 60 years of Electronics Weekly and looking ahead to the future of the industry.
Read the Electronics Weekly @ 60 supplement »
Read the first ever Electronics Weekly online: 7th September 1960. We've scanned the very first edition so you can enjoy it.
Read the very first edition »
Tune into this Xilinx interview: Responding to platform-based embedded design
Tune into this podcast to hear from Chetan Khona (Director Industrial, Vision, Healthcare & Sciences at Xilinx) about how Xilinx and the semiconductor industry is responding to customer demands.
By using this website you are consenting to the use of cookies. Electronics Weekly is owned by Metropolis International Group Limited, a member of the Metropolis Group; you can view our privacy and cookies policy here.