Overvoltage cutoff circuit - Page 1
Forum section: electronics / projects

author: NOON Design
author_rank: Contributor
timestamp: December 26, 2020, 02:44:18 am
content: Hi everyone, hope you're all managing to find some enjoyment in the holiday break.I'm designing a power supply for a vintage valve (tube) microphone that uses a very special and now unobtanium valve. It requires 4V @100mA and 120V @1mA. The 120V supply is easily done with a TL783. The 4V heater supply is extremely sensitive to noise though.I have prototyped some circuits and worked out that an LM317 style linear regulator with a cap multiplier input and all the other usual low noise tricks is not quiet enough, there is still a significant white noise floor. The LT3045 is magic though and it sounds magnificently quiet. However I have no previous experience with this chip and don't know how much I can trust it and what the typical failure modes are so I'd like to add some protection. If the 4V rail goes much higher it will cook the valve quite quickly. It's also nice to only turn on the 120V after the 4V rail has had some time to warm up the valve, but not strictly necessary. I'm working on the worst case assumption that the LT3045 melts and goes short circuit sending the full unregulated voltage through.I've sketched out an old fashioned comparator driving a relay circuit to cut off the 4V supply if the voltage goes high and another one that switches on the 120V after the 4V rail has come up to speed. I was wondeing about modern alternatives though and have seen there are lots of 'power supervisor' type chips that have these kind of functions. (e.g. MAX6399 etc)I am totally unfamiliar with them though and would appreciate some input from anyone that has used them about a good type to use for this sort of project and advantages / disadvantages compared to the old fashioned comparator and relay cicruit.

author: graybeard
author_rank: Frequent Contributor
timestamp: December 26, 2020, 03:24:52 am
content: Build a crowbar circuit with a SCR that shorts the output if it goes over Voltage. The SCR will act much faster than a relay and stay shorted until the power is cycled.Use a comparitor to trigger the scr.You will need a current limiting or foldback circuit to make the crowbar work.

author: NOON Design
author_rank: Contributor
timestamp: December 26, 2020, 04:01:08 am
content: Absolute speed is not critical like with silicon as there is some thermal lag in the heater, a relay should be well fast enough for the purpose. SCR crowbar circuits have always seemed like very blunt instruments to me and quite old fashioned now.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 26, 2020, 09:25:55 pm
content: Linear stabilizers and step-down converters have a risk of supplying the full input voltage in case of a malfunction. Crowbar protection on SCR is not a bad option, but it is rough and not so fast. The most reliable method is a good old transformer in an isolated source. Any fault in the primary circuit will not pass the voltage to the load.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: December 26, 2020, 09:51:14 pm
content: The reliability of modern parts makes such a 1960s approach anachronistic. Modern regulators have thermal shutdown, OVP, enable pins, etc.Not sure why this crowbar idea still hangs around.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 26, 2020, 10:08:15 pm
content: Quote from: Alex Eisenhut on December 26, 2020, 09:51:14 pmIn addition, they have defects, have insects on the PCB, have corrosion and moisture on the contacts, and some other reasons that will not allow them to perform all their protection functions.All right, but I'd prefer the old anarchism when flying into space.

author: magic
author_rank: Super Contributor
timestamp: December 27, 2020, 11:26:49 am
content: If the tube can survive a voltage slightly higher than usual, like 4.5V or 4.7V, simply put a Zener diode or TVS in parallel with it and fuse the input to the regulator. Two components and it's done and pretty bullet proof.Quote from: NOON Design on December 26, 2020, 02:44:18 amThis configuration seems a bit backwards: LM317 is good at rejecting mains ripple but adds much white noise, while capacitance multiplier adds less noise and is equally (poorly) effective at removing input noise across all audio frequencies.For a low tech solution, I would consider a TL431 wired as 4V shunt regulator with a diode in series with it, then an RC filter, and then an NPN emitter follower (or Sziklai pair) which subtracts the diode voltage and outputs 4V again. Possibly an LM7805/09/12 in front of the whole thing.

author: NOON Design
author_rank: Contributor
timestamp: December 27, 2020, 08:49:09 pm
content: Quote from: magic on December 27, 2020, 11:26:49 amThis is what I'm considering as the simplest approach, with the comparator / relay approach as a slightly faster and more tuneable mid-level solution and one of the modern power management chips as a fancy solution, but nobody has commented on any of those chips yet.Quote from: magic on December 27, 2020, 11:26:49 amThat's what I've discovered through trial and error. The LM317 is plenty quiet enough for normal audio circuits and I've used them hundreds if not thousands of times before. I wasn't sure just how sensitive this circuit was and started with the known reliable approach. Turns out it's extremely sensitive! The LT3045 iscrazyquiet across the spectrum, I just don't trust it entirely yet.Quote from: magic on December 27, 2020, 11:26:49 amThis sounds like a slightly more modern approach to the original discrete regulator circuit that was used with these microphones. They originally included a small battery as a voltage reference!

author: floobydust
author_rank: Super Contributor
timestamp: December 27, 2020, 09:53:57 pm
content: Your problems don't make sense to me - a noisy filament supply doesn't affect mic noise as you are experiencing. Although, we have no circuit or tube p/n or mic model.LM317's are fine for filaments, they do vary in quality some are noisy but output capacitance swamps most of that.Filaments are biased up to say 60VDC wrt the cathode to lower noise, and you must have bypass caps for that as well or it will hiss.If you need protection for the tube's filaments, a series string of 1N4002's works fine to clamp any overvoltages. Keep it simple. Soft-start is good as well, especially if there could be a hot plug-in of the mic to an already on PSU.Helping on an Altec coke bottle mic power supplyP518A(M11), using 5840 pentode and for B+ 300V the basic multistage RC filter for low hum and ripple, I add a string of zeners to regulate B+ at a mid RC stage.TL783has a high minimum load spec. 15mA. so you need a hot resistor for that. LM317 for the filaments. No issues with noise.I recommend adding a medical mains EMI filter (no Y-caps at stage 1) and some ferrite beads because RFI sensitivity can be a problem. Vintage condenser mic power supplies had an electrostatic shield in the power transformer, so modern toroids are absolutely terrible there absent that. Just tons of mains EMI comes through the power transformer's interwinding capacitance. Hammond split-bobbin power transformers269 seriesare pretty good, low capacitance pri-sec.

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 27, 2020, 10:00:26 pm
content: What is the tube number, is it directly heated, and is the noise voltage available?If it is an old 4V tube, its noise voltage could be higher than the indirectly heated ones.The prices asked in some domains for these old tubes rather high.Can you replace the tube with a 12ax7 and use ordinary power supplies?The filament tolerance is 12.6 or 6.3 V +/- 10%, and the noise voltage specs assume just a 50/60Hz supplyI am resto-modding an old mixer using 12ax7 and 12DW7, and just purchased somegeneric, new, ecc803 and ecc832 triodes for about $13 each.The mic preamps are just biased by the grid charge and plate current ~ 0.2 mA , so component count is minimal.Actually I am converting some mic inputs to grounded grid to better match the keyboard and guitar outputs.I made a spreadsheet of some pre-amp input noise voltages, and measured the actual output levels of some instruments here:

author: NOON Design
author_rank: Contributor
timestamp: December 27, 2020, 10:24:27 pm
content: It's a Neumann KM56 with the Rev A circuit which has no cathode resistor so the filament ground and cathode are directly connected. Later revisions added a cathode resistor which made it far less demanding on the power supply but apparently it doesn't sound as nice and it's a risky job to modify without damaging the valve.The AC701 valve was specially designed for this mic range and it has some unique features which means there are no drop-in equivalents, despite what some people have tried and think works. If you can get a replacement valve ($$$$$+) it's highly unlikely to be within spec. The internal layout is a marvel of German engineering of the 1950s. The mic body is thinner than a 12AX7!I have an old EI mains transformer with shield so I don't expect any RFI issues.The TL783 works well as a HV regulator for the 120V, although I'd be wary of using it for anything higher. The voltage setting resistors draw enough current to keep it stable and I have a couple of cap discharge bleeder resistors to give it a little more work to do. They have to be bigger than your standard 1/4W and get a bit warm but you have to expect some heat in valve circuits.This is all a bit off track from my original question though, I have the regulators sorted, just looking at overvoltage protection schemes.

author: floobydust
author_rank: Super Contributor
timestamp: December 27, 2020, 10:43:48 pm
content: AKG C12A uses constant-current for the filament and usual RC three-stage filter for 120V B+.Older C12 PSU seems to have a zener clamp to prevent overvoltage with an unplugged mic and hot plug-in. Note the RF cap across the filament.Telefunken AC701K tube i.e. in Schoeps M221B is 4V 0.1A and 120V 0.37mA so at 0.1A overvoltage protection could be a simple circuit- string of diodes, or transistor plus zener shunt reg. SCR crowbars do false-trigger from ESD. Lots of options.Where is your fuse? As a failed filament regulator trips your protection circuit and then what. Everybody sits there and cooks. Next the power transformer is overloaded, so an additional secondary fuse I would have.Some condenser mics undervoltage the filament possibly for lower noise or better space-charge.edit unable to upload C12 schematic grrr