I'm working on a neat idea for my model railroad.
I need to convert a 0->1VDC input source (20 ma max from a DAC) to 0->12VDC 3.5 amps. I'm thinking this is a job for a op-amp. But it's been a while.
Any advice appreciated.
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!
That's more or less a transistor throttle, the typical op amp won't be able to drive 3.5 amps. Probably need a FET on the final stage, with heat sink, to handle 3.5 amps comfortably.
It's been WAY too long for me to figure out the specifics, but basically you'd be looking at an input, depending on the actual use you might want to optoisolate it, and then a stage or two if amplification and a final driver. Unless you just need it to go on and off. That'd be a lot easier. Op amp or comparator or even a 555, driving a solid state relay of sufficnent capacity for the 3.5 amp load.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Simple physics won't allow you to gain BOTH volts and amps from a single source. You can boost the voltage from 0>1 to 0>12 volts, but your current will be proportionately diminished.
Likewise you can raise the current, but your voltage will be proportionately diminished. No such thing as something for nothing.
Unless you use the original supply to control a higher rated supply, you're not going to gain both voltage AND current together. Unless something has recently been discovered that I'm not aware of, to gain one, you'll lose the other - no win-win with electricity.
Mark.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
Actually, this is almost a didoe-type block detector - the current detected across the drop of a diode or two is amplified and powers a control circuit, and while it's common for these to be simple open collector current sinks running rail ro rail, ther eare a few that are set up as current sources to drive lamps and LEDs directly. Needs one more stage to amplify the current from the 100ma range to 3.5 amps.
Does have me wondering, now that I read it again, 0-1V input? Not too many things are that low, drop across a pair of diodes is 1.2-1.4 volts, TTL and CMOS logic are much wider swings than that. Current transformer detecting a flow of current? Perhaps. But then we are back to the block detector circuit, and it's a matter of adjusting the design so that the output is not swung rail to rail but rather is proportional.
My wild guess is the 12V 3.5A output is to control LED lighting...
I think the OP's hint is ample: The input will be driven from a digital to analog converter output.
A better question is what kind of load is it going to drive? Would a PWM output be OK (it can be filtered)? Is the load current generally proportional to the output voltage, or can high current be needed even at low voltage? With a linear solution (no PWM) the power dissipation when driving low volts and high amps will be daunting (read: big heat-sink).
What is the resolution of the DAC? Do you need that much resolution (scaled of course) on the output? It may matter if you want PWM output.
There are integrated power amp ICs that might do the job, depending on your needs.
Need more info to make good suggestions.
Andy
I've apparently completely failed to understand what the original poster is asking ....
Randy you hit the spike on the head. It is for LED's.
http://www.ledwholesalers.com/store/index.php?act=viewCat&catId=100
These LED strips are bright enough and efficient enough to light a layout. And there's a huge variety of colors. From Bright White Sunlight (Early morning light) to Full Spectrum (High Color Rending Index Midday sun) to incandent warm white (evening setting sun) to night time, (blue)
I have a 4 channel DAC that puts out 0->1V. I want to drive this in an op-amp (or series of op amps) and line these strips up. Morning sunlight on the east side, Midday straight above, and Incandecent warm white on the west side. And night time lined throughout east->west.
The power source for the LED's would be a 12VDC 50 amp supply that I use as a general feed on my layout. I just need the op-amp to regulate that output. I need to start searching the tech documents on op amps and see what they say.
The specs on most of these is 5 meters and around 24 Watts (roughly 2 amps) But I'll need to run about 10 meters worth for each channel (LED strip)
I can easily read the fast clock on the digitrax system and simulate the sun rising and setting on the layout with slowly shifting lights. Even the shadows would move. I just need the driver circuit. Multiple op-amps wired up in parallel maybe?
One of the things that always spoiled the illusion of good layouts is seeing hard shadows from light sources pop up on backdrops. This could also help with that.
Very creative idea!!!
I looked at the website. I think a better avenue for you might be to use one of their dimmable LED power supplies, and find a computer controllable dimmer for it (google is your friend).
Directly driving LEDs requires controlling (regulating) the current instead of the voltage. LED light output varies linearly with current, but exponentially with voltage. While there are linear and PWM power amplifiers that can be configured to regulate the current output, the design of a safe, stable, reliable control circuit is not trivial.
Please don't take this the wrong way, but some of your questions suggest you may not be ready to develop an LED driver solution on your own, hence the dimmer recommendation above.
Well I could get 4 of these....each would power 30 meters (~150 feet) of LEDs and then use a computer controlled x-10/crestron computer controlled dimmers. Seems like a bit of a long work around, but if it works....
http://www.ledwholesalers.com/store/index.php?act=viewProd&productId=840
Compared to the computer-to-DAC interface, plus a custom driver board design and build, I'd consider it a short-cut rather than a work-around. And I design electronic HW like that for a living!
Have fun!
AN op amp is pretty much not what you want. Can you skip the DA converter and grab the digital input directly? PWM is the solution here, somilar to what this guy did:
watch?v=mbXjKfGtag
Or a beefed-up version of the Aux Box, because really what this ends up being is a high current DCC decoder. PWM control on the 12V side is better than varying the 120AC side into a transformer.
I don't know off the top of my head what the response time of typical LEDs is. The OP is using this for video or still photography, so "good enough for visual" might not be good enough. Filtering the PWM output would help if the LEDs respond too quickly and interfere with the shutter/exposure control. That is assuming this is powered from the 12V supply he originally suggested, not a current-controlled LED supply.
"Better" means lots of things... Off the shelf with a warranty is one definition. Yet another toy to tinker with to make it work right is another...
On the other hand, a single string of any given temperature whiteness, at a fixed brightness, for each time/angle, together with some editing to adjust white balance, contrast and brightness after the fact might also work.
I'd be curious to see the internals fo the 8 amp dimmer Micro Mark has. It goes int he low voltage side. PWM is pretty much the only way you can dim LEDS, the brightness is current related, not voltage, and the working range is VEY small - you can dim them to a point, but beyond a minimum current, they just don;t light.
It all depends on the PWM implementation - TCS decoders can dim LEDs very well, Digitrax decoders really can't. Both use PWM to vary the function outputs. TCS has a CV to adjust this for incandescent bulbs or LEDs, and when set for LED it works great.
For $9 it might be worth it to check out the Micro Mark device. If the potentiometer is driving another circuit, it's not a huge deal to implement a digitally controlled potentiometer in its place. Or really go out there - servo motor to turn the shaft.
This one: http://www.micromark.com/led-strip-dimmer,10721.html
Had this come up earlier I probably would have thrown one on my last order just to look at it.
V*V/R = Watts. So I see where the non linear part comes in when you linearly reduce voltage.
I have several options here:
Use something similar to infineon. I can use a standard dimmer with a dimmer compatible supply for the LEDs, or I can use infineon's dimmers specifically for LEDs. (Smarthome.com has them) I would also have to get the developers kit. This option is the fanciest, but most pricey by a long shot.
Use the micromarks dimmer control. But I would need to replace the varistor with a digitally controlled one. No clue where to find those or how to hook one up to a computer.
Use something similar to a DCC decoder issue commands via loconet to the decoder, and modify the output through a (J)FET possibly?. But FET's increase current based on the input. And DCC decoders are PWMs I could modify the output of the PWM with a capacitor to smooth the current pulses before feeding into a FET maybe.... The current of each FET though would have to be precalculated based off the power and number of each LED strip attached to it.
Thanks Randy, Alan. I'm going to look into this further.
There are some high current DCC motor decoders, but I don't know if any of them go above 5 amps. I think some of the Aristo RC recevers for large scale get up around 10 amps - they use PWM as well. The difference is that with PWM you can control much large currents with smaller components, because the drives in a PWM circuit are operated at essentially full on or full off, so the driver doesn't have to dissipate much wasted power. A analog driver, especially on the dimmer settings, has to dissipate all the excess current not being delivered, usually as heat, and so you need a high wattage rating and usually a hefty heat sink.
The fact that the Micro Mark unit can control up to 8 amps and is a rather small box almost guarantees it uses PWM.
COuple of more links I found (I'm kinda interested in this as I think by the time I build my 'ultimate' layout, I will be lighting it with LED strings like this and I would really like to have multiple dimmign control so I can simulate a whole day/night cycle)
No details but ideas: http://forums.parallax.com/showthread.php?138613-Controlling-High-Current-LED-PWM
This one uses an Aurdino and a chi from TI, plus amplifiers to increase the current capacity of each channel: http://sonicrobots.com/2012/04/24/the-pwm-controller-tlc-5940-the-arduino-and-a-high-current-output-circuit/
The key seems to be maintaining constant current, as a current change to the LEDs will change the color temperature of the light, wheras PWM doesn't have this effect.