Simple Current Limiting - Page 1
Forum section: electronics / beginners

author: monika
author_rank: Newbie
timestamp: April 25, 2023, 08:02:39 pm
content: Hello all,I'm looking for some help with the design of a really simple current limiting circuit. It will need to be adjustable up to 60A 12v and was thinking of something along the lines of this:Problem here being that it is not adjustable, but is quite simpleI'm guessing at that kind of current, T1 would have to be a MOSFET... Rsense will have to be a high power resistor too.Can anyone assist with the making this adjustable or perhaps even something else entirely?ThanksMonika.

author: Peabody
author_rank: Super Contributor
timestamp: April 25, 2023, 08:47:46 pm
content: T2 will not turn on at all unless its base voltage is about 0.6V above its emitter. So if you're dropping 0.6V across Rsense, at 60A you would be dissipating 36W of heat in Rsense. Also, whatever you use for T1 will have a similar problem since it will only be partially on when limiting current, and will therefore be dissipating lots of heat.Are you sure you need to limit current?

author: Benta
author_rank: Super Contributor
timestamp: April 25, 2023, 10:02:09 pm
content: Your limiter will potentially have to dissipate 720 W. I'm sorry, but this not a case of "a MOSFET", but some really hefty power devices plus cooling.

author: langwadt
author_rank: Super Contributor
timestamp: April 25, 2023, 10:15:24 pm
content: https://xyproblem.info/

author: Zero999
author_rank: Super Contributor
timestamp: April 25, 2023, 10:28:49 pm
content: Can it be latching? That way it will never have 12V across it, at the full current. I'd just use a circuit breaker, or PTC resistor AKA polyfuse. Failing that, you could replace T1 in your circuit with an SCR, that way it will latch and auto-reset, when the current is interrupted.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: April 26, 2023, 12:23:19 am
content: An example of a single IC:https://www.ti.com/lit/ds/symlink/tps2hb16-q1.pdfYou can take a look at the block diagram how it’s constructed.

author: Jwillis
author_rank: Super Contributor
timestamp: April 26, 2023, 01:11:54 am
content: Quote from: Benta on April 25, 2023, 10:02:09 pmYou could do it with a FDL100N50Fhttps://www.mouser.ca/datasheet/2/308/1/FDL100N50F_D-2312204.pdfbut as you say you would need a hefty heat sink.power = current2 x resistance = 60 x 60 x 43m Ohms = 154.8 watts

author: james_s
author_rank: Super Contributor
timestamp: April 26, 2023, 06:49:45 am
content: What exactly are you trying to do? A linear circuit handling 60 amps is possible but not trivial, and depending on the compliance range you need, you will have a large amount of heat to get rid of.

author: monika
author_rank: Newbie
timestamp: April 26, 2023, 09:47:58 pm
content: Quote from: james_s on April 26, 2023, 06:49:45 amI have a heat and am looking to adjust the current to find out the optimum heat for the particular application hence the requirement for an extremely simple and uncomplicated design.Once I've found the ideal setting, I'll then look to a more appropriate solution.Thanks

author: monika
author_rank: Newbie
timestamp: April 26, 2023, 09:53:24 pm
content: Quote from: dobsonr741 on April 26, 2023, 12:23:19 amInteresting - I'll have to take a look at this

author: monika
author_rank: Newbie
timestamp: April 26, 2023, 09:55:09 pm
content: Quote from: Jwillis on April 26, 2023, 01:11:54 amI suspected some cooling device would be required. Quite reasonable power though considering - I've seen worse

author: dobsonr741
author_rank: Frequent Contributor
timestamp: April 26, 2023, 10:00:18 pm
content: What is the nominal current? Is it 60A or less?In any case, choose a mosfet with lower rds on, for example 5mOhm. Use multiple mosfets in parallel.

author: james_s
author_rank: Super Contributor
timestamp: April 26, 2023, 11:40:08 pm
content: Quote from: monika on April 26, 2023, 09:47:58 pmSo this is a resistive load? In that case automatic closed loop control is probably not necessary. You may be able to use a small arc welder to control the current, or you could control the voltage which will control the current according to Ohms law. A step down transformer fed by a variac could accomplish this.

author: Kim Christensen
author_rank: Super Contributor
timestamp: April 27, 2023, 12:03:40 am
content: If you're just trying to adjust a heater element's heat output, just PWM control it.Maybe instead of using 12V @ 60A (720W) why not use a dimmer on a regular 240V heater element?

author: Zero999
author_rank: Super Contributor
timestamp: April 27, 2023, 10:28:04 am
content: Quote from: monika on April 26, 2023, 09:47:58 pmThe problem is, when using a linear regulator, such as in the circuit you attached to the original post, the transistor acts like a variable resistor, meaning it dissipates a lot of power. The worst case is, limiting the current to half the full load, which would mean half of the power is being dissipated by the transistor. In your case, the full load power is 60*12 = 720W, so when the current is 30A, the total power in the circuit will be half, giving a total of 360W, with half, 180W being dissipated by the heater and the other 360W by the transistor. Using PWM and a MOSFET with a low on resistance of just 5mR would result in a power dissipation in the transistor of 0.005*602= 18W, which would still require a heat sink, but is much more reasonable.

author: MrAl
author_rank: Super Contributor
timestamp: April 27, 2023, 11:37:50 am
content: Quote from: monika on April 25, 2023, 08:02:39 pmHi Minika,When dealing with the kind of power you are dealing with the modern idea is to go with a switching type circuit. A switcher can be much more efficient.Yes a linear is simpler, by far, but the kind of power losses associated with linears make them impractical for higher power circuits.Did you know even simple wall warts had undergone a transformation from passive, linear, to a switching technology. That's to increase efficiency. The devices may only have to handle 6 watts yet they are still going to switchers.

author: Terry Bites
author_rank: Super Contributor
timestamp: April 27, 2023, 12:15:58 pm
content: R1 has to be big enough to bias T1 fully on and small enough to (hopefully) limit the current through T2 to an acceptable level.There's conflict of interest here. It’s a poor design.The idea is that the current through R2 creates a voltage. As this aproaches 0.7, on T2 turns on which robs T1 of base bias current. This will throttle it back T1 until an equilibrium state is reached. but what about all that heat?It looks plausible with Ilim=0.7/ Rsense , but in practice it will fail misserably. On a good day it will sit there simmering and then boiling as Ilim is reached.The current path from T1 C>E can flow unimpeded to the load via T2 C>E. T2 will then melt into a short circuit blob and kill T1 with too.What protects the protection, not much. See Horowitz and Hill (3Ed) chapter 5 page 957 and comments on figure 9.2d.You could make life easier using an LM317 with or without a boost transistor egwww.homemade-circuits.com/lm317-with-outboard-current-boost/and many more.Take the path of least resistance!www.electronics-notes.com/articles/analogue_circuits/power-supply-electronics/current-limiter-circuit.phpCalculate the currents and power dissipation under normal and fault conditions before starting on a design.Ask yourself "Is there a single fault condition in my circuit that will ruin my day and cost me dear."Always put a fuse in series with the limiter input. Fires are a right pain.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 27, 2023, 01:49:55 pm
content: What is "a heat"?You need to tell us what you are actually doing, there is absolutely no point in the discussion otherwise.

author: Zero999
author_rank: Super Contributor
timestamp: April 27, 2023, 04:41:55 pm
content: Here's a simple PWM circuit I designed many years ago to dim an old fashioned incandescent lamp. You'll of course need to use a bigger transistor than the IRL540. Many options have been mentioned in this thread. As it's a heater, there's no point in switching it at such a high frequency, so the timing capacitor can be upped to 1µF and a 1M potentiometer used. That will give a frequency of 1.4Hz, which is perfectly acceptable for a heater.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 27, 2023, 04:44:26 pm
content: Heater? A good old electromechanical thermostat. That thing which comes with a capillary tube you put somewhere you want to sense, you adjust the target temperature, and then it clicks ON-OFF regularly.

author: MrAl
author_rank: Super Contributor
timestamp: April 27, 2023, 07:40:53 pm
content: Quote from: Zero999 on April 27, 2023, 04:41:55 pmHi,What is an incandescent lamp? (ha ha)I think 1.4Hz will be too low if this is a resistive type heater then that may give it too much time to cool off during the off periods. That means higher peak currents even though it would cool off only a little. Of course it also depends on the thermal mass which we dont know yet. Maybe 10Hz, or 20Hz, or maybe 1kHz.

author: Zero999
author_rank: Super Contributor
timestamp: April 27, 2023, 08:04:39 pm
content: Quote from: MrAl on April 27, 2023, 07:40:53 pmI don't think so. It depends on the heater of course. My electric oven doesn't switch anywhere near 1Hz. Heck, even my soldering iron switches at no more than a few Hz, let alone 1kHz. 1Hz should be fine for a 720W heater. Most resistive heaters also don't draw huge current surges, since they run much cooler than incandescent lamps.

author: MrAl
author_rank: Super Contributor
timestamp: April 28, 2023, 02:14:51 pm
content: Quote from: Zero999 on April 27, 2023, 08:04:39 pmHi,You could be right but it's easy enough to test.I do have to wonder why you picked 1.4Hz now though even if it does work. Why not 1.3 or 1.5Hz.Also, 10Hz can't be worse than 1.4Hz unless you have some idea why.Sometimes the effect on other devices can be a problem too. I know some soldering irons work at low Hz like maybe even 0.1 Hz. The secondary problem is it keeps making a noise as it turns on and off, sounds like a dinosaur breathing. Its very annoying.Also, for a higher power device like 500 watts, in some part of this house the lights would dim then get brighter, dim then brighter, etc. That happens with a microwave oven pulsing on and off. Also very annoying.This leads me to think that 100Hz min would be good.

author: Zero999
author_rank: Super Contributor
timestamp: April 28, 2023, 05:15:34 pm
content: Quote from: MrAl on April 28, 2023, 02:14:51 pmThe main downsides of higher frequencies are: more RFI and higher switching losses. Go higher, if you like, but there's no point for a heater.There wasn't any reason why I suggested 1.4Hz, other than it's what that 555 generates with a 1M potentiometer and 1µF capacitor. There would be no point in going for 1.3Hz, or 1.5Hz, because the tolerance of the capacitor and potentiometer are too loose and is why I wouldn't bother with 10Hz, but 14Hz.Sound shouldn't be a problem with a resistive load. It's likely it's the transformer in your soldering iron which is making the noise.If your lights noticeably dim when a 500W heater turns on, then you must have a problem with your wiring. I can turn on my oven or microwave in my house, which use more power than that and my lights don't dim, even the crappy CFLs and LEDs with capacitive dropper power supplies.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 28, 2023, 05:29:44 pm
content: Quote from: monika on April 26, 2023, 09:47:58 pmThis is what variable power supplies are for. A simple option is a variac and a step down transformer as suggested by james_s. A light dimmer switch which is generally some sort of phase angle controller is another option. If you really want to use DC instead of AC, there are bench supplies that can do what you need such as this one from BK precision:https://www.bkprecision.com/products/power-supplies/1900BYou can find much cheaper no-name supplies on amazon or ali express, and in your application you can parallel a couple of them to hit the 60A if you find cheap 20 or 30A supplies with CC/CV operation.

author: MrAl
author_rank: Super Contributor
timestamp: April 29, 2023, 01:56:27 pm
content: Quote from: Zero999 on April 28, 2023, 05:15:34 pmHi,I am not saying you are wrong, just that maybe it should be tested.About the 500 watt heater and lights dimming issue though, i had a feeling you were going to say that (ha ha).See, when you have an issue like that, and im sure this isnt the only house like this, you dont rewire the entire house just so you can run a heater at 1.4Hz or so. I know this house has an issue but it could be with the power coming into the house also on that side of the line as the power in this area is very strange too. It's a little more complicated. It's been this way for over 25 years and there are only some moderate problems that are not that hard to get over without rewiring anything.Also, if 10Hz is too high then how do we know that 1.4Hz isnt too high. Why not go with 0.5Hz then.I guess all i was wondering was how you picked 1.4Hz as that's an unusual number i think rather than 1Hz, 1.5Hz, or 2Hz. I thought maybe there was some scientific reason for it. Now i see i guess it was just the value of resistors you had at the time or something like that. That's not a problem.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 29, 2023, 02:06:20 pm
content: 1.3Hz, 10Hz or 0.5Hz all sound fine for "a 700W heater". That thing probably has enough thermal mass so that anything beyond 0.1Hz makes absolutely no difference whatsoever.But of course, it depends. For example, inkjet printers work by heating up a microscopic ink droplet. It takes some microseconds (if even that) to do so, so you need control bandwidth of maybe 1 000 000Hz. On the other hand, consider an annual thermal energy storage excavated into bedrock, with millions of liters of water. There you run the heaters every year, so bandwidth is 0.0000000317Hz. Simple electric ovens that use mechanical thermostats typically cycle around 0.003Hz or so.So there is quite a lot of variance between a "heater" and a "heater". As the OP is just playing games with us and not telling what they are actually doing, there is no point in guessing.

author: MrAl
author_rank: Super Contributor
timestamp: April 30, 2023, 02:20:25 am
content: Hi,I agree with most of that. There is one more thing to consider.Some space heaters make a slight crackling sound when they first turn on. That is due to the metals expanding when the elements gets hot. I think if it turned on and off every 10 seconds then it would keep making that noise, which i think would be objectionable.Maybe 1Hz to 2Hz would be ok though i never tried that. The long one i had made that noise, and the built in ones make that i had in a house some time back made that noise every time the heat turned on (ha ha). Wasn't a big deal but i would rather not have it like that if i could have it run constantly at some low duty cycle or whatever kept the rooms warm enough without turning on and off all the time.The newer microwave i saw a friend had did not turn on and off at all. Mine turns on and off at settings below 4 (1 to 2) and above 2 (3 to 10) it regulates the power in a linear fashion. Luckily i like to use the oven at power 3 most of the time, but sometimes like to use 1 and 2 also, which causes the oven to turn on and off, on for maybe 12 seconds, off for 24 seconds, or something like that on power 2. The friends oven never pulses like that no matter what the setting is.

author: EPAIII
author_rank: Super Contributor
timestamp: May 01, 2023, 08:15:28 am
content: Just thinking:A Hall effect device would get past the heat problem in the sensing. And it would be isolated from the main circuit so the circuitry may be simpler.An OP amp with adjustable gain would provide adjust-ability.Then you need to decide what to do when the current is too high. Complete cutoff could be via a relay, mechanical or solid state. But if you want to allow it to still flow, just not over 60 Amps or whatever set point or with some fold-back to a lower current, then you need something with a serious power handling capacity. And probably some serious cooling.As others have asked, just what are you trying to do? Stating the problem in precise terms is the first step toward getting an answer.Or this may be completely off base. Just thinking.