What constitutes a "capacitive" load?

Thread Starter

SiCEngineer

Joined May 22, 2019
299
I am working on a power supply design but I suppose this is a general question that most people will be able to answer.

I am in need of purchasing a load switch from a company and they have assumed that my load is capacitive. Now, in real life, it would be a complex load with cap/ind/res impedances. But I am doing a PhD and using a dummy load of 300kOhms to represent a low power output which is to be switched.
I am confused because, of course, there is some capacitance placed across the output load resistance to reduce ripple and provide voltage hold-up during the switch off times. However, the "load" itself is just a resistor.
In this case, does simply having capacitance across the load resistor make it a technically resistive-capacative load, or is it purely resistive?

Thanks!
 

MrChips

Joined Oct 2, 2009
22,531
With any reactive load, the current through the load will not be in phase with the applied voltage.
If the current leads the voltage then the load is capacitive.

Remember "ELI the ICE man" as a memory aid.

Voltage (E) leads the current in inductive load.
Current leads the voltage in capacitive load.
 

KeithWalker

Joined Jul 10, 2017
1,405

Papabravo

Joined Feb 24, 2006
14,867
In a reactive load, the inductive reactance and the capacitive reactance have the opposite sign. So if the inductive reactance is 10 Ω, and the capacitive reactance is 12 Ω. then the load is capacitive.
 

Audioguru again

Joined Oct 21, 2019
2,405
Since the load is a 300k ohms resistor then the current is almost nothing and any switch with gold-plated contacts will work fine.
Current-limiting in the circuit will limit the turn-on current through the switch in the "voltage hold-up" capacitor.
 

Thread Starter

SiCEngineer

Joined May 22, 2019
299
Hi All.

Yes, the load current is small. Issue is that the load voltage is higher, multi-kV, so the load switch is a specicalist one from a company in Germany. Because of the high voltage I am keeping the load cap small, it is a Mica 100nF-200nF capacitor that is placed across this load in particular. Voltage is 3kV.

The company said the load current does not cause any differences in the switching loss and only the voltage, capacitance and load switch frequency matter. For my power supply there is a total power of around 500W, yet they are quoting me for their highest power switch with a 4kW radiator. It seems like it is quite overkill.

Btw - made a mistake with the load resistor. The switched resistor is 15kOhm or so and consumes over 500W.. The 300kOhm resistor is a fixed resistor which consumes about 30W only.
 

WBahn

Joined Mar 31, 2012
26,398
I am working on a power supply design but I suppose this is a general question that most people will be able to answer.

I am in need of purchasing a load switch from a company and they have assumed that my load is capacitive. Now, in real life, it would be a complex load with cap/ind/res impedances. But I am doing a PhD and using a dummy load of 300kOhms to represent a low power output which is to be switched.
I am confused because, of course, there is some capacitance placed across the output load resistance to reduce ripple and provide voltage hold-up during the switch off times. However, the "load" itself is just a resistor.
In this case, does simply having capacitance across the load resistor make it a technically resistive-capacative load, or is it purely resistive?

Thanks!
They are talking about the net reactive behavior of the load. To a large degree, the inductive and capacitive portions of the load will cancel each other out leaving you with a net that is one or the other (or possibly so little that it doesn't matter).

The reason they are assuming that your load is capacitive is that switching inductive loads presents specific challenges due to the inductive kickback that can cause arcing across the switch.
 
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