capacitor charging circuit didn't work :(

[daviddoria]

guys, i had another thread where we came up with :

http://bandtank.com/charge5.jpg

however, i built it and it didn't work . nothing happens at all when i turn it on. any tips on where to start trouble shooting??

 

[TillEulenspiegel]

D2 is facing the wrong way, for 1.

 

[Sebi]

Yes, but i recommend a power transistor for coil driving...

 

[daviddoria]

sebi, was D2 actually the wrong way? what does D2 do exactly?

 

[Exo]

D2 protects the 555 (or the transistor in the 2nd scematic) from the high reverse voltage created when the magnetic field of the coil collapses (when it is shutdown). When the coil is On the diode does not conduct and does nothing, but when the coil is shut off it will generate a 'spike' with a reversed polarity wich could damage the 555. the diode will conduct this spike and safely let it flow away...

but if the diode is in the wrong way it will be a short circuit when the coil is active

 

[daviddoria]

and it is indeed backwards in the schematic drawn? also , the diode on the secondary... i dont understand how to determine which way current is flowing in the secondary to see which way to put that diode..

 

[ljcox]

and it is indeed backwards in the schematic drawn? also , the diode on the secondary... i dont understand how to determine which way current is flowing in the secondary to see which way to put that diode..

I suggest you read the post
"Why would you use a clamping diode when using a relay?"
This explains the diode across the coil.

As for the diode in the secondary, you need to know how the transformer is wound. The convention is to draw a dot beside the start of the windings. If a current goes into the primary at the dot end, the current will come out of the dot end of the secondary.

The magnetic flux generated by the current in the secondary is opposite to the flux generated by the current in the primary. This is Lenz's Law, which states:- "Induced EMFs and their resultant currents are in such a direction as to oppose the motion that produced them". The "motion" in the case of a transformer is the changing flux due to the primary current.

Len

 

[jem]

Hi:

Indeed D2 is facing the wrong way in the original schematic as it will short the transformer primary. As I suggested before, if you simply remove D2, and replace R3 with 100 Ohms, you should be able to get the circuit working. There is no need for D2 if driving directly from the output of the 555 since it can both sink and source current.

Sebi's circuit should also work. In this case, D2 IS required because, as mentioned before, the collapsing field in the transformer primary will induce potentially damaging voltage spikes.

Jem

 

[Sebi]

Jem, i mean the back EMF sometimes rise up to 300V, the output transistors have max40V Uce, so definitely need the diode.