Question on powering my analog and digital circuit - Page 1
Forum section: electronics / projects

author: EricNava
author_rank: Contributor
timestamp: February 17, 2021, 02:06:44 am
content: Hi all, I have a project I am starting, and I would like some advice as how to power some components.I am working on my own soldering station to use JBC tips, so the basic operation will be to measure the thermocouple in the iron tip using an instrumentation amplifier, and also control the ac power delivered to the heater in the tip with a solid state relay.The station will use a mains transformer with dual 24V secondaries. One secondary is to power the heater cartridge and another to power the station electronics.I plan to rectify and smooth one of the 24V secondaries and then regulate it down to the logic level voltage.I am trying to decide on the overall best way to power the electronics of the station. I am mainly concerned with power supply noise and EMI. Since the instrumentation amplifier is a sensitive analog device, I want to minimise possible sources of noise.Normally I would use a SMPS to convert 24V down to the 3.3V for the electronics, but I am worried the noise from the switching would cause problems with the amplifier.(I know that layout plays a huge part in the EMI and noise of a switching regulator, but even good examples still produce more noise than a linear regulator from my understanding)Is that correct, or am I worrying over nothing?I could use a linear regulator for the 24V to 3.3V conversion which I think would produce much less noise, although that would be very inefficient. Especially since the display I will use can draw about 700ma at 3.3V.Another potential option would be to have another small transformer for 24V AC to around 5V AC and then rectify and smooth that and use a linear regulator for the 5V to 3.3V. That would make the efficiency less of a problem, and avoid using a SMPS.Perhaps just using a well laid out step down converter like thishttps://www.ti.com/product/LMR36015would not cause major issues.The instrumentation amplifier I want to use is:https://www.ti.com/product/INA333

author: bob91343
author_rank: Super Contributor
timestamp: February 17, 2021, 05:22:09 am
content: I agree that you should use a linear regulator from a lower voltage source, be it transforming the 24V or the main power line.

author: EEauroro
author_rank: Contributor
timestamp: February 17, 2021, 08:20:03 am
content: With 0.7A current draw, using an LDO to drop 33V (after rectifying 24VAC) to 3.3V will result in 21W of power dissipation. So choose an appropriate heatsink for the LDO.Another approach is to use a SMPS to step down 33V to 5V and use an LDO to drop 5V to 3.3V to power your analog circuits. Thank would be my approach. You can add additional filtering if needs be (ferrites/inductors). Select those that have good performance at the operating frequency of your SMPS

author: EricNava
author_rank: Contributor
timestamp: February 17, 2021, 08:56:05 am
content: Quote from: EEauroro on February 17, 2021, 08:20:03 amThanks for that suggestion, I will likely do that. I will also have a look at transforming the 24V AC down to lower voltage and using a linear regulator for the final stage. Depends on how much small transformers cost vs a DC step down circuit.Quote from: bob91343 on February 17, 2021, 05:22:09 amYeah seems like using a linear regulator is the better idea