Re: Resistor pulse handling?

Chris Carlen wrote:

Greetings:

I wish to sample a raw 75V 125kHz H-bridge output. The two H outputs will go to a differential amplifer/attenuator to create a single ended, low-level sample of the output voltage. Trouble is, I don't want the raw signals running along my PCB from the high powered department to the low level zone and radiating spikes all over the place. Thus, I'd like to slow the edges just a bit before routing them long distances.

So use an RC. Trouble is, for instance a 1.25MHz cutoff filter with a 500 ohm resistor could potentially dissipate 11.25W in the ressitor on transitions, even though the RMS current is only 17.7mA.

The question is, can a simple 0.25W 1210 resistor handle the transients?

If I make the resistors larger, then they contribute to errors in the differential amp gain by being close to its input resistance. Well just make them a part of the input resistors you say? Ok, but then doesn't the AC CMRR suffer if the caps aren't matched?

Looking at the Panasonic ERJ 1% series, they allow only 2.5x rated power for short term overload. But I suspect this isn't really the applicable spec for this situation. There is nothing in the datato help one figure out the device's tolerance to repetitive VERY short term overload, where average power is still well within the continuous rating.


Comments appreciated.


Good day!



Since you definitely are not considering carbon composition resistors, then decades of experience with them is not useful.
Perhaps the following compromise can be made.
1) start with a 100 ohm resistor from the source to node 1 that has a (filter) capacitor to ground; set the capacitor value to ten percent of the original calculated value. Peak pulse dissipation is fairly low, as there is less energy in the harmonics.
2) from node 1 to node 2 use a 200 ohm resistor and at node 2 use a capacitor that is 40 percent of the original calculated value. Do not know energy (spectrum) content of what flows thru resistor thru capacitor, but some whiz could do the math. I would dare say that energy may be in the same ballpark.
3) from node 2 to node 3 use a 200 ohm resistor and at node 3 use a capacitor that is 80 percent of the original calculated value. Do not know energy (spectrum) content of what flows thru resistor thru capacitor, but some whiz again could do the math. Guess in similar ballpark.
The idea is to dissipate the fastest stuff first, and do the rest later; spreading the energy in steps.
The values may need adjustment, but the idea is given for evaluation, not as an exact solution.
**
Alternate method: use a ferrite bead in series with the source to the bypass node; ther aer SMT versions as well as leaded versions.


.

Relevant Pages

  • Re: Capacitor discharge question
    ... power circuits. ... concerned about the capacitor being damaged. ... capacitor discharged into .1ohms what peak energy rating on the resistor is ...
    (sci.electronics.design)
  • Re: Resistor pulse handling?
    ... start with a 100 ohm resistor from the source to node 1 that has a capacitor to ground; set the capacitor value to ten percent of the original calculated value. ... Do not know energy content of what flows thru resistor thru capacitor, but some whiz could do the math. ... I would dare say that energy may be in the same ballpark. ...
    (sci.electronics.design)
  • Re: Energy loss in a capacitor
    ... capacitor in connected to an equal uncharged capacitor through a ... the final energy in the capacitors is less than the initial ... This loss in energy is dissipated in the resistor as heat ... of Hertz on radio waves did just that. ...
    (sci.physics.research)
  • Energy loss in a capacitor
    ... capacitor in connected to an equal uncharged capacitor through a ... the final energy in the capacitors is less than the initial ... This loss in energy is dissipated in the resistor as heat ... Electromagnetism entail preference for Joule Heating over radiation? ...
    (sci.physics.research)
  • Re: Cap. Discharge Res.
    ... I am referring to the capacitor across the ... I am assuming that either the appliance has a faulty resistor connected ... Any cap that is placed across the mains in a piece of equipment, ... should not in theory require any resistor to discharge it, ...
    (sci.electronics.repair)