high voltage low power resistor
high voltage low power resistor
(OP)
I have a RC compensation network on a high voltage op amp (Cirrus PA78). It consists of a 3.3K resistor in series with a 33pF capacitor that cycles at 1000 Hz maximum with a 210V maximum swing. The dynamic power dissipation in the resistor is f*C*V^2, or 1.45 mW. If I put 210V across a 3.3K resistor the static dissipation is 13W! There is no staic dissipation in this application though because this resistor is in series with a capacitor.
So I need a high voltage resistor but it doesn't need to be able to dissipate a lot of power; a bit of a contradiction. If I pick a resistor with a limiting element voltage higher than 210V and a power rating better than 1.45mW will it be reliable? Something like the Rohm KTR family seems to fit this.
Thanks,
John D
So I need a high voltage resistor but it doesn't need to be able to dissipate a lot of power; a bit of a contradiction. If I pick a resistor with a limiting element voltage higher than 210V and a power rating better than 1.45mW will it be reliable? Something like the Rohm KTR family seems to fit this.
Thanks,
John D





RE: high voltage low power resistor
The 1206 resistors I typically use (vishay/dale) are rated 200V, 0805 are 150V. In this situation I would use two 0805 or two 1206 resistors in series.
RE: high voltage low power resistor
Keith Cress
kcress - http://www.flaminsystems.com
RE: high voltage low power resistor
Keith Cress
kcress - http://www.flaminsystems.com
RE: high voltage low power resistor
The capacitor value 33pF is pretty darn low, almost within "stray" territory. And 1kHz isn't exactly RF (but beware harmonics).
RE: high voltage low power resistor
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If we learn from our mistakes I'm getting a great education!
RE: high voltage low power resistor
VE1BLL has a good question. As the 210V is across the RC, the actual voltage is dependent upon the signal rise time. My worst case is about 9 uS, so i=C*dv/dt (33pF*210V/9uS) gives 0.72mA, and i^2*R is 1.7mW, very similar to the actual dynamic power in the capacitor.
I have a space constrained design, so I'm trying to keep the solution as small as possible. it looks like one 2010 can do it, or two 0805 resistors depending up on the PCB space available.
John D
RE: high voltage low power resistor
RE: high voltage low power resistor
So the peak voltage across the resistor itself about 2.4 volts (?).
RE: high voltage low power resistor
If my worst case current was 0.72mA the max voltage was 2.4V (<< 150V) and the max power was 1.7mW (<< 125mW), why did this resistor fail? It has zero static power dissipation (open circuit at DC due to the series capacitor), and low dynamic power dissipation. Why did it fail?
A capacitor failure could have brought this on, but the series cap is rated for 3000V (AVX 1808HA330KAT1A) so that seems unlikely. I'll need to test that though.
Thanks for the help,
John D
RE: high voltage low power resistor
RE: high voltage low power resistor
Or - forget about it and just use two resistors in series from now on. Used the time saved to drink beer.
RE: high voltage low power resistor