Archive for February, 2014

28
Feb
14

Transient Voltage Suppressors (TVS)

These devices can be thought of as high-current capable zener diodes. A TVS will be rated in terms of it’s reverse breakdown voltage which in practice is the maximum voltage you want to allow before a protection device, like a fuse, kicks-in. You place a TVS shunted across a DC voltage rail with the cathode on the positive voltage rail and the anode connected to the power supply common, or the reverse of this for negative voltage rails. The protection device will normally be in-line (in series) to remove power from the circuit being protected.

ON Semiconductor makes a series called the MiniMOSORB line that has impressive current handling capabilities. For example, the SA12A part is capable of limiting the voltage to about 14V and can withstand about 25A of current. So for example, a mobile device powered by 12-13.8 volts can be protected from overvoltage or reversing the supply leads (its still a diode) even though it may be protected by a 15A fuse; i.e. the TVS will present a short and ensure that the fuse is blown. You can find TVS diodes for sale at many supplier web sites and on eBay where sellers like myself have them for sale.

In effect, if your circuits cannot tolerate a higher than normal voltage spike that is transient or permanent, caused by a voltage regulation fault for example, then a TVS is a great way to protect the circuit from an overvoltage condition.

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04
Feb
14

Testing Unknown RF Transformers

Sometimes you have a shielded transformer in your scrap box, or have acquired some from a surplus distributor, and you’re not quite sure what you have. Even with a marked, currently stocked part, you may want to more closely examine its frequency characteristics or in-circuit Q factor.

There is a very basic way of determining what the intended frequency and passband characteristics were intended to be. For this you will need an oscilloscope and a signal source with a reliably close manual indicator, or a digital frequency display. Ideally the signal source should extend up to 10 or 20 MHz. The idea is to manually dial the frequency adjustment of the generator while driving a sine wave into the primary of the transformer while observing the output across the secondary using the scope display.

The transformers commonly found are going to be adjustable slug tuned or fixed, and will have 4-6 connections. Quite commonly, the primary will be a single, un-tapped winding, with the secondary being center-tapped. In the case of balanced modulator or mixer transformers, this may be immaterial because the two windings can or will serve either role. It’s really the application of the transformer that matters if the turns ratio is something other than 1:1, or a center tap is not required, etc.

You may want to check what you think is a primary or secondary with an ohmmeter just to be sure, but if the piece is really a single adjustable coil, you’ll soon discover it.

Starting at 100 kHz, you increase the frequency of your generator while looking for a peaking effect in the display of the sine wave as seen by the scope across the secondary. If the transformer is broadband, is will be a lower Q transformer that will peak, but very slowly, then decrease in magnitude as you roll pass the center of its passband. If the unit is high Q, and the turns ratio is more than 1:1, you will need to slow your frequency tuning a bit as the peak will be much more sudden as the transformer hits its point of resonance. Note this frequency. If the transformer is slug tuned, try moving the slug up or down and re-check the frequency; you should now see the shift in the passband as a result of this adjustment. Of course if the transformer is not slug tuned, your application circuit may contain a variable capacitor for fine adjustments anyway, or is broadband enough that the isolation across the transformer is all you needed.

You can also make careful measurements to determine the turns ratio of the transformer.

At HF frequencies, it is reasonable to observe the primary and secondary peak to peak voltage readings on your scope and draw a simple conclusion. If the primary P-P voltage is say 1 volt and the secondary is about 4 volts, you have stepped-up the voltage by a factor of 4, so you have a 1:4 ratio. Likewise, if the P-P voltages are about the same, you can conclude there is a 1:1 ratio. Again, the fixed transformers can be tested in the same manner. Obviously if you have some know parts from Mini-Circuits, or your testing a bi-filar wound toroid for example, then the turns ratio will be known.

Testing for Q factor takes a bit more patience and better equipment really; i.e. a scope with cursors that can help you easily find the 3dB point on either side of your transformer. Of course this is not a demanding requirement for a new or used oscilloscope today.

Follow the link for a refresher, but to calculate the Q of the transformer, we are going to need to measure its 3dB bandwidth in addition to the center or resonant frequency measurement we made above. The Q then can be calculated by dividing the 1/2 power or 3 dB bandwidth number into the resonant frequency. In this case we are going to use the (0.707 x the peak voltage) method to determine our 3 dB point in terms of voltage. If you use a convenient secondary output P-P voltage value of 1, then you are looking for the points where the voltage comes down to .707 volts P-P.

For example, if you find your transformer peaks at 9 MHz and your 3 dB point spanned 600 kHz, you would calculate a Q of 15.

04
Feb
14

Hamvention 2013

The misty view of the Flea Market early Friday Morning.

20130517_084307

Hamvention 2013 was again a great place to meet people and find some bargains. There was a Amp Hour meetup that I could not attend, but I did run into Chris Gammell there and we had time to chat in the market. This year I found some nice used SMA jumper cables out in the flea market, and bought some great parts inside in the expo area. I also attended FDIM, and enjoyed that very much; so much I decided to return to FDIM and Hamvention in 2014.