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I want to set up for a 274 rectifier and I have read that for a cap input filter the first cap should only be 4uf? What is the recommended value of the capacitor if I use a choke input filter of about 5 henries? thanx for the help, Dak
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The actual requirement is to limit the peak repetitive current. You can model the PSU in PSUD to find this, though it does not contain a specific 274A/B model. I see no spec in the 1936 data sheet 274A) but the VadeMecum page says 525mA peak current. PSUD gives me 505mA with 4uF at 450+450v, 180+180 ohms per diode, 150mADC output, and using the 5V4G.
Not turn on surge current. That's a different matter.
Question, will a huge value C1, in a critical choke input filter cause high peak repetitive currents?
BTW, I'm not advocating the use of a huge value C1.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
If the choke has critical or greater inductance, then the current through the choke (and hence the rectifier) is roughly a sine wave superimposed on a steady bias. The current then is always more than zero and less than twice the average current. The following capacitor does not have any effect on this.
If 5Hy is critical inductance for your current draw and voltage (the DCR of the choke factors in as well) then C1 can be very large without bothering the 274b.P.S. I'm not willing to tightly define "very large" but IMO 100uf or 200uf should not be a problem at all.
BTW, the data sheet does show 4uf as the maximum capacitance for a cap input filter in conjunction with 180 ohms impedance in each plate circuit.
If you increase the impedance you can increase the capacitance.
If you decrease the impedance you must decrease the capacitance.
For a critical choke input filter the impedance requirement is negated.
A 5Hy choke has 3.77k ohms of reactance at 120Hz.
If it's not the same, I believe there is at least a direct relationship between the math that determines the maximum cap value vs. the impedance requirement WRT a cap input filter and the math that determines the maximum cap value (of C1) vs. the reactance of the input choke WRT an choke input filter.
For all I know that too is part of the meaning of "critical inductance" and you can use as big a C1 value as you want as long as the choke meets critical inductance?
P.S.S. The critical inductance value is the value needed to keep each of the two diodes contained in the 274b conducting for their whole half cycle.
Because the diodes are each on for a much longer time (vs. a cap input filter) the current through the 274b can stay low (just a little over load current) and still replenish C1 without drawing too much current through the 274b.
Making sure there isn't too much current through the 274b is what all these requirements are in place for.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 10/10/14 10/10/14 10/10/14 10/10/14
Hi Tre, so , you are saying that with a choke input filter the 4uf is not a necessary requirement. I am planning to do LCLC filter, with a value of about 50uf for the first cap and around 200uf for the second cap. cheers, Dak
The 4uf is not intended to be a requirement. It's intended to be a maximum uf value.4uf is the largest cap you can place directly after the 274b when using a cap input filter.
You're not using a cap input filter. You're using a choke input filter.
You don't have a cap directly following the 274b so there's no need to concern yourself with what it's maximum value might be.
You, instead have a choke directly following the 274b and you do need to concern yourself with what it's minimum value has to be for it to be of critical value.
As long as you are using a critical inductance choke input filter, 50uf will be fine.
The math to calculate what constitutes critical inductance is,
Inductance = ((voltage/current)+DCR of choke)/1000
Inductance in Hy, current in amps, DCR in ohms
One example of a 5Hy input choke being of critical value would be a supply of 325 volts at 65ma (.065 amps) with 50 ohms of DCR in the choke.
Most designers will increase the Hy value just to be safe, maybe even double it.
I don't know the voltage of your supply and I don't know the idle current of your load (circuit) or the DCR of your choke.
You should plug your own numbers into the formula and see what you get.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 10/10/14
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