Fullscreen HTML5Med Res (??MB)HTML4

Advertising Index Altronics.................................27-30 Blackmagic Design....................... 7 Dave Thompson........................ 103 DigiKey Electronics....................... 3 Emona Instruments.................. IBC Jaycar............................. IFC, 51-54 Keith Rippon Kit Assembly....... 103 LD Electronics........................... 103 LEDsales................................... 103 Microchip Technology.............OBC Mouser Electronics....................... 4 OurPCB Australia.......................... 5 PCBWay......................................... 9 PMD Way................................... 103 Radio Valves - Dieter Dauner.... 103 Silicon Chip Back Issues........... 25 Silicon Chip Binders.................. 79 Silicon Chip Pico BackPack...... 78 Silicon Chip Shop........ 71, 94, 100 Silicon Chip Subscriptions........ 31 The Loudspeaker Kit.com............ 8 Used Gear - Jim Rowe.............. 103 Wagner Electronics..................... 93 Notes and Errata Maxwell’s Equations, November 2024: on p91, Gauss’ full name is incorrectly given as Henrich Gauss instead of Carl Friedrich Gauss. Watering System Controller, August 2023: a bug in the WiFi stack used in the original WebMite firmware can cause spurious reboots of the Controller. We recommend you update to the latest firmware version (released January 2025) which fixes this problem. Next Issue: the March 2024 issue is due on sale in newsagents by Thursday, February 27th. Expect postal delivery of subscription copies in Australia between February 26th and March 14th. 104 Silicon Chip controllers for the larger motor, although that would be more expensive than modifying a capacitor start motor (which might not work anyway). Any thoughts? (J. C., Auckland, New Zealand) ● Andrew Levido responds: You have a couple of good options. It seems like you have pretty good understanding of motors and understand the risks in the following advice. You probably can use an unmodified capacitor start motor with a VSD as long as you ramp up past the centrifugal switch opening speed and remain above the closing speed thereafter. I believe there is a fair bit of hysteresis built into the centrifugal switch to prevent it from chattering. You would have to measure the two speeds (closed and open) for each motor; I suspect they are pretty variable between models. Alternatively, you could switch the start winding and capacitor out externally with a suitable switch or contactor once the motor has started. A manual switch or a simple timer would probably be enough to control it. Then you could reduce the speed as far as you like without any risk of burning out the start winding (but you would still need to monitor it to make sure it doesn’t stall). The downside I see here is reduced torque at low speeds. I have no idea how much of a problem this would be for a lathe. Still, I suspect once the work is spinning, the torque required to keep it going is much lower than that required to get it up to speed. However, the speed regulation may be poor at low speed – I honestly don’t know. I think you would have to try it and see. I don’t suggest you switch the motor between mains power and the speed controller, as the instantaneous current demand at switch-over could trip the over-current protection. It’d be better to let the motor be spun up gradually by the speed controller with the start winding in-circuit, then switch it out. Also see the letter in the Mailbag section last month from Ian Thompson (p6, January 2025), who has an alternative method for controlling the speed of induction motors with centrifugal switches. Eliminating hum in 12V 20W Stereo Amplifier I got around to building the Compact Australia's electronics magazine 12V 20W Stereo Amplifier kit (May 2010; siliconchip.au/Article/152) from the Altronics K5136 kit. Overall, I’m surprised how good the sound is, and it can drive the two Klipsch speakers I have quite well. The only thing I’m not sure about is that it seems to have a bit of a ‘hum’ at the loudspeakers when switched on, even with the volume low, if there is no input signal. Where does the hum come from and can I eliminate it? I used heavy duty shielding on input cables. I attached the wire across the three potentiometers, they are grounded together and it’s in a metal box. The potentiometers and wire are grounded to the negative 12V DC input as per the instructions. I notice that when I put my finger on the volume potentiometer, the hum almost goes away. Could it be some sorting of grounding problem? (E. M., Hawthorn, Vic) ● If the design had an inherent hum problem, it would have shown up in the performance measurements/graphs published in the article as a poor signal-to-noise ratio or high distortion. So we’re pretty sure it isn’t a circuit problem. Also, it’s DC-­powered, so there’s no reason for mains hum to be present unless the DC supply is poorly regulated. Based on your description, it sounds like it may be being picked up by the potentiometer bodies or the case. First, did you scrape off the passivation from the pot bodies before soldering the wire to them? Please be careful doing this as the dust can be toxic (eg, do that outside and make sure it doesn’t blow towards you). If you didn’t, it’s possible there is no electrical connection to the grounding wire. If the bodies are definitely grounded, it’s likely the hum is coming into circuit ground from the power supply. Do you have a different power supply you can try? Do you have an Earthed metal object (like a desktop computer case) that you can connect to circuit ground via a clip lead, to see whether Earthing it helps? We also suggest checking if the hum is present without any input signal connected, to verify that it isn’t being injected via the signal ground connection from the signal source. Note: E. M. replied to say that Earthing the negative output terminal of the switch-mode power supply eliminated the hum. SC siliconchip.com.au