Announcement

Collapse
No announcement yet.

OT Question About Current Limiting

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • OT Question About Current Limiting

    Hey folks ... I turn once again to the vast knowledge of my machinist friends to see what advice you can offer on building some short circuit protection.

    Suppose I have a voltage source (think battery - either 12v or 24v) and I want to turn this into a current source limited to 10 amps. I was planning on using a pair of PNP power transistors as pictured in the link. However, after some back of the envelope calculations, I have determined I need a 0.07 ohm 10 watt resistor. It seems that these typically run $4-$8 a pop. Combined with the cost of the transistors and this starts to become expensive*.

    http://en.wikipedia.org/wiki/File:Cu...imiter_PNP.PNG


    *To clarify, this is for a commercial application so every little bit matters. Furthermore, this gadget already has something like 60 relays and 48 BJTs and 12 MOSFETs. Space is also a concern. I want something that I can fit on a board fairly easily, although I realize it will need to be fairly large to handle the currents involved.

    So, what options do I have for turning a battery into a current source? Really, I just want protection to prevent an accidental short circuit from damaging certain components. While a fuse would be sufficient, in this application, I want something that "resets" itself as soon as the short is removed. Really a current limited source is ideal for a couple of reasons, but that's the main one.

  • #2
    Look up "crowbar"

    Comment


    • #3
      Originally posted by J. R. Williams View Post
      Look up "crowbar"
      Why? A crowbar circuit shorts the voltage output, it does not limit the current. It is so named since it is analogous to dropping a literal crowbar across the terminals to short a voltage source. We use "crowbar" circuits to short out capacitor banks but once they are activated, they do not automatically reset.

      As far as I know, it fails both my requirements. It neither resets itself nor does it limit current ... it only limits voltage.

      Comment


      • #4
        Perhaps you can turn this on top of its head......

        Rather than SENSING current in order to limit it, SET the current.... Make the circuit try to produce 10A all the time..... which actually will have no effect....Evidently some other circuit controls the current, which you wish to limit. So the effect of attempting to produce a 10A current in this way will be to allow any current UP TO 10A to flow, while in the case of anything larger, it will be limited to the setting of the current source.

        You want a 10A current source. One with low total voltage drop unless the setpoint current is reached.

        The fault in this is the same as in the case of the current limiter.... you can dissipate a large power in the transistors..... 10A x 12V, or 120W possibly, or 10A x 24V (240W) in the case of a 24V battery.

        It is best if you can put this in the negative side, and use an NPN, or an N-channel mosfet, or standard IGBT. That would allow the use of a handy ground referenced opamp circuit.

        Resistors of 0.01 ohm are obtainable.... they will produce a drop of 0.1V at 10A, at a dissipation of 1W. Make the circuit a 10A current source, with an opamp as a sensing amplifier. With a mosfet or IGBT, the drive at this low frequency application will be minimal. A mosfet can be found with a very low RDS(on) which will mean very little voltage drop until the limit is reached.

        If the circuit MUST be in the positive lead, a p-channel mosfet can be used... but it may be necessary to "float" the opamp supply.

        You can use the same idea of an opamp current sense amplifier to run a regular current limiter, of course.
        CNC machines only go through the motions.

        Ideas expressed may be mine, or from anyone else in the universe.
        Not responsible for clerical errors. Or those made by lay people either.
        Number formats and units may be chosen at random depending on what day it is.
        I reserve the right to use a number system with any integer base without prior notice.
        Generalizations are understood to be "often" true, but not true in every case.

        Comment


        • #5
          Eleven inches of 28 gauge copper wire is 0.07 ohms with a lot more than 10 watts dissipation if open wound. I use resistors like that frequently and they cost almost nothing. Just find what you have in the wire resistance tables and calculate accordingly. A 120 watt current limiter will need a heatsink that is dependent on the amount of current the load draws in normal operation.

          Simple current limiting isn't the best choice for device protection. To prevent thermal damage you need foldback limiting. When the current limit is exceeded the limiter cuts the output to a very low level that it uses to monitor when the excess load goes away. Then it automatically restores the output to full. This is how IC voltage regulators work and they can be wired to act as current limiters with an emitter follower arrangement power transistor. Look in the application notes for common voltage regulators such as the 78XX series.
          Free software for calculating bolt circles and similar: Click Here

          Comment


          • #6
            A current limiting circuit is about the simplest circuit you could build- one three terminal regulator, a power transistor, and a resistor or two. For low currents, just the regulator and one resistor is all you need.

            The problem with a limiting circuit is that the power transistor is dissipating power based on the current level multiplied by the voltage across it. In the case of a short circuit on the load, the voltage across the power transistor is essentially the power supply voltage- that multiplied by the current represents a lot of power and heat. If the short remains for any length of time, things can go bad very quickly. If your limiting circuit can handle the power on a continuous basis, then it can survive. The same may or may not be true of the power supply.

            At any rate, the limiting circuit will lose some of the voltage from the power supply, so ideally it would be placed in the power supply circuit ahead of the voltage feedback point. It can actually be the first thing in the chain, direct from the rectifiers and main filter cap.

            If you're running from a battery source, say 12v or 24v, you may not be able to tolerate losing any voltage- then the job can still be done, but the circuit is more complex.

            Evan beat me to it- but I'll add one thing- you can make a resistor from just about any piece of conductor. Stainless sheet makes a good resistor, and you just have to calculate the resistivity based on the thickness and width of the strip, then the length. Chances are it will have lots of surface area to dissipate the heat. Of course, you're looking at 7 watts here, which isn't much really. You can fold it into an M shape and bend a couple tabs on the ends to bolt it to a circuit board- and that's your connections as well.

            If you use the three terminal regulator, you might be able to take advantage of the temperature cutoff feature of it- if your pass transistors get above the cutoff temperature, the regulator ( which would be mounted as close to the transistors as possible) would self-protect, thus also cutting off the power transistors. I know this works, but it's not something I rely on in any of my designs, and not something I would really recommend.
            Last edited by darryl; 09-17-2012, 01:06 AM.
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

            Comment


            • #7
              Evan - I thought about winding my own resistor like you describe, but I wasn't sure how practical it would be in large numbers. Maybe I could wind a long coil around an armature in the lathe and then cut a bunch of short pieces from it. I don't need a very accurate limit; the circuit should handle up to 12 amps with out any damage.

              Basically, all the current limiter does is protect some 10 amp relays from fusing together in the event of a short. Also it will prevent 12 MOSFETs from being damaged from over-current, although they are rated for 49 amps and will have heat sinks so I'm not too concerned about them. The rest of the electronics shouldn't be affected by a short.

              Maybe that deserves more of an explanation: This is for pyrotechnics firing system. I am working with a well known manufacturer to develop a new computer controlled firing system that is affordable, reliable and very versatile. What I am concerned with is accidental shorts "in the field". Evan probably knows what I mean, but for those who are unfamiliar with the topic: we use e-matches to ignite fireworks. To make a long story short, there is a very real likelihood that some of these e-matches get crossed and will short. That rat's nest of orange is a module with e-matches connected!



              On most systems, a short will either damage the firing system or it blows a fuse/trips a breaker and the show stops until it's fixed. That's a big problem for computer controlled systems, especially if it's a pyro-musical. The idea is to keep the show going and just skip the shorted cue.

              JT - Unfortunately, I think it does need to be "positive lead".

              Sounds like all three of you are saying more or less the same thing. I need to learn about using linear regulators or op-amps in conjunction with a transistor to control the current.

              EDIT:

              Hmm ... I had a look at a 78XX regulator datasheet. Figure 10 in the application notes looks just like the circuit I posted but with a linear voltage regulator. I don't want that - I want the voltage to be free to change. The user may fire either from 12 volts or 24 volts but in both cases, the relays can't handle much more than 10 amps.

              http://www.datasheetcatalog.org/data...ola/MC7809.pdf


              The other thought I had was to use a comparator and a current sense-resistor. Then, when the current reaches some preset level, the comparator would trigger an interrupt service routine in the computer. This would shut off the firing voltage and skip to the next cue before turning the firing voltage on again. However, a typical "on" time might be 150 mS and I'm not sure that this method of triggering the computer to control a relay would be any faster than if I just "waited" the short out. The relays can take a pretty big current pulse for a very short time, so the goal is to just prevent that pulse time from being long enough to damage the contacts.
              Last edited by Fasttrack; 09-17-2012, 01:48 AM.

              Comment


              • #8
                Use figure 9, but wire the regulator as per figure 7. In other words, the collector of the transistor still goes to the output terminal of the regulator, but the actual output comes from the other side of the resistor, which is connected to the control terminal. The voltage regulating function of the regulator is disabled in this circuit, so the maximum voltage is still passed through (minus lost voltage to operate the circuit). Even a 5 volt regulator will work in this circuit- it does not limit the output voltage to 5 volts, as the control terminal doesn't go to ground. In the event of a short it does, but then the circuit is limiting current anyway, so the voltage specification of the regulator operating as a current limiter never comes into play.

                I do have a problem with the voltage loss though. Because the regulator chip needs a minimum voltage to operate, that is subtracted from the raw power supply voltage and the output voltage will be low by that amount, plus the amount across the output resistor, which will be another 1.25 volts. From a 12v supply, you might only get 9 volts on the output. This problem has come up for me several times. It might be that the circuit you posted has a lower voltage drop. I'll look at that again-

                Ok, that two transistor circuit shown will have less voltage drop than the regulator chip circuit. The current cutoff knee won't be very sharp, but then you don't need it to be for simple short circuit protection. The 'issue' statement made with that circuit is in error- if the base voltage of the pass transistor goes above Vcc, the transistor is cut off, not saturated. The circuit works as shown though.

                A considerable current has to flow through R1 in order to saturate the pass transistor and reduce the drop across it to a low value. R1 would have to carry at least 100 ma and maybe more like 200 ma in order to supply enough turn-on current to the pass transistor. For 200 ma, the value would have to be about 50 ohms or so- I'd start with a 62 ohm resistor and lower that value if you find your output voltage to be less than 11 volts or so under the normal load. It will have to be rated at 3 watts so it doesn't burn up, and the circuit will be drawing the 200 ma or so all the time, whether the output is under load or not.

                I would recommend using a P channel mosfet instead of the bipolar pass transistor. Not only will the current lost through R1 be a lot less, the current cutoff operation will be sharper, and there will be less voltage drop right up to the point where the cutoff begins. R1 can be about 1k in that case, for less loss in it, and an easier job for the 'protection' transistor. You could reduce your .07 ohm resistor to about .055 or so as well.
                Last edited by darryl; 09-17-2012, 02:38 AM.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

                Comment


                • #9
                  Originally posted by Fasttrack View Post
                  JT - Unfortunately, I think it does need to be "positive lead".
                  No problem, just more complicated......

                  However, it could still be done by putting it "below ground", and it will be invisible to the circuit as far as a voltage to ground, circuit output is still ground referenced. Many older power supplies had everything in the "below ground" position so they could use NPN pass transistors and still have a ground referenced output. Topologically, it is the same as in positive lead.... the battery or other power source is not directly grounded, so the circuit regulates the voltage between battery positive and ground... or in this case, it regulates the current that can pass into the battery negative lead.

                  Bottom line it is do-able

                  Look up "PWR4412" on Digi-Key...... specify 0.01 ohm..... they have thousands of these resistors, and the parts are quite reliable. You have a choice of wattages.

                  Comparator limiter, current regulator, whatever.
                  CNC machines only go through the motions.

                  Ideas expressed may be mine, or from anyone else in the universe.
                  Not responsible for clerical errors. Or those made by lay people either.
                  Number formats and units may be chosen at random depending on what day it is.
                  I reserve the right to use a number system with any integer base without prior notice.
                  Generalizations are understood to be "often" true, but not true in every case.

                  Comment


                  • #10
                    Originally posted by darryl View Post
                    Use figure 9, but wire the regulator as per figure 7.
                    D'oh. Thanks... I didn't even look at figure 7 because I didn't see a transistor in the schematic
                    The voltage drop is going to be a problem. In addition to the figure you came up with, it will pass at least one more diode before reaching the e-matches. Typically, e-matches get wired in series if you want to fire multiple things simultaneously. I want enough voltage to always fire something on the order of 5 matches. That means I'll have a load resistance of ~8.5 ohms and the all-fire current is typically 1.25 amps. At 12 volts and one diode, this is just enough to reliably pop 5 matches. With the current limiter ... well that drops me down to only 3-4 matches on the 12 volt setting.



                    Maybe it is time to start looking more seriously about building in a switching power supply so even a 12 volt battery provides a firing voltage of, say, 32 volts and limited at 6 amps or so. To be safe, I figure I need 1.5 amps per series of matches, but I'd like to be able to fire multiple chains at once, which is why all the components and the limiter is designed for higher current.

                    Comment


                    • #11
                      Originally posted by J Tiers View Post
                      No problem, just more complicated......

                      However, it could still be done by putting it "below ground", and it will be invisible to the circuit as far as a voltage to ground, circuit output is still ground referenced. Many older power supplies had everything in the "below ground" position so they could use NPN pass transistors and still have a ground referenced output. Topologically, it is the same as in positive lead.... the battery or other power source is not directly grounded, so the circuit regulates the voltage between battery positive and ground... or in this case, it regulates the current that can pass into the battery negative lead.

                      I'm not sure I completely understand what "below ground" means but the reason why I want it positive lead is because the ground is switched for different slats. It's a matrix switching method, so 60 relays can control 576 cues with a greatly reduced amount of cabling. Also, I'm not sure how this below ground method would affect my micro controller. What I want is a system where the micro controller remains active during the short. We use pretty big lead-acid batteries so even with a 10 amp "short", the battery can still power the micro controller.


                      I guess I could just stack the 0.01 ohm resistors in series to get the desired resistance ... I just want to minimize the number of solder points since we pay for those when it comes time to make the boards.

                      Comment


                      • #12
                        What you need to use is a PTC Fuse. This is a small device that vastly increases its resistance when it heats up a bit. It is essentially a resettable fuse.

                        Some good information is available at: http://www2.electronicproducts.com/Fuse_or_PTC-article-farr_littelfuse_aug2008-html.aspx

                        These are often used in the power window circuit of older (non computer controlled) cars. If the switch is held on after the motor stalls, the fuse goes to a high resistance state and protects the motor.

                        Update: Here's a device from DigiKey that might work for you. $0.81 each. http://www.digikey.com/product-detai...000-ND/1045835
                        Last edited by ftl; 09-17-2012, 02:32 AM.

                        Comment


                        • #13
                          One more thing I wanted to say- the 2N2955 is not a particularly rugged transistor, not as good as it's N channel complement, the 3055. I don't trust it to handle a high current pulse once it's already hot. I would definitely go with the P channel mosfet instead, and pick one with a high current rating and at least 100 volt capability- more if a 24 volt supply is being used. I don't know if there's much in the way of inductance in the load, but just looking at that mess of wires makes me think that you would want the most reliability from your circuit. I personally don't think it's overkill to go for a 150 or 200 volt part there. These days it's not much cost-wise either.
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

                          Comment


                          • #14
                            Originally posted by ftl View Post
                            What you need to use is a PTC Fuse. This is a small device that vastly increases its resistance when it heats up a bit. It is essentially a resettable fuse.

                            Some good information is available at: http://www2.electronicproducts.com/Fuse_or_PTC-article-farr_littelfuse_aug2008-html.aspx

                            These are often used in the power window circuit of older (non computer controlled) cars. If the switch is held on after the motor stalls, the fuse goes to a high resistance state and protects the motor.

                            Update: Here's a device from DigiKey that might work for you. $0.81 each. http://www.digikey.com/product-detai...000-ND/1045835
                            Boy, the simplicity is very appealing, but that would require the circuit to be shut off to cool down. I would like something that is fairly fast and can sense the short so once the short is removed, the current is "restored" within about 100 mS.

                            Comment


                            • #15
                              Originally posted by Fasttrack View Post
                              Boy, the simplicity is very appealing, but that would require the circuit to be shut off to cool down. I would like something that is fairly fast and can sense the short so once the short is removed, the current is "restored" within about 100 mS.
                              Put a suitable lightbulb in series!

                              Low resistance when cold, high resistance when hot, visual indication of an over current situation, fast reset, what more could you ask for?

                              I used to build model train controllers with the output protected by a bulb and have often used them as a crude constant current source.
                              Paul Compton
                              www.morini-mania.co.uk
                              http://www.youtube.com/user/EVguru

                              Comment

                              Working...
                              X