Overloading a power supply?


#1

Hi everyone…I saw a vid where a guy explain,“Dont overload you power supplies” (in a eurorack case) …so:my question for us here is : what can happen to the modules if the PSupply is overload,i mean if theres not enough power for the modules? Are they all gonna badly work or gonna do stupid things? …Thanks you a lot!


#2

For the sake of simplicity, let’s only focus on the +12V rail. Let’s say your power supply is able to deliver 10W of power for this rail. Your modules are drawing 1000mA. This would require 12W of power.

Depending on the power supply:

  • Case 1: a fuse blows somewhere. The designer of the PSU could have “hardcoded” the 833mA (= 10W / 12V) limit somewhere with a fuse.
  • Case 2: your power supply just overheats, with the risk of burning components and reducing their lifespan (especially electrolytics).
  • Case 3: the output voltage is reduced to stay at the maximum power limit. In this case, this would correspond to an output voltage of 10V.
  • Case 4: (with transformer/rectifier/regulator linear power supplies, reaching the maximum capacity of the rectifier’s caps). The output voltage is no longer a constant signal, but a cut-up piece of sine-wave going between 8V and 12V, with an average value of 10V.

What would be the consequences of an incorrect output voltage?

Digital

The MCU and the digital section of digital modules run at 3.3V, usually generated through a linear or switched-mode regulator embedded in the module. In this case, running the module on 10V or a wobbly voltage will have no adverse effect: the linear regulator would simply dissipate less power and would happily generate 3.3V (cool!), the switched-mode regulator will switch with a different duty cycle and will maintain a 3.3V output.

Op-amps and co

In a gross approximation, op-amps, OTAs and integrated VCAs (2164s) are current-steering devices: their behaviour is fairly independent of their supply voltages (as long as the supply voltage is in the right range to allow their internal current sources to operate). A TL07x can run from +/- 3.5V to +/- 18V. What happens when you run them on a lower voltage (say +/-10 V instead of +/-12V) than planned by the designer? You simply get less headroom. This might cause clipping. For example, Plaits’ output is roughly between -8V and +8V. If you run it on +/-5V, you’ll get a clipped waveform!

The devil: references!

And now the thing that actually causes problems: voltage references. There are some circuits (such as the V/O scaling front-end of an analog filter, or the amplifier that converts a CV to a narrower range so that it can be processed by the ADC of a microcontroller) that require a source of a fixed, known DC voltage, say -5V.

There are two ways of getting this DC voltage.

The “wrong” way (Doepfer, many cheap modules) is to just use one of the supply rails and a voltage divider. In this case, if the supply voltage is tainted by noise, wobbliness, or is at the wrong level, the circuit will not operate normally. In the case of a VCO or filter, the range could be shifted (say it’ll go from 0.2 to 200 Hz instead of 20 to 20kHz). With an overloaded rectifier-based power supply, you’ll hear the mains frequency modulate the filter or VCO frequency!

The “correct” way (Mutable Instruments) is to use voltage reference ICs. This eliminates the noise, and allow the circuit to withstand variations in supply voltage (say 11.5V or 12.5V instead of 12V). However, this is not completely robust. Many of my modules use a -10V voltage reference, with a current setting resistor computed for a minimum supply voltage of -11.5V. As long as the supply voltage is below -11.5V, the reference generates a perfect -10V and is not prone to any noise or wobbling. But if the negative supply voltage goes above -11.5V, the voltage reference will no longer deliver -10V and the same problem (shifted or modulated parameters) will occur. Now the $1000 question: why don’t I use -5V references and don’t use a lower current setting resistor to tolerate a wider range of variations on the negative supply? The reason is that it would waste more current on the -12V rail in normal operating conditions.


#3

Would there be any “easy” way for digital modules to read the reference voltages, and give a warning. Guess it would just a require an extra adc channel.
I could imagine it being a nifty little feature in modules, since most people are completely working blind, when it comes to power issues.


#4

Yes it would be doable, but I would argue that this should be the job of the power supply (shut-down or warning when the maximum current load is reached).


#5

Waaow! Thats gold plated answers for me! thank you…!


#6

What happened when I overloaded my Makenoise Shared System Case (not very hard to do) was there was random unwanted bursts of noise, modules sounded weird, and I had suicidal thoughts. Definitely overspec on power when you can.


#7

Another problem can be that one rail (e.g. the 12V rail) can’t deliver the required power and a fuse blows - but the other rails (-12V and 5V) stay intact. A well designed module would malfunction but won’t be damaged by this. However, on cheap modules, circuits can be destroyed in this scenario - I remember the V2164 VCA chip will die when only one supply rail is present if it’s not protected by the circuit around it. (Can someone confirm this? I can’t find any info anymore but I remember seeing a discussion about this problem somewhere)

Bottom line: don’t overload your power supply. Calculate the required mA with modulargrid. If anything behaves weirdly, shut down your system and check your power requirements


#8

http://www.electronic-sea.net/SSM2164.html