SS Vibrator Replacement

 

 

One problem with restoring early DC supply radios like Farm Sets or Auto radios, is the mechanical vibrator. Usually they are toast or work sporadically and are very noisy. After getting a couple of sets from a friend to restore for him, I decided to look around for a good substitute and decided to make my own.

The replacement shown here is a combination of circuits I found around the internet and information from an electronic design engineer friend. I decided to make it small enough to fit back in the existing vibrator container and to be plug & play. Two versions were designed, one for positive ground sets, the other for negative ground sets. As shown they are for 6 volt systems but can be modified easily for other voltages.

 

 

Click here for larger image and parts list.

This is the circuit of the positive ground replacement. The CD4047 IC is used to generate a 100 Hz square wave to drive the N-Channel MOSFET's. One problem that occurred during the initial testing was a much higher output voltage than was expected, this was traced to the fact that with the original mechanical vibrator there is a "dead zone" time, where no voltage is applied to the transformer primary. Using the IC to simulate the contact switching there is no "dead zone" time, so the average voltage applied is much higher than with the mechanical switching. There were two ways I could compensate for this, the first would be to add another circuit to simulate the "dead zone" time, but this would add complexity and increase the size which might interfere with the replacement fitting into the original container. The second option was much simpler, just reduce the voltage to the transformer primary, using one or more silicon diodes. Each diode will drop the input voltage by about .7 volts, in the units I built two diodes were enough to bring the secondary down to the correct voltage.

The MOSFET's I used are 100V 33A units in TO-220 packages, they do not require any heat sink in this application, after running under load for 30 minutes they are cool to the touch. Use P-Channel devices for the negative ground unit and N-Channel devices for the positive ground unit.  The total cost of all the parts was less than $10. 

If you don't need a synchronous vibrator just eliminate the rectifier diodes and use a four pin base.

 

This is the first test unit I built. All components mount on the one side of the board, the 2200uf capacitor is mounted on the rear. It is all point to point wiring, a small printed circuit board could be made easily

 

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