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Intro to Effectsblog

March 16, 2014
  • Choosing a USB Power Delivery Power Bank

    USB was originally designed as a replacement for the various serial and parallel connectors for computer peripherals. Back in the mid 1990’s, portable computers were becoming more popular and the industry wanted a standard interface that used a smaller connector.

    As well as carrying data, USB also added the ability to carry power. A single cable could be used both to communicate and power a device. In the first version of the USB specification, power was limited to a quite small 100mA at 5V. The power limits of USB have been gradually updated with new versions of the specifications, but they have still been quite limited for modern devices such as battery packs. To address this, the USB standards body introduced a new USB specification called Power Delivery (USB-PD).

    Power Delivery brings two big upgrades.

    1. Maximum power increases from 7.5W to 100W.

    Although the previous generation has an additional specification called Battery Charging (USB-BC) that can support 25W, this isn’t available on all devices. Even then, PD still brings four times as much power.

    2. Variable voltages.

    Previous generations are limited to 5V, and getting to higher voltages from a USB connection required additional boost converters. USB-PD can support different voltages such as 5V, 9V, 12V and 20V.

    Getting here required a couple of significant changes. Firstly, Power Delivery requires a USB-C cable. Secondly, there needs to be some intelligence in the devices, and in some cases the cable too, to negotiate the power features.

    Previous to USB-C, you’ll recall that a USB cable has different connectors at each end. You usually do not see a USB-A – USB-A cable. One of the key reasons for this is to prevent two power sources from being connected together. A USB-C cable is the same at both ends. To prevent two power sources being connected together, PD devices communicate with each other to determine which should provide the power. The specification provides for role switching which means if your device supports it, it can be both a power source and a power sink. Think about connecting a power bank to a laptop. With role switching, the power bank can either power the laptop, or the laptop can recharge the power bank – with the same cable. Nice.

    Voltage is also handled by the Power Delivery negotiation. When you connect a power bank to a charger, the charger communicates it’s capabilities to the power bank. It may say that it can support 5, 9 and 12V and what it’s current limit for each voltage is. The power bank then tells the charger what it prefers. For example, it may confirm that it requires 12V @2A. The charger then enables this and the power bank begins charging at the requested voltage.

    An important thing to remember here is that although the specification supports a range of different voltages and currents, most devices will not support all of them.  100W chargers are still rare, and most battery packs will only support a subset, depending on their size and price point. So when you are choosing a battery or charger, it’s important to check that it can provide the voltages and currents that you need to power your device.

    The Mission 529M is a USB-PD power converter for guitar effects and similar devices. The 529M performs the PD negotiation with a power source such as a portable power bank or wall charger and provides the power on a 2.1mm center pin negative connector. You select the voltage that you want the 529M to provide on the effects pedal power output using a small switch on the underside. The factory default is set to 9V. The 529M can support 5 different voltages and you’ll need to check these against the battery or charger that you want to use.

    • 529 6V – requires USB-PD 5V
    • 529 9V – requires USB-PD 9V
    • 529 12V – requires USB-PD 12V
    • 529 15V – requires USB-PD 15V
    • 529 18V – requires USB-PD 20V

    If we check the specifications for the Mission 529 USB battery pack we see it supports the following voltages on it’s USB-C PD output:

    • 5 V, 3 A
    • 9 V, 2 A
    • 12 V, 1.5 A

    So, with this battery we can select 6V, 9V or 12V using the voltage selector switch on the 529M. This battery does not support 15 or 20V so we won’t be able to use those. If we request a voltage on the 529M that the source does not support, the output will remain at the nearest voltage level below that it does support. In this case if we select 15V or 18V, the output will remain at 12V.

    Also be aware of the current limit. We can see that this battery pack supports 2A at 9V, so we would need to make sure that the total current draw from our connected devices does not exceed that.

    If, we want the 529M to provide higher voltages or more current at the output, we’ll need to use a battery that can support it. The Naztech 60W Super Speed which we also recommend has the following specs:

    • 5V, 3A
    • 9V, 3A
    • 12V, 3A
    • 15V, 3A
    • 20V, 3A

    So this battery would support all of the available 529M voltages up to 3A. It’s larger and costs a bit more though, so you’d need to decide if the extra cost and weight is acceptable. If you are building a small fly rig and don’t use any pedals that require anything other than 9V, then you may choose the smaller battery. If you are building a large pedal board and need 18V or 3A, then the larger battery is likely the better option.

    So before you buy a PD battery pack or charger, just remember to check the specifications to make sure it meets your requirements. Hopefully this arms you with the information to choose the best one for your needs. One of the great things about USB batteries is the features keep increasing and the price keeps lowering, so if your needs change in the future, all you’ll have to do is just swap out the battery.

  • Batteries: Which One is the Best?

    I’m not sure of the reason the de-facto standard for effects pedal power became the 9V battery. Many low current pedals such as buffers, boosts, and distortions could easily be designed to run equally well on the more common AA battery type if we so desired.

    I’ll hazard a guess that history has a lot to do with it. If I recall correctly, the Boss, Electro-Harmonix, and other pedals I used in the 80’s pretty much all used the 9V battery. I imagine this same history has a lot to do with why we are also stuck with the evil center pin negative DC power connector on most pedals. I’m sure Roland must have had a good reason for using this back in the day on the iconic Boss effects, but from a product design standpoint, it’s a pain in the ass.

    Anyway, let’s get started. We are going to try to figure out the best choice of battery for effects pedals and how long they will last in each of our devices. To do this, we will need a few bits of information. Roughly in order of significance, these are:

    1. Battery chemistry
    2. Device current draw
    3. Device cut off voltage

    Battery chemistry defines the chemical makeup of the battery. The most common chemistry types for consumer primary cells are Zinc-Carbon (or Carbon-Zinc, or just Zinc; it’s the same thing), Alkaline, and Lithium. Let’s get a bit of terminology out of the way first. A primary cell is a single-use or disposable battery. These are designed to be used once and then disposed of, preferably recycled. A secondary cell is a rechargeable battery, and we’ll get to those later.

    The naming of the chemistry is all rather confusing. Zinc-Carbon cells do contain carbon, but it’s the reaction between zinc and manganese dioxide that forms the basis of the battery. We really should call them Zinc-Manganese batteries, but nobody ever does. Alkaline batteries also use Zinc and Manganese so they could be called the same thing. However, we call them Alkaline because they use a base electrolyte rather than the acid electrolyte used in Zinc batteries. Lithium batteries use a Lithium anode but are not the same as Lithium-Ion, which are secondary batteries. Got it yet? No?

    Zinc-Carbon batteries are the cheap ones you can buy in boxes of 50 for $19.99 on eBay. They are often called Heavy Duty, or Super Heavy Duty, neither of which means anything. They have a lower capacity than alkaline batteries, resulting in a shorter usable life. The body of the battery is made of zinc and forms the anode. The acid electrolyte eats into the zinc over a fairly short time giving these types of batteries a much shorter shelf life, and they are more prone to leaking. This type of battery is OK for something like, say a TV remote, but best avoided for effects pedals. You can use them in a pinch, but don’t leave them in the pedal unused for long periods.

    Alkaline batteries are the most commonly used in effects pedals. These are the Duracell and Energizer batteries that most of us use day to day.

    Lithium batteries are relatively new and quite expensive. We’ll do some calculations later and see if they make sense to use in effects pedals.

    Current draw is a nominal figure that defines how much current will flow through the device during operation. Depending on the pedal design, this can change during use, but most pedal manufacturers will publish a figure for current draw, and we can use this to calculate our battery life.

    If the pedal uses DC-DC converters, which digital devices usually do, it will have a Cutoff Voltage. This is the point at which the voltage from the battery gets low enough that the pedal stops working. These pedals will work the same all the way down to the cut off voltage, and then just stop. Other types of design may not have a hard cutoff voltage as such, they can continue working but the performance will change. It’s unusual to see a cutoff voltage published in the specs. Fortunately, there are some common industry practices around this, so we can get an estimate for our calculations.

    Head over to your favorite search engine and search for a datasheet on your battery; reputable manufacturers will publish these. If you can’t find your exact make, an equivalent will do. I use the Duracell 6LR61. Find the specs on your pedal from the User Guide or manufacturers web site and look for the current draw. For ease of demonstration I picked a few from the Roland Boss product line, and looked up the current draw on the spec sheets.

    • DS-1 Distortion – 4mA
    • OD-3 Overdrive – 9mA
    • DD-7 Digital Delay – 55mA

    There’s no exact cutoff voltage listed for these so we’ll have to estimate. It’s common industry practice for 9v battery operated products to work at least down to about 7v, so we’ll use that for our calculations. On the 6LR61 datasheet we are going to look for the constant current discharge graphs. Let’s start with the OD-3, which has a draw of 9mA. The red line on the graph is close at 10mA so we’ll use that. Draw a line across from the 7v cutoff voltage until it intersects the 10mA line. Then draw a line down to read off the service hours.

    Life of a 9v Alkaline battery in the OD-3 Overdrive

    From this we can see the approximate life of a 9v Alkaline battery in the OD-3 Overdrive is about 50 hours. Pretty neat. Let’s try some more. The DD-7 has a higher current draw of 55mA. We’ll need to go to the second chart from the datasheet for that. The closest line is 50mA, so again we’ll start at 7v cutoff, draw across to the 50mA graph and then read off the service hours.

    Life of a 9v Alkaline battery in the DD-7

    From this we can estimate about 7 hours life from the same battery in our DD-7.

    What if there is no chart for the current draw of our device? Well we can approximate it by drawing our own line. The DS-1 has a pretty low current draw for an effects pedal at 4mA. Let’s draw our own estimated graph based on the information we have.

    Life of a 9v Alkaline battery in the DS-1

    Here we can see a rough estimate of the battery life in the DS-1 would be about 200 hours.
    Modern programmable digital pedals and multi-effects with DSP’s, micro-controllers and digital displays consume quite a bit more power. A Strymon Timeline for example has a recommended minimum power supply current rating of 300mA, which would give us a battery life of less than 30 minutes. That explains why these types of pedals don’t run on primary batteries!
    A few Lithium Alkaline batteries are available billed as offering twice the capacity. Let’s take a quick look and see if they would be a good choice for effects pedals.

    Comparing the life of 9v Lithium battery vs Zinc-Carbon and Alkaline equivalent

    Here’s a chart comparing the life of 9v Lithium battery vs Zinc-Carbon and Alkaline equivalents at 50mA continuous discharge. If you recall, we used 50mA as our number for the Boss DD-7, so let’s do a quick comparison. The graph for the alkaline battery is probably an average, rather than the specific chart we looked at for the 6LR61 so the numbers are a little different, but they are in the same ball-park.
    This chart shows a service life of around 6 hours for 50mA at 6.6v cut out voltage. We got around 7 hours at 7v on the specific battery model, so it’s close enough.

    The Lithium battery is showing around 15 hours vs 6 hours for the alkaline. That’s 2.5 times the service life, which sounds pretty good. So we should start using Lithium batteries in all our effects pedals and get double or more the life, right? Well here’s the problem: Battery Junction has 9v Duralocks (essentially the 6LR61 in our tests) at $1.15, whereas the lowest cost Lithium 9v are $6. So in our DD-7 we’d get 2.5 times the life for 5.75 times the cost.

    There may be some cases where a Lithium battery makes sense. If you needed to run our example DD-7 on battery for a day at a festival with minimal opportunity to change batteries and it was worth the cost to avoid the possibility of failure? Maybe. You can also see that the discharge curve is much flatter; which means if you have a pedal with a very high cutoff voltage, above 7v for example, the Lithium might make sense, but such products are unusual.

    So there we have it, the old favorite alkaline 9v remains the best choice in most cases. The range of service life is quite interesting. Just with the three pedals in our example, we have almost 30 times difference in battery life. If you need to run your effects on batteries, it’s definitely worth making the calculations to figure out how long you should expect in each device.
    Note that we have a margin of error in our examples. If you need to be more precise, factors such as operating temperature and the battery’s internal resistance need to be taken into account. Devices with voltage regulators will draw more current as the voltage in the battery decreases and this will also impact the figures. A manufacturer’s current draw figure is a nominal value that should be used as a guideline. Even so, these details are only going to make a few percentage points difference. Unless you are designing a pedal for sale and are concerned with optimizing it for energy efficiency, using the techniques here should be quite adequate for most.

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