Bipolar current limit

OK this is perhaps overkill.. see footnote. So I've got an indicator LED (bi-color) the input is the error term from a thermal loop, and I turn that into an LED current.

I was thinking it might be nice to turn up the gain on the current, but then I worried about burning out the LED with too much current from the opamp.* So I drew this up... and then added the diodes to stop the transistors "running backwards". And the cap to stop the oscillations when the transistors turn on.

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Don't laugh too loud. Is there some easier way to do this?

George H.

*Right one easy answer would be to find an opamp with ~20 mA maximum current. (uA741 ??? :^) Do you have any suggestions? All our "in stock opamps are higher currents.
Reply to
George Herold
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Assuming that this is for a visual indicator, just use a rail-rail output op-amp with a series resistor. In normal operation the op-amp is within its voltage limits and current is as commanded. When the input is big, the op-amp "asks" for too much current, hits the rail, the resistor limits the current, and Bob's yer Uncle.

There's a lot of reasons not to want to let an op-amp hit the rail, but as long as you have one that does so in a reasonably well-behaved manner you should be OK.

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Tim Wescott 
Control systems, embedded software and circuit design 
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Tim Wescott

I've done that, but the one issue is the turn on voltage of the LED. I've got a few volt dead space near zero volts. (I want to say again that perhaps this is a silly idea, and I'll bag the whole thing.) So by feeding back of the current I eliminate that dead space. Then I dreamed of more gain.. so when the loop is closed but a little unstable maybe you could see the oscillations in the error term in the intensity of the LED... or if near zero by having it change color. But then some student leaves it in high gain with maximum error and the current from the opamp heats up the led. (potential failure...)

So I wanted to limit the current to the LED. (I never can seem to put enough background into my questions...)

Hmm well I run opamps into the rails all the time. For slow stuff, I've never had problems.

In this case I'm talking about running opamps at their current limit. I've never done that (on purpose) before.

George H.

I
Reply to
George Herold

I'm not saying don't do the current feedback -- I'm just saying that if you do that AND put a resistor in series with the LED, then when the op- amp can it'll regulate current, and when it can't, the resistor will cleanly limit current.

You could even make the current-sense resistor into your current-limit resistor, and get plenty of signal thereby.

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Tim Wescott 
Wescott Design Services 
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Tim Wescott

Looks like an OpAmp "snuffer" to me... there is no bound on the current you are pulling current directly from the output. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

Hmm yeah that's my 1 k ohm series resistor.. well 1k to 0 k. when the opamp is railed, 15V -> 15ma... then when changes get smaller so ~1V or smaller signals 1V ~15 mA.

George H.

Reply to
George Herold

This:

Let R = maxcurrent / (Vcc - Vfwd)

It assumes that Vcc = -Vee, and that the two diodes' Vfwd is reasonably close, but it'll get the job done. Can be easily rearranged for a single supply, if you have enough overhead voltage. If you need more gain put a resistor in the leg between R and the V- input, with another resistor to ground.

Vcc o |\| .------|-\ | | >----o----. o--------)------|+/ | | | |/| | | | o - V -> | Vee ^ - | | | | o----' | | '--------------o | .-. | | R | | '-' | | === GND (created by AACircuit v1.28.7 beta 02/28/13

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Tim Wescott 
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Tim Wescott

Right. I'm not sure where the voltage drop is.. (opamp or transistor) but the current is limited by whatever the opamp delivers.

All my opamps (in stock) are ~30-40 mA max output current. They are all rated to have the outputs shorted continuously. (It's not something, I've looked at carefully.) I thought that current might kill the leds.. even the spec of 25 mA max probably limits the lifetime. So I'm snuffing the opamps above ~14 mA (0.7 V is about where it turns on hard.)

George H.

Reply to
George Herold

Right.. did my dropbox link not work?

George H.

Reply to
George Herold

Yes it did -- but you have a bunch of extra stuff in that schematic that's not necessary if you size R correctly.

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Tim Wescott 
Wescott Design Services 
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Tim Wescott

I want to be able to change R..photons/ volt... (though leds are not all that great at low current) from ~1k ohm to ~50 ohms...

Speaking of which I'm using a green /red bipolar led, not all that great at matching the eyes response... (the green is much brighter than the red, eye-wise) would red/orange... green/blue* be better?

George H.

Reply to
George Herold

I think you have the brighness control in the wrong place. You can use a cheaper pot and/or get more reliable operation when you're not putting 10mA though the wiper

Once you move that, current limit consists of building a gain stage and letting the op-amp reach for the rails. it doesn't need to be a rail-to-rail op-amp it just needs to be one that remains sensible whist making an effort.

eg: --. B

Reply to
Jasen Betts

If you use a Howland current source (1 op amp plus one sense resistor plus one quad R pack) you can just put the pot as a "volume control" in front of it.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

Ahh... OK I think that works! (Seems fine on paper.) Thanks! You know I often get stuck down some rabbit hole of a design, and instead of digging deeper I need to back out and crawl down some other hole.

George H.

Reply to
George Herold

Thanks Phil, I've never used a Howland current source. Is there some advantage over the more traditional circuit? (Like Jasen's above.) The Howland keeps the load grounded... but I don't see how that helps me here...

George H.

Reply to
George Herold

Either one works.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

What I drew is a voltage follower on the voltage on R. If you want more current/volt, then you can either reduce R or you can make the thing into a voltage amplifier on the voltage on R by putting a resistor from V- to ground, and a resistor from the junction of R and the photodiode to V-.

Oh, a green/red LED matches my eye response just fine -- I can't tell the difference between them.

I'm not sure what would be better.

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Tim Wescott 
Wescott Design Services 
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Tim Wescott

Would this do what you want?

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

Right that's what I started with.. I then made a wrong turn. Jasen straightened me out. (I've now got a 50 ohm sense resistor and ~700 ohms in series with the LED.. Which limits maximum current to ~15-20 mA or so.)

Oh Jasen, if you are ever in Buffalo, I owe you a beer, or your beverage of choice.

George H.

Reply to
George Herold

OK, that lets you scale full LED brightness to some smaller voltage input, independent of the current limit.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

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