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Beginners / Bulk input caps and impedance
« Last post by paulca on Today at 10:49:46 am »A split out question from my other project. Trying to pick out some of the basics I might be missing.
If I put a cap directly across a DC barrel jack input, then when I connect the power to it, I will get a spark and the supply will basically see a 0 Ohm short for a fraction of a second and a load which seems to want to pull an ideally infinite amount of current.
So, it seems to make sense to put at least a "token" value resistor in series before it. That or one of them magical to me, inductors.
As it is a bulk input cap, it has to be able to pass at least the average current of the load.
If I place an upper limit on the PSU of 200mA and choose a quarter watt resistor, then using ohms law....
I want a resistor that will drop 0.25W maximum across it at 200mA under normal operating conditions.
P / I = V
0.25W / 0.200A = 1.25V
R = V / I
R = 1.25V / 0.2A
R = 6.25Ohm
Now, for the "empty cap" short. Initially the resistor will be dropping all 36V of the input. So the "instantaneous" current through the resistor is limited to
I = V/R
I = 36V/6.25O
I = 5.76A
P = V*I
P = 36 * 5.76
P = WAY TOO MUCH
However. It's transient. How long does it take for that volt drop to reduce as the cap charges?
Well, we know that initially we would be putting 5.76A into the capacitor. This I believe can be input into the capacitor charge equation and produce a charge time / curve.
I think this is the end of the line and as far as I can go, without dusting off the simultaneous equations I haven't explored in quite a while!
As the cap charges the voltage drop across the resistor reduces... so the current passed with reduce as well.
With 5.7A in and 0.2A out and, say, a 220uF cap. How does one calculate the total transient power dropped across the resistor and how do you determine if it's "sensible"?
Can I assume that a quarter watt resistor will take a few micro seconds of 100W+?
If I put a cap directly across a DC barrel jack input, then when I connect the power to it, I will get a spark and the supply will basically see a 0 Ohm short for a fraction of a second and a load which seems to want to pull an ideally infinite amount of current.
So, it seems to make sense to put at least a "token" value resistor in series before it. That or one of them magical to me, inductors.
As it is a bulk input cap, it has to be able to pass at least the average current of the load.
If I place an upper limit on the PSU of 200mA and choose a quarter watt resistor, then using ohms law....
I want a resistor that will drop 0.25W maximum across it at 200mA under normal operating conditions.
P / I = V
0.25W / 0.200A = 1.25V
R = V / I
R = 1.25V / 0.2A
R = 6.25Ohm
Now, for the "empty cap" short. Initially the resistor will be dropping all 36V of the input. So the "instantaneous" current through the resistor is limited to
I = V/R
I = 36V/6.25O
I = 5.76A
P = V*I
P = 36 * 5.76
P = WAY TOO MUCH
However. It's transient. How long does it take for that volt drop to reduce as the cap charges?
Well, we know that initially we would be putting 5.76A into the capacitor. This I believe can be input into the capacitor charge equation and produce a charge time / curve.
I think this is the end of the line and as far as I can go, without dusting off the simultaneous equations I haven't explored in quite a while!
As the cap charges the voltage drop across the resistor reduces... so the current passed with reduce as well.
With 5.7A in and 0.2A out and, say, a 220uF cap. How does one calculate the total transient power dropped across the resistor and how do you determine if it's "sensible"?
Can I assume that a quarter watt resistor will take a few micro seconds of 100W+?
?