[sdiy] Module power - regulated or filtered (passively)?

[rsdio at audiobanshee.com]

Great answer!

Schottky are great because they have a low forward voltage drop, and that allows more headroom.

I believe that I’ve seen MOSFET transistors for protection, but how would they be used against polarity reversal? Does anyone have example circuits?

Since some of our favorite SSM and/or CEM chips get very unhappy with only one of the bipolar supply voltages present, I think it’s possible to use MOSFET switches to shut off one supply when the other one is missing. If anyone has circuits for this solution, it would be fun to review them.

If a series resistor for filtering is appropriate, then it seems like it might also serve as protection against over current and/or over voltage. There are comparators that say all you need to interface inputs to higher voltages than the supply rails is to put the appropriate series resistance in line. Of course, that’s signal protection, not supply filtering.


All things considered, I suppose that part of my question is: “How much can we depend on the regulation of the Euro power supply?” I often wonder why we really need to re-regulate (other than the fact that cheap supplies might bring out the worst in all modules).

Brian


On Jan 15, 2020, at 5:53 AM, Spiros Makris <spirosmakris92 at gmail.com> wrote:
> Hello everyone,
> The answers given are mostly true by themselves, but all those methods address different problems associated with a power supply section.
> So, trying to make the distinction here, the main tasks you need to accomplice so that the module stays safe:
> 1. Guard against inverse polarity. Just a shrouded connector is not a robust solution, you need electronic protection against it. MOSFET transistors or shottky diodes placed in series are the best options in most cases
> 2. Protect the module from an overcurrent condition. That could be a user caused sort circuit or a failed part of the system, which is not caused by reverse polarity. To be fair, in most sanely designed modules that is unlikely to happen, so it could very well be omitted. AFAIK, most commercial modules don't even bother. Fortunately, you can estimate the maximum safe current draw with a fair amount of certainty, so you can pick an appropriate fuse (most probably rated under 200mA) if you so wish.
> 3. Protect from an overvoltage condition. Again, this is a very unlikely fault in most modular systems that follow one of the defined standards. Even then, most of our electronics go up to 36V, therefore you need a major screw up for things to smoke.
> 
> 
> The functions that relate to how well your module will operate (regarding noise, oscillator drift etc) are:
> 1. Provide regulated and reasonably "clean" supply voltages for the bulk of the system's circuits. That includes opamps or other ICs that benefit from maximum headroom.
> 2. Provide regulated, "clean" and  accurate  voltages that will be used as a reference (tuning controls etc) or supplied to circuits with high supply noise sensitivity. Usually those are 2.5V-5V.
> 
> A supply voltage is a DC voltage + various AC stuff you don't want there, like 50/100Hz hum, noise from the regulators, high frequency ripple ("hum") from switching mode regulators, slow fluctuations because of temperature drift or other factors. Your goal is to put all of the problems within spec, ie suppress them enough that they won't be noticed.
> You should regard any output that doesn't come from a circuit dubbed as "precision" a voltage that is only reasonably stable and with at least some AC noise. So your case power supply might say 12V and come from a 7812 regulator, but you should never assume it to be exactly 12V (or any other measured value). 
> 
> Opamps generally don't require much in terms of supply filtering because they have power supply rejection (called PSRR- power supply rejection ratio). Slowly drifting DC is no problem to them either because the gain and bandwidth of various stages is always set in place with passive components and is not depended on the power supply. Of course, the supply voltage has to be larger than whatever you want to amplify. Other than correct bypassing of each IC, it's rather pointless to do more for them in most scenarios. If the board includes +/-10V (or 9V) regulators for other reasons, it will indeed be better to use those. Otherwise, the benefits hardly justify the cost in board surface, heat generation and efficiency loss.
> 
> Single-ended transistor circuits will always be prone to supply noise. It's always a matter of how much it matters in that specific context and how much that noise will be amplified. For example, a single transistor overdrive circuit will have a gain of 1 for supply noise. Essentially, whatever ripple there is in the supply will be passed to the output intact! Now in such cases, a long term stable voltage is of little importance- what really matters is that we keep the frequency of those fluctuations low enough that they will stay unnoticed (you really can't hear an 12V go to 11V within one hour or more). Therefore, we have two options: we either use a regulator IC or a passive filter and our goal is to filter frequencies over 50Hz, which is the lowest frequency of ripple/noise that could appear in a system. Generally speaking, on low current draws (<10mA), a simple resistor and capacitor can be satisfactory, but comes at a considerable cost in terms of size, because a low resistor means you have to use a pretty big capacitor for this to have any point. However, it can theoretically have very low voltage drops. I would argue that in a system with 24V total supply voltage headroom is hardly an issue, so using the regulator IC is the best choice in technical terms for most cases I can think of.
> 
> Control potentiometers and circuits related to CV are the most important and usually the most sensitive. Unlike amplitude, you can detect even slight detuning very easily, and the result can be detrimental. You have to both provide a precise voltage (a "reference"), and a clean one (low noise, in any frequency). Common examples are the tuning knob of a VCO, the Vee supply of an OTA (especially when biased with a resistor and not a current source), the Positive supply of an exponential converter made with NPN transistors (supply fluctuations are translated to reference current and pass directly to your VCO output frequency) etc. Those are all cases where long term drift causes very audible results and makes you retune you oscillators or appears as FM/AM modulation.
> To set things straight, a resistor and a capacitor can only suppress some high-frequency components but don't regulate at all. Common voltage regulators don't do the trick either because they are not designed to be stable over temperature, or over time. Your 5V reg can read 5.1V tomorrow and still be within spec. Instead, you have to use a reference voltage generator, that is advertised as "precision" and look at its spec sheet. There you can find numbers that justify if it will be good enough, or you need to aim for something even better. Depending on the IC used, those voltages could also supply whole circuits and not just control or other references. Always check with the datasheet to verify that the current draw you need is compatible with the stability you are asking for.
> 
> There are many dimensions to this question and one has to get a grasp of how a circuit is affected by a supply voltage. You can simulate for that by putting an AC source in series with your DC voltage supply, grounding the inputs (if there are any) and scoping the output.
> 
> Spiros
>