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Improved DRSSTC Current Monitor Circuit

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Marko

Sat Jun 10 2006, 09:47PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

The complex part of the circuit is just for display purposes and has no important function other than lighting up a few LEDs. And as i stated before, you could eliminate those op-amps completely and use just a couple of diodes or'd together. I'll still finishing up this document, so i'll probably add a few "simpler" alternatives as well.

This IS the part of circuit i'm talking about. 'thing about lighting few leds', current snse circuit used to provide constant voltage for LM3914 input.
As I figured out OCD is the same for all of us, I didn't say nothing for it.

Reaching

Sat Jun 10 2006, 10:00PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

mhh, why do you think that my circuit will not work properly? i looked at your shematic and copied the important parts, such as the comparator etc which is nearly the same as in your circuit except the shorter pulse on the ocd out on the ne555 to clear the sync flipflop on the driver board. R7 and R8 is a voltage divider by ~8 to provide a exact current reading on the bargraph. for example, if you get 1 volt on the comparator input, you should have 100A primary current, this gives 0,12volts on the input pin 5 of the lm3914 so the first led lights on, cause the voltage range of the lm3914 goes from 0-1,2volts to light all 10 leds.nothing too complex here.

btw dan, what should i change to make it work properly in drsstc duty, on the breadboard it works just fine.

Marko

Sat Jun 10 2006, 10:10PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

The circuit will work, but leds will undergo 'PWM' effect dependent on interrupter duty cycle.

Maybe a filter cap on input or something should help a bit, but I don't know how good is speed going to be.Å¾

You can always try-and-see.

Dan's circuit sems to use active filter, current setpoint linked to OCD etc. for a bit higher conponent count.

Steve Conner

Sat Jun 10 2006, 10:37PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

wrote ...
1. If you use a large time constant (enough to provide proper hold-up for the display to provide accurate readings over entire PRF range), then it will be TOO SLOW for current limiting. A few hundred bursts could go by with a 5-10 sec time constant filter before the current limiting kicks in.

This is true. I got round that in a sneaky way though. I do the current limiting with a window comparator working on the unrectified CT signal, so it's as fast as can be since there's no filter whatsoever. So if the current goes over, the drive will be stopped on the very next cycle.

For the meter, I used a precision rectifier with a very fast attack time constant (0.1uF/10 ohm) and a very slow decay (0.1uF/10 Meg ohm). So it grabs the peak current within about 2 cycles and holds it for long enough to give a steady reading above about 10bps. It was designed for an analog meter but would also drive a bargraph fine.

HV Enthusiast

Sat Jun 10 2006, 11:45PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

That method alone gives you at least twice a fast response alone as the conventional rectified comparator circuits.

I was playing with the notion of using a precision rectifier, but decided i didn't need it since signals below 1V (50A to 100A in the coils i've done) really don't mean much as far as DRSSTCs go.

Hazmatt_(The Underdog)

Sat Jun 10 2006, 11:49PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Thank you Dan for posting the current limiter.

I have not yet attempted building a SSTC of any variety and am clueless at such high currents. These types of posts and schematics really help me see what these systems should be doing and how to approach problems like HF current.

I hope to have a small SSTC halfbridge going soon. Only trouble is that its not in my budget, nothing is.

So, Ill just take care of my Linear Systems class, sit tight, and try to cease some thunder some day.

Steve Conner

Sat Jun 10 2006, 11:57PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

If you'd wanted to build Tesla coils, you should have taken the "Completely Useless But Kind Of Fun Systems" class instead.

HV Enthusiast

Sun Jun 11 2006, 12:04AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Actually, most of my classes I took at university would fit under the category:

"Completely Useless and DEFINITELY Not Fun"

Heck, the only useful class i took in high school was typing.

Hazmatt_(The Underdog)

Sun Jun 11 2006, 02:46AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Steve, I can't just build a TC for a class. In fact, the last class 400D, a design class, wouldn't have made it possible.

You think "Sure you could its design"

Sorry bud. This is a TEAM effort. I wanted to do something as important as power supplies, well, nothing doing. I got thrown into a group to solve their power problem.

Coming up with a viable electric 4 wheel off-road vehicle was good, and I got an A, but I would have rather done a power supply.

The point is, even if you have a design class, its all by group and I was out voted 4:1 and wasn't allowed to do a project by myself.

Bonus: since it was all calculations, it didn't cost me anything.

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