solid state vibrator

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ID: 219792
solid state vibrator 
05.May.10 13:04
10925
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Jan Terranea (NL)
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Jan Terranea

This design I have in use to replace a 120 hertz mechanical nonsynchronous vibrator in a small case to feed mobile radio-equipment from12 volts DC.

There are 2 vibrator types: series-drive with 3 pins connected, and shunt-drive with positive dc voltage at pin 4.

As the mulivibrator needs positive DC it must be connected as a shunt-drive vibrator.

(In case of a synchronous vibrator the ht-rectifying part is done by 2 HT-diodes.)

My previous solid state series-drive transistor vibrator got too hot after some time at a load of 60 mA.

 A friend advised me to try switching power mosfets and he gave me some HUF75345S3S.

These fets can stand to about 20 volts between gate and source, and to about 55 volts between gate and source.
So at 24 volts power input additional measures will be needed to protect these fets.
These switching fets do not consume any current at their gates, and their internal resistance between source and gate in the open-state is almost zero, so they only warm up during te period that the gate-tension is about 3 to 4 volts positive, and that is the reason why the switching transistors can be small, but they really need to have a high amplification.
1-2 must be 10 times ore more to the value of no. 3-4 to obtain good square waves,
1-2 togethet with 7-8 determine the switchting (vibrating) frequency (r-c time)
      to make about 120 HZ values are:1-2 = 12 k, 7-8=470 nF.
 5-6 are protecting diodes to the transistor bases at 12 volts DC-input.
13-14 are switchting Fets, that open at about 3-4 volts, so this vibrator does not work
      if the input voltage gets too low to firmly open the fets.
15-16 lead to the double LT-winding on the transformer, connected to vibrator-pin 1and 2;
      pin 3 is connected to –DC, pin 4 is connected to +DC.
 
Jan Terranea, may 2010.
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solid state vibrator 
08.Jan.23 21:08
10925 from 13534

Christian ADAM (F)
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Christian ADAM

I did check your solid state vibrator design using the same switching power mosfets HUF75345S3S as recommended by you. For the the small transistors, I used BC337.

The result is very good. Most important point to be be highligted : the mosfets even without heat sink do not heat with a current of several amps...This allows a very compact card board: see hereunder ...

Thank you for this usefull tip !

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Final execution, an example of mechanical implementation 
05.Mar.23 10:54
11360 from 13534

Christian ADAM (F)
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Christian ADAM

After having tested the proposed circuit, it was time for the final mechanical implementation.

Here under a photo showing the electronic replacement of a PHILIPS 7857 vibrator for the 1937 PHILIPS autoradio 249B. The aluminium suport frame is not needed as a heat sink but is a mechanical support of PCB and allows connecting everything together.

The data sheet of the MOSFET indicates Ron maxi = 0,7 mOhm. Id did check this and found less than 0,5 !!! This explains why no heat sink is necessary.

Circuit was designed for 12V but I could also test it on the 6V version (248B). This circuit works perfectly without modification for 6 or 12V. 

Old vibrator and his electronic replacement

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Final execution, an example of mechanical implementation 
15.Jan.24 14:33
12831 from 13534

Gidi Verheijen (NL)
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Gidi Verheijen

Jan and Christian,

See the succesfull final execution for a 6 V Blaupunkt car radio.

Thanks for your info.

Gidi Verheijen

 

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Final execution, an example of mechanical implementation 
16.Jan.24 11:57
12918 from 13534

Michael Watterson (IRL)
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I've also restored equipment that uses a synchronous vibrator. That's where the AC rectification o f the secondary HT is done by an additional pair of contacts. Though this is possible with FETs, the simple solution is to wire those vibrator pins with 1N4007 rectifiers. UF4007 is only needed for SMPSUs, the vibrator is slow enough. Also unlike replacing selenium or valve rectifiers there is no need for a series HT resistor.

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