circuits

[ben111]

hi.

i just want to know what causes a circuit not to work if it is build bigger than the size stated in the circuit diagram.i just buid a tv signal amplifier  but when i used it on my tv, the signal was even worst.it was about trice the original size.
  i build it on a cartboard paper as i did not have a pcb.


  i tried buying one(4.5by5.3 in dimension) online but it was very expensive at about 26dollars but including shipping, but 10pieces will cost $36 but one is not $3.6.there is a money sendine company called money gram in my country cameroon.the minimum amout of money you can send is about $8.if there is someone that can make me a pcb and post it at this price,i will be very greatful as it looks like my circuit will not work without it.


  i have a BFR72(or 92 )transistor.it has very short leads.i am afraid i will over heat it while building the circuit as i have only a 60W shoulder.are the transistors very resistant to heat?if not what can i do to make the connections without destroying it.
  thanks

 

[audioguru2]

Radio and TV circuits must be compact to reduce stray capacitance and inductance in the wiring.

Your soldering iron is too powerful for electronics. Connect a rectifier in series with it to cut its power in half. Use a switch to short the rectifier for full power.

 

[ben111]

thanks. 
i know about the stay capacitance but i want to know how they come about due to the large circuits.
  i did potential dividers in high school.lets say i want to reduce the voltage to the sholder by half, do i have to connect two resistors in parallel,then connect the shoulder across one?
    do you know anybody that loves making pcbs?










(forgotten how to spell shoulder)

 

[audioguru2]

Radio circuits don't work if their capacitance or inductance is too high:

1) Wires on a circuit beside each other have capacitance between them. The capacitance is higher when the wires are longer on a bigger circuit layout.

2) Wires have inductance that is higher when the wires are longer on a bigger circuit layout.

Your shoulder is at the top of your arm. A soldering iron heats parts so that they melt solder.

Your soldering iron has too much power at 60W. You should reduce it to 30W (half). If you reduce its voltage to half then its current will also be reduced to half so its power is only one quarter which is too low.

If you connect a resistor in series with it to reduce its power to 30W, then the resistor must be huge to also dissipate 30W. A series rectifier diode cuts the power in half without getting hot.

 

[ben111]

is there only one type of series rectifier diode?
most part sellers here are very dull.all they want is tha name and number of a part.can you give that to me so i can try to find?
  thanks for the spelling.

 

[audioguru2]

If your soldering iron doesn't have a transformer and isn't a soldering gun, then a 1N4005 rectifier will cut its power in half. Connect the rectifier in series with one of its two wires and connect a switch across the rectifier to switch to full power.

 

[aniruddha]

If the size of ckt is quite large then the effect of stray capacitances and inductances could be reduced by
1- Twisting the connecting wires.
2- Connecting a very small (about 1pF) capacitance in parallel close to the IC pin.

 

[audioguru2]

If the size of ckt is quite large then the effect of stray capacitances and inductances could be reduced by
1- Twisting the connecting wires.
2- Connecting a very small (about 1pF) capacitance in parallel close to the IC pin.

I don't think so.
Twisted wires are very close together which makes higher stray capacitance between them.
Adding capacitors increases capacitance.

Stray capacitance is reduced by reducing the size of the circuit.

 

[ben111]

can the connecting of small capacitance capacitors not instead help?
what if the circuit is so big that the connecting wires cannot  give rise to any capacitance between them?

 

[audioguru2]

Connecting small capacitors adds capacitance when no more is wanted.
A large circuit layout has the wires spaced far apart for low stray capacitance between the wires but high capacitance to anything that gets near.

Each long wire is a transmitting and a receiving antenna to and from all the other wires, also theyhave high inductance along the entire length that is not wanted.

 

[MP1]

Ben,
If you will grip the transistor leads with needle-nose pliers while soldering, the needle-nose pliers will act as a heat sink and keep the transistor from getting over heated.
In regards to the circuit layout problem, are you sure the problem is capacitance? I don't know what your circuit looks like, but did you put it in a grounded metal box and try re-flowing the connections to insure you do not have a bad solder joint?

MP

 

[ben111]

I think i did the connections very well.i mounted the circuit on a cartboard paper but i did not earth the circuit.though i did not solder the transistor to the wires i wrapped the wires around the leads of the transistors very firmly  so it is imposible that there is an open circuit.

 

[MP1]

You might want to first try soldering all connections and shielding the circuit in a metal box. I don't know if this is the cure, but I am sure it will help improve the signal.

MP

 

[ben111]

do you think i really need to solder the components together?
what about the earthing?
i took the point earth as the negative terminnal  of the battery as they are  on the same wire.
you can look at the circuit in the RF projects , there you will see tv signal amplifier.
thanks for your help.

 

[MP1]

I assume you are discussing this project:
http://www.electronics-lab.com/projects/rf/003/index.html
In the instructions, it tells you that soldering the components together is critical. Also, note that since you are dealing with RF frequencies, you must shield your board so that stray signals do not interfere. There is a connection for antenna. This should be the only place that RF signals are introduced into the circuit. I highly recommend that you solder all components and house it in a metal box which is connected to your ground or commons on the circuit.

MP

 

[ben111]

thanks.
i will put it in a box.
  since i did not have a pcb, i bought a plaque a essai as it is called in french.it has many long strips of copper on it with many holes along  the length of each path.
i tried soldering some components to the board but it was so so so so difficult to do that i only soldered one lead  of a resistor and it was by mistake and i couldn't repeat the mistake.
  the project says you should put the soldering iron on the lead then put the soldering wire at the point where the lead emerges from the copper side of the board and that the soldering wire will melt taking heat from the lead and that this proccess should take not more than five seconds.i have a 60WATT soldering iron.when i do these the soldering wire does not melt and i fine myself heating the resistor for more than 15 seconds.
    i do not know if the problem is with the soldering wire(ie does not easily melts) or i am doing it badly.
  even when i melt the wire  directly with the solder,it does not adhere to the plaque d'essai.
    please tell me what to do and please also tell me other methode of soldering.
thanks.

 

[MP1]

Ben,
You probably have oxidation on the copper strips on the board. This is easy to overcome. Go to your local electronics store and get a small bottle of liquid solder flux or a solder flux pen. Put a liberal amount of flux on all surfaces that you want the solder to adhere to before applying heat. You should succeed with a nice even flowing solder joint that flows perfect on all parts. Flux is one of the best things you will ever encounter in the world of soldering. Use a little isopropyl alcohol to clean the flux from your board when finished. If you do not clean the flux off the board after you are finished, dirt will stick to it and after some time, you could end up with a short between pads or components. Thus cleanliness is important.
I hope these pointers help you.

MP

 

[ben111]

thanks for your help.
i hope that if i find the flux it will make the soldering easier.
i also hope it is not expensive.

 

[ben111]

hi.
i bought a new soldering wire and it is so so good.it solders very well it acyually has much flux in its core.
  please MP(or anyone) can you go back to the project(RF circuits:then Tv Signal Amplifier) and read the last chapter  and please tell me what a feed through capacitor is and how i should used it.
thanks