Current Limiting Device/Circuit

[AeroJDK]

I have an application where I need to deliver a maximum of 30W to a motor at all times. For example, if I'm using a 5V power supply, I'd like the load to be able to draw up to 6A. If the load attempts to draw more than 6A, the power supply would begin to "dump" the extra current, mainting 5V @ 6A to the load. This would occur until the "dump" current was great enough to cause a fuse to pop, thus ceasing the power to the load.

I understand that to make this work I'd need a supply that's greater than 30W because of the extra current draw, but is there an in-line circuit or even a piece of hardware (programmable power supply, etc.) that would work in this type of application? I've been researching standard 30W power supplies, but all seem to have an over-current capability even though they're "current limited" and thus they can provide greater than 30W to the load. Constant current power supplies seem to work fine until the maximum current is reached, at which point the voltage will begin to drop as more power is required from the load.

Any help that you guys could provide would be greatly appreciated...I'm open to suggestions!

Jonathan

[mtucker]

I see what you are trying to do. Like you mentioned, you can buy a bench power supply that lets you set the current limit to 6A (or you can fairly easily make an add on circuit), but when you try to draw more than 6A the voltage will drop.
They drop the voltage becasue that is the only way to limit the current to the load. In other words, let say you have 5V and your load is 1 ohm (5A flow). Now your load tries to draw more current. It does so by lowering its input impedance. Lets say it is 0.5 ohms now. If you kept the voltage at 5V, you then have 10A flowing. The only way to keep the current down to 6A with the 0.5 ohm load is to drop the voltage to 3V.

You could go with a constant power limit that limits the power to a maximum of 30W, but your voltage will still have to drop to limit the delivered power.

Matt

[AeroJDK]

Right...V = I * R and assuming I is held constant, if R is changing, V is going to change along with it. I've been doing some more research and come across "Power Zeners" and "Shunt Regulators"...might there be anything with those that would allow me to do what I'm trying? The following post eluded to it

http://www.rcgroups.com/forums/showthread.php?t=535808&highlight=Power+Zener

but I'm still a little confused if that would, in fact, allow me to do what I'm attempting.

Jonathan

[deh6]

Very interesting problem.

The characteristics of the motor with its load seems to be an important factor. I assume you are using the typical pm motor.

I think the problem is constant power, but it has to be within both voltage AND current limiting.

Consider the case where the load is removed (or even goes negative!—airplane diving, cart goes down a hill, etc.). This “constant power” supply will try to put 30 watts into the motor. Without load the motor will have to absorb (heat) all the power, which it will do by a “runaway” rpm increase. The motor may overheat/burnup. Under a normal rpm/load situation most of the power is going into the load with only a portion into heating. Voltage limiting is needed to limit RPMs with attendant losses to a level that won't overheat the motor.

The main limit on the motor is current, which results in I^2*R losses in the windings, commutator, brushes, etc. If the torque required by the load goes up, the current must increase. Current limit is needed for this situation.

This leaves a “window” of voltage and current for which constant power is desired. I'll wrestle with this tonight. First off, it looks like for the general case it may not be all that simple (e.g. a zener, etc.). However, in a specific case it may not be all that complicated.

Can you say more about the characteristics of the load?

[vintage1]

You can do this in analogue, but its dirty.

You need to take current and voltage signals and multiply them together, and use the result to apply negative feedback to the circuitry. I've seen this done in re-entrant characteristic PSU's where the idea is to limit the POWER that the power supply is dumping to protect the actual power stages in it..in fact a typical regulator works a bit like this.

Now I was a bit of a non linear analogue circuit design specialist in my yoof, and let me tell you that accurate voltage multipliers are like hens teeth..especially DC ones. You basically have tow elements you can use..semiconductor junctions, that are approximately exponential/log in their V->I characteristics, and FET transistors that obey a square law under similar conditions. Both have huge temperature coefficients.

The most useful one to start with is the semiconductor junctions..take the voltage, and current, (sensed with a resistor) and use them to generate variable current sources that are fed to a pair of diodes. take the two voltages and add them together, with an analogue opamp, and if they exceed a certain level, start to decrease the output voltage.

In practice the BEST way to get the two diodes to behave is to use say a 4 transistor on a chip type carefully, use the base emitter junctions of two for measurement, the base emitter junction of a third to sense temperature (fed from a constant current it will show a negative temperature coefficient) and the 4th transistor to HEAT THE CHIP UP TO A DEFINED TEMPERATURE.

You would need to switch on ten to 15 minutes before using, and calibrate fairly carefully, but it would work.

There are a couple of temperature compensated analogue multiplier chips around as well..CA 3080 springs to mind as a possible.

http://www.intersil.com/cda/deviceinfo/0,1477,CA3080,00.html

Of course a PIC with a couple of A2D's is will do a far better job with far fewer components, which is why I no longer do analogue non linear design..apart from RF mixers, there simply isn't any call for it..;)

[deh6]

AeroJDK,

I need to deliver a maximum of 30W to a motor at all times I re-read all the posts (a number of times). I'm not clear exactly what you wish to accomplish.

If it is to protect the motor, then a current source power supply, the current set to the 6 amps, and an external shunt voltage regulator (such as the zener+transistor, heat sink, etc.) would limit the maximum power. At no-load the shunt regulator takes virtually all the power, and as the load on the motor increases the current drawn by the motor increases until the voltage falls below the setting of the shunt regulator (5v).

Constant current power supplies seem to work fine until the maximum current is reached, at which point the voltage will begin to drop as more power is required from the load. However, this statement makes a whole new problem. Namely, do you wish to increase the current to the motor when the voltage drops below (I presume) 5 volts. As far as the motor is concerned, the main limiting factor is current, which shows up as heating of the windings, etc. If this is the case, then set a constant current supply at max motor current. If indeed you wish to maintain constant power, then the problem is more complicated. (As I was typing this Vintage posted. I, too, was thinking along the lines of a micro w ADC to do the control. However, if the characterstics of the load are known, e.g. a prop, it might be a bit easier.)

[Cirip]

Reading a few times the thread I am tempted to believe that the initial problem is flawed. The original poster said:
If the load attempts to draw more than 6A, the power supply would begin to "dump" the extra current, mainting 5V @ 6A to the load
Now, if the load takes 30W from 5V, then the load resistance must be R=5/6=0.833 Ohm.
If the load wants more than 6A, the reason is that the equivalent resistance is lower, but in order to maintain 30W dissipated, the current must go higher, therefore the 6A limit is no longer valid.

Also, the statements "maintaining 5V @ 6A" and "If the load attempts to draw more than 6A" are incompatible in my opinion. There is only one load that draws 6A at 5V and that is 0.8333 Ohms.

So, the question seems to be: What does the OP need? Current limit or constant power?

Cirip

[AeroJDK]

I know this is changing what I originally said, but what if I'm looking for a simpler solution...what about putting a fuse post power supply. If I'm running the power supply in constant voltage mode @ 5V, could I put a 6.5A fuse inline so that once 5V * 60.5A = 32.5W is reached, the fuse blows? Does there exist such specific fuses that will fast-blow at a certain current draw if it reaches that for a few miliseconds? Maybe that's asking too much...but maybe someone knows of something simliar out there.

Thanks again for everyone's continued help...

Jonathan

[jeffs555]

I doubt that you will find a fuse that works like that. You could accomplish the same thing with a crowbar circuit after the fuse, and trigger it on overcurrent.

Like others have said, if you would describe why you want to do this and what you are trying to accomplish you would get much better responses.

[mtucker]

Fuses aren't the most accurate devices. You can put 7A through a 6A fuse for a long time (hours if not days) and it will not blow. Normally you have to go near twice the rated current for them to blow quickly.

You could try a crowbar circuit, but unless your power supply can put out more than twice the rated fuse current the fuse may still be slow to blow.

Sounds like you might want to use a pass transistor that is on when the current is less than 6A and then opens up and latches off when the current goes above 6A. Since your load is inductive, you will want to put a diode around the transistor to protect it from back emf (just like you normally see on the relay coils.

Matt