Archive for December 7, 2013

Current meter and current control

Subsequent poking around the the internet, looking at tons of schematics, I seem to be on the right track when it comes to connecting REF LO to IN LOW and will try that out once I get back to my lab. I am hopeful enough that I can continue with other modules. I was hoping to make it to A1 Parts since I am conveniently in Etobicoke today, but I feel like I’d rather not deal with public transit more than I have to in favour of a nice relaxing afternoon with my lovely lady. I’d probably be more likely to go if I drove. I intended to pick up a number of things including a 24V transformer (see last post on the control voltage board), an automatic wire stripper, and a panel cutout nibbler tool. Then I got to thinking about current shunts.

Of course, the voltmeter will need it’s current complement – the ammeter. I’ve been having a hell of a time not only finding good parts for one locally, but even figuring out the best components to get and the best configuration for it.

I’ve been following Dave Jone’s great videos on the EEVBlog YouTube channel, and on many posts he suggests using a parallel array of 10 1% resistors instead of a honking high-wattage single resistor for greater accuracy. While shopping for other parts last week I was disappointed that the values I needed were out of stock in my usual haunts (10 x 10Ω 1% resistors, which yield 1Ω in parallel). Though I could get the parts easily through DigiKey and the like, it had me take a step back and consider my other options. A search on DigiKey revealed lots of current shunt options including fancy chassis mount 4-wire temperature corrected ones. Sounds interesting.

The issue is that I want to place as little resistance and capacitance on the output as possible. Dave’s designs include a 1Ω shunt which makes the math easy but would dissipate 9W of power under maximum load, which is a bit high. For safety and to reduce the temperature induced error, my 10Ω resistors would need to be at least 2W (1W would give me only 10% headroom and would heat up like hell). The easy solution of course is to reduce the resistance to 0.1Ω or even 0.01Ω. Though this is easily possible, it would involve a complete recalibration and I would lose out on my 1V = 1A constant current control. For this reason, I think I will leave the 1Ω (ESR) resistor array there on the high side for the constant current sense and order acceptable parts from DigiKey. For this I have the option of getting my 10x 10Ω 1% 2W resistors, or a single 1Ω 1% resistor in a TO-220 package which has the advantage of me being able to heatsink it, reducing the temperature drift. A bit of a toss up. With the 10 resistor array, I have an opportunity to achieve a higher overall accuracy since the ESR would essentially be the average of the 10 values. The single one I can heatsink and will involve soldering two terminals instead of 20. Decisions, decisions.

The other issue comes in as to where I place my two shunts or even if there should be two shunts. Dave’s design in his power supply video (check his channel, most informative and helpful there), he places his current sense on the high side upstream from the voltage regulator and seems quite keen to avoid any sort of series resistance on the output side (no doubt for a good reason). I will follow him here as I barely have an idea of what I’m doing and prevents a lot of nasty problems I’d rather not deal with. That’s easy.

The only stick in the mud with that is that it’s position would also sense any power consumption of the voltage regulator and pass transistor as well as any losses in the circuitry so my 1V Iset will equal 1A … but not directly on the output. I can trim the control voltage to cancel out this modest difference and get it close to being what happens (effectively) on the output, but I am doubtful that it will track linearly with voltage and current changes. I highly suspect this is something I will just have to live with as it prevents a lot of other nastiness I most definitely don’t want to deal with. So it goes. I also kind of wanted to have a display for my Iset rather than some crusty, inaccurate panel markings for the knobs. The same issue applies – I would have to compensate for the current drawn by the regulators even if they are small.

Using CircuitLab, I simulated this up and it indeed indicates that I would have to set Iset at least 10mV higher to achieve the correct cutoff to the load, not even counting the limiting curve of the darlington transistor as I have it set up. So, if I metered the Iset voltage, I would have to trim the input to the meter as well to compensate. Though I just had the bright idea of building in a dummy load to use with Iset, which would actually be useful for calibration and other uses, that’s just too complicated a solution. I want to be able to set (and to see my setting) whether or not the load is connected to the output. It seems I will have to settle for trimming Iset a bit higher, and the input to the meter lower, to compensate. This way, I could use a 4T rotary switch to select the voltmeter display input for V+/0, V-/0, V+/V- and Iset adjusted to cancel the power consumption of the regulator etc. This is good so I can see Iset and Iout at the same time. Some kind of panel indicator for indicating this reading is in A instead of V would probably help too, if I am feeling that anal about it.

So that’s pretty clear. Though not ideal, it will have to do for my first power supply. I have to be mindful that I can’t go too crazy with it or I will never finish the project.

The next issue, however, is monitoring of the actual output current. This display of course must only display the current delivered (or sinked from) the load. This means putting the sense resistor (shunt) on or very near the output. Now, this is where I really do not want a significant resistance. Even 1Ω means not only 9W of power dissipated, but a 24Ω load becomes a 25Ω load as far as the constant current sense sees. This would necessitate me keeping this series resistance as low as possible, at least by 1 order of magnitude. This will also mean I have to make sure the input configuration on the meter reads at the correct scale as well. This seems a solvable problem and just a matter of simple arithmetic. How well and how accurate the display will be reading a lower input range needs some investigation (especially before I go and buy expensive shunts).

the ICL7107 seems to directly take a range of 200mV in 3.5 digits or 199.9 giving us a resolution of 100µV. Using a 100mΩ shunt resistor would mean that 100µV = 1mA which is expected. Leave the decimal point disconnected and the display will read directly in mA and go up to 1999mA. However, this is really pushing it when it comes to the ICL7107. I can’t find anywhere in the datasheet where it specifies it’s accuracy, but one can assume from the scale that it would be ±0.5mA (±50µV) under ideal conditions.

Applying the same simple calculations, if I change the shunt to 1Ω give me 1mV = 1mA a nice round number. Now the display will read up to 199.9mA. If I go the other way and use a 10mΩ shunt, this would mean 10µV = 1mA knocking the mA off the scale and giving me a max of 19.99A.

In going through the calculations I realize that I will probably be all right with the output resistance of this shunt using the three above. For higher currents, there will be lower resistance. For the maximum output possible on the lowest setting, which is 200mA (199.9mA actually), I get P=I2*R= 40mW which is peanuts. At full output and highest range, 3A and 20A (19.99A) respectively, the power dissipated would be 90mW. In the middle range, 1A load will cause shunt to dissipate 100mW. Far more acceptable that my back of the envelope (and as it turns out, unrealistic) calculation of 9W!

This did bring up an interesting issue here. I wrote in previous posts that the voltmeter schematic had a flaw on it that was causing non-linear readings. As it turns out, I got the Ammeter schematic from the same place and I would have to perform the same fix on the ammeter as the circuit beyond the input is identical. What the calculations above did show me was that the stated resolutions for each of the three ranges seems to be off by a factor of ten. The article attached to this schematic states that using the 1Ω shunt give you a display resolution of 1mA. This would mean that the IC would “see” 100µV/mA. My calculations show that it actually sees 1000µV/mA (1mV/mA) and since the 7107 reads directly in 100s of µV, the display should read “001.0” for 1mA, not “0001”. Unless I’ve missed something or messed up. If you are reading this and can fault my math above – let me know.

Obviously, I will have to build and test it to see if my assertions hold water. Like anyone, my calculations could be wrong and his right. The sucky thing is, if I am right then I will have to find a way of switching between three shunts – something that can take a max of 3A of DC current running through it. If he’s right, I just use a DPDT switch no worries. If I am right, I will need a 3T (ON-ON-ON) switch which will not only be harder to find, but more expensive I’m sure. One thing I’m absolutely sure of is not wanting to add yet more relays to the system or have cascading DT switch which is just silly.

Regardless, instead of following the schematic and using 5W resistors, considering my maximum possible power dissipation of any of my shunts could be 9W if I have the wrong range selected, I’d much rather spring for 20W shunts (either single or parallel array) just to make sure that if I have the wrong scale setting on the Ammeter, I don’t melt anything. Makes sense as that sounds like exactly the sort of mistake I would make.

Update

Here’s fun, as it turns out – we are both right! I had a hunch and checked the datasheet again to be sure the 7107 does indeed read 200.0mV full scale directly. The answer is it does, provided the voltage reference is set to 100mV. In his application (and most likely mine) the voltage reference is set to 1V which explains the order of magnitude difference in our calculations for the scale factor. With that in mind, I think I will opt for having two shunts, switchable by a SPDT switch which means less parts and less cost (hurrah!). Actually, this would have been my final choice in either case since I do not need a display of less than 1mA or greater than 3A so the only practical ranges I need are the 2A (1.999A) and the 20A (19.99A). If I set Vref to 1V then it follows I will need a 1Ω shunt for the 2A scale, and 0.1Ω shunt for the 20A scale.

But wait! I can do one better. Actually, it answers the initial question and problem I started with before digressing. If I do it my way, and set the reference voltage to 100mV then I can use a 0.1Ω shunt for 1.999A and 0.01 shunt for 19.99A. This accomplishes my original goal of minimizing the output resistance and power dissipation. I can even revise my power calculations, under maximum load of 3A, the 0.1Ω and 0.01Ω shunts will dissipate 900mW and 90mW respectively. Much better than 9W and this allows me to opt for 2W resistors instead of 20W ones. Not only is this a far cheaper solution, but there are more parts available in that range. This might also have applications for the scale dividers for the voltmeter, as mentioned in previous posts.

Voltmeter isolated supply

So it became apparent when fiddling with the voltmeter that some slight alteration to my schematics for the power supply would be necessary. The datasheet for the 7107 as well as many posts tell me that it is best to have an isolated supply for the meters. This makes a great deal of sense, this way the meters will be unaffected by whatever happens to the power supply it’s measuring.

My original idea was to tap off the V+, V-, and 0V from the main power supply circuit just after the AC filtering board and just before the Voltage/Current regulator board. Here is the latest schematic with my original idea:

Control Voltage board v2.1

Control Voltage board v2.1

To make an isolated supply, I will have to separate these two using a transformer, meaning it has to be on the AC side. I know what you are thinking (as I thought myself) “oh what a pain, I need another transformer/rectifier/filter” but really, it’s not so bad, and in the name of accuracy and better function why not? It’s easy enough to get a transformer, a bridge rectifier and a couple of caps soldered together. Since it’s not used for the main power supply, the current rating can be far less. In addition, I can use it to generate other control voltages I need allowing for a guaranteed clean signal and control of the main power board. So, everybody wins!

I even thought of a way to reduce the number of components and go with a single output transformer so I believe at the end of the day, the benefits and cost will balance nicely. The 7107 requires a dual symmetrical ±5V power supply. In the original schematic above, I use a cascade of positive and negative voltage regulators to achieve this. As it turns out, I happen to have another Intersil part – ICL7660 – which can make a symmetrical supply from a single one no problem. So right off the bat I can leave out three voltage regulators, not bad!

I will have to perform a few more tests to be sure though. I believe I read that the 7660 can sink up to 45mA so I need to monitor the -5V rail on my voltmeter to see if two meters would exceed 80% of this figure. Doubtful since the displays are driven off the +5V supply. Still worth checking though in case I have the current path wrong.

So what I need to do is move the 12 power diodes to the main board (these are there to compensate for the slightly higher voltage from the main transformer and to lower the differential voltage a bit to save heat and preserve linearity of the variable voltage regulator) and instead connect the control voltage board to a smaller, isolated power supply I will make for it. Here’s the laundry list:

  • 24V, 1A transformer, single output
  • Small rectifier and filter caps to replace the diodes and connect to the transformer

As I mentioned above, this supply is not just for the displays, but also provides two voltage references which I would like to be independent and accurate as these will set the Voltage and Current Limit on the main board. If I get a large drain on the main board, the control voltages will be unaffected. The more I think about this, the more I like it. Also, it will power the 4 relays ensuring proper operation.

I haven’t had time to draw up a new schematic for this scheme, but here’s the flow:

  • AC 115-117V transformer gets rectified and filtered to about 26-28Vdc
  • LM7824 steps down and regulates this to 24V providing the reference voltage for Vset and the input to the 12V regulator
  • LM7812 steps down from 24V and controls all the 12V bits and bobs I have including the 4 relays and powers the soft start and short circuit protection circuits on the output board and provides input for the 5V supply
  • LM7805 steps down from 12V powering the +5V rail for the meters and feeds into the ICL7660 for inverting
  • ICL7660 inverts the voltage for the -5V rail for the meters

Looks like it may just work. My only concern is the relays providing too low an impedance path to ground which may screw everything up. That’s what testing is for! I’ll post an updated schematic when I have a moment to create one and illustrate this idea a bit better.

Time also to raid my parts, I know i have a bridge rectifier and caps for it and I’m hoping one of my salvaged transformers will do the job. If not, I’ll buy one.

Voltmeter troubleshooting

Tests reveal something interesting

As a follow up to my previous post, I had a few minutes to mess with the erratically behaving voltmeter today and I believe a few quick tests might have found the problem – two problems anyway. The solution is not yet clear, but I have some idea on how to proceed now at least.

In reading the very helpful Tips for using Single-Chip 3½ Digit A/D Converters – thank you Intersil – I performed a couple of tests using my multimeter and scope

By the way, to those other hobbyists out there – you really can’t live without a scope, even a basic analog one like mine. Helps to see what the hell you are doing.

Testing voltages

So my first thought was to probe around and collect some voltage readings to see how effective (or not) my input voltage divider is. As it turns out, mostly, it works! Within a certain amount of error of course, but I can trim that out easily. I measured the voltage coming in to the IN HI pin of the IC and it returned values as expect in all ranges except for 2V. So there’s problem number one: I need to put a proper resistor value between IN HI and IN LOW for the 2V range. I’ll solve that later.

What was most illuminating was the 20V range. As before, a 5V input revealed close to expected voltages at that decade setting on the IC pin. It did also using a 12V input BUT at 12V input, on the 20V setting – the number was off by half it seemed. Curious.The correct voltage was reaching the IC IN HI pin, but why was it displaying something different?

In looking at the Tips datasheet (linked above) one of the questions in the accuracy section asked about a non-linear voltage at higher input levels. Well damn, that sounds like what’s happening. From the data sheet:

Accuracy Problems
Problem – Above a certain input voltage level, the displayed reading does not linearly track the input.
Action – Observe the waveform at the output of the integrator stage (pin 27) of the A/D converter. There should be no clipping at the positive and negative peaks of the ramped waveform. The value of RINT or CINT may be too small, or the oscillator frequency may be too low, allowing the integrator to saturate. See previous section on component value selection.

So the integrator could be the culprit. I tried to check the waveforms with my scope and I have a hard time interpreting the results. My scope is old and hack calibrated so I’m not quite sure what I should be looking at. I probed the integrator pin and bounced up and down on DC coupling like a low frequency square wave. I probably should have it on AC coupling which revealed a very short ramp up long sustain and quick drop. The datasheet showed an example waveform that looks just like a triangle wave to me (ramp up and down) so this is not quite what I was expecting. Either I get a low frequency square wave (dc coupling) or something completely wrong (AC coupling). It said to watch for clipping, so this could be what’s happening. A flat sustain I guess could be interpreted as extreme clipping. Anyone want to chime in?

So now I have a possible plan of action. I may breadboard up another unit to make easy changes and experiments without having to solder and de-solder the built one, then apply the changes to the built one. It also suggested the oscillator frequency might be too low, but as far as I can see on the scope, it is correct and producing nice waveforms. Here’s what I have to do:

  • Re-check voltages to make sure the voltage divider is not at fault
  • Check value and replace the caps on the integrator pins of the IC, possibly with a bit higher value
  • Probe the integrator to see if that produces the nice ramp-up/ramp-down waveform

Hopefully fixing the integrator will solve the problem and I hope I didn’t fry three ICs doing this.

Later that day…

OK well this is encouraging. I googled around (what I do when I’m obsessed with a nagging problem) and came across something I didn’t see before. Two other gents, using the same schematic are having very similar issues. Best part is they solved it (at least for them, who knows about me yet). Read here

So the coles notes version is that not only do I have to trim up my divider scheme on the input but I am probably missing a connection! I did think of this fix last night but I didn’t try it for some reason or other. It makes sense to because this ties REF LO, IN LOW, and ANALOG COMMON together giving us a fixed reference point. Both claimed that it fixed the issue and it started behaving properly which is good news!

What I find confusing is that it isn’t clear if I am supposed to connect IN LO to ANALOG COMMON and to ground, or if I’m only allowed to connect IN LO to ANALOG COMMON or ground (not both). The official datasheet seems to suggest I can’t do both but the forum comment claims success with connecting both. For safety reasons (the ICs are $3.80 a pop) I will try just connecting IN LOW to ANALOG COMMON/REF LO and see if I have success. If not, i’ll try connecting the lot to ground also.

As I am at my girlfriend’s place for a few days I will be unable to test it (oh why can’t I have everything in the same place?) but it will also give me a much needed break from swearing at the thing.

Here is my updated fix-it list:

  • Connect pin 32 (REF LO) to IN LOW
  • Smoke test power on
  • Check that the REF HI pin reads exactly 1V from and adjust if not
  • Conduct test measurements and rejoice if fixed, howl if not

If this turns out to be the culprit i’ll be so pleased as I don’t have to mess with swapping out caps, playing with the integrator, or checking every bloody joint on the board. Wish me luck!

Voltmeter hell

By the way – it draws about 70mA, not bad 🙂

It’s the sort of thing that makes you want to set your painstakingly assembled workbench on fire.

In my last post, I postulated that I could eliminate the calibration problems with my ICL7107 voltmeter by giving it an isolated power supply and altering the output resisters to a more sane configuration. Sadly, I’ve done both and it’s exactly the same result! Bastard.

This is the sort of thing where you have to take a step back before taking a hammer to the thing – it’s really frustrating. Yes – I know I should be patient and work through it logically – and I will, but what an annoying problem.

The 200V range seems to give me good results regardless which is great actually, I can use that to find fault with the others. I calibrated the ref voltage to 1V no problem, the 200V range seems to work whether it’s 1.4V, 5V or 12V no worries. It’s the other ranges I would like to introduce to a rabid tiger. The 20V I can get somewhat close to the 200V at lower voltages but it reduces to half at 12V and is off the map in the 2V range. By the way, the 2V range is just whacky and bears no resemblance I can see to reality.

Yes, I’ve tried other ICs, I have a whole collection I’ve been swapping out to no effect.

Obviously, I’ve botched the job and need to troubleshoot every bloody connection. Best thing to do is to breadboard up another one and play with it because really – I’m sick of bodge soldering. It’s just wrecking my board at this point.

There is a solution. I will find it. Most likely it’s my output resistors forming wonky voltage dividers again, perhaps switching to 1% and getting close to those magic numbers I put in the last post would help. Other than that, I can only think that I have a dead cap (i’ve already replaced half of them) or have something mis-wired that I missed. Either or both are possible.

Time to hit the books, well – the datasheet – and find a solution. Stay with me, I’m not by any means abandoning it, it’s made me mad now which means I will solve it.

Voltmeter

A power supply needs a display

In my second instalment of the power supply project, I’ve constructed the volts display. I’m kind of working back to front, as I’ve already been though much designing and I also built the AC-DC module which I will detail in a post to come soon.

What I’ve built is just a voltmeter and a display for it on a single piece of protoboard using the popular ICL7107 IC which is a voltmeter, ADC, and LED display driver in a monster of a 40-pin dip package from Intersil. This was quite a godsend as the power supply was already getting quite complicated and I wanted a simple, one-chip solution for the volts and amps display.

I had previously breadboarded it up and it works well. Actually, very little hassle considering what it’s actually doing. I did, however, run into problems with obtaining the IC briefly. I payed a visit to Creatron at Spadina and College (in Toronto) and they only had one in stock for $17! I stupidly bought it thinking “ok, it’s an expensive IC” but as soon as I got home, I looked it up online and found it for $2 something at Jameco. Bah! I was so pissed.

A couple of weeks later, I went to Supremetronic (for those who don’t know, it used to be on Queen Street West and moved to College St. then amalgamated with the Home Hardware. It now lives in the basement of the Home Hardware at College and Spadina) to grab some more parts. They not only had the ICL7107, but had it for $3 and change! So I bought four of them. Lesson learned: shop around.

I still like Creatron, especially for their nice protoboards, but it really does pay to shop around. Also, I have this horrible need to shop at brick and mortars for some reason. It’s more fun.

Back on topic.

So last night I actually go to solder the thing up. It was a bit fiddly to get it all to fit on two sides of one protoboard. Lots of ugly resistor forests which hide under the elevated 7-segment displays. I chose 1″ 7-segs just because I could and they look cool.

The font view, note the crappy soldering

The font view, note the crappy soldering

I modified the circuit to have a switchable range: 0-200V (199.9V), 0-20V (19.99V) and 0-2V (1.999). The last one I’ll use purely for calibrating it as the supply has a minimum voltage of 1.2V the last range is sort of useless for day to day use. I wired it via an 8pin pcb connector to a 2P3T band switch to change the range and set the decimal point. Lovely.

Rear view. Monster of a DIP

Rear view. Monster of a DIP

Took me about 8 hours of soldering, the last two of that finding and fixing cold or otherwise inadequate solder joints. I’m not the best at soldering as you can see, it’s a fine art I’m still developing skill in. When I first powered it on, it was intermittent and displayed junk characters. Fixing the joints stabilized it nicely. The oscillator is working fine, the auto-zero is auto-zeroing.

It’s not without it’s problems however. I did note that the calibration seemed off for the different ranges as well as it measured voltage non-linearly. A big problem and most perplexing. As with most things, the reasons and solutions are rather simple.

When test measuring, I made the beginners mistake of measuring it’s own supply. So when the calibration trimpot was set to a reference voltage of 1V, 5V read ok on the 200 and 20V range (with minor error) but 12V (again measured from the same supply) was horribly off on the 20V range. What’s more, the 2V range was off by a mile, displaying numbers about 1/2 of what they should be in the wrong range. Drove me mad.

I started reflowing joints and yanking out caps and resistors trying to find the problem. I realized only later that the problem wasn’t me or my build, but the schematic I was using. The must frustrating “trap for young players” (as Dave Jones from the EEVBlog would say) is that schematics you find online are frequently wrong, or the idea is right but whomever drew up the schematic forgot something or got a part value wrong. The error then gets multiplied as you modify the circuit to suit your application. Shit happens.

I used the following schematic which was deceptively simple and did (sort of) work on breadboard. Works reasonably well for the 200V and 20V range as presented provided the following:

  • you don’t be a smartass and replace the 1M input resistor with a 10k thinking that it will set the range to 2V (100k sets it to 20V)
  • you don’t measure the voltage of the same supply (or without a high impedance input and even then expect some error)

These two traps are something an experienced design engineer or seasoned hobbyist would know, but I’m just learning. After a couple hours swearing at the thing and bodging in new caps (unnecessarily) I realized that I can actually troubleshoot this. Going through it logically, the IC is working fine. The clock works, the displays work, the measurements are just off. The reference voltage reads 1V as it should so it MUST be the voltage divider on the input!

Looking at the schematic, and comparing it to the official data sheet, I notice a discrepancy. This guy (he isn’t mentioned on that page) modified the scale by changing the 1M input resister, rather than the resistor between the IN HI and IN LOW pins. This will work, after a fashion, but check what happens when you want a 2V scale. 200V scale is 1M, 20V is 100k, so I thought “duh i’ll put a 10k in there for 2V”. Logical right? Not really. The two resistors form a voltage divider which needs to scale the input by a factors of 10. So 1M/10k is 100:1, 100k/10k is 10:1, and 10k/10k is … 1:1! No wonder I was getting such an off reading, it was dividing the input by half instead of 10. Thus, it was half the proper reading and in the wrong range. Yes, I’m an idiot.

Fortunately, to correct the problem all I have to do is move a few resistors around and move a couple of jumper wires. Fortunately, I saw someone with a similar problem and a kind gent (goes by Ron H) posted a solution for correct values for the resistor that will yield the same ranges here. he even provided exact (I assume calculated) values to further reduce error. The new values are: 200V = 10.1k, 20V = 111k, 2V = infinite (open). I’ll put in the next closest values with 1% and/or series resistors to get close to that. Best part – it keeps the input high impedance (1MΩ) as any voltmeter should be!

I’ll post the results of my tinkering in a future (soon to come) post.

The power supply project

Progress… it’s good

Okay! So I’ve finally got back into electronics (hence why I haven’t used this blog much, been in my basement lab, recently set up) and revitalized a few projects from 2009ish that I’ve been dying to build. Top of the list is a bench power supply. Back then, I hacked an old computer PSU to make a quick, variable output power supply and it’s “ok”. Turns out “ok” just isn’t good enough.

To start with the thing is bloody noisy. It’s an old switchmode and my scope says it has a huge ripple of 50-80mV! That just … sucks if I’m trying to do anything serious and especially sucks if I am trying to do anything digital (though I’m still very much in analog mode at the moment). Sure I could have added a ton of filter caps on the output to reduce it, maybe a common-mode choke but really, who wants to do all that mess? A better design from the start will work better and the plus is I can say I built an accurate lab power supply.

So, I researched about a billion plans to come up with a workable, linear power supply design. The basic specifications are as follows:

  • Adjustable voltage output of 1.2-30Vdc
  • Adjustable current limiting of 1mA to 3A
  • Symmetrical +/- and tracking
  • Fine, linear control of the voltage and current
  • Adequate circuit and mains protection
  • Long life and easy repairability

Not insurmountable. In fact, done many times before by practically every electronics hobbyist. Sort of like a carpenter building his own workbench, the electronics hobbyist builds his (or her) own test equipment. Sure, I could buy a commercial one that fits these criteria (or most of them) for around $200 (and I’ve already spent more than that) but I want it to be mine. My design, fitted to my needs.

I’ve started writing actually late into the design of it, and have actually started building modules of it I was sure about. I’ve created this category to track this, and my other electronics build projects.

Though I realize very few people will read this blog, I hope the few that do can chime in with suggestions, I am still very much a novice and if you see anything glaring or have something helpful to share with me, that would be most welcome.