AC adapter for use with sensors

Other Hardware Sensors
1

Does anyone know of a source for a small U.S. style three prong AC to 9VDC adapter to power Arduino boards. It should have a 2.1 mm barrel connector.

I would like the AC ground prong connected to negative DC. This is how larger three prong adapters work with devices like notebook computers.

Common two prong 9V adapters are isolated from AC but "float" with almost 50 VAC if you put a scope probe on the barrel. This means it is necessary to ground the Arduino for many sensor applications.

Providing a ground is often a pain and a three prong grounded AC adapter would be much better.

I have even found that touching the USB connector on the Arduino board can cause I2C errors when it is powered by a two prong adapter.

I used an Uno with an Adafruit Data logging shield for tests. I powered the Arduino with a 9V two prong adapter.

I touched the outer part of the USB connector with a ground wire several time and the following sketch fails with an I2C error, red LED lit, after an average of ten touches.

#include <Wire.h>
#include <RTClib.h>

uint8_t const GRN_LED_PIN = 2;
uint8_t const RED_LED_PIN = 3;

RTC_DS1307 RTC;

DateTime now;

int32_t lastTime;

void setup() {
  pinMode(GRN_LED_PIN, OUTPUT);
  pinMode(RED_LED_PIN, OUTPUT);
  Wire.begin();
  RTC.begin();
  now = RTC.now();
  lastTime = now.unixtime();
}
void loop() {
  int32_t t;
  do {
    now = RTC.now();
    t = now.unixtime();
  } while (t == lastTime);
  
  if ((t - lastTime) != 1) {
    digitalWrite(RED_LED_PIN, HIGH);
    while(1);
  }
  lastTime = t;
  digitalWrite(GRN_LED_PIN, t & 1);
}
2

Bump up -

I'm interested to see if someone doesn't have an answer here, too.

3

This is total overkill, but it does provide a 3-wire AC connection.
Need a 9V regulator with it.
http://www.mpja.com/12V-17A-Phihong-Power-Supply/productinfo/18091+PS/

This one appears to have a 3-wire AC connection also.
http://www.mpja.com/12V-35A-Ilan-Power-Supply/productinfo/16741+PS/

4

After a search I found this three prong 9V adapter PSAA18U-090-R Phihong | Mouser Europe.

I have ordered one and will post an evaluation after it arrives.

5

I'd be interested to find out if the ground on that Phihong adapter is a pass-through or not. I.e. if you connect a ohm-meter to the outside metal contact of the barrel and the ground pin on the incoming power connector on the brick, do you get a low-resistance connection or not?

A somewhat related question is: Why do switchmode power supply manufacturers frequently eschew providing this sort of grounding? Is the floating voltage something you can ignore as long as the device is sufficiently insulated / the currents are sufficiently low?

I agree that being well-grounded is a good thing... many not so fond memories of older metal Apple laptops (like the titanium G4) giving me zaps continuously if I was well-grounded and only using a 2-prong adapter on the power brick. Switching a a grounded power cord always solved that problem.

6

The Phihong adapter arrived from Mouser.

It has a pass-through ground.

Voltage is 9.075 volts open and 9.070 volts measured at the connector when plugged in to an Arduino. I bought two adapters and the second one is 9.027 volts, as measured by my bench meter when connected to the Arduino.

My meter is good to 0.025 % for VDC and I had it calibrated recently.

Very low ripple and noise on a scope.

Now I can measure signals of a few micro-volts reliably with my MCP3424 differential ADC. The LSB for this ADC is 1.95 micro-volt when in 18-bit mode with the PGA set to a gain of 8.

Noise from the cheap two prong adapter killed the measurement so I am now very happy with the three prong Phihong.

7

Nice find on that supply.
I haven't done much (any) analog stuff with ATMega yet. Will have to check one of those out sometime.

8

fat16lib:
The Phihong adapter arrived from Mouser. It has a pass-through ground...Very low ripple and noise on a scope.

Great news. And now for a question that has been vexing me:
If you were designing a device to get power from an on-board switchmode power supply like the VOF6 from CUI, would you make a point of connecting a AC ground (assuming you're using a 3-wire AC cord) to the GND on the DC side of the power supply? Would you fuse such a connection in case the outlet is hopelessly miswired or the AC ground develops a high potential for some other reason? Or perhaps use a very thin trace coupled with a 0805 fusible resistor with a low resistance for said connection?

For that matter, is it good design practice to always tie the purported AC ground and Neutral lines together on a PCB or to leave them separate? While I should be able to do this (assuming the outlet is wired correctly), experience in the field has taught me otherwise.

9

This article from Wikipedia suggests using a 2,200pF, 2kV capacitor to couple the PCB GND to the 120V mains ground. Sound reasonable?

10

The Wikipedia article assumes you want the PCB to be DC isolated. This article appears to be mostly aimed at audio systems so AC hum is emphasized.

In my case I want the Arduino board to be connected to the 120 AC ground so the above adapter is perfect. This is the only ground in my system.

Many notebook adapters have about a one meg-ohm resistor and a large cap from the 120 AC ground to the outer barrel. This is in the spirit of the Wikipedia article. I have not found a 9V adapter of this type but this type adapter could be useful to avoid multiple hard grounds in some applications.

Ground system design for large systems is a real art but thankfully my little Aduino ADC system is now working well with the Phihong adapter.

11

Hi and thanks for the reply. Would the resistor and cap you are describing for laptop power supplies be wired in parallel or in series? I would assume parallel but want to be sure.

12

The caps are parallel with the resistor in notebook adapters.

The caps kill noise and AC hum the resistor tries to keeps the DC voltage near ground but prevents ground loops and is safe if you touch high voltage and the notebook at the same time.

13

Any particular wattages to observe for the resistor? I.e. would a standard 1/4W resistor suffice? Thanks!