Voltage Regulator issue, 24V to 12V



  • Hi,
    I have upgraded my delta printer to Duet, and also changed the PSU to a 24V Meanwell 600W.

    I am still running a couple of items on the effector platform from 12V (Tricklaser LED ring, and Tricklaser Hot End LED indicator). To regulate 24V to 12V I am using Pololu buck regulators https://www.pololu.com/product/2846. The LED ring is powered by the buck regulator which is in parallel with the hot end fan. It works well.

    The problem is with my hot end LED module (http://www.tricklaser.com/LED-hot-end-heating-indicator-module-RM-LED-HAT.htm), I hooked this up the same way as my LED lightring, except it is taking 24V from across the 24V hot end wires.

    I figured (basic electronics knowledge only) that the regulator would drop the voltage to 12V and KCL would ensure the hot end LED module would only draw as much current as it needed through the regulator. Unfortunately as soon as I powered the hot end up there was a pop, and I saw the buck regulators soul escaping as smoke. The LED module connected to it is fine. Did I misunderstand how these things work? Would the fact that the hotend is drawing significant current somehow force the buck regulator connected across it (note, not in series with it) to dissipate much more power than it is capable of?

    I'm hoping that someone more capable with electronics than myself can tell me if I did something stupid with my wiring, or if it all should have been OK (it's always possible that I got some solder across two pads or something when I hooked up the buck regulator).

    Here's a photo of the setup:


    This looks OK to my untrained eye, others might see some glaring fault…



  • Errrr…...ummmm

    In the normal world you would connect the 24 volts to say something like a buck regulator which would drop it to 12 volts and then connect the Output of the regulator to whatever your trying to power @ 12 volts.

    If you just want an LED to light up then something like a 4700 ohm (aka 4.7k) resistor would do the trick in Series with the LED and powered from the 24 volts. The resistor might get a little warm however, but not too bad.

    • _____ RRR ____ LED ____ -

    I'm a bit on the tired side so forgive me for not reading all the details in full. Bottom line is if you just want to run the LED from 24 volts use the 4.7k resistor. Actually 2.2k - probably 20k would work, just changes the brightness. Too low and you just get heat , smoke, etc.

    Edit: looking at the device your trying to light up you can still just use a resistor by the looks of it, you'll perhaps need to go down to say a 2.2k and perhaps half watt instead of 1/4 watt. personally I would just try the 4.7k resistor 1/4 watt and see how she runs.

    Ohh and I doubt a switching regulator would work off a PWM driven heater very well. Go with a resistor solution. A linear regulator would probably work but why bother. Keep it simple.

    Jeff


  • administrators

    The issue with the design as you have it is that the hotend circuit switches (via PWM) the low side so the voltage across the BUCK +LED circuit will switch rapidly from 24V to 0V .

    If you want to use the 24V already present at the hotend to power the LED (to reduce the wiring?) you need to provide a seperate GND line so it's a constant 24V. At that point you can either use the BUCK or a resistor to drop the voltage/limit the current through the LED.



  • @T3P3Tony:

    The issue with the design as you have it is that the hotend circuit switches (via PWM) the low side so the voltage across the BUCK +LED circuit will switch rapidly from 24V to 0V .

    If you want to use the 24V already present at the hotend to power the LED (to reduce the wiring?) you need to provide a seperate GND line so it's a constant 24V. At that point you can either use the BUCK or a resistor to drop the voltage/limit the current through the LED.

    To avoid having to run another ground wire (a PITA if I want to run a wire inside one of the towers) can I use the ground wire from some other component on the effector to ground the buck regulator? So, the 24V positive from the hotend to the buck regulator's VIN, then connect GND on the buck regulator to the hot end fan GND wire? I don't really want to go a pure resistor solution because a.) I'm stubborn and I've started doing it this way and b.) seems like that resistor would have to dissipate a fair bit of power? Might not be too much, I'll find out what the LEDs use…



  • Though now I think about it, why would rapid switching kill the buck regulator? So long as the voltage on the input side stayed within limits (0-36V for this regulator). Do they just not tolerate rapidly changing loads?


  • administrators

    I am not saying that the BUCK damage was caused by the PWM, just that the combination of a PWM input to a BUCK (which itself is doing PWM) may not work properly with the LEDs.

    You can use another ground wire as long as it can ahdle the combined current, but don't use the analog ground for the thermistor(VSSA)



  • Thanks, I'm trying to figure out the forward current for the piddly SMD LEDs on this LED module, so that I can test a resistor solution. Out of curiosity, when a hotend is being brought up to a setpoint temperature would PWM be in use, or would it just be constantly on until approaching a the setpoint then rapidly switching to maintain?

    Because the regulator blew within two seconds of starting the hotend heating. Trying to develop my understanding of how this stuff works 🙂


  • administrators

    In the past it the hotend would be fully on until it got close to the temperature and then PWM would kick in based on the PI'd curve.

    Dc42 recently added the thermal modelling code which I have not investigated yet (it just worked! :)). I am not sure if this changes the behaviour of the PWM further out from achieving the desired temperature.



  • I soo would not go with a buck regulator on the switched hot end, if it was in bang bang mode..maybe but not PID and PWM regulated, even if it's fast bang bang I wouldn't do it. The only correct way would be to have enough input capacitance as to smooth out the PWM and then you loose the LED flashing. A resistor isn't going to get all that warm especially considering it appears the indicator system already has dropper resistors in place for the 12v system. So really just need to limit the current enough so the 24v looks like 12v. A linear regulator say an 78L12 probably would do the trick better then the buck regulator.

    Jeff



  • Thanks, I'm tinkering with resistors now, but note that even <100 Ohm doesn't result in the LEDs lighting. I put a multimeter across the hotend wires and it showed ~-8V, not 24 🙂 I suppose this is something to do with the PWM on the hotend.

    I'm going to try using a resistor from the hotend 24V supply and using the ground wire for the hotend fan (which I have as not using PWM), see if that works. I have other applications in my printer for the buck regulator, so nothing will be wasted.



  • @COMMLXIV:

    Thanks, I'm tinkering with resistors now, but note that even <100 Ohm doesn't result in the LEDs lighting. I put a multimeter across the hotend wires and it showed ~-8V, not 24 🙂 I suppose this is something to do with the PWM on the hotend.

    I'm going to try using a resistor from the hotend 24V supply and using the ground wire for the hotend fan (which I have as not using PWM), see if that works. I have other applications in my printer for the buck regulator, so nothing will be wasted.

    Wow, my apologies on being so far out on calculations. It sounds like the bed heating indicator has some good voltage drop already via the resistors onboard. If you did surface mount soldering you could just change those resistors out with higher values making it a 24v unit instead of 12v. You could try a linear regulator as well such as an LM78L12 or LM7812, They will react much better to the PWM.

    Jeff


  • administrators

    Something isn't right. Unless they are VERY high power LEDs or long LED strips, I would expect them to light up using a 100 ohm series resistor and 24V supply. Although the resistor would quickly get hot and perhaps burn out unless it is rated for ~2W or greater power dissipation.



  • @dc42:

    Something isn't right. Unless they are VERY high power LEDs or long LED strips, I would expect them to light up using a 100 ohm series resistor and 24V supply. Although the resistor would quickly get hot and perhaps burn out unless it is rated for ~2W or greater power dissipation.

    It looks to me like the device is just 3 LED's with some limiting resistors, makes no sense to me either. The PWM unless it's at a really low duty cycle should be easily lighting up the LED's.

    Jeff


  • administrators

    Are you sure the LEDs still work if you connect them to a 12V supply?



  • If you are going to apply pwm to a 12v device running from a 24v to 12v converter then run the converter input positive from the 24v positive eg the hot end heater output or pwm fan output, then the converter input negative to the psu negative. Then send the converter output positive to the 12v device and then ground the device to the 24v negative pin of the connector your using ie hot end heater or pwm fan. This way the converter gets permanent 24v but the output can be pulsed to achieve dimming for leds or temperature regulation.

    For leds or multiple 12v fans wire them in series, that's the easiest option.

    You need a decent sized converter for a hot end heater 5A capable.



  • @dc42:

    Are you sure the LEDs still work if you connect them to a 12V supply?

    Yep, checked a couple of times, worked my way down from >8kOhm to 100Ohm, no light, but the LED array worked after, so it's not like a bunch of current got through and cooked it during my tests. I'll try again from high resistance values but using a different ground connection, maybe the hotend fan.

    The hotend is working nicely as a hotend, which is the most important thing. The hotend LED array is ultimately just bling, so if things get too hard I'll put it in a box somewhere and use it for a later project.


  • administrators

    Try connecting it with the series resistor direct to the 24V supply while you experiment with the resistor value. I suggest you start with about 500 ohms.


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