FM transmitter (mod4)

[Hero999]

An LM7812 needs an input voltage of at least 14V.

Lead acid batteries should not be charged about 14.5V and shouldn't be left connected to >13.8V once fully charged.

The LM7809 or even an LM7808 will work.

 

[chingyg]

oh crap, I forgot about that. what if I take out the lead acid battery?
will the 12V work then?

 

[Hero999]

If you increase the value of R8 to 82k you should be able to power it directly from a lead acid battery. If you decide to use a battery use a 100mA fuse or at lease put a 10Ω resistor in series with the whole circuit.

If it's run from a 12V supply the LM2931A 5.0 can be replaced with a cheap LM7805 or LM78L05 regulator.

 

[chingyg]

So you don't have to change Q3, just change R8, add fuse and power from that wall adapter with the 7812 and the capacitors shown in the circuit in my previous post?

It would enhance performance right?

 

[Hero999]

As I told you before, you don't need the LM7812.

If the output of that all adaptor is 17V, you'll need to disconnect the battery when its voltage rises above 14.5V to prevent it from overcharging.

 

[chingyg]

No what I mean is that I'd leave the battery out of the circuit. and the extra 7812 wont do any harm will it?

 

[Hero999]

You can keep the LM7812 as long the output of the adaptor is >14V otherwise the LM7812 might behave erratically.

 

[chingyg]

alright. thanks for the lesson, now I understand why I keep on frying 7812 in the past years now hahaha.

 

[Hero999]

You won't fry an LM7812 is the input voltage is too low. It just won't regulate properly and might oscillate.

You'll only fry it, if you do something silly like connect it up backwards or to a higher voltage than 35V.

 

[chingyg]

oh, well. I used to use it for a 32V input.

 

[Hero999]

That shouldn't kill it unless there are high voltage spikes.

Was this a vehicle power supply by any chance?

 

[transistor495]

Input current limiting is very essential if you connect to a 12v LA battery. Else Q3 will fry and smoke within seconds.

 

[Hero999]

Would you recommend adding an emitter resistor for 12V operation?

May be 10R will do.

 

[mvs sarma]

oh crap, I forgot about that. what if I take out the lead acid battery?
will the 12V work then?

why not 6V battery? even the bias of last transistor can be shifted to regulated voltage, leaving collector only to unregulated. Value of R8 can be re-calculated for the same collector current.

 

[chingyg]

That shouldn't kill it unless there are high voltage spikes.

Was this a vehicle power supply by any chance?

Its some cheap homemade transformer supply, I used it to build ignition coil drivers. but those thing gives you horrible headache, from the sound or the ozone.

 

[chingyg]

Would you recommend adding an emitter resistor for 12V operation?

May be 10R will do.

what about 10R in series with the whole circuit? then use a heatsinked transistor for Q3, but what would you recommend, something like 2sc1162

 

[audioguru]

With a brand new 9V battery, Q3 in my FM transmitter draws about 50mA and dissipates 0.45W which makes it warm.

Q3 might fry with a 12V or 13.8V supply even if the value of its base resistor is increased.
Use the original circuit powered from a 7809 regulator.

 

[chingyg]

ok, I am going to try to build one with 7810 then. I'll post my schematics up soon.

 

[chingyg]

BTW can I use 2N2222A for the 2N3904's coz i got a bunch of 2N2222A lying around at home.

 

[transistor495]

2N3866 is a specialized VHF power amplifier. I think it will boost your signals with just a 10R emitter resistor in the above circuit. Add a proper heat sink.

**broken link removed**

I made one kind of that and tested and put it inside my junkbox . I must say it emits ultra strong radiation that severely overloads cheap receivers nearby.

It is a costly transistor.