A new, improved, oven for the PD 2005 Precision power supply - Page 1
Forum section: electronics / projects

author: MBY
author_rank: Regular Contributor
timestamp: June 04, 2012, 11:26:49 am
content: As you may have read in the threadhttps://www.eevblog.com/forum/product-reviews-photos-and-discussion/looking-for-goodinexpensive-bench-power-supply/my PD2005 PSU has let some of its magic smoke away. Thankfully, all of the "unobtainium" parts of the electronics seems to work, only the stability oven heater has failed. The oven contains all the reference stuff and to eliminate temperature drift, this electronics is heated to a specific (to me unknown) temperature. The heater is simple: a heating filament in series with a bi-metal thermostat. The heater runs directly on 115 VAC mains (I use a 230->115 VAC transformer as Sweden has 230 VAC in the mains).First, I wanted to keep things simple and build my new heater to run directly from 115 VAC. The schematics attached shows that concept. A couple of resistors and zeners and a LDO creates 5 VDC for the electronics and a mcu is to be controlling the power on the heater (heater not shown, but I probably gonna use power resistors rather than a filament).But this idea was not so simple after all. You see, I wanted to be able to control the oven lamp, so I needed another bridge rectifier and a optocoupler. The problem is that a optocoupler need several mA, and several mA means I need to burn away non-trivial amounts of heat in the "resistor step-down".The heater is controlled in a CC-style circuit with a PWM to set the level. Also the heater is turned on via a AC-copuled high side transistor, to lessen the probability of a software bug that turn the heater full on forever.Another problem with this design is that most of high voltage FETs runs in depletion mode, meaning they need "beyond rail" drive on the gate (what I mean is that to turn off a depletion mode N-FET you may need a voltage lower than source).All in all, that makes for a complicated circuit. So I'm gonna redesign this for low voltage use, so I will use another wall wart inside the 2005 (its plenty of room) to drive the heater from something like 12 volts.All suggestions is appreciated.The schema for my first (scrapped) design attached.Edit: If you own a Power Designs model 2005 precision power source you can help! There is a few measurements on a working unit that I could use. The temperature and power of the oven is two unknowns.

author: SeanB
author_rank: Super Contributor
timestamp: June 04, 2012, 05:30:59 pm
content: I would guess the setpoint is between 70-80C. This would be doable with a 10W wirewound resistor, as it will be able to get this hot on it's surface without insulation.Try to wind your new element to be around 8-10R when cold, and then use the insulation of the original to act as a housing. For a controller just use a 70C normaslly closed thermal switch, it will operate at around 75C to disconnect the heater, simple and probably will fit in the position of the original. If the original is still working then reuse it anyway, most are capable of switching 1A or so.

author: muvideo
author_rank: Frequent Contributor
timestamp: June 04, 2012, 05:44:18 pm
content: If this can be of any help, or you want just to play:this is a very simple oven heater/controller I built some monthes ago.It is without hysteresis, but it is a proportional one.The values are for 45°C with a 15-18V workingvoltage, at that voltage the maximum heating poweris something around 5-7W. The heating element is Q1.RV2 is a trimmer that regulates the range of interestand RV1 regulates the setpoint more finely.If you put a trimmer in series with R4 you can tunethe integral term, the one i built is slow enoughthat doesnt oscillate.Fabio.P.S. The reason I used TL431 was simply because I had manyPC psu broken...

author: robrenz
author_rank: Super Contributor
timestamp: June 04, 2012, 06:37:52 pm
content: I suggest not changing from the wound filament style heater. It gives a much more uniform temperature input. What good is a super precision PID control if you have hotspots at the discrete resistors. The resistance wire is cheap on ebay. I have used this supplier often with good results.http://www.ebay.com/itm/165-ft-Nichrome-Heating-Resistance-Wires-5-sizes-kit-/310212043161?pt=LH_DefaultDomain_0&hash=item483a142199They ship from The EU to me in the US.

author: codeboy2k
author_rank: Super Contributor
timestamp: June 04, 2012, 07:31:08 pm
content: Quote from: MBY on June 04, 2012, 11:26:49 amok.. simple. right. got it..Quoteok... so far so good...QuoteI choked and spewed food at this point. Sorry to get you all messy. You don't need an MCU to control a PSU reference heater. The simple PI controller that muvideo posted is a great starting point. Don't overcomplicate it with MCU PID control. And you almost never need the derivitive (D) term. PI is usually sufficient and easy to do in hardware.You can easily get enough nichrome wire from an old toaster, $1 at a garage sale, or a hair dryer, $5.00. Or the local junk dealer.. or Salvation Army, etc...No need to buy it from eBay.

author: MBY
author_rank: Regular Contributor
timestamp: June 04, 2012, 07:51:00 pm
content: Quote from: robrenz on June 04, 2012, 06:37:52 pmThe problem is the mechanics. The filament needs to be isolated and at the same time conduct heat to the inner tube. A few word on the ovens mechanical design: The outermost part is a shield tube, and inside there was the filament attached to a inner, thicker tube. The inner tube holds the electronics. The isolation between the two tubes seems to be some sort of glass wool.I didn't say I would use a few resistors. I plan on use basically the same "tracks" but with small thru-hole resistors instead and a lot of them. But if I use nichrome/kanthal filament or resistors is a little bit besides the point, as the electronics will be the same. The inner tube "will be a good conductor. Because I don't want to adhere something to this inner tube, I will anyway make my own tube in copper to thread on the inner one.

author: MBY
author_rank: Regular Contributor
timestamp: June 04, 2012, 08:05:36 pm
content: Quote from: codeboy2k on June 04, 2012, 07:31:08 pmWell, a single MCU is hardly to "complicate" things. Why not? The problem is not that I cannot find resistor wire. It has almost nothing to do with it. What I use in the end is dependent on mechanics. I have lot of good old TL431 (used in muvideo's design above) but they are all in a TO-92 package. They don't cost much, but if I have to find SOT-23 ones, I still need to buy something. Of course PI may be perfectly adequate, but a mcu is sime enough. One "dumb" thing about the 2005 is that the oven lamp indicates "oven on". I want it to indicate "temperature regulated" and/or "heater error". A mcu in a SO8 package is perfectly fine here I think, as I can do more intuitive indications and I already has the board real estate. The PCB must be thin (only smds) and cannot extend more than a dozen or so mm in width. It can, however be several cm in length. The current design uses two SO8 packages and a couple of SOT-23.And I may very well use nichrome wire, but that depends on the mechanics. I don't want another episode of filament short circuit to the inner tube. Resistor heating is a feature, not a "cannot find nichrome"-situation.

author: MBY
author_rank: Regular Contributor
timestamp: June 04, 2012, 08:12:41 pm
content: Quote from: muvideo on June 04, 2012, 05:44:18 pmTL431 is a nice one and I use them as often as I can! But the ones I got is in TO-92 package and I need smaller things. But your design is neat but not entirely usable for me as the heat source is the transistor. I need many sources of heat to avoid "heat spots" even if the inner tubing and my planned copper tube will spread the heat. Of course, heat will be generated in my transistors as well, especially at lower setting (I don't want to use PWM as I don't want to create noise, so the analog control will burn some heat in the transistor, but that a side effect).But maybe a small adjustments to the circuit can make resistors (R7 in your schematics) the primary source of heat.

author: muvideo
author_rank: Frequent Contributor
timestamp: June 04, 2012, 08:39:52 pm
content: Quote from: MBY on June 04, 2012, 08:12:41 pmR7 and D1 limit the current in Q1, perhaps you can try to put the wire heater between Q1 collector and ground.As for hot spots, Q1 and R7 are problem candidates.Fabio.

author: robrenz
author_rank: Super Contributor
timestamp: June 05, 2012, 12:36:22 am
content: Just a suggestion. Wrap the inner tube with a layer of Kapton tape to electrically insulate from the resistance wire. Attach insulated leads to the correct length of resistance wire. Wind the element over the Kapton tape at the correct spacing and put another layer of Kapton tape over the windings to hold the wire spacing and also over the insulated leads junctions. From the looks of the stuff that was on the original heater wires the Kapton won't be any worse thermal conductivity.Another thought, If the slow oscillations we are seeing are from the heater on off cycles. a PWM heater drive with a fixed Hz would help diminish that.

author: MBY
author_rank: Regular Contributor
timestamp: June 05, 2012, 08:03:39 am
content: Good tip, robrenz. I have kapton tape and thought of using it. But it does not conduct heat well, does it? Maybe its not critical, as you said, the old stuff may not have been ideal either. But I don't know what the material was. Maybe some sort of paint that has degraded to a powder.PWM is the most obvious solution but I wonder if it may cause noise. So I designed a analog control. But PWM is not out of the question entirely. Also, as some have noted, a analog HW PI-loop is sufficient. I have not ruled that out, but I'm still thinking in terms of MCU because I want to control the oven lamp to indicate heater status (error, temperature stable, etc).No I have scrapped the design running of 115 VAC directly. It got to messy. So I will include a own transformer an run off everything at low voltage. A side effect is that the oven lamp has to be replaced by a LED.

author: robrenz
author_rank: Super Contributor
timestamp: June 05, 2012, 10:08:32 am
content: Quote from: MBY on June 05, 2012, 08:03:39 amI was thinking an analog input PWM IC with a analog HW control loop not a MCU. There are some used for motor drives IIRC.

author: MBY
author_rank: Regular Contributor
timestamp: June 05, 2012, 11:05:29 am
content: Quote from: robrenz on June 05, 2012, 10:08:32 amAh, I figured. However, I found that the worst case power dissipation of the power controlling transistor will be "only" about 1.8 watt (and it will be trivial to avoid that operation area) if I use a analog control instead of PWM (actually, a PWM->DC-converter) based on a 7.5 watt heater. It may be sufficient to create a "hot spot", but I don't think it will be an issue. Bu hey, its easy to disconnect the PWM->DC step and use PWM directly, MCU or no MCU. See the pwm control part of the scematics (even if this design is obsolete now, the heater drive part is essentially unchanged).

author: MBY
author_rank: Regular Contributor
timestamp: June 06, 2012, 01:51:53 pm
content: Ok, now I'm about to home-etch my first "low voltage" design. I still going with an MCU as I have explained before (I want some "intelligent" oven lamp control), an ATtiny45/85.As you can see in the schematics I have somewhat redundant control over the heater. First, an AC-coupled high side switch to make sure only a PWM signal can activate the heater. Second, the power control using a simple pwm->DC level converter and a common CC-style circuit to drive the heater. The downside is heat in Q1, but if there will be a problem (I don't think so, I did the math) I just patch the PCB to make use of pwm directly.I haven't decided exactly what to use as a heater, but I need about 20 ohms to get around 7 watts. I think 7 watts is reasonable. Either nichrome wire or maybe 20+ resistors. I even had a crazy idea of using loooots of small 0603 in a sandwich between two DIY copper tubes, but I don't think I can afford the space needed. I tested a portion of the old heater and it seems to handle the bigger current of lower voltage needed but if I want anything of this design it is to _lower_ the intermittent temperatures if possible, not rise it.Never mind the really idiotic drawn schematics. It was a quick and dirty job and I used components already in the library. I personally don't like the way the mcu is drawn at all and I usually use named nets and "off page" gaps, but I didn't bother this time (I'm relative new to KiCad as well and haven't learned it yet. All my old libs and such is for Cadsoft Eagle).The layout is captured from the "3D view" in pcbnew (KiCad). The same story here: I could not bother spending hours to get a decent ground plane or unify the use of different packages (this ugly beast uses 0603, 0805 and 1206 depending on what I have in my possession). It was important to make it single sided, as I have 0.6 mm single side PCBs, and size really matter here as I plan to stick the whole thing between the inner and outer tube of the oven. That explains the elongated PCB, less than 18 mm side but near 70 mm in length.Edit: The ISP connector is a micro-match. A good thing of this land pattern is that its easy to "touch-program" without needing to actually mount it.Edit2: Some of the values of the components are from the old design and will be adjusted.

author: muvideo
author_rank: Frequent Contributor
timestamp: June 06, 2012, 02:45:09 pm
content: Given the fact that you want to activate the heater with an AC signal from the mcu, you can alsobuild a simple charge pump and charge the gate of an Nmos, the mos will be connected like theone you have drawn (that is Pmos but connnected like an Nmos, check itnow I have togo but I dont understand the way you connected Q2, seem that the source voltage will be lower thaninput voltage... I'll check better later).Also I would invert R25 with TH1 so the adc values will go up with higher temperaturesonly a "cosmetic" effect.There ia a nice murata tool to chose quickly the series resistor so you can esily put thethemistor voltage in an easy range for the adc (I usually start with a series resistorvalue in the range of the thermistor working resistance).http://www.murata.com/products/design_support/mcnvs/index.htmlFabio.

author: MBY
author_rank: Regular Contributor
timestamp: June 06, 2012, 03:01:28 pm
content: Quote from: muvideo on June 06, 2012, 02:45:09 pmGreat tool! It seems to work ok in wine (running linux here). I have a junk box full of NTCs but I have no data on them, but they seems to be fairly standard with a room temp resistance of about 10k.You are absolute right, the high side PFET is reversed. I hate the KiCad symbols, I always usually use the implicit DS-diode to orient the device, sloppy me.Anyway, a charge pump is a good idea, but since I need to pump 5 VDC to 12+, I need several steps, so I think the common NFET-PFET circuit is easy enough. Since I already routed the board and succeeded with a single side PCB without extra wiring I will only change "bugs" and really bad circuits, such as the wrong-way PFET.But, when I think about it, a "charge pump" will be pretty easy. You see, I don't need to generate any voltage, only sink. A "sink pump" should be easy. Will check that out.Apart from being in the wrong direction, it is nothing strange with Q2. This is a very common way of making a high side switch.

author: muvideo
author_rank: Frequent Contributor
timestamp: June 06, 2012, 05:07:12 pm
content: The murata tool:it's very nice, if you go in setting you can activate other simulations like for example selfheating effect. For a micro design it's very useful "Temp. Approximation", that suggests theapproximation polynomial and shows the error. I suggest you to characterize the thermistormesuring it at 3 temp points far apart and use an online calculator for extracting the beta from these.So you will have the temp conversion almost done as you first write the fw (probably foryou it's enough to figure the working temp and use a linear interpolation).The strangeness i found was because of me:at first sight I thought that the mosfet was a P part with D and S swapped in the schematic,later I made a quick google search that returned a seller (Future) reporting it as an N channel,so I didnt understand. Now reading the datasheet it's clear that the first thought was correctThe charge pump:I was thinking to something like this:The advantage is less parts and an Nmosfet that it's easier to find with good parameters(I mean in general N vs P, dont mind the parnumber in my schematic, it was choose only for the simulation).You can add a small capacitor on the gate, depending on the output frequency of themcu pin and refine the component values.It should work, but I didnt tested itFabio.

author: MBY
author_rank: Regular Contributor
timestamp: June 06, 2012, 06:15:48 pm
content: Quote from: muvideo on June 06, 2012, 05:07:12 pmYes, it seems to work. Thanks for your help, I like the idea. But I selected components mainly on what I already possess and as the PCB is routed I think I stick with the current design.

author: MBY
author_rank: Regular Contributor
timestamp: June 10, 2012, 12:57:07 am
content: Ok, I have etched and mounted the PCB and checked the electrical. It fits in the old heater mount, but I still don't have any new heater. Time to start on the firmware.The wire on the pic is not a fix, its only to strengthen the ground plane. I'm sure it will work just fine without it. I did not found the DPAK transistor I plan to use, so I took a MPSA13 NPN darlington in a TO-92 just for testing. It handles a lot for the package, but not enough. Will be enough for testing though.The "resistor bank" is NOT the heater, its only the current sense. Slightly over-engineered. The non-pupulated 1206 (under the wire) is where the NTC goes, but it will use leads (I will place the NTC into the "electronics package" inside the inner tube). This PCB is to be mounted along with the heater between the outer shied (the red tube in the picture) and the inner one. Also the zener is not mounted as I could not find it, but its not needed really, just some overvoltage protection for the LDO that takes no more than 16 volts.It may seem to be strange to mount the PCB together with the heater(!), but I think I figured all out. If space or temperature does not permit it in the end, there is of course possible to mount the PCB on the outside.

author: saturation
author_rank: Super Contributor
timestamp: June 10, 2012, 12:24:52 pm
content: Thanks for all the fine work, MBY, been following your progress keenly. As you mentioned earlier, when its up and running I'd be curious to see how much noise it injects into the outputs.

author: MBY
author_rank: Regular Contributor
timestamp: June 11, 2012, 08:16:59 am
content: Yes, the noise will be interesting to see. I worked little with the firmware and started with a clean "P" controller or "droop controller". It is only slightly better than thermostat on-off with hysteresis. But I soon went for a full PID (maybe I drop the D term later on). One problem is that Tiny85 only got a 8 bit PWM. I found that a really fast updating control loop acts as a overlayed low frequency PWM. But I haven't got the full PID to work yet and I know why. The input range is 1024 steps, the output range is 256 and the integral part must exist in a much bigger interval not to be completely "damped" by windup-protection.In later years I have become aware of that broking the "float taboo" is often helping a lot in these circumstances. So, I started to use floating point math (please don't flame me on this, I'm perfectly aware of using left shifts and fix precision integers but I also know that it may actually _save_ code space to use floats. I have seen it many times. Floats is not a n00b decision).Anyhow, when I was leaved the project for work I still hadn't got the PID to work but I have only spent like one hour trying to make it work. I will resume the project in a day or two.A short word on the power supply. I don't want to load the transformer of the PD 2005 (and its off when the PSU is off) so I will use a 230->24 VAC @ 1.5 A ordinary transformer witch gives 12 v at 115 volts and sufficient current. Hopefully that also reduces noise. But I also have thoughts of using a SMPS with PFC (the power factor can be bad with a common transformer when a rectifier is used). Any ideas? Should I pursue a (PD2005 internally mounted) SMPS or go with the trafo?Saturation: Do you have an idea about the temperature (or the heater power) of the original heating system? Of course, I do not need to replicate that in detail, as a recalibration theoretically could use any temperature at all, as long as it stable, but it would be nice to know (I primarily want to settle for a little lower temperature to spare the electronics).I'm amazed that the electronics survived. Damn it was hot! I almost thought the transistors were germanium, but it cannot be, it was so hot! It took ages to get the temperature down and I had time to smell the smoke from meters away, turn it off, unplug it, take it down from the shelf, go find a screwdriver, open the upper cover, the lower one, spray-freeze it from above, from below (the base), notice it to warm up again (the thermostat clicked a few times back and forth), and it was still to hot to touch and smelled "funny". It must have been at least 5 minutes but may have been 15! And when I first noticed something was wrong it must have been several minutes of not-so-regulated heater action. I still don't know what happened. The only data I have is that the oven lamp started to light brighter a couple of minutes before disaster, as it happens that I had mounted a LDR on it to log the oven duty cycle. Well, if someone wants to know the duty cycle they have to do it themselves as my PD2005 gonna be unique.

author: T4P
author_rank: Super Contributor
timestamp: June 11, 2012, 11:38:55 am
content: I'll say SMPS, remember to use some`extra filtering as i don't think the china smps's have much noise filtering

author: MBY
author_rank: Regular Contributor
timestamp: June 11, 2012, 11:43:46 am
content: Yep, I will need one with PFC and of some quality as it would go (nearly) 24/7 unattended. I will test with my trafo first and if I experience noise I shall try a SMPS. I will have to find a good one, as apart from line noise I cannot tolerate much EMI either (as it will be mounted inside the chassis of the PD2005).

author: SeanB
author_rank: Super Contributor
timestamp: June 11, 2012, 06:00:55 pm
content: Use the magnetic, more reliable, and less EMI. Also easier to filter out the bridge rectifier noise and it will last decades like the original. Most SMPS designs will die after a few years of operation.

author: MBY
author_rank: Regular Contributor
timestamp: June 18, 2012, 06:58:52 pm
content: Hi again! I haven't have the time to work on the project lately but when I leaved it the last time the PID worked perfectly. I did a mini-oven with thru-hole resistors and some kapton tape and stuck the NTC inside it. Of course, it has not the thermal inertia or power as the real deal, but that will be adjusted by adjusting the PID constants. And yes, the D therm is rather superfluous but I think its a bit cool to have it anyway.The new oven will be about 6.5 watts I think and I settled for heating wire (not the original one) and kapton as isolator as suggested in this thread.But, as I shortly mentioned a while ago (maybe in the other thread) the heating filament goes all the way down to the base of the heater assembly, so I cannot reach it or replace it with ordinary wire. And, as I'm using lower voltage and higher current, I cannot stick to my original plan to use only "I/O"s already present as pins in the socket. Therefore, I need to make a small hole in the outer shield to stick the cables inside. That means there is no longer any need to have the control electronics _in_ the oven. So maybe I do another spin at the board to fix the two "bugs" I detected. It would be nice with current feedback to ADC, using the U2B opamp. I also realised I never did post the second schematics, so here it is:I'm sorry it is a crappy print screen. KiCad doesn't have a function to export/print to pictures (besides SVG). Some comments: The six 43R resistors in the current shunt is actually 10, not 6. I'ts only a matter of mounting "towers". I also did have to swap some pins to enable a debugging SW (USI-driven) uart. If I do another spin I might as well put in a bigger MCU as the space can grow outside the heater assembly. A cap on the NTC was removed - stupid me, of course you have trouble programming with a 100 nF load on SCK!Edit: The rectifier on the left is on a own board. I draw and routed them on the same PCB, and cutted it to two pieces. That way I saved some PCB material.

author: MBY
author_rank: Regular Contributor
timestamp: July 01, 2012, 11:59:13 am
content: Ok, time for a update...I have worked with the firmware and I'm quite happy with the function now. But PID-optimization is slow and boring as hell and I have to redo it every time something changes, like thermal isolation, setpoint, drive voltage, etc, etc.Now I'm doing another spin. I have changed the controller to a ATMega328 (Mega88 is enough, but 328 is what I have) to get a 16 bit PWM. I also added current sense feedback to the ADC and improved some other things. I was using a software/USI simplex UART with the Tiny85 for debugging. Now I have HW USART and also test pads for I2C if it ever will be used.As I talked about before, I do not longer aim at putting the PCB inside the heater, so the PCB area could be greatly extended simplifying routing, measurement and patches.I also added a second LED "interface" so I can drive a bicolor LED and a optocoupler feedback from the PD2005 so the controller knows whenever the PSU is on or off. This way, I can optimize the PID further still as the electronics inside the heater adds a fair amount of heat.My design is floating (dedicated transformer), so a optocoupler seemed logical.I actually already has routed the board, but not etched it. Probably I will make some final adjustment like improving the ground plane. I also promised to add something to commemorate the first or two first forum members that can answer my questions in the other thread. In the other thread, robrenz thankfully gave me some ohms and temp measurements, so I (or he) must figure out something (small text or artwork) on the copper layer before I can etch.Attached are the new schematics and KiCads 3D-view of the board. The DIL-6 on the board is actually a DIL-4 (PC/EL817), the 3D model is wrong.Again, its a PITA to do a plot and convert to a picture that this forum takes for upload, so only print screens.Edit: The HC49 style package is actually not a crystal but a SMD switch that happens to fit on the same footprint. My idea was to set a setpoint with a press of a button by using a common K-type thermometer and press when the desired temperature is reached.

author: grenert
author_rank: Frequent Contributor
timestamp: July 02, 2012, 03:06:14 pm
content: It is interesting to see that this PS has a custom ovenized reference circuit. Is it the same in both the 2005 and 2020? The Power Designs C500 (100V 500mA model) just uses an LM199. I wonder which is more stable.

author: robrenz
author_rank: Super Contributor
timestamp: July 02, 2012, 04:12:30 pm
content: Quote from: grenert on July 02, 2012, 03:06:14 pmNo custom oven on the 2020

author: MBY
author_rank: Regular Contributor
timestamp: July 03, 2012, 10:12:28 am
content: Quote from: robrenz on July 02, 2012, 04:12:30 pmThat got me thinking. What does the 2020 use? Do you know other units that use a custom oven? I was thinking to document my project well and release it as "open source hardware" to let others improve their PD PSUs. But, of course, that prerequisite that the stability on my PD2005 actually improves. If it worsens or give no benefit then there is not much point besides fixing a broken oven.Any ideas on text or artwork on my PCB to hail you!?I'm thinking of something like 10*10 mm (approx 0.4*0.4 inches). Remember that I will etch the board at home, so very fine details may be destroyed in the process. I can use a picture (bw of course) or a text (edit: or perhaps a dummy/nonsense component as an Easter egg).

author: MBY
author_rank: Regular Contributor
timestamp: July 06, 2012, 12:24:25 pm
content: I'm going of the internet for a week or so as I'm switching ISP as well as LAN layout. But I haven't forgot the project, it is still running.I probably add another LED control line (why just control two LEDs when you as well can drive a full RGB one?) and maybe other features (I'm thinking of double NTCs for redundancy and maybe other HW redundancies. This puppy should be designed for unattended 24/7 running and must err in the right ways).I'm also thinking of replace the PD 2005s main transformer to some 230 VAC model. I have bad experiences with old transformers. Thankfully I have always happened to be at home when the fire starts, but that's no guarantee.Edit: Another thing I did was making a new outer shield to be mounted on the actual shield. Made by what? A Red Bull bottle and PU foam! I will cut out the foam to make a chamber to mount on top of the shield.Will report back in a week or two!

author: MBY
author_rank: Regular Contributor
timestamp: July 28, 2012, 03:07:21 am
content: So, it has been much more than a week, but I haven't forgot about the project! I currently don't have a lab, so I can only do computer related stuff now (cad, SW).Anyhow, attached is the latest schematic. I have added a second NTC as a HW protection using the second half of LM358. I'm currently tries to fit this addition to the PCB in Pcbnew (KiCad).Sorry for the poor layout of things in the schematics. KiCad is a real PITA when you want to move blocks around to make new room. The result is that you stop caring and just squeeze in a new cap, resistor or whatever and leave it so with overlapping designators etc.

author: MBY
author_rank: Regular Contributor
timestamp: August 30, 2012, 10:29:48 pm
content: Ok, things always take more time than anticipated... But I had lot to do the last months. Finally my new lab is coming around. I have only a half dozen moving boxes to go through before I will start up the oven project again!See you all in a few days I hope!

author: MBY
author_rank: Regular Contributor
timestamp: September 18, 2012, 07:18:25 pm
content: Ok, time for an update. I have etched the PCB but was impatient enough to make two basic and idiotic errors, so I had to make patchwork almost immediately. First, I forget to do a last flooding and DRC before printing the graphics. A last time change made the ground plane and VCC connected. Argh!The second idiotic mistake was in the design process. I forgot that the Mega48/88/168/328 family does not have a selectable 2.56 V AREF but only the usual bandgap (1.1 V). Argh 2! So I had to cut traces for fixing error 1 and piggyback a TL431 (they sure come in handy often, 2.5 V was perfect!) to AREF.But now I'm up and running with my new PCB, enjoying 16 bit PWM and real USART, two LED/lamp drive lines and feedback both from NTC and from current. And also my SW independent second NTC interlock.I'm currently converting the SW from Tiny85 to Mega328. No pictures at the moment, but the design is basically as I showed in earlier posts. PCB is much bigger than the Tiny85 board as I no longer plan to use the PCB inside the oven.Edit: Language...

author: robrenz
author_rank: Super Contributor
timestamp: September 18, 2012, 07:52:38 pm
content: Glad to see you are still at it. I have not had time to do anything with mine yet but my first project will be to make a run of 10 turn 1 ohm pots for the 0 to 1mV scale. They dont exist as far as I know but we need them to get the microvolt adjustability that these suppies are capable of. I also stole a Preamble/LeCroy 1855 100Mhz differential amplifier for $300.00 on ebay. (see my sometimes it pays to be stupid thread) This has 10x gain and will allow meaningfull ripple measurements to be made on these supplies.

author: MBY
author_rank: Regular Contributor
timestamp: October 20, 2012, 04:44:11 pm
content: Time for a status report!Now things starting to come together. I have remounted the oven to the PD2005 and started testing. The PID loop regulates very well, much cleaner than the old thermostat. By experiments I have established that the old thermostat were shutting the heater power when the temperature is 70 degrees C and turning it on again at 65 degrees. So there is a hysteresis of 5 degrees. My PID loop keeps the temperature within about 0.3 degrees. The only problem is that the electronics in the oven creates rather much heat in itself, forcing different PID settings depending on if the PD2005 is on or off. Still, there is a tendency to put the temperature up or down by turning the PD2005 on or off. So the "always on" heater is a bit of a dud. But the same goes for the original design. The way to see it, is perhaps to regard the heater (when PD2005 is off) as a "pre-heater", shortening the time for the unit to reach stable temperatures.My setpoint is 60 degrees, not 65-70. I can adjust the setpoint up to 85 degrees but I think 60 degrees is good enough as its a compromise between power requirements, "effort" for the PID, stable temperature and longevity.I'm using two NTCs. One of them is used by the MCU controlled PID, the other one is a interlock using only analog circuits to shut the power when the temperature is off (interestingly, the hysteresis here seems to be about 5 degrees C), adjusted by a pot to about 70 degrees.I had a hard time to fit it all in the unit. My first design where a small board intended to sit in the oven housing, but in my second design I've scrapped that idea and made a board about 75*90 mm. Then, the new transformer needed to be fitted, as well as the power board (rectifiers and caps).I ended up using felt as insulation. The felt is inside the oven, and another layer on a paper roll (toilet paper!) that fitted perfectly on the oven.The stability seems to be improved. Stupid enough, I didn't record the stability enough, so I will ponder this thread to get clues about how bad or good it was before. But during several hours, the max and min recordings seems to be 200 µV apart in CV mode (no load). I will do additional tests under load and CC mode.Below is some pictures. Note the fitting of the transformer (is a 230->24 VAC @ 1500 mA used as a 115 -> 12 V. The max current used is just under 500 mA). The front panel neon light for the oven is replaced with a RGB diode. Blue is "too cold", red is "too hot", green is "hunky-dory", combinations of the colors indicates various error conditions, pre-heating and such). Also note the dial cover that was originally white but very worn. I'm used my girlfriends nail polish with a "Power Designish" orange color. I think its rather neat!From the top of the oven (big red cylinder) comes all the signals from the heater. The tick green wires are power, red and green enamel are the two NTCs and the thicker white is an ordinary K-sensor to a external thermometer. Also note the outer felt isolation. Upper in the picture you can see the control board. Between the transformer and the front is the little power board. Its not secured yet and only sits there. I'm using a network cable to connect the oven to the control board.On the control board (main pcb) you can see my little patch board I mentioned in a earlier post, with the TL431 reference.Everything is rather ugly, a rats nest, but it works!

author: MBY
author_rank: Regular Contributor
timestamp: October 28, 2012, 05:06:56 pm
content: Okay, another update with some stability figures. My first impression was that the stability goes up when the temperature does the same but I was wrong.A lower setpoint seems to improve stability. I have tested temperatures of 46.3 degrees C and 70.0 degrees C.Code:~95 mA:SetpointK-tempDM3061 avgDM3061 min/maxFluke 45 min/maxMeas timeAdscansd/d45049.493.077 mA92.5296/93.610192.485/93.59810 min41.0805/1.11345049.392.395192.1742/92.639592.130/92.5863 min40.4653/0.45670570.092.69691.8280/93.390391.795/93.33710 min41.5623/1.542~10V:SetpointK-tempDM3061 avgDM3061 min/maxFluke 45 min/maxMeas timeAdscansd/d70570.09.995339.99497/9.995459.9953/9.9956~5 min4480/300µV40046.39.996339.99631/9.996359.9956/9.9958~ 20 min440/200µVSome comments: The setpoint is in ADC units, K-temp is temperature reading on the K-type thermocouple I placed in the oven core. The avg and min/max readings are of course DMM readings from my Rigol DM3061 (6.5 digit) and Fluke 45 (5 digit). "Meas time" is the time I gave the values to stabilize and "Adscans" is something that relates to the SW (the ADC-samples per value). "d/d" is just the max-min deltas for the two DMMs.Now I'm collecting max and min values for much longer times, a couple of hours. Edit: I don't like the discrepancies between the DM3061 and the Fluke 45 in the d/d column. I may have a noise problem and I need to assert if the noise is from the PD2005 or something else.

author: robrenz
author_rank: Super Contributor
timestamp: October 29, 2012, 12:23:42 pm
content: Does the Rigol Allow setting the number of power line cycles NPLC to different values? I used 100 NPLC on the measurements you saw in the other Power Designs thread. That averages out a lot of the noise.

author: MBY
author_rank: Regular Contributor
timestamp: October 29, 2012, 12:59:13 pm
content: Quote from: robrenz on October 29, 2012, 12:23:42 pmNo, not to my knowledge, but the sample time can be adjusted with no reference to NPLC. You can speed up the measurement and the display indicates from 4½ digits to 6½ digits of accuracy (but always show 6½ digits). However, short time noise does not appear to come from the PD2005 as it does not vanish when the unit is powered off (along with the oven controller).

author: robrenz
author_rank: Super Contributor
timestamp: October 29, 2012, 01:18:01 pm
content: You might try shorting the inputs the Rigol right at the meter jacks and recording min, max, avg over similar time spans to see what the meter stability/noise is.

author: MBY
author_rank: Regular Contributor
timestamp: October 31, 2012, 02:07:58 pm
content: Quote from: robrenz on October 29, 2012, 01:18:01 pmYes, good idea. I'm just checked both the Rigol and the Fluke for some hours and not a single LSB digit noise on the Rigol (20V range), but +/- 2 counts on the Fluke 45 (10V range).

author: robrenz
author_rank: Super Contributor
timestamp: October 31, 2012, 02:45:22 pm
content: IMO you should lock both meters in thier lowest range for the highest resolution to do the meter stability test. Then set your PD to a voltage that is still within that lowest range to do a stability test. That will give you the most resolution on your stability tests. The 20 volt range does not have enough resolution to see the noise and drift of the meter.

author: MBY
author_rank: Regular Contributor
timestamp: November 02, 2012, 10:48:16 am
content: Quote from: robrenz on October 31, 2012, 02:45:22 pmThat is a valid point! But I'm not really interested in the stability beyond 1-10 µV but will do the test anyway because you never know what will turn up. Actually I did this very test before, but I didn't record the results (and I never measured the PD2005 at this range).But, what I know this far: The noise is not from the PD2005 as it appears even with it (and the heater) turned off. The noise is rather big on my scopes, about 50 mV PP (AC coupled), so I'm looking for some noise source. Probably some switched wall wart (those f**king things always spur noise on everything!). I also have to check the termination of the measurements and some other stuff. However, the 50 mV PP doesn't show on the DMMs (too fast probably) and since my DMMs show very little noise when the PD2005 is off I suppose that drift is real.Now I have done some SW changes, added a bootloader so I can fit everything and close the chassis and still update the SW with only the serial port on the outside.

author: MBY
author_rank: Regular Contributor
timestamp: November 03, 2012, 07:23:41 pm
content: Ok, I checked the DM3061 on the 240.0000 mV range. Measured min and max was +50/-150 nV after several hours (a whole night).Now I have a very strange problem. My oven has regulated very good both when the PD2005 is on and when its off. No problem. Error part in the PID is zero almost all the time. Now, I have assembled everything, did a calibration and was ready to put the cover on when I discovered that the error jumps around as if it was a loose connection or something, but ONLY when the PD2005 is off. My design is floating and the only connection between the oven controller and the PD2005 is via an optocoupler (so I know when its on and can preset the integral part according to that).What is going on? No problem when the PD2005 is on, but in off state the NTC seems to jump around. I've tried the most obvious things like measure ohms and voltages between floating things and reversed the power outlet.(And I "successfully" involuntary tested AC on the ohms range on a DMM as well as the residual-current device [unsure about the terminology, in Swedish we say "jordfelsbrytare" meaning "ground fault breaker"] in my apartment! I'm an idiot! Edit: And, yes my UPS also. Everything worked.)