FM transmitter (mod4)

[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?

 

[audioguru]

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]

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]

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]

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

 

[audioguru]

Put the 3.3k and the 220 ohm resistor in series across 5V. Then the 220 ohm resistor has only 0.31V, not 5V.
If Q2 is turned on as hard as it can, with its collector at +5V then its emitter would be +4.3V, not +5V.
Something was shorting the emitter to the collector or to +5V.

Measure the resistance between where the emitter was, to ground. 220 ohms?

Is the 0V of the 5V part of the circuit connected to the 0V of the supply and to the 0V of other part of the circuit?

 

[bananasiong]

Put the 3.3k and the 220 ohm resistor in series across 5V. Then the 220 ohm resistor has only 0.31V, not 5V.
If Q2 is turned on as hard as it can, with its collector at +5V then its emitter would be +4.3V, not +5V.
Something was shorting the emitter to the collector or to +5V.

Measure the resistance between where the emitter was, to ground. 220 ohms?

Is the 0V of the 5V part of the circuit connected to the 0V of the supply and to the 0V of other part of the circuit?

Thanks for all these!! After troubleshooting shooting shooting shooting and shooting, I found that I missed to cut one of the tracks! But I don't know how come it was working at the 1st time Must be very careful if using veroboard.

 

[bananasiong]

Hi,
Now I've fixed my transmitter, I'm sure that its quality is good enough as I've listen from real auto-tuning FM radio. My super regenerative receiver doesn't give good sound, I can't even hear clearly. Is there anything to do with center tap at which turn of the coil?

Thanks.

 

[audioguru]

I am glad that you shooted and shooted until you fixed your transmitter.

The super-regen doesn't have an FM detector so it sounds bad with FM. It probably also sounds bad with AM. It is too simple to be a real radio.

 

[bananasiong]

I am glad that you shooted and shooted until you fixed your transmitter.

Hehe, thanks.

The super-regen doesn't have an FM detector so it sounds bad with FM. It probably also sounds bad with AM. It is too simple to be a real radio.

Okay, it is not a radio, but I'll try shooooothing it
Thanks

 

[Hero999]

I take it you want a reciever so you can make an intercom or a walki talki.

You could buy a cheap FM radio, then if you want use the parts to build a better radio.

 

[bananasiong]

Yes, I have a cheap radio. But there is an IC inside which is not available here. I need to remove it from the radio. Besides, it is not only a FM radio but including AM radio as well. I can't just copy the circuit right? I just want to try the maximum quality of a super regenerative.

Thanks

 

[audioguru]

I just want to try the maximum quality of a super regenerative.

A super-regen doesn't have any quality. It is cheap, cheap, cheap, cheap, cheap, cheap, cheap, cheap, cheap, cheap, cheap, cheap, etc.

 

[bananasiong]

A super-regen doesn't have any quality. It is cheap, cheap, cheap, cheap, cheap, cheap, cheap, cheap, cheap, cheap, cheap, cheap, etc.

LOL.. that's cheap.. hehe
But I noticed that the coil does affect the reception. Previously I used 6 turns with spacing between turns, I couldn't get any station. But now I'm using a 10 turns, each turn closed to another turn, then I can receive from my transmitter and I can get 1 or 2 stations.. hehe

 

[bananasiong]

Hi,
LM317 questions again, I've read the datasheet. The graph shows that when it is 25'C and when the load current is 1A, the drop out voltage is around 2 volts. Am I right with what I've mentioned above?
I'm using a 9-0-9 transformer and rectifier using only 2 diodes with center tapping. Why is a higher filter capacitor needed when the load current is higher? Is it because of when the load current is getting higher, drop out voltage will be higher also, then the bigger capacitor can reduce ripple, so the voltage regulator can regulates well. Right?

Thanks

 

[audioguru]

LM317 questions again, I've read the datasheet. The graph shows that when it is 25'C and when the load current is 1A, the drop out voltage is around 2 volts.

The dropout voltage on the graph is what they measured from some they tested. It is not a guarantee. The guaranteed spec's were made with an input voltage 3V to 5V higher than the output voltage.
At the dropout voltage then the regulation is poor because the output voltage has dropped 100mV instead of being regulated.

I'm using a 9-0-9 transformer and rectifier using only 2 diodes with center tapping. Why is a higher filter capacitor needed when the load current is higher? Is it because of when the load current is getting higher, drop out voltage will be higher also, then the bigger capacitor can reduce ripple, so the voltage regulator can regulates well. Right?

Yes. The input voltage at the lowest part of ripple from the filter capacitor must be higher than the output voltage plus the dropout voltage. Higher current increases the amount of ripple so its lowest voltage might be too low unless the filter capacitor is big enough.

 

[Hero999]

Hi,
LM317 questions again, I've read the datasheet. The graph shows that when it is 25'C and when the load current is 1A, the drop out voltage is around 2 volts. Am I right with what I've mentioned above?

But it's lower at lower currents.

What current does your FM transmitter draw?

Much less than 1A, at 20mA the droput is 1.5V and at 100mA it's about 1.6A. Your FM transmitter probably uses about 50mA so anywhere between these figures is acceptable.

You have a point audioguru, these are typical values and not the worst case. I didn't see anywhere on the datasheet where it suggested 3V.

 

[audioguru]

I didn't see anywhere on the datasheet where it suggested 3V.

The datasheet list a condition of having Vin - Vout of 3V and of 5V for some of its important regulating spec ratings.

 

[bananasiong]

But it's lower at lower currents.

What current does your FM transmitter draw?

Much less than 1A, at 20mA the dropout is 1.5V and at 100mA it's about 1.6A. Your FM transmitter probably uses about 50mA so anywhere between these figures is acceptable.

Yes, the transmitter uses less current. I just want to see whether I can read the datasheet correctly or not. Thanks . If I use this power supply (not using it with my FM transmitter) for higher current, LM317 will not be efficient since the output of the rectifier circuit and across filter capacitor is not enough. I should replace it with a low drop out voltage regulator such as lm2940-9? Is there any adjustable low drop out voltage regulator?

Thanks

 

[bananasiong]

Hi,
I found this, ncp1117. When the output current is 1A, the drop out voltage is only 1.1 volts. The maximum current is 1A, how if higher? Will it burn or just not functioning?
Is there any adjustable low drop out voltage regulator? I don't know whether I can get this here or not.

Thanks