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November 2007 Archive

 
27 November 2007

I've written before about instability in some 3-lead voltage regulators (TL750L - read by clicking here) if not bypassed as discussed in the data sheet. My battery monitor uses a Microchip MCP1525 precision 2.5V regulator as an A/D converter reference, and it also requires bypassing the output pin with a 1 to 10 μF capacitor.

At this stage, I'm using a solderless breadboard to develop the microcontroller portion of the circuit, including the MCP1525 reference.

Omitting this bypass capacitor causes the regulator to become an oscillator, not a voltage regulator:
 

MCP1525 without bypass capacitor on regulated output pin.


I've also mentioned before that it's important to watch scope probe connections, as long ground leads may result in erroneous data. To illustrate this point, I added a 2.2 μF / 35 V tantalum bypass capacitor to the MCP 1525 regulator and looked at the 2.5 V output using a Tektronix 10X model P6106 oscilloscope probe using the stock 6" ground lead and a Tektronix 2246 100 MHz analog oscilloscope. Not the desired result, as there is a noticeable burst oscillation riding on the output:
 
MCP 1525 with 2.2 μF bypass capacitor and 6" ground lead. AC coupled to illustrate the noise.

Connection with long ground lead.

Is this a real problem with the MCP 1525, or is the oscilloscope probe picking up trash from other parts of the development board, such as the logic-to-RS232 converter chip? This particular part is suspect because it has an on-board flying capacitor voltage converter to develop the ±15V RS232 interface. (This chip can't be removed from the breadboard as it's a surface mount part.)

To verify my suspicion that the probe's ground lead was picking up noise from other circuitry, I removed the probe sleeve and ground lead, substituting a short length of bus wire wrapped around the probe's ground tip, as illustrated below.
 

Shortening the ground lead makes it clear that the noise is not generated by the MCP1525, as there just a trace of the noise present now:
 
MCP 1525 Output with short ground lead to oscilloscope probe. AC coupled to illustrate the noise.
 
25 November 2007

I ran discharge tests at C/5 for two new 1200 mAh Panasonic P120CSJS NiCd cells. The cells are rated at 1200 mAh, average 1300 mAh when discharged at a C/5 rate. As the data below demonstrates, the two cells I tested are quite close to Panasonic's 1300 mAh average performance data.
 

24 November 2007

I measured a second Panasonic AAA NiMH cell and found it quite similar to the first one. Both are about two years old and have never been used in equipment, although I've occasionally put both on the Panasonic charger. Usually these cells are rated at 5 hour discharge rate (called C/5), or 150 mA for the nominal 750 mAh capacity.
 

Panasonic provides a discharge curve for a slightly lower rated (700 mAh) cell model at a 140 mA rate, C/5. My two cells don't hold up as well, but that's not unexpected.
 
24 November 2007

I've decided to put the updated panadapter, Z93, on hold for the moment, and work on a couple simpler projects. The reason for this decision is that N8LP is working on a low-end panadapter and Elecraft is working on a high end panadapter, leaving little to no market for the design I've been contemplating.

A year or so ago, I breadboarded a simple battery evaluator, to discharge a battery at a defined current and measure the output voltage. A significantly improved version of this evaluator is my new project. At the moment, I have a enough of the new design working to make some simple measurements, such as the one below.
 

My present plan is to combine the battery evaluator design and measured data of typical rechargeable and primary cells into an article for QEX. I will also offer the battery evaluator as a kit, available sometime this spring. The kit will include Windows plotting software.

If you would find a battery evaluator a useful addition to your test gear, drop me a note. My current thinking is that the evaluator will test up to two batteries simultaneously, with a maximum discharge rate of about 1A, and a maximum battery voltage of 12 volts. A variety of discharge scenarios will be permitted, including time-varying discharge.

The present design is oriented to small batteries´┐ŻAAA, AA and D cells, for example, as well as specialized batteries such as those used in hand-held VHF/UHF equipment. However, if there is sufficient interest, an optional high-power discharge module could be added, with a maximum rate of 10 to 20 amperes.

In working on the design over the last few days, I've found that six NiMH AA batteries in my stock are essentially dead. The best of the lot holds a fraction of its rated charge.

 
16 November 2007

I received the accessory heating elements for smaller wires (#24 - #36 AWG) for my Hakko FT-800 thermal stripper, pictured below. The heating elements snap into the tweezers-style hand piece, so changing between smaller and larger wire sizes is relatively fast.

By the way, the photo below and the Hakko review photos were taken with a new Nikon D80 digital camera, and an 18-135 mm Nikkor zoom lens. The image quality is hard to appreciate from the reduced size images on this site, but it's quite impressive when seen at full resolution. (I've also added this photo to my FT-800 review page.)
 

 
14 November 2007

I've sent out a test version of Z100 firmware release 2.2 today. This version adds high/low error tuning status to the tone decoder mode and also permits wide (approximately 50 Hz) and narrow (approximately 25 Hz) tone decoder bandwidths, via the 25/50 Hz slide switch. The high/low is limited to single, fixed position LEDs to left and right of center and are also blanked out when the error exceeds 250 Hz. The idea is not to duplicate the normal quasi-spectrum display, but rather to provide an assist for anyone unable to judge high/low. This new mode is activated when Position 1 is selected in the rotary switch. Normal tone decoder mode is Position 0.

After feedback from the test customer, and any necessary fixes, I'll make 2.2 available for all Z100 owners.

I now have three articles for QEX during 2008. They are:

  • The Z100. I've gone through the first edits with the ARRL and expect it to make the January or February QEX.
  • Ferrite loops. Accepted, but no publication date yet.
  • Carbon film, carbon composition and metal oxide resistors. Accepted, but no publication date yet.
 
06 November 2007

Hakko has a special promotional price on its FT-800 thermal wire strippers, with $100 off the list price. I've wanted a thermal stripper for some time, so I bought one last week. It arrived yesterday and I've had a chance to use try it out. I've posted a review that you may find in the page index at the top left or by clicking here.
 

04 November 2007

After a long break from electronic and radio projects, I hope to get back to normal activities.

As usual, I've moved the October 2007 Updates to an archive page linked in the table at the top of this page or  by clicking here.
 

04 November 2007

I've added an updated Z90 firmware release, version 610, to the Software Updates page. The revised firmware permits you to reverse the sweep direction, thereby placing signals in their correct orientation when the Z90 is used with equipment that inverts the IF frequency, such as the K2 on most bands. Download the firmware at the Software Updates page