Add VoltMeister 100 part 1

content/posts/2014-11-24-voltmeister-100-atx-bench-power-supply-part-1.md

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+---
+title: "VoltMeister 100: Bench Power Supply - Part 1"
+created_at: 2014-11-24 16:05:04 +0100
+kind: article
+tags:
+  - electronics
+  - voltmeister
+  - power supply
+summary: |
+  A bench power supply is one of the essential tools of any electronics hobbyist.
+  Although you can buy a such a unit for less that € 50, it's way more fun to build 
+  one yourself.
+---
+As an electronics hobbyist one of the most essential tools is a bench power
+supply. I don't have one yet, so I'm currently stuck with using simple wall warts. 
+This is fine for powering an Arduino, but it gets more tacky when dealing with things
+like op-amps. 
+
+Because I only have limited experience with Arduino and a rudimentary understanding
+of electronics, I decided not to make my first project about using 230VAC directly
+and creating an adjustable power supply. Instead, I opted for a more common approach:
+use an existing ATX computer power supply to deliver the four common output voltages
+of these supplies: 3.3V, 5V, and ±12V.
+
+This is Part 1 of a two part write-up of how I designed and built my first bench
+power supply.
+
+ * [Part 1 - Design, schematic and PCB](https://ariejan.net/2014/11/24/voltmeister-100-atx-bench-power-supply-part-1)
+ * Part 2 - Enclosure and finished build - _coming soon_
+
+## The ATX Power Supply
+
+Modern computers all comform to the ATX standard, set by Intel in 1995. Part of this 
+standard is the _power supply_. ATX power supplies are easily available and interchangable.
+
+Older models ATX power supplies have a 20-pin connector, new ones have a 24-pin connector.
+
+![24 Pin ATX Connector](/img/24-pin_ATX_power_connector.jpg "24-Pin ATX Connector. Image from Wikipedia.")
+
+The 24-pin configuration adds four pins, one for each supply voltage and GND (Ground).
+A 20-pin connector does fit a 24-pin socket. Yay for standards!
+
+An ATX power supply offers a total of four voltages. However, they each have different power
+ratings. There's probably a sticker on your supply telling you the exact details.
+
+![Asus Power Supply Ratings](/img/asus-atx-power-supply-ratings.jpg "Asus SL-22A Power Supply Ratings")
+
+The supply I'll be using in this project comes from an old Asus S-Presso. As the
+label indicates, it can supply different voltages with different power ratings:
+
+ * 3.3V at 17A
+ * 5V at 13A
+ * 12V at 16A
+
+That's quite beefy (except the -12V), especialy if you're mostly toying around with 
+microcontrollers and microprocessors.
+
+There are also two other notable voltages. -12V and 5VSB.
+
+The -12V is ideal if you're doing things with op-amps that need a positive and
+negative reference. But, as you can see, the -12V rail only supports
+up to 0.3 amps. This means you can't combine the ±12V rails to create
+a 24V output that draws lots of current.
+
+The 5VSB provides 5V at a maximum of 2A _when the power supply is on stand by_.
+When you have the power supply plugged into mains, but it's not running, the
+power supply still delivers 5V over this _stand by_ line! Your computer normally
+uses this to wake from sleep and such things.
+
+## Design goals
+
+I've set the following design goals for the VoltMeister 100:
+
+ * Use a small-size ATX power supply (Asus SL-22A)
+ * Output 3.3V, 5V, 12V and -12V
+ * Show stand-by and power-on conditions
+ * Limit output current to 2.5A
+ * Build a nice enclosure so it can sit on my bench :-)
+
+The ATX power supply does a good job of limiting current and shutting down in 
+short-circuit situations. However, 16 amps on 12V is quite a lot of power
+and will probably cause a lot of ICs to release their magic smoke. Since there
+is nothing I can think of right now that'd need more than 1 amp from this supply,
+I'm want to limit the output current the somewhat safer level of 2.5 amps (for each
+voltage).
+
+## ATX Connector Pin-out
+
+ATX Power supplies have a 20-pin or 24-pin connector. These are compatible
+connectors and the 20-pin connector fits the 24-pin connector.
+
+![24 Pin ATX Pin-out](/img/ATX_PS_signals.svg.png "24-Pin ATX Connector Pin-out")
+
+Let's walk through these quickly. All `COM` pins are common ground and can be used
+with any supply voltage. 
+
+ * Pins 1,2, 12 and 13 provide 3.3V
+ * Pins 4, 6, 21, 22 and 23 provide 5V
+ * Pins 10 and 11 provide 12V
+ * Pin 14 provides -12V
+ * Pin 9 provides 5VSB, always available when the supply is connected to mains
+ * Pin 16 is the power switch, connect it to GND to turn the supply on.
+ * Pin 8 (PWR_ON) supplies 5V when the power supply is on _and_ providing stable voltages.
+ * Pin 20 is marked as _not connected_. In the past the ATX spec placed an optional -5V here, but has since been removed all together. Don't rely on -5V to be available.
+
+The most important thing to note here is the `PWR_ON` pin. It supplies 5V when 
+the supply is providing a stable output. `PWR_ON` does not come on instantly, as 
+it takes some time for the ATX supply to stabalize. Although this process is quite
+fast, there is a noticable delay of about half a second between switching the supply
+on and the `PWR_ON` going high.
+
+## Schematic
+
+All the hard work of converting mains 230VAC to a more suitable 3.3/5/±12V is done by
+the ATX supply. The only custom things left to do are:
+
+ * Accept a 20 or 24 pin ATX connector
+ * Provide a connection for the power-on and standy-by LEDs
+ * Provide a connection for a power switch
+ * Provide connections for 3.3/5/±12V outputs and GND
+ * Limit current to 2.5A for each output voltage
+
+The schematic for this is rather straight forward:
+
+![Schematic](/img/voltmeister-100-revc.png "VoltMeister 100 Rev C Schematic")
+
+The power LED is directly hooked up to pin 8, `PWR_ON` with a series resistor to limit
+the current through the LED and make it not annoyingly bright. The same goes for the stand-by LED. 
+
+The power switch is directly connected to pin 16 and GND, pulling pin 16 low when switched on.
+
+The four output voltages, together with GND, are routed to output pins so they're easy to hook
+up to the actual banana sockets in the case.
+
+Each supply voltage is fitted with a 2.5A resettable fuse (or PTC). Anything up to 2.5A is fine, 
+above that the fuse will start to act as a circuit breaker. When the faulty situation is resolved,
+the PTCs will reset and you use the supply again.
+
+_Note: you may notice there is a 2.5 amp PTC on the -12V output. That's weird because the 
+power supply limits this voltage to 0.3amps anyway. Keep in mind that other power supplies
+may be rated for higher current on the -12V rail and will need this PTC to keep me safe._
+
+## PCB
+
+Because the ATX connector (MOLEX 39-28-8240) uses a 4.2mm pitch spacing it does not fit 
+on a protoboard. So, let's design a PCB! Here's my the C revision of my PCB design:
+
+![VoltMeister 100 PCB](/img/voltmeister-revc-pcb.png "VoltMeister 100 Rev C PCB")
+
+I've created wide traces for the supply voltages, as up to 5A needs to be able to
+flow through them. (5A is the trip-current of the PTCs). Each voltage, together with
+GND is exposed through screw terminals. There are pinheads for the LEDs and the switch.
+
+I've opted for SMD parts because I wanted like to give SMD soldering a try.
+
+## Lesson learned
+
+ * ATX power supplies are easy to work with, but also quite powerful. Be very careful if you open one up.
+ * I should mark the +/- for the LED and terminals more clearly
+ * Consider the connection points on the PCB, it turned out that it would have been easier if the LED/Button connections were on the same side as the voltage output terminals.
+
+## Round-up
+
+In this first part you've learned about the ATX power supply I used for my first 
+bench power supply project: **VoltMeister 100**. With the standard ATX connectors
+it's easy to design a simple schematic to let everything behave correctly. 
+
+From the schematic it's a short way to designa simple PCB.
+
+Next up it's ordering all the necessary parts, soldering everything and putting
+it in a nice case. More on that in Part 2 of this series.