Impressive. The video is a few months old and yet not a single downvote. You not only explain it in great but not uneeded detail but also in a way a dumb guy like me understands it. Bravo
Good presentation! The cap at the 317 input is not needed. Did this circuit 35 years ago with TIP50, not a Fet. The 317 is not an independent regulator, it is a part of the MOSFET gate circuit. The parts better be chassis mounted TO220 style. Curret limiting with power resisors on the input also helps for low current circuit. The Zener voltage should be high enough to allow 5V across the 317 and enough to turn on the FET, with the PTC add margin for its normal droput. 10 - 12 V may be OK. A 10uF cap on the FET gate OR the 317 adj. pin improves regulation. A large value resistor can set the startup slope with the 10uF cap on the gate. The Zener connects the output to the FET gate, basically AC grounding the FET gate (with a 10uF to gnd) making the 317 work the FET source as a common gate config. The FET drain works the power supply as its load.
This is *exactly* what I was looking for. Excellent series! Thanks a lot. I was looking for a good way to regulate voltage in a hifi tube amp requiring about 425V. This is excellent.
A bit less known way to implement current limit is to use matched transistor pair to regulate mosfet. This way you have even less voltage drop. I normally use voltage divider on the gate and use NTC as the pull-up to implement thermal limit.
I've solved this problem before using foldback current limiting, you can get away with pretty aggressive ratios(nominal current lim vs short circuit) with these higher voltages. Only real drawback you have to be conscious of what your load looks like in order to make it through startup. Might be worth looking into if you want something a little snappier than the PTC.
Funny, you're having exactly the same issues I am having with a similar project. Btw I like to use something like a TL431 instead for current limiting. It's a lot more stable and you get a lot more control. For worst case full short situations, I decided to switch off the voltage completely. This can be done with a PMOS or relay.
Oh, btw for the overcurrent protection, I then use a comparator with a pretty big hysteresis. So one basically has to restart the power supply to reset the circuit. I thought about the PTC myself, but it's rather slow (because of the stored heat in the heatsink) and not always very predictable.
I was thinking about multiple LM317 regulator stacks to achieve 325V regulation from 330V, but this gave me some ideas, but would use something like a BUX80 bipolar for input, your circuit looking good, needs fullwave and decent smoothing on the input though, you can at least put a 160uF 400V cap on the input (2cm diameter, 2cm tall-Elna) to get those higher currents at better regulation.
Thanks for these great video's and providing honest insight while learning & burning stuff. ( We all do that. :D ) You could try adding fold-back on "stage 1" => Fast short circuit protection with a defined Isc.
Great video! Would this circuit survive, if you switch it hardly (very steep voltage ramp) to a loaded capacitor or other low resistive high voltage source? Would the linear voltage regulator be able to control the high voltage mosfet that save?
I have built a similar circuit and it does not survive a hard short circuit on it's output. The 317 breaks and so far I found no solution to fix this design problem. In my opinion the voltage over the LM317 is too high for a very short moment, until the current limit comes into action. So now I would like to see another video from Fesz, with a short circuit demo. (Hope he has got some spare parts left ...).
To be honest, I was trying to focus on the linear regulator part more. Also my transformer has a single high voltage output, so the only way to get both a positive and negative voltage out of it was with this 2 diode schematic. You can't use 2 rectifier bridges with the output ground interconnected on a single transformer winding; that would only work with 2 separate windings (so not with a single winding with a median tap).
I love your videos. I’m just a hobbyist, so forgive me if this is a dumb question - but why couldn’t you use one of those self-resetting fuses for overcurrent protection?
That will also work, I think the same basic operating principle is used in both the PTC and the fuses you mentioned. I used the PTC since its something more people are familiar with and it was a component I already had lying around.
What do you think might happen if instead you used a resistor instead of the PTC and thermocouple it to the LM 317 and allow the heat of the resistor to trigger the over temperature protection in the lm317?
Hi. I learned a lot today thank you. Q: When voltage across the 10R reaches ~.6V, the power FET turns off. But the circuit is still using 30mA? Does this mean that it is not completely off, and gets hot because 30mA is still going through it? So 150V * .03A = 4.5W being dissipated by the power FET?
Exactly. This sort of current limit doesn't turn the circuit off but rather it just limits the current. The point is that the current stops increasing at ~30mA rather than the up to 300mA that we would get by relying on the LM317.
Thank you! I'm just a tube-amp building hobbyist. I've tried so many different Maida style regulators, all of which had their issues, and none of which I fully understood. I really want to build your version. I will sign up for your Patreon, as I don't feel right just lifting your schematic, you are very good. I really need a HV regulator board I can rely on to get voltages from 120 to 400 volts. A question I have though... With a regulator like this. Is it still best practice to keep the raw DC input somewhat close over the desired output voltage, so that overall you're not dropping a huge difference. That waste has to go somewhere right? Typically how many volts should my raw DC be over my desired output with this regulator sot runs good but doesn't have a huge waste?
For any linear regulator, it is mandatory to keep the input voltage higher than the output - I would like to point out that "raw DC" (if by this you mean what you get after rectifying and filtering with capacitors), still looks like an offset sawtooth and the voltage that needs to be taken into account is the minimum of the sawtooth (not the avearge). I will have to check again what the minimum voltage drop is for this particular schematic, but its the threshold voltage of the MosFet plus the minimum requirement of the LM317, maybe an extra ~0.5-1V just for good measure.
@@FesZElectronics Thanks for the quick response! In the video a wide range of output voltages was shown because its variable, that's why I asked. For my use I would set a fixed output voltage to the tube anodes. 1 or 2 volts is easy, I was thinking of 10V or so for good measure. In my case the tube plates need 340 volts (not sure on how much current two channels will draw yet). So for my bridge + first reservoir capacitor stage output I'd model a transformer that lands me around 350V after the reservoir, it looks like I have 260V toroid transformer that will work.
Regardless of whether its fixed or variable, the same method to calculate the minimum voltage drop is true; its just that for the variable version, the minimum input is calculated based on the maximum output voltage. For 260V AC - you should get a peak of 368V; but after rectification and filtration you should still be above the 350. I would just like to point out that its also important to take into account the transformer series resistance - if this is large, you will not be getting the 368V peak, but rather something lower when large currents are being drawn. Anyway, if you are making a PCB, you can leave an empty footprint in parallel with the upper feedback resistor to change its value (add another one in parallel to lower it) - so you can set 340V for example on the output.
It could be interesting to se a direct short on output at max current. Does it still survive ? I have struggled with the same problem, and build in a thyristor and a relay to breake the short. But then i have to shut down the whole supply to get back.
With the PTC in the base emitter of your current limiting transistor won't it damage the base emitter juction due to the high voltage across the PTC, even if it is only temporary?
The PTC resistance changes quite slowly - since it is heated up from an external source; so you will not really get sudden changes in voltage. This means the transistors will always have time to react and limit the input current, and should not get damaged.
@@FesZElectronics thank you for your reply...I was just thinking of an increasing voltage on the base emitter junction. It is sometimes hard to visualise what happens.......you have an increasing resistance and a reduing current, I was visualising there might be a point where that voltage could, in theory, be enough to damage the transistor. It needs a lot more thinking through on my part, the circuit is much more dynamic than I first thought. Again thank you for the reply...it is much appreciated as is your You Tube posts.
The PTC has a limited life, it is used as a protection device on circuits so I don't think you can keep cycling this without it eventually stop working.
You are right, the PTC is not something that should be used as a current limit during normal operation. My idea was to do something as reliable as the over temperature shutdown in the LM317 - which ends up putting the die well above its maximum rated temperature. So I just wanted to make a mechanism that protects the circuit during rare events, so it doesn't brake instantaneously.
How about just coupling the Regulator to the transistor on the heat sink? This way if the transistor overheats, the regulator will pick up the temperature swing and shutdown.
I did try doing that, it was my initial method, but by the time the regulator picked up the over temperature (internally set to ~180C) the transistor was way above its maximum operating junction temp...
Can you please guide me ? I have been learning ltSpice from your videos and its a very good journey. I am an undergrad. I want to know apart from college academics what else to do to have an impressive resume ? Im in electronics and comm. Engg JNU India
My best advice would be - practice what you learn. Try to create or build projects for yourself where you apply the things you learn about. That will teach you a lot about debugging and well practical applications. Just because something works on paper doesn't mean in also works in real life.
Well regarding the parts issue, I try to buy as few components as possible since if I don't need them on the moment, they will just be forgotten in a box somewhere. So with this circuit I got the minimum number of LM317s and transistors which was 3pcs. I of course did not plan on braking all of them, so that is why I ran out of parts... Anyway, if you want to support my work, Patron is the best way.
What exactly do you think I should detail more? I usually try to keep videos short, so they don't become boring, but I could always insist on certain topic more.
@@FesZElectronics I personally interested in detailed schema walkthrough, review why something works or doesn't (from physics or electronics point of view), data-sheets parameters meanings and advices how to choose right values (and reasons why). I'm trying to learn to deeply understand why a circuit behaves particular way and physics behind it, unfortunately don't have time for full time studying, so I'm trying to learn from various information sources (including your videos). So I personally think dipper and more detailed explanations are better, because it is already plenty of videos in youtube with high-level circuit demos, but very little deep-dive circuit walkthrough and as far as I understand from various forums many people looking for this kind of information. Your videos are very good, and if you will be making them even more detailed, they will be even better. Thank you.
the current limitation of those parts, like the lm317, consists of destroying it with a remaining high resistor. have a try on a current mirror and a proper regulator.
