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Improved DRSSTC Current Monitor Circuit

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LAN_403

HV Enthusiast

Fri Jun 09 2006, 05:09PM

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

Here is the first revision release of my improved DRSSTC Current Monitor Circuit.

Features:

- Bar Graph Meter
- Range up to 1000A
- Uses inexpensive commercial current transformer (or wind your own)
- Real time viewing of peak currents within the primary circuit
- Active current limiting
- Maximum current setpoint indicator

The complete document and design can be downloaded here:

http://www.easternvoltageresearch.com/currentmonitor02.pdf (4.6MB PDF)

Dan

Reaching

Sat Jun 10 2006, 10:36AM

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

wow dan, nice circuit and a very good idea with the bargraph display, exactly what i was searching for to give my teslathon drsstc something special.

but why these horrible expensive comparators?
iÂ´ll try to see if i can build up this circuit propely with lm393 cheapo comps or so, for my needs it dont have to be 0,1A exact.

HV Enthusiast

Sat Jun 10 2006, 12:17PM

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

Reaching wrote ...

but why these horrible expensive comparators?

Reaching,

The only comparator used in this circuit is the LM311 which is a generic comparator and pretty inexpensive. I assume you are talking about the LT1630 op-amps.

Yes, i forgot these were quite expensive. These were originally chosen for my prototype because the op-amp needed to be a true rail-to-rail input/output since i was dealing with small signals, but for this particular design, a cheapo LM741 will do the trick just fine, although be a bit slower and not work as well when ran near one of the rails.

But, yes, the LM741 or similar generic op-amp that is capable of running single-ended should be fine.

The only thing to really worry about when choosing your op-amp is how far off the supply rails it can operate. Try to choose something that can operate as close to the rails as possible for maximum performance, especially for low currents.

I'll update my datasheet to include the cheaper op-amps later today. Thanks for noting that.

Please note that I am releasing a whole series of downloadable DRSSTC papers and design guides which will be added regularly to the Amateur Design Resource section of our website. Check back often!!!

Steve Conner

Sat Jun 10 2006, 05:42PM

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

If you don't like expensive op-amps, my current monitor/limiter does all that Dan's does using a TL074 and half of a LM339.

The only drawback of my circuit is that it's not at all obvious how it works, but trust me it does read and limit the actual peak primary current accurately. Right down to zero amps, too.

It doesn't have the "view setpoint" feature, but you can add that with one diode and one SPST push button. (again I leave this as an exercise for the reader :P)

If you're looking for other rail-to-rail op-amps to substitute in Dan's circuit, try the LMC6484 or LMC660.

Marko

Sat Jun 10 2006, 05:55PM

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

The only drawback of my circuit is that it's not at all obvious how it works

Huh?

Nice addon for a DRSSTC.
I wonder if anything more simple than that can be done, like LM3914 (for LED's) with rectifier and
burden (like OCD) with some passive calibration?

Probably circuits won't be hard to build but they end to have bigger number of conponents than entire coil (steve ward's at least

)

Reaching

Sat Jun 10 2006, 06:39PM

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

im still on breadboarding this circuit and the first part works fine, but the lm741, lm3914 story wont work, dont know why,too many resistors . now im looking for a way to simple connect the lm3914 to view the primary current.

okay, i got it, but handle with care, its still a breadboard design, but works fine. the only thing you cant do with it is to view the overcurrent setting on the bar graph, but you can connect a digital voltmeter on U Ref to view the setting

Marko

Sat Jun 10 2006, 08:11PM

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

okay, i got it, but handle with care, its still a breadboard design, but works fine

Exactly what I tought of.
You could maybe make R7 and R8 variable for calibrating/offseting the meter (just for led's this seems fair), and voltage on burden depends on current directly.

Maybe you can be evil and use 3914 led output itself as easily readable overcurrent protection, altough it won't be fine-tunable and practical as comparator.

I wonder what was the reason for more-complex design?

HV Enthusiast

Sat Jun 10 2006, 09:10PM

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

wrote ...

I wonder if anything more simple than that can be done, like LM3914 (for LED's) with rectifier and
burden (like OCD) with some passive calibration?

Sure. If all you want is an LED current meter, a single diode, cap, and an LM3914 IC is all you need. However, for this particular design, two separate filters are required for the current sample. One is sized to reset every pulse burst, while the LED display is designed to hold for a second or two (orders of hundreds of bursts). Remember, if you use only one RC filter (for hold-up) and size it too large, the current limiting will be too slow to work properly.

wrote ...

It doesn't have the "view setpoint" feature, but you can add that with one diode and one SPST push button.

You'll need to correct for the diode drop though, unless your full range current voltage is large enough that the Vfwd of the diode is neglectable.

wrote ...

Probably circuits won't be hard to build but they end to have bigger number of conponents than entire coil (steve ward's at least

Firkragg,

If you compare just the active current limiting portion circuits of Steve Wards vs. this circuit, Steve Ward actually uses more components. Neither circuit is better or worse, they both accomplish the same exact thing.

wrote ...

okay, i got it, but handle with care, its still a breadboard design, but works fine. the only thing you cant do with it is to view the overcurrent setting on the bar graph, but you can connect a digital voltmeter on U Ref to view the setting

Sounds like you have an error in your breadboard circuit. I'm using the same circuit in my DRSSTC IV system now and it works perfectly (setpoint indicator)

wrote ...

Exactly what I tought of.
You could maybe make R7 and R8 variable for calibrating/offseting the meter (just for led's this seems fair), and voltage on burden depends on current directly.

This does work, but given Reaching's circuit as is, it will only provide a true reading during 100% duty cycle. The 100us time constant at the input to the comparator is much too slow provide proper hold-up between bursts, especially at lower PRFs. I use about a 5-10 sec time constant on my RC filter (10uF / 1Meg) for the LED display.

As stated before, you really need two separate filters for proper operation.

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.

2. If the time constant is set to very short, the active current limiting will react fast enough, but the hold-up time will be too short to maintain a proper level (again at entire PRF range) to be accurate on the LED display meter.

Hence the need for the buffer op-amp circuits in there.

They act more as buffers, than scaling / offset.

That is the reason for the more-complex design.

Of course, you could buffer with diodes as well (come up with a pair of diodes) into two separate circuits.

BTW, if you are afraid of a few extra components in your DRSSTC design, then you should probably stick to traditional spark gap coils!!!

Marko

Sat Jun 10 2006, 09:29PM

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

BTW, if you are afraid of a few extra components in your DRSSTC design, then you should probably stick to traditional spark gap coils!!!

I actually more tought for steve conner's current sense circuit, and I never want complicating things that aren't complicated. Your one isn't that bad really.

HV Enthusiast

Sat Jun 10 2006, 09:38PM

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

Firkragg,

The current limit circuit i use is about as simple as they get. Its no more complicated than anyone elses current limit circuit including both the Steve's. Just a current transformer, rectifier, and comparator. Heck, its actually less complicated than Steve Conners as he is using (2) active components.

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.

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