Power supplies for modern MCU Geiger counters

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8 months 6 days ago #6820 by Juzzie
This may help - https://www.arrow.com/en/research-and-events/articles/using-capacitors-to-filter-electrical-noise

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8 months 5 days ago #6821 by FSM19
I've had similar problems in the past. The input impedance of the counting circuit is high, and tiny spikes on the power supply can make things go haywire. I now use  a linear supply, the mains input is filtered. Then a conventional 12 V stabilised circuit, followed by a 7805 with more filtering than, shown on the datasheets (100nF + 100uF across the output and a small common mode choke - not sure of the value I wound it on a salvaged ferrite  toroid of unknown properties). This set up has been running for more than a year now.
 

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7 months 3 weeks ago #6825 by njc
You would definitely benefit from a better-filtered PSU or a linear, however you might try some series-L, shunt-C low pass filtering. Firstly though, you want some capacitance on the input and the output of the 7805 if you don't already. I like to put say 47-100uF and something smaller in parallel like .01 or .1 to help with RFI too (I'm a ham so there's always RF around!). If that's insufficient, you should try some series inductance in between shunt caps right at the input to the 7805 (a 'pi' filter). For these types of projects a fair amount of noise is usually acceptable, unlike with audio circuits.

Of course if your DC earth (we call it "ground" in the US) is tied to the AC mains then that could be the root cause of all of this and not the ~60mVpp ripple. You might be able to test this by operating on a battery and tying the DC (-) to your mains ground (er, earth) or the neutral.

Cheers!
Nick W1NJC

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7 months 3 weeks ago - 7 months 3 weeks ago #6826 by Simomax
I am going to build a nice linear PSU for small electronics. I just want a simple and reliable solution for 5V and 12V at 1 amp each. I have all the parts, it's just a matter of digging them out and having some time. I can't be bothered fannying about with crappy SMPs and making filters and things to make good of that which is essentially, bad. And will probably only last 3-5 years before it go pop. I had one go up in smoke a few weeks ago when trying different power supplies. It was plugged in, working, and after a few minutes I could smell it, and then I saw a bubble appearing on the case. It was one of the two(?) 400v smoothing caps had gone dry, heated up and melted the plastic on the inside. I think it carried on working as there was another smoothing cap on the mains side. I don't like cheap SMPs. Never really have done, but certainly don't now. There are some good SMPs though, but they are expensive and usually too big for small electronics. By Big, I mean I don't need 5V at 20A for running a couple of Geiger counters. One amp is enough.

     

 

I was going to throw a linear PSU together a couple of weekends ago, but the transformer I was going to use is too high a voltage so when rectified and smoothed it resulted in about 29vdc. That's too high for my liking to go into a 7805 and a 7812 and would just cause unnecessary heat. I have a bunch of old transformers in the workshop, but the workshop is like a crashed spaceship at the moment, so getting to things is a matter of moving lots of other things. Its a bit like one of those slide puzzles where you only have one square empty. It's a pain. I need more room!   My plans are set on a linear. Quite basic, just the transformer, bridge rectifier, decent smoothing cap, 2500uF - 3500uF should be nice for a couple of amps at 50HZ ripple. So then that right into a 7805 and a 7812 with some small ceramics to remove any small noise around the regulators. And that is about it. I will get around to it soon, after I have finished being so busy....
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