Constant current - any use this ?
- Thread starter manie
- Start date
BeanieBots
Moderator
Never used it myself but read the datasheet, there's a gotcha!
Have a close look at the current vs voltage graph.
You need ~5v before it's at the 20mA nominal spec, so not much use direct on a PICAXE output.
Linear constant current is a bit archaic these days, especially for LED driving. It's just SO inefficient. There are now many switching drivers for such applications that only require a single inductor to give ~80% efficiency.
Could be useful as a quick and simple NiMh/NiCd charger or possibly a long distance sensor drive supply but again, there are better methods now.
Do you have a particular app in mind?
Have a close look at the current vs voltage graph.
You need ~5v before it's at the 20mA nominal spec, so not much use direct on a PICAXE output.
Linear constant current is a bit archaic these days, especially for LED driving. It's just SO inefficient. There are now many switching drivers for such applications that only require a single inductor to give ~80% efficiency.
Could be useful as a quick and simple NiMh/NiCd charger or possibly a long distance sensor drive supply but again, there are better methods now.
Do you have a particular app in mind?
hippy
Ex-Staff (retired)
Only two things I can think of ...
* Constant current for battery charging.
* Current-loop communications ... if you've got an ASR33 Teletype and a couple of furlong of cable. http://en.wikipedia.org/wiki/ASR-33_Teletype.
The later might also be useful for driving opto interfaces over a long distance, eg, MIDI, or where high-side +V might be variable but you only want to switch 20mA through something. Basically that's still using it as a constant current LED driver, just with the LED more remote.
* Constant current for battery charging.
* Current-loop communications ... if you've got an ASR33 Teletype and a couple of furlong of cable. http://en.wikipedia.org/wiki/ASR-33_Teletype.
The later might also be useful for driving opto interfaces over a long distance, eg, MIDI, or where high-side +V might be variable but you only want to switch 20mA through something. Basically that's still using it as a constant current LED driver, just with the LED more remote.
Constant current tips
If you don’t mind using 3 resistors then it’s not too difficult to generate a constant current using a transistor but you need to be able to access a stable voltage (eg. PICAXE 5 V supply or output pin). The base of the transistor is driven from a potential divider from this supply and the third resistor is connected between the transistor’s emitter and ground. For example, a base voltage of 2.5V and a 1K emitter resistor will set the collector current at a constant 2 mA.
JFET transistors don’t even need resistors to generate a constant current, just connect the gate to ground. However, this does not allow you to adjust (or shut-down via the PICAXE) the constant current output.
If you don’t mind using 3 resistors then it’s not too difficult to generate a constant current using a transistor but you need to be able to access a stable voltage (eg. PICAXE 5 V supply or output pin). The base of the transistor is driven from a potential divider from this supply and the third resistor is connected between the transistor’s emitter and ground. For example, a base voltage of 2.5V and a 1K emitter resistor will set the collector current at a constant 2 mA.
JFET transistors don’t even need resistors to generate a constant current, just connect the gate to ground. However, this does not allow you to adjust (or shut-down via the PICAXE) the constant current output.
BeanieBots
Moderator
Constant current sources do have their place in analogue design applications but they are typically in the order of 10's of uA. Rarely over 1mA and certainly not 20mA.
The transistor method described by MFB is fine for most apps but how 'contstant' the current is depends on the transistor hfe and the total impedance of the base drive circuit.
If you're really interested, have a look at the internal design of op-amps. They often have several internal constant current sources but these are often made using multi-junction transistors which are not available as descrete components.
To be honest, I really cannot think of an application these days that could not be done better another way.
The transistor method described by MFB is fine for most apps but how 'contstant' the current is depends on the transistor hfe and the total impedance of the base drive circuit.
If you're really interested, have a look at the internal design of op-amps. They often have several internal constant current sources but these are often made using multi-junction transistors which are not available as descrete components.
To be honest, I really cannot think of an application these days that could not be done better another way.
Hey BB ! I still have to search out everything if I get an idea to do whatever... but the help from this forum is always phenomonal ! If I need it in future I'm sure I'll get something done and learn in the process. Even at +60 one is NEVER too old and I've certainly exceeded ANY learning curve from the past in this past year....
BeanieBots
Moderator
So true manie. You're never too old to learn.
I also find this forum to be particularly helpful and a veritable goldmine of information.
I also find this forum to be particularly helpful and a veritable goldmine of information.
fernando_g
Senior Member
Reading the datasheet, one can see that this device is intended for automotive apps.
Since automotive apps are cost sensitive, this device better not cost much more than a 1watt resistor.
I could think of other apps, like trickle charging batteries.
Don't know...This could fit the category of oddball devices just like the National Semi's single cell flasher. LM3909?
Since automotive apps are cost sensitive, this device better not cost much more than a 1watt resistor.
I could think of other apps, like trickle charging batteries.
Don't know...This could fit the category of oddball devices just like the National Semi's single cell flasher. LM3909?
Test results
BeanieBots, Further to your comments about the limit usefulness of a single transistor constant current circuit, I tested a 2N2904 with two 4.7K resistors forming a potential divider that feeds 2.5V to the base from a regulated 5V supply. The results for an 887 Ohm resistor in series with the emitter were a current of 1.996mA at a collector voltage of 2.5 volts, increasing to only 2.005mA at 12 volts. That should be 'constant' enough for many applications.
BeanieBots, Further to your comments about the limit usefulness of a single transistor constant current circuit, I tested a 2N2904 with two 4.7K resistors forming a potential divider that feeds 2.5V to the base from a regulated 5V supply. The results for an 887 Ohm resistor in series with the emitter were a current of 1.996mA at a collector voltage of 2.5 volts, increasing to only 2.005mA at 12 volts. That should be 'constant' enough for many applications.
There are many apps for constant current sources.
I think BB knows that, but I think was referring to the limitations in certain applications.
It is quite easy to make a constant current supply from transitors, linear or switched regulators and, perish the thought, even from a PIC controlled switcher.
Even the originally mentioned component may have a 'place' somwhere. No, it's not efficient, but it IS small.
For driving power LEDs it is (IMHO) pretty silly to drive them using a linear CC source. Highly inefficient. Switcher based devices are cheap, powerful and efficient - it's a no-brainer unless switchers frighten you ? (What do you want? Heat or light?
)
I think BB knows that, but I think was referring to the limitations in certain applications.
It is quite easy to make a constant current supply from transitors, linear or switched regulators and, perish the thought, even from a PIC controlled switcher.
Even the originally mentioned component may have a 'place' somwhere. No, it's not efficient, but it IS small.
For driving power LEDs it is (IMHO) pretty silly to drive them using a linear CC source. Highly inefficient. Switcher based devices are cheap, powerful and efficient - it's a no-brainer unless switchers frighten you ? (What do you want? Heat or light?
)BeanieBots
Moderator
Oh manie, you'd love them
They are the perfect FET poppers
Seriously, a very useful suite of circuits to become aquanted with. No end of uses and variations on a theme. Many custom chips are now available to take all the hard work out of designing one.
One of my favourites (the 2577-ADJ) doesn't even let you get near the FET drive. It's all internal. You actually have to put a little effort in if you wan't to destroy it
They are the perfect FET poppers
Seriously, a very useful suite of circuits to become aquanted with. No end of uses and variations on a theme. Many custom chips are now available to take all the hard work out of designing one.
One of my favourites (the 2577-ADJ) doesn't even let you get near the FET drive. It's all internal. You actually have to put a little effort in if you wan't to destroy it
BeanieBots
Moderator
Yep, Nat Semi.
One of the devices covered by the free "Simple Switchers" design software that Nat Semi provides.
http://www.national.com/analog/power/switchers_made_simple
One of the devices covered by the free "Simple Switchers" design software that Nat Semi provides.
http://www.national.com/analog/power/switchers_made_simple
BeanieBots
Moderator
Go for it manie.
Very little effort to get something that works.
Lots of effort to get it working really well/efficient.
Layout and quality magnetics are important if you want really good results but even a sloppy build should actually work.
Start with something simple like a buck or boost converter. (2577 is ideal for boost).
Then move on to flyback and winding your own transformers.
THEN....
Get a PICAXE to modify the feedback loop
Before you know it, you'll be doing MPPT control
Very little effort to get something that works.
Lots of effort to get it working really well/efficient.
Layout and quality magnetics are important if you want really good results but even a sloppy build should actually work.
Start with something simple like a buck or boost converter. (2577 is ideal for boost).
Then move on to flyback and winding your own transformers.
THEN....
Get a PICAXE to modify the feedback loop
Before you know it, you'll be doing MPPT control
Buzby
Senior Member
A 3904 might be tough, but is it as tough as a 20M ?
Two weeks ago I put the power on to my bread-boarded circuit, and something started to smell HOT !!!.
Was it the 78L05, or a transistor ?.
No, I burnt my finger when I got to the 20M !. Power pins the wrong way round.
Quick power off, swap wires, power on, download and run. No ill effects - apparently.
How did it survive ?. I would have thought every transistor would have popped, the the resonator gone way out of calibration, but it's still running now.
Obviously I'm not going to use this chip for anything important, it might just be stressed to it's limit, but it's a tough little critter !.
Two weeks ago I put the power on to my bread-boarded circuit, and something started to smell HOT !!!.
Was it the 78L05, or a transistor ?.
No, I burnt my finger when I got to the 20M !. Power pins the wrong way round.
Quick power off, swap wires, power on, download and run. No ill effects - apparently.
How did it survive ?. I would have thought every transistor would have popped, the the resonator gone way out of calibration, but it's still running now.
Obviously I'm not going to use this chip for anything important, it might just be stressed to it's limit, but it's a tough little critter !.
Yes, we can get luck as well as unlucky.
Your comment "Obviously I'm not going to use this chip for anything important, it might just be stressed to it's limit" is exactly right.
I did a silly on a PCB with a dsPIC. It still worked but one ADC channel had popped. The one I wanted of course. But having a working chip with a burnt peripheral meant lots of wasted time - and a lesson learnt...
I'm sure there must be a mnemonic for all this - burnt rhymes with learnt, burning rhymes with learning and cool rhymes with fool. I wonder if I can squeeze in a banana?
Anyway, this MPPT? I've never heard of it. What is it manie?
Your comment "Obviously I'm not going to use this chip for anything important, it might just be stressed to it's limit" is exactly right.
I did a silly on a PCB with a dsPIC. It still worked but one ADC channel had popped. The one I wanted of course. But having a working chip with a burnt peripheral meant lots of wasted time - and a lesson learnt...
I'm sure there must be a mnemonic for all this - burnt rhymes with learnt, burning rhymes with learning and cool rhymes with fool. I wonder if I can squeeze in a banana?
Anyway, this MPPT? I've never heard of it. What is it manie?
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BeanieBots
Moderator
Chips can be surprisingly tollerant of reverse voltage.
The clamp diodes alone will offer 2*diode drop voltage protection. However, there is no current limit so with a beefy power supply, the current will be very high and melt the chip very quickly. It can be so quick that the chip can actually explode.
It was probably the current limit of your supply that saved you.
The clamp diodes alone will offer 2*diode drop voltage protection. However, there is no current limit so with a beefy power supply, the current will be very high and melt the chip very quickly. It can be so quick that the chip can actually explode.
It was probably the current limit of your supply that saved you.
I didn't actually say that you did mention it...
I thought you had built an MPPT thingy before... so you could tell us... I probably got that wrong .. old age ... but , hey, what the heck
It's Friday, the weekend starts... the pubs are nearly open...
If you smell your PICAXE burning,
Read the Book and do some learning.
If you find your board has burnt,
You may have read but you have not learnt.
There, I knew I could get some lousy rhyming in somewhere
I thought you had built an MPPT thingy before... so you could tell us... I probably got that wrong .. old age ... but , hey, what the heck
It's Friday, the weekend starts... the pubs are nearly open...
If you smell your PICAXE burning,
Read the Book and do some learning.
If you find your board has burnt,
You may have read but you have not learnt.
There, I knew I could get some lousy rhyming in somewhere
hippy
Ex-Staff (retired)
MPPT ... http://lmgtfy.com/?q=mppt
BeanieBots
Moderator
Nice poem Dippy
Manie, t'was I that first mentioned MPPT in post #19.
Thought it might be a nice little project for manie's first switcher project
One of the uses for a (PICAXE variable) constant current supply/source.
Manie, t'was I that first mentioned MPPT in post #19.
Thought it might be a nice little project for manie's first switcher project
One of the uses for a (PICAXE variable) constant current supply/source.
BB: Could do that sometime but I've also learnt this: Although exciting to learn/do new things, first try and complete projects already doing ALMOST well, such as chook-house... On top of that is still the "Displacement Measurement" project that needs speeding up. Thanks, for now I'm busy (and still learning) enough to cope with it all. Its like me asking you to take up Amateur Astronomy, learn EVERYTHING (about : different telescopes/nomenclature/FL/FOV/CA/Aberation/flocking/collimation/alignment/Alt-Az/Eq/tracking/PEC/Constellation/Dec/RA/WR stars/Var/.....) and then we start Atro-Imaging..... In between you have to get to know our own Moon....in detail ! ALL THAT in 18 months....... OUCH !
Ah Astronomy. I have a strange desire to get a smallish telescope.
I'm interested in doing a little bit of photography.
Nothing grand. Nothing finalised or even thought about in any detail.
I've been casually looking at different offerings and various reviews.
My shortlist includes.
A 10 inch from Orion Optics UK. e.g. http://www.orionoptics.co.uk/CT/ctrange.html
or a cheaper http://www.orionoptics.co.uk/EUROPA/europa250f4.8del.html
- or a similar model from Meade.
A few comparative reviews I have read suggested that the Orion Optics is significantly superior optically in a given price range.
Any thoughts based on your experience?
And no, I'm not expecting Hubble quality so I'm being realistic.
I will avoid any gags about Uranus (which was, in fact, discovered by some smutty schoolboys in the 18th Century).
I need a telescope powerful enough to see Patrick Moore orbiting the moon.
Thanks.
I'm interested in doing a little bit of photography.
Nothing grand. Nothing finalised or even thought about in any detail.
I've been casually looking at different offerings and various reviews.
My shortlist includes.
A 10 inch from Orion Optics UK. e.g. http://www.orionoptics.co.uk/CT/ctrange.html
or a cheaper http://www.orionoptics.co.uk/EUROPA/europa250f4.8del.html
- or a similar model from Meade.
A few comparative reviews I have read suggested that the Orion Optics is significantly superior optically in a given price range.
Any thoughts based on your experience?
And no, I'm not expecting Hubble quality so I'm being realistic.
I will avoid any gags about Uranus (which was, in fact, discovered by some smutty schoolboys in the 18th Century).
I need a telescope powerful enough to see Patrick Moore orbiting the moon.
Thanks.
Dippy: This is WAY...................OFF TOPIC, and this forum could just be swamped now there are so many views out there. PM me if you like... Southern Skies appetizer attached.........
Edit: Nice looking Carbon tube job that ! Even in S.A. terms it is quite expensive !
Edit: Nice looking Carbon tube job that ! Even in S.A. terms it is quite expensive !
Attachments
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OK.
This looked like a dead thread...
PM me your opinions if you get a chance. I am a telescope virgin.
The only thing I do know is that I'm happy to spend an extra quid for quality.
So that rules out a few makes from a certain 'neck of the woods'.
I'd be grateful. No hurry.
Thanks in advance.
PS. Oly noticed after logging on: Nice piccy. One of yours?
This looked like a dead thread...
PM me your opinions if you get a chance. I am a telescope virgin.
The only thing I do know is that I'm happy to spend an extra quid for quality.
So that rules out a few makes from a certain 'neck of the woods'.
I'd be grateful. No hurry.
Thanks in advance.
PS. Oly noticed after logging on: Nice piccy. One of yours?