Monthly Archives: May 2013

Digital thermometer came in the mail today

It took forever to get here, almost two weeks. I ordered it from Amazon. It comes with two probes, one that can go up to 260°C and another that can handle 1000°C. I’ll be using it to measure temperatures of ICs in prototypes as well as to measure what temps are coming out of my hot air tool at various distances and flow rates.

Below is a picture of the setup. I’m measuring the temperature of a voltage regulator in a prototype. It’s throwing out over 60°C with a 5.5V input and a 300mA load on the 3.3V output. That’s too hot and it’s right on the edge of thermal shutdown. I won’t be using this regulator in the final design. It has no tab or pad underneath the IC, nowhere for heat to go.

temp_setup

A close up shot of the sensor on the IC. I’m holding it down with polyimide tape.

temp_probe

This meter is nice, only drawback is that it doesn’t log.

Current measurement device

The current project I’m working on has some issues that in the past I haven’t had to worry about too much, mainly power consumption requirements. In past projects I was able to get away with simply adding up the max power consumption values for each component in the device and then select a power supply that exceeds the minimum requirement. In this particular project the device has to have the option to run off of USB power but available USB power is under the maximum amount of current that the device can consume depending on what the user is doing with it. It’s ok if the user circuits can’t run off of USB power if they draw too much, but at no point should the device fail to be able to reprogram the CPLD when plugged into the USB bus. The solution is to have the micro controller manage the power input to the CPLD with a MOSFET. When the device powers up it checks to see if it’s running on USB power, if it is, it attempts to communicate with the PC via the FTDI220 to see if the user wants to reprogram the CPLD. If that is the case it supplies power to the CPLD and immediately goes into programming mode.

As I’m working through the prototyping phase, I want to be able to see what the current consumption is like with the device in it’s entirety and individual components and document as I go along. This allows me to verify that I understand the data sheets and that my power management circuitry is within spec. I can do a lot with my multimeter but I’d like to be able to use it with my oscilloscope and more importantly log data with my computer. None of the three multimeters I have output a voltage representing the current or have an interface for logging data.

To solve some of these issues I built a current meter that outputs a voltage based on the current flowing through it. With it I can view the output on my oscilloscope and log data with my USB daq.

cmeter_bottom

cmeter_top

It’s based around a hall effect based current sensor IC from Allegro Microsystems (ACS711), an opamp (AD8542) and a 3.3v LDO. It picks up a lot of noise unless it’s in a metal box (particularly from fluorescent lights).

cmeter_box

Below is a graph of current consumption from a 100uF electrolytic capacitor. The graph is clipped at 1.6V.

capcharge

Lots of EMI issues, especially with the gain jacked up. It’s useable in multiples of 50mA. At 10 the signal gets lost in the noise. I built this in one evening. With some work it could be a much more useful device, but I don’t have time right now to make it better.