Somebody please double check my homework.

I got four 2400 MFD electrolytic caps and just selected a 10 ohm 200
watt power resistor from the scrap bin. I get 0.48 seconds to charge
these caps through the resistor using a very simple formula I found:

CHARGE TIME = 5*R*C

I'm setting a time delay relay to .5 seconds before shorting out the
resistor.

Karl

On Oct 17, 6:36*pm, Karl Townsend
wrote:
Somebody please double check my homework.


Karl

You get an A. If you happen to have a smaller wattage resistor, it
would work. It is only in the circuit for half a second. But
certainly no problem using what was in your scrap pile.

Dan

On Oct 17, 5:44*pm, " wrote:
On Oct 17, 6:36*pm, Karl Townsend
wrote:

Somebody please double check my homework.

Karl

You get an A. *If you happen to have a smaller wattage resistor, it
would work. *It is only in the circuit for half a second. *But
certainly no problem using what was in your scrap pile.

* * * * * * * * * * * * * * * * * * * * * * * * * Dan

Don't forget that once you remove that resistor, the rectifiers are
only conducting for a very short part of the cycle. Once the
capacitors are charged up the incoming power has to go to nearly full
voltage before they start to re-charge the capacitors. So, the current
Spikes can be quite high for a very short period of time, 120 times a
second.
Since all the power is being consumed over a very short period of
time, and at high amperage, a higher percentage of the electricity you
are paying for is used to heat the wires.

You can reduce that, and the inrush current, and reduce you output
ripple with a choke between the rectifiers and the capacitors.

On Sun, 17 Oct 2010 17:04:01 -0700 (PDT), Cross-Slide
wrote:

On Oct 17, 5:44*pm, " wrote:
On Oct 17, 6:36*pm, Karl Townsend
wrote:

Somebody please double check my homework.

Karl

You get an A. *If you happen to have a smaller wattage resistor, it
would work. *It is only in the circuit for half a second. *But
certainly no problem using what was in your scrap pile.

* * * * * * * * * * * * * * * * * * * * * * * * * Dan

Don't forget that once you remove that resistor, the rectifiers are
only conducting for a very short part of the cycle. Once the
capacitors are charged up the incoming power has to go to nearly full
voltage before they start to re-charge the capacitors. So, the current
Spikes can be quite high for a very short period of time, 120 times a
second.
Since all the power is being consumed over a very short period of
time, and at high amperage, a higher percentage of the electricity you
are paying for is used to heat the wires.

You can reduce that, and the inrush current, and reduce you output
ripple with a choke between the rectifiers and the capacitors.

According to the servo amp manufacturer, a choke is not necessary and
this is on the high end of recommended capacitance. Now, I have no
clue on wire size needed to prevent the insulation from melting. I'm
going with four #12, one to each cap, and will check for excessive
heating after startup.

Karl

Karl Townsend wrote:

On Sun, 17 Oct 2010 17:04:01 -0700 (PDT), Cross-Slide
wrote:

On Oct 17, 5:44 pm, " wrote:
On Oct 17, 6:36 pm, Karl Townsend
wrote:

Somebody please double check my homework.

Karl

You get an A. If you happen to have a smaller wattage resistor, it
would work. It is only in the circuit for half a second. But
certainly no problem using what was in your scrap pile.

Dan

Don't forget that once you remove that resistor, the rectifiers are
only conducting for a very short part of the cycle. Once the
capacitors are charged up the incoming power has to go to nearly full
voltage before they start to re-charge the capacitors. So, the current
Spikes can be quite high for a very short period of time, 120 times a
second.
Since all the power is being consumed over a very short period of
time, and at high amperage, a higher percentage of the electricity you
are paying for is used to heat the wires.

You can reduce that, and the inrush current, and reduce you output
ripple with a choke between the rectifiers and the capacitors.

According to the servo amp manufacturer, a choke is not necessary and
this is on the high end of recommended capacitance. Now, I have no
clue on wire size needed to prevent the insulation from melting. I'm
going with four #12, one to each cap, and will check for excessive
heating after startup.


The higher the capacitance, the higher the peak current through the
diodes. The servo amp manufacturer doesn't care about your 'power
factor', either.


--
Politicians should only get paid if the budget is balanced, and there is
enough left over to pay them.

On Sun, 17 Oct 2010 17:36:42 -0500, Karl Townsend
wrote:

Somebody please double check my homework.

I got four 2400 MFD electrolytic caps and just selected a 10 ohm 200
watt power resistor from the scrap bin. I get 0.48 seconds to charge
these caps through the resistor using a very simple formula I found:

CHARGE TIME = 5*R*C

I'm setting a time delay relay to .5 seconds before shorting out the
resistor.

Karl

The cap will charge to 99.3% of applied voltage in 5 time constants.