• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

LTSpice and linear regulator

Status
Not open for further replies.

Sceadwian

Banned
Was just going to play around in LTspice with a linear regulator, dropped a few components and immediately got an unexpected result. It's a 12 volt regulator model (supplied directly from linear) I was surprised to see the intial voltage spikes to 16 volts, granted the output is unloaded. Using a large output cap starts eating the voltage peak and applying a load does as well. I'm using the startup directive which means it starts at 0 volts and goes to 20 volts in 20us. Do real world linear regulators do this as well? Can anyone shed some light on the responce?
 

Attachments

crutschow

Well-Known Member
Most Helpful Member
It's likely that that closely approximates the operation of the real regulator. The regulator takes a finite time for it's circuits to settle to the final value. During that time there can be a small overshoot in the output. But in a real system with an output capacitor and load that should not be a problem

That's why I always simulate circuits before I build them. It often shows up unexpected behavior.
 

Sceadwian

Banned
I expected some overshoot, but unloaded it's 4 volts over the output. Unloaded the over voltage lasts 20ms, loaded with a 100ohm resistor it's like 80µs and a volt lower. Bigger caps almost completly squash it. Live and learn I guess. Nice to see a simulator showing close to real world results
 
Last edited:

crutschow

Well-Known Member
Most Helpful Member
Many of the Spice models give close to real world results (but not all unfortunately).
 

Sceadwian

Banned
Most of the inaccuracies I've run across so far are from parasitic capacitance and inductance, and 'behavioral' models as opposed to real spice simulation models.
 
Last edited:

Speakerguy

Active Member
Actually, nevermind my last post. It's a simulation artifact. You've got it set to start all voltage sources at zero with the 'startup' in the sim command. If you make the voltage source a pulse that has a 10ms ramp from 0 to 20V and then stay at 20V for indefinitely long (on time for the pulse longer than your sim time) you will see it goes away and all you get is a very small amount of ringing as the reg turns on. It also goes away if you simply delete the startup command and have the external source start at 20 and stay there. The spike is just an artifact from the voltage source starting at zero and going to 20 in almost zero time flat. Even just a 1ms ramp time is enough to keep the turn on spike below 0.1V.
 
Last edited:

Sceadwian

Banned
I'm using the startup command, it's defaulted to 10us though. I"m curious though when you flick a switch on a real circuit what does the regulator actually see? If it's a switch I'm sure it's noisy, if it's a switch powering a transistor that actually turns the regulator on, I'm guessing it's a pretty sharp turn on.
 
Last edited:

Speakerguy

Active Member
True, if it's a semiconductor switch then it becomes worrisome if that is truly the real-world result and not a simulation artifact. So much is going to depend on input capacitance, trace inductance, parasitics, etc that it would be hard to say what it would look like in the real world. But you're right, it could be something to worry about.I usually have so much bulk capacitance on the input side for filtering that I guess I never really thought of it.
 
Last edited:

Sceadwian

Banned
I bet yer lights dim when you turn your power supplies on =) Mind you this is not bad, cause any PC power supply I turn on causes at least a few cycles of voltage dip. Just gotta avoid blowing the fuses or tripping breakers.
 

Speakerguy

Active Member
Well, on the audio amps yes they do dim the lights :) Current inrush on those toroids is huge, not to mention the beer can caps.

I find GFCI's to be the biggest pain, but they are worth it.
 
Last edited:

Roff

Well-Known Member
Well, on the audio amps yes they do dim the lights :) Current inrush on those toroids is huge, not to mention the beer can caps.

I find GFCI's to be the biggest pain, but they are worth it.
Are you saying that you audio amp trips your GFCI?
 

ericgibbs

Well-Known Member
Most Helpful Member
hi SC,
If you replace the Vreg with a 7812, the excessive overshoot disappears.

RB,
tried the input cap [10uF] in the original circuit, no effect.

Suggests, the 'overshoot' is an artifact of the LT1086-12 model.

EDIT:
I set the plot background to white and the plot line width to wide, in LTSpice, makes it easier to see for us old guys.;)

If you ever print your plots, you will save a lot of black ink.!
 

Attachments

Last edited:

OutToLunch

New Member
to get a better idea of what may be happening, you need to compress the simulation time to look only at the area of interest - i would say no more 100usec of simulation would suffice. You would also need to include the input voltage and the input current along with the output voltage.
 

ericgibbs

Well-Known Member
Most Helpful Member
to get a better idea of what may be happening, you need to compress the simulation time to look only at the area of interest - i would say no more 100usec of simulation would suffice. You would also need to include the input voltage and the input current along with the output voltage.

hi,
I did that in a number of runs using the 7812, never got the 16V long overshoot that SC saw.

Also tried various load and current plots, again, nothing as high as SC.

I believe is a problem in that LT1086 model.

If you have LTS I could post the *asc.:)
 

Roff

Well-Known Member
The startup spike could be an artifact of the model, or it could be an accurate representation of actual performance. Hardware is the only way to know for sure.
 

Speakerguy

Active Member
Are you saying that you audio amp trips your GFCI?
Oh no, just that it dims the lights. The big one that causes the dimming is actually a commercial unit (Adcom GFA-555, 200Wx2 @ 8ohm). That amp is getting to be old enough I'm starting to think about re-capping it, it's pushing 15 years easy (not sure of manufacture date as I bought it used). I did have an electric smoker at one time (the only kind permitted by my apt complex) that tripped the GFCI every time I plugged it in though.

I just mentioned GFCI's as I have aquariums and GFCI's truly are a lifesaver, even if they sometimes do have nuisance tripping problems. Breakers and fuses are far to slow imo.
 
Last edited:
Status
Not open for further replies.

EE World Online Articles

Loading
Top