# LTSpice, power ESR, capacitance ESR, Charge Graph

Discussion in 'Circuit Simulation & PCB Design' started by ACharnley, Oct 25, 2017.

1. ### ACharnleyMember

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Hi,

Just learning LTSpice.

Here I have a 50F capacitor with circa 20mOhm ESR and a power supply of 5V with a high 12ohm ESR.

I'm expecting a charge graph over several seconds but as you can see, the power supply ESR appears to have no effect?

Regards,

Andrew

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2. ### MikeMlWell-Known MemberMost Helpful Member

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You have a 50 FARAD capacitor, not a 50uF one....

Your time constant will be 1000000 times longer than you expect. The dV/dt during 10s (your simulation duration) will be uV! When simulating anything, you always should do an estimate in your head. If we charge 50uF through 10Ω, wouldn't you expect the time-constant to be 500uS? So simulate for 5000us or 5ms.

It is bad practice to give the same electrical node two different names. In your case, V+ and Vout. Pick one, or the other.

Also, if you expect to show the charging current transient from zero Volts to 5V, then you must instruct the simulator not to "precharge" the capacitor by doing its normal DC bias solution ahead of the Transient simulation.

Run the attached circuit with and without the "startup" in the .tran statement and see the difference on V(out). Note the "Start external DC supply voltages at 0V" check box in the "Edit Simulation Command".

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3. ### ACharnleyMember

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Th 50F value is correct (supercapacitors). The "startup" command is what was missing, thank you!

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5. ### MikeMlWell-Known MemberMost Helpful Member

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Where can you buy Super Capacitors with 12mΩ ESR?

6. ### ACharnleyMember

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See Nesscap eshsr-0100c0-002r7.

It's 25mohm for the pair, max.