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Why step start?

PostPosted: Tue Apr 15, 2003 11:58 am
by Guest
:?:
I don't have a step start in my E-rig and since November no problems .....
Am I just pushing my luck.....some other e-rigs that I know also don't use step start. So why is it often said that it must be done?

Thanks Dave KE1AV

Why step start . . .

PostPosted: Tue Apr 15, 2003 12:45 pm
by steve_qix
Hi Dave,

The step start is not a necessity, but for big transmitters or very good power supplies,it is nice.

When you first turn on a power supply, the filter capacitors are completely discharged. In theory, there is an infinite amount of inrush current during the first instant when the supply is activated. Of course, the current is not infinite, as there is resistance in the wiring, transformer, house circuits, etc.

By using a step-start, you reduce or eliminate this inrush current. For what we're doing at 350 watts, it probably is not a problem, but for more than this, a step start really helps.

Regards,

Steve

PostPosted: Tue Apr 15, 2003 9:44 pm
by N9NEO
Consider the circuit to be RLC
R from house and feeder wiring, transformer wiring...
L for most part from transformer. Also must lump in reflected impedance from primary side of transformer, but for most calculations this can be ignored. C is your bus cap. Assume about 0.001H for inductance and probably close to 0.1 ohms. These should be in ballpark to what you have at socket. If another transformer between socket and caps you must adjust Z to include this. Calculate the frequency of rlc circuit, and see how this relates to 60Hz.

Now since caps on output side of diode bridge voltage will ring up and stay there. If low Z source (high kVA) can get large charging current. Diode will fail. If low Z and high Q (L/R ratio large) cap voltage will try to ring up to twice line voltage. If cap voltage rating is high then they will in fact ring to a very high voltage approaching 2X expected. If cap voltage is lower then cap will act like a zener voltage, gets very leaky when overvoltaged, and limit the peak voltage.

Can perhaps use a NTC thermister to limit Icharge. Like Steve said, will have to pay attention when doing larger supplies.

73
Bob