Power draw of the ER-301

Out of habit I made sure to switch this module to its 5V setting via the bus. What I’m wondering is what is the 5V draw and is there a change with the 300mA +12V and 20mA on the -12V?

Here are the current consumption measurements for when the 5V source is set to BUS:

120mA @ +12V
20mA @ -12V
320mA @ +5V


Am i correct in assuming that the BUS setting is the more appropriate if one has a 5v rail?

It won’t matter either way to the ER-301 because all rails are regulated and filtered before they get to any sensitive circuitry. The switch is really just there to give you options. The many variables in a eurorack power system make it exceedingly difficult to give hard rules but here is a try:

Is your 5V rail produced by simply linearly regulating down the 12V rail?

Yes - Then use the REG setting because the regulator inside the ER-301 will be much more efficient and produce a lot less heat. Stop here.
No - Keep going.

Does your 5V rail have at least 320mA of unused capacity?

Yes - Then use the BUS setting. Stop here.
No - Keep going.

Does your 12V rail have at least 300mA of unused capacity?

Yes - Then use the REG setting. Stop here.
No - You need a bigger power solution for your case. Stop here.

Do not switch the 5V source while the module is powered up. You might cause arcing in the switch which will reduce its operating life.


Thankyou for the detailed response Brian,

As you may be aware from the name/emails on your file, I’m yet to receive my 301, but it helps to have such clear and concise information in preparation.

I am unfamiliar with this distinction. Sounds like something that should be available from psu specs, but I don’t remember reading about it.

I initially was going to mention that, but then realised that my 5v current on both cases was 300ma or under, so my question moot!

If your 5V rail is derived from regulating down the 12V rail then it will usually be a feature of the distribution board that you are using. The most common example is the TipTop Passive Bus Board:

(Red arrow indicates the linear regulator that is generating 5V from the 12V rail.)


Linear refers to a method of voltage regulation that produces a very clean output but at the expense of wasting a lot of power. In the case of regulating 12V down to 5V, a linear regulator will “throw away” 60% of the power as heat. It’s counterpart is a switching regulator which produces a less clean output (in relative terms) but at a much higher efficiency (usually around 80%, so only 20% is wasted as heat). Also, since the 5V rail is used for powering digital circuitry the cleanliness of a linear regulator is not necessary.

I’m also a fan of having the regulator as close as possible to the component(s) that are consuming the current. It put’s less strain on your ground path. So I personally tend to always just use the REG setting. However, if you have good quality grounding (between modules and PSU) then this is a non-issue. See how as you get into it the specifics the variables kind of explode? :grin:

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Again, many thanks.

Another concise and very helpful explanation for me. After years of living on a marginal solar electric system I understand the inefficiencies of changing currents from one voltage to another. It always irked me to be converting 12v DC to 240AC via a noisy modified sine wave inverter to plug in a laptop in order to convert the voltage back to 13.8V DC!

As with solar power to a non engineer, the details of Eurorack powering pretty far from simple to the uninitiated!

Long and the short of it for me, I think, stick to REG.

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Re: this on the wiki: “If you plug in a module other than the ER-301 backwards then the +12V, +5V, and ground rails will all be shorted together.”

I’m interested in knowing if this is the case with all bus boards, even the current production Make Noise board which is reverse power protected? I’m not sure what reverse power protection means for this board technically speaking–as in where specifically the failsafe feature lays in the electrical path of the system, and how that may or may not affect my ER-301 if I accidentally plug a different module in backwards. Looking to access the 5V rail in my case in the coming weeks and it would be nice to find an answer on this one. :deciduous_tree:

ModularGrid has the ER-301 listed as 300mA on the +12V rail, 20mA on the -12V rail, and no 5V.

Is that correct for the Rev 10 hardware which doesn’t have the 5V bus switch?

I’m about to move everything into a new case and am very close to the -12V capacity according to ModularGrid, and am planning a couple of changes for safety’s sake. But I wanted to make sure these numbers are correct :slight_smile:

Doesn’t quite agree with the wiki, but close.


I put an email intio Pittsburgh tonight for clarification concerning the now not manufactured but in circulation MOVE PGH.

At the mo I have the 301 running the REG config but wonder, would it be better, considering the 5v draw, running the BUS mode. This would free up draw on the 12v rail. Seems the 301 like a lot of digital modules prefers a 5v rail. As long as I don’t accidentally incorrectly plug any other modules onto a rail , no harm will come to the sound computer. That’s the thing…

The Pittsburgh MOVE 104 steps down the 15v from the brick to the +5v. It does not linearly step down the 12v rail. That’s what they said. And it provides 500mA on the 5v rail. So, based on the series of question answers above, I should switch to the BUS. Gulp!