FM transmitter (mod4) - Page 11 - Electronic Circuits Projects Diagrams Free

bananasiong · 20th November 2006, 12:18 AM

Quote:

Originally Posted by Hero999

It depends on the current draw but you only have one diode drop so assuming a current of 100mA and 1N4001s you'll get 11.98V.

You don't need a low dropout regulator since an LM317 will have a dropout voltage of about 1.6V at 100mA.

Since there is no difference then I'll choose the one with center tap.

LM317 is an adjustable voltage regulator, I've just simply gone through the datasheet, this is the way to calculate $V_{{o}}=V_{{ref}} (1+ {\frac {R_{{2}}}{{R}_{{1}}})$ and it says that the product of adjustment current and R2 can be neglected.
So can I just do calculation to get 9 volts?

Thanks

audioguru · 20th November 2006, 12:30 AM

The dropout voltage of an LM317 regulator is when it is no longer regulating and its output has dropped 0.1V. Most of them have a dropout voltage of 1.6V at 100mA but with others it is higher but how high is not shown. They have their spec's listed with an input voltage 3.0V and 5.0V higher than their output.

bananasiong · 20th November 2006, 12:34 AM

Quote:

Originally Posted by audioguru

The dropout voltage of an LM317 regulator is when it is no longer regulating and its output has dropped 0.1V. Most of them have a dropout voltage of 1.6V at 100mA but with others it is higher but how high is not shown. They have their spec's listed with an input voltage 3.0V and 5.0V higher than their output.

So, if I use a 9-0-9 transformer, it might not be working for current higher than 100mA?

audioguru · 20th November 2006, 12:56 AM

If you use a 9-0-9 transformer then the peak is 12.7V and the voltage at the main filter capacitor is 12V. There will be a little ripple but the transformer's voltage will be higher than its rating because it will not be fully loaded. Then there will be plenty of extra voltage to avoid dropout.

Make an LM317 circuit with 470uF for the main filter capacitor for a 100mA load. Use 1000uF or more for higher load current. The 100uF capacitor at its output is already in the transmitter's circuit.

bananasiong · 20th November 2006, 01:03 AM

Quote:

Originally Posted by audioguru

If you use a 9-0-9 transformer then the peak is 12.7V and the voltage at the main filter capacitor is 12V. There will be a little ripple but the transformer's voltage will be higher than its rating because it will not be fully loaded. Then there will be plenty of extra voltage to avoid dropout.

Make an LM317 circuit with 470uF for the main filter capacitor for a 100mA load. Use 1000uF or more for higher load current. The 100uF capacitor at its output is already in the transmitter's circuit.

Oh, then I can remain the 9 volts transformer, with center tap rectifier. Is 100mA enough for FM transmitter, super regenerative and PIC microcontroller?
Is protection diode needed as shown in the datasheet?

Thanks

bananasiong · 20th November 2006, 01:45 AM

The emitter voltage of Q2 is 5 volts after replacing a new transistor, same as the collector voltage.

audioguru · 20th November 2006, 01:52 AM

Why bother connecting a transmitter and a receiver to the same power supply? If the transmitter can hear the receiver then there will be acoustical feedback howling.

bananasiong · 20th November 2006, 01:57 AM

Quote:

Originally Posted by audioguru

Why bother connecting a transmitter and a receiver to the same power supply? If the transmitter can hear the receiver then there will be acoustical feedback howling.

Nono.. I mean the same type of power supply but not connecting both the tx and rx together.
Voltage at the emitter of Q2 is 5 volts but not 2 volts..

audioguru · 20th November 2006, 02:05 AM

C7, the pcb or the transistor is shorted since the emitter has the same voltage as the collector. Maybe R7 is open or is disconnected from ground or from the emitter.

The datasheet for the LM317 says that a protection diode is needed from output to input since the output capacitor is big, to protect in case the input is shorted. Add it if you want but I don't think anything is going to short the input.

bananasiong · 20th November 2006, 02:36 AM

Quote:

Originally Posted by audioguru

C7, the pcb or the transistor is shorted since the emitter has the same voltage as the collector. Maybe R7 is open or is disconnected from ground or from the emitter.

The collector and the emitter is not shorted, I've measured with digital multimeter in null setting. I've also resoldered R7, both ends are connected well.

Quote:

The datasheet for the LM317 says that a protection diode is needed from output to input since the output capacitor is big, to protect in case the input is shorted. Add it if you want but I don't think anything is going to short the input.

Okay, thanks.

bananasiong · 20th November 2006, 05:06 PM

Hi,
I've tried even replacing new C7, as well as C5 and R7. But how come the collector voltage is same as the emitter?

audioguru · 20th November 2006, 05:23 PM

Quote:

Originally Posted by bananasiong

I've tried even replacing new C7, as well as C5 and R7. But how come the collector voltage is same as the emitter?

The collector is connected to +5V with a piece of wire (the coil) so +5V is normal. The emitter should be at about +2V.
Something must be shorting the emitter to either the collector or to the +5V supply. With a low voltage ohm-meter measure the resistance from the emitter to the collector. It should be very high. If you don't have a low voltage ohm-meter then remove Q2 and measure the resistance of where its emitter and collector wires were connected. The 5V regulator might messup the measurement if an ohm-meter with a low voltage isn't used, then also remove the regulator to make the measurement.

bananasiong · 21st November 2006, 12:33 AM

Quote:

Originally Posted by audioguru

The collector is connected to +5V with a piece of wire (the coil) so +5V is normal. The emitter should be at about +2V.
Something must be shorting the emitter to either the collector or to the +5V supply. With a low voltage ohm-meter measure the resistance from the emitter to the collector. It should be very high. If you don't have a low voltage ohm-meter then remove Q2 and measure the resistance of where its emitter and collector wires were connected. The 5V regulator might messup the measurement if an ohm-meter with a low voltage isn't used, then also remove the regulator to make the measurement.

What is a low voltage ohm meter? I can get a digital multimeter which can check whether two terminals are shorted or not.

audioguru · 21st November 2006, 01:04 AM

My digital meter measures OHMS with a 200mV low voltage source so it doesn't turn on transistors and diodes. It also has a DIODE TEST where it has a higher voltage and measures the forward voltage of a transistor junction, diode or LED.

If your meter has a voltage that is high enough to turn on the transitor then you will have a wrong reading of OHMS.

bananasiong · 21st November 2006, 04:54 AM

I've removed Q2 and measured the resistance where the emitter an the collector were connected, it is aroun 3.3k

hm:

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20th November 2006, 12:30 AM	(permalink)
audioguru	The dropout voltage of an LM317 regulator is when it is no longer regulating and its output has dropped 0.1V. Most of them have a dropout voltage of 1.6V at 100mA but with others it is higher but how high is not shown. They have their spec's listed with an input voltage 3.0V and 5.0V higher than their output. __________________ Uncle $crooge

20th November 2006, 01:45 AM	(permalink)
bananasiong	The emitter voltage of Q2 is 5 volts after replacing a new transistor, same as the collector voltage. __________________ Superman returns..

20th November 2006, 01:52 AM	(permalink)
audioguru	Why bother connecting a transmitter and a receiver to the same power supply? If the transmitter can hear the receiver then there will be acoustical feedback howling. __________________ Uncle $crooge

20th November 2006, 02:05 AM	(permalink)
audioguru	C7, the pcb or the transistor is shorted since the emitter has the same voltage as the collector. Maybe R7 is open or is disconnected from ground or from the emitter. The datasheet for the LM317 says that a protection diode is needed from output to input since the output capacitor is big, to protect in case the input is shorted. Add it if you want but I don't think anything is going to short the input. __________________ Uncle $crooge

20th November 2006, 05:06 PM	(permalink)
bananasiong	Hi, I've tried even replacing new C7, as well as C5 and R7. But how come the collector voltage is same as the emitter? __________________ Superman returns..

21st November 2006, 01:04 AM	(permalink)
audioguru	My digital meter measures OHMS with a 200mV low voltage source so it doesn't turn on transistors and diodes. It also has a DIODE TEST where it has a higher voltage and measures the forward voltage of a transistor junction, diode or LED. If your meter has a voltage that is high enough to turn on the transitor then you will have a wrong reading of OHMS. __________________ Uncle $crooge

21st November 2006, 04:54 AM	(permalink)
bananasiong	I've removed Q2 and measured the resistance where the emitter an the collector were connected, it is aroun 3.3khm: __________________ Superman returns..