Constant current?

Andrew Cowan

Senior Member
I am thinking about getting a new power supply, and I am wondering, when would you want a constant current power supply?

I know LEDs want constant current, but all the power supplies I have seen have this as a feature. Is this just for testing LEDs?

I am assuming constant current means the voltage varies automatically to keep the current constant...

Or is it like a 'max current' function, where it only decreases the voltage if the current gets too high - and it won't increase the voltage if the current gets too low... If it is this, how quickly does it respond to an overcurrent situation?

Thanks,

Andrew
 

BeanieBots

Moderator
If it's a "bench" power supply, then it means the current won't be exceeded and the response will be as good as it is at maintaining voltage under variable load. Is it useful? It's ESSENTIAL for prototype working. That's been said here a thousand times but nobody takes any notice, shortly followed by "have I destroyed my PICAXE".

Besides that, constant current can have many uses. PV simulation, battery charging and all sorts of testing.
 

papaof2

Senior Member
If it's a "bench" power supply, then it means the current won't be exceeded and the response will be as good as it is at maintaining voltage under variable load. Is it useful? It's ESSENTIAL for prototype working. That's been said here a thousand times but nobody takes any notice, shortly followed by "have I destroyed my PICAXE".

Besides that, constant current can have many uses. PV simulation, battery charging and all sorts of testing.
Although I expect to see "current limited" instead of "constant current" in this context, as in
adjustable, voltage regulated, current limited power supply

On the other hand, this may just be another instance of the colonies being separated by a not-quite-common language ;-)

I find a current limited supply ideal for charging batteries (SLA, AGM, NiCd, NiMH, Li-xxx) and have used one to recover a Li-ion PDA battery that was too far discharged for its own charger to be able to recharge it. The device just beeped when connected to the original charger, but 10ma for a couple of hours brought the battery voltage up enough for the original charger to recognize the battery as "OK to charge". Not recommended for novices - you do need to know the safe charge level for each battery type & size.

John
 

Andrew Cowan

Senior Member
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kevrus

New Member
Well in case anyone is interested, but maybe off at a tangent, airfield runway lighting is run from a constant current source to maintain equal brilliancy on all lamps thus eliminating voltage drop over long cable runs of several kilometers, although the constant current sources used are usually of a larger size than a bench mounted unit, (although bench units are used for individual lamp testing)...
 

boriz

Senior Member
I see no reference to “constant current” anyplace on that page. But the panel shows two LEDs labelled CC and CV which I assume mean Constant Current and Constant Voltage.

There is however no apparent switch or control to change modes. So I suspect this is a current limiting PSU. It prolly defaults to CV mode (you can see that on the pic) and when the load exceeds the preset current, it switches to CC mode. So it seems the voltage does not increase to maintain the preset current level.

I’m guessing that you preset the current by shorting the output to trigger CC mode, then dial in the current limit.
 

evanh

Senior Member
CC/CV regulation doesn't switch as such. Both functions are active at all times but which function that is limiting is dependant on the load conditions.

If the load voltage rises to the voltage limit then CV will own it. If the load current rises to the current limit then CC will own it.

However, being able to accurately and precisely set a small level is not always possible, so do treat these flexible devices with some salt.
 

BeanieBots

Moderator
Indeed, that IS a current limited power supply.
Once the current is reached, it operates in constant current mode with an LED indicator stating which mode it is in.
 

BeanieBots

Moderator
It's what you set it to.
Not obvious how to determine the setting but you could just short out the output and set it that way.
 

moxhamj

New Member
The constant current can be set in lots of ways. It can be set by the current going into a transistor base. It can be set with an op amp circuit. There are contant current chips you can buy. I learnt about constant current supplies when I built my first bench power supply.

How do you know what it is till you try it? Well, hopefully you designed the circuit so you know in advance - eg by setting a resistor. But if you want to just suck it and see, you can test any constant current generator by shorting it, and then measure the current.

On a bench power supply, I have one that you can dial up the volts. Then you push a button which shorts the output, and dial up the milliamps/amps on another meter. Then release the button. If I dial up 5V on the volts and 20mA on the mA, I know that the supply will give 5V to any load until that load draws more than 20mA. Then it will switch to current limiting (a led on the supply indicates this).

But for a constant current device, not a power supply, but a dedicated constant current device, it will supply that current no matter what the load. Put 1 ohm on the output and it will give 100mA. Put 100k on the output and it will supply 100mA (no matter how many volts that might entail, though clearly there are limits to 'ideal' constant current or constant volt supplies).

For an ideal supply, it would supply constant current no matter what. If you shorted the output it would supply 100mA. If you left the outputs unconnected, it would increase the volts to millions of volts till a spark appeared and then would regulate the current to 100mA.

Interestingly, the heart is a constant current device. If volts are pressure, and current is flow, the body needs a constant flow of blood (about 5 litres per minute at rest). V=IR and R is the resistance of flow along the pipes. So if you eat too much fat, it clogs up the arteries. Resistance rises. Flow must stay constant, so the pressure must rise to compensate. Higher pressure = high blood pressure.

Anyway, must go. Morning tea with scones and jam and cream and cake has just arrived...
 
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evanh

Senior Member
Constant current means the same thing as a current limit. It's exactly the same for voltage.

And yes, to preset the current level you short the output. Otherwise the voltage limit will be reached. Some power supplies can disable the output to set both the voltage and the current levels.
 

westaust55

Moderator
Although I expect to see "current limited" instead of "constant current" in this context, as in
adjustable, voltage regulated, current limited power supply

On the other hand, this may just be another instance of the colonies being separated by a not-quite-common language ;-)

I find a current limited supply ideal for charging batteries (SLA, AGM, NiCd, NiMH, Li-xxx) and have used one to recover a Li-ion PDA battery that was too far discharged for its own charger to be able to recharge it. The device just beeped when connected to the original charger, but 10ma for a couple of hours brought the battery voltage up enough for the original charger to recognize the battery as "OK to charge". Not recommended for novices - you do need to know the safe charge level for each battery type & size.

John
papaof2,

Agree with you.
Constant current means the current is held constant (as opposed to limited) and the voltage varies to maintain the current.
Otherwise there is constant voltage where the voltage is regulated to remain constant irrespective of the current.

Current Limit means fixing the upper end current (say to prevent overload) but the current can vary between 0 and the limit
 

Dippy

Moderator
Proper Bench power supplies behave exactly as said by BB in post#8.
i.e. you set your current LIMIT and when that is reached it behaves as a Constant Current source.
Though if you overlaod rapidly and hugely it gives the impression tha ithas tripped, but it hasn't.


"So you don't know what the max current allowed until it reaches it?"
Andrew,
if you get yourself a QUALITY PSU like those made by Thurlby Thandar (aka TTI) you can P-R-E-S-E-T the current limit BEFORE connecting up.
Top quality gear doesn't come cheap, so sit down when looking at the prices. Check the spec. You get ,usually, what you pay for. Proper engineers don't use Beijing Bangers.
And, of course, blowing up PCBs and processors can be quite annoying too :) Time and money my boy.
 
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westaust55

Moderator
Well in case anyone is interested, but maybe off at a tangent, airfield runway lighting is run from a constant current source to maintain equal brilliancy on all lamps thus eliminating voltage drop over long cable runs of several kilometers, although the constant current sources used are usually of a larger size than a bench mounted unit, (although bench units are used for individual lamp testing)...
You are right there Kevrus.
In fact they are often 3 or more-level or variable systems so the lights are bright at dusk and dawn when ambient light is still there. Maybe one (or more) systems for runway lights and another (set) for taxiway lights.


But as those runway lamps have a short life at high intensity, they are reduced in current and brightness as the night progresses (becomes more dark) so they are still very visible but extends the life greatly.

Supply Voltage is typically of the order of 1000's of volts and they use a HV cable in a loop with small transformers at each light so failure of one lamp does not cause the entire loop to fail (unlike many Christmas tree light sets).
 
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Dippy

Moderator
Blimey, he knows everything about everything.

"Constant current means the same thing as a current limit."
- really?
 

westaust55

Moderator
Blimey, he knows everything about everything.

"Constant current means the same thing as a current limit."
- really?
No, not everything, just the equipment that I do design work on for the projects I work on all over the world.

So, is there a problem with having knowledge :confused: :confused: :confused:


The current limit is there to protect the supply from excess loads that would overheat or destroy the supply at high currents or short circuits. It does not set/define a lower limit. At best it is a special case of a constant current.

A constant current supply maintains the current at a predefined point which may be well below the current limt. The current will be exactly that current, not higher or lower.
 
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kevrus

New Member
To continue my rambling about airfield lighting...

The larger primary voltage on airfield circuits can operate up to 5Kv, and as Westaust said, there are isolating transformers on this series circuit.
Airfield lighting is split into many circuits....approach, supplementary approach, threshold, TDZ (touchdown zone) runway centreline, runway edge, runway stop end, taxiway centreline, taxiway edge, papis, stop bars, apron edge, signs and wigwags,and each circuit is split across more than one constant current source to eliminate total circuit failure.
There are seven standard preset brilliancy levels, these bieng 100%, 80%, 30%, 10%, 3%, 1%, 0.3%, which incidentally do not correspond to current levels of the same percentage. A commonly used max design current is 6.6A (100%), 80% lamp brilliancy is achieved at 6.3A, 30% at 5.4A, 10% at 4.5A, 3% at 3.9A, 1% at 2.9A.
Interestingly, although possibly obvious, the lamps do not have a voltage rating, just a wattage and current rating, I.E. 200w@6.6A, 100w@6.6A, 45w@6.6A.

I must stop rambling now
 

moxhamj

New Member
Kevrus - this is very interesting rambling!

One of the features of a lightbulb (which incidentally make good "poor man's" current limiters in themselves) is that they start off with a very low resistance which rises as they come on. A lightbulb will often blow when it is first turned on. So would driving from a constant current limit this inrush current, and hence allow the bulbs to last longer?
 

westaust55

Moderator
Incandescent bulbs

Okay not Kevrus but here is an answer.

In theory, yes it should. With incandescent bulbs (those with filaments), the filaments become fragile as metal is eroded and re-deposited from the filament and due to mechanical forces between the coils.

The filaments are typically in the form of a coil and hence an air cored inductor. Thus there is a substantial force between the coils due to the ampere-turns created with the high current at the instant of turn-on.
 

kevrus

New Member
Dr_acula, thats a good question, I would imagine that the life expectancy would increase because the inrush would be limited to the lamp rating/CCR (constant current regulator )setting and not determined by the lamp resistance.
With the airfield CCRs there is an adjustable 'ramp up' time which would also increase life expectancy, although this can't be set to slow as the response time for recovery from failure is stipulated in airfied regs, typically from 0.5 secs to 2 secs depending on the circuit. They will also trip if the current exceeds a high limit value, typically set to 6.75A for a 6.6A circuit.
 

LizzieB

Senior Member
Definitely! 12 volt 100watt halogen microscope lamps run from a current regulated supply last way longer than ones on a regular variable voltage supply.
 

LizzieB

Senior Member
Pretty normal for a high power microscope. If you want a very small area to yield a bright image when magnified, then you need to start with a lot of light on that very small area.

Not all specimens are nicely transparent, or nicely reflective, silicon wafers for example. Polarising and other optical filters may be used that reduce the available light.

IR filters can handle the heat issue.
 

kevrus

New Member
Lizzie, do the microscopes use a remote light source with fibreoptics to transmit the light without the heat?
 

LizzieB

Senior Member
There are many different types and configurations of microscopes designed for an infinite variety of applications, and with all kinds of different lighting systems. Yes, fibre optic illuminators are used for some purposes.
 

westaust55

Moderator
100W Microscope lamps!

Do you wear sunglasses?

Does the sample fry?
Heat wise, cannot be any worse than photographic slide projectors with 100Watt or 150Watt QI lamps only a few cm from the plastic slide film. Admittedly they do have fans.
 
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