+5V direct from power supply?

Hi all - I’m building an Atmega based euro module that only needs +5V.

What is the current common practice for powering it? Directly from the power supply’s +5V or should I regulate down from +12V to keep it safe? I’ve seen older discussions where direct +5V can be a problem (spikes?) depending on the power supply, and I know Mutables are now only regulated down which I wouldn’t mind doing. I personally use an Intellijel tps80w with a +5v that works fine.

Any advice which way to go?


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If someone else is going to use it, and particularly if you’re going to sell it, you should probably get your 5v from 12v, otherwise you can do whatever you want.

If you’re fine with a disclaimer about requiring stable 5v power, then that’s a possibility.
But I doubt that many people know how stable their 5v rail is.
I’m quite confident in my 5v rail, but I obviously can’t be confident in anyone else’s rig, since I didn’t build that - so if I was selling a module that would be destroyed by a unstable 5v rail, I’d probably go for getting it from +12v, or -12v. This does increase the power drawn from the 12v rail though.

If you want to use the +5V rail, put a transorb between the +5V point and ground (Vishay SA5.0A for example). Do it, or in the long term, all your modules are going to be killed, mostly by µZeus. This is a serious problem that cost me above 10k€.

The inconvenient of using the +5V rail (even with the protection):

  • Bigger connector
  • Reverse polarity protection is trickier (to make it really work you need to leave some GND pins unconnected, otherwise you can’t prevent some supply rails to be shorted to ground when the connector is plugged backwards)
  • There are many power boards without +5V.
  • There are many power boards with an underpowered +5V that can’t even be replaced or disconnected.

It’s sad, because I love the idea of a dedicated supply for digital modules - it’s just not implemented in a reliable and consistent way in Eurorack.

There are modules (4ms, noise engineering) with a jumper on the back allowing you to select either the external +5V rail, or an internally generated +5V. That’s a nice solution too if you’re willing to assist customers in deciding on which choice is best.

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What’s the problem with the µZeus? I’ve read about problems before, but noone seems to actually say what’s wrong with it. And the high ratings on different web shops don’t give any useful hints as well.

I’m guessing spikes on the 5V rail?

I don’t know exactly what in their design causes the problem. At first I thought it’s just because it’s the most popular - but in the case of fried Grids the ratio of µZeus to other PSUs is above 10 to 1 and I don’t think this accurately reflects the market, so there might be something specific to it.

The issue is a short transient above +5V when the system is powered on.

@pichenettes : Out of curiosity, did you encounter the same issue of frying modules from a spike with Meanwell PSU’s ? I’m in the middle of building this which is just a CD4013 and a LS14 directly powered from the +5 rail.

No report of fried Grids with meanwells. But maybe because they are not very common?

OK, thanks for all the advice. I’ll just regulate down from 12v. It’s a shame having 5v right there and not using it…

Interesting about the µZeus. If any of you are members of the Synthtech Modular group on Facebook, Paul recently wrote a detailed post on what he thinks is wrong with its design. It seems many manufacturers have suffered from it.

@Olivier, which MI modules, apart from Grids, use the 5V Euro buss?

I have 2x uZeus in my system, and my Grids is still alive and well, but there have been occasions when it doesn’t appear to be working, on turning on my system.

Could be a seperate issue though.


I wish there were more modules with a jumper to chose to use the +5V rail.
There are more and more PSUs which support +5V and at the same time it seems less and less modules are using it.
Most of the modern uController are using 3.3V or less and you have to regulate the 5V down to 3.3V. So I think the danger of frying the uController is not as high as with the old designs where the 5V is going directly to the uController.

Somewhat related since we’re talking of 5V and Grids, what is the purpose of the large concentration of ground vias below LM1117? Just so that more ground signal passes though? Branches has that too.

All MI stuff now has built in 5V regulation sourced from 12V, None have the need for a 5V supply.

Besides the electrical connection of the ground planes, I’d guess the the vias provide some thermal coupling for better heat dissipation also.

Deciding if/how to use +5V rail has been bugging me recently too.

6581punk I know. But some of the older modules did use the 5V supply.pichenettes has mentioned Grids was one of them, but I’m wondering which other ones might also have done the same.


> @Olivier, which MI modules, apart from Grids, use the 5V Euro buss?

Braids and Edges (made before 2015) also used the +5V bus, but not directly - it was regulated down to 3.3V.

> what is the purpose of the large concentration of ground vias below LM1117?

They help spread to the other side of the board the heat from the regulator (and absorbed through the ground plane).

@pichenettes so they’re not so vulnerable to 5V spikes, whereas Grids used the Euro 5V unregulated (because it has a 5V MCU, I guess)?


Yes, in Grids the +5V goes straight to the MCU, and it doesn’t tolerate much in terms of overvoltage, even transient.

Thanks for pointing me to the SA5.0A. I’ve made an adapter board that converts any digital module to +5V bus, protecting it from overvoltage.