Tag Archive for power supply

Panel Pooper

Institutionalized Dumpster Dive

Over the weekend, I had occasion (in other words I made an occasion) to visit A1 parts and surplus in Etobicoke. I have mentioned it before as a candyland for the junk enthusiast and this is essentially correct. Despite its remote location, it is well worth the visit, just be prepared to dig and expect no help from the rather cantankerous guy at the counter. Total comic book guy for the electronics store which I find endearing rather than offensive. Anyway, if you ever end up there, be prepared to wade through tons of unsorted junk, find a few gems, and score a few surplus deals.

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Dummy load acquires stability

Just a quick test and a quick solution

Had a few minutes to just check up on why the load would change when the voltage does. A simple test of trying a higher voltage power supply seemed to do the trick. Stayed within 10mA of where it should, and probably did better than that, but my multimeter can’t measure that. The new power supply is a 200mA 12V wall wort which is very unregulated. I measure 16V off the bugger with no load. Makes me wonder if what is printed on the box is merely a suggestion. Still, nice to have the headroom. Now the opamps have enough swing to really kick that MOSFET into regulating the current flow reliably.

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Encouraging results

Some quick tests work out quite well

Had a tiny pocket of time with which to test a few things with my aforementioned dummy load I just build. In looking through my collection of junked wall wort power supplies I found a low power one, a 9V 210mA. I wanted to check and see if the voltage headroom for the opamp would be worse. As it turns out, the stated 9V is actually 11.3ishV but whatever. I knew the supply would only need to deliver a few milliamps so I ventured to see how much exactly.

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Dummy Load revisted

The dummy load returns and I do some cooking

I do very much enjoy watching youtube channels on electronics. It has probably been the single most instructive resource for me and led to real understanding of what I am actually doing.

Some time ago, I watched Dave Jone’s excellent episode on building a constant current sink – a “dummy load” which is an essential piece of test gear for testing out power supplies. So much less fiddly than messing with loosely spec’d power resistors, I had always had the intention of building one but never quite got around to it.

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Safety Outlet

A simple build for much peace of mind.

I recently came across a new (to me anyway) electronics youtube channel presented by one Peter Oakes. I love collecting these channels, many great designers out there and lots of good information to absorb. I may do a post just on my favourite ones in future. Anyway, he is making both a power supply and dummy load so I ventured through his videos to glean lovely pearls of wisdom.

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Bleeding it dry

Shocking!

Cor blimey I nearly blew my head off. Shorting capacitors, particularly large ones is a very scary experience. I had removed the rectification board from the mains board of the power supply project. Read more

Constructive Feedback

The Zener pre-regulator returns, and is improved

So nice to have finally set up my workbench again and I’m a flurry of pliers and screwdrivers. Following up on a semi-meh-kinda success (but sort-of fail) is a resounding success! Just what the engineer ordered.

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The Milliohm Meter gets a tune up

Inexactness always bothered me. In the electronics hobby it is to be expected, given the millions of tiny variables we assume don’t exist just make our calculations take less than a year and our designs to be reasonably constructible at home.

I’m referring specifically to my milliohm meter adaptor which I’ve been fiddling with (see here and here oh and here). Last time, I cursed the inexactness of my cheap Chinese multimeter, and not having a 2A range so I could be reasonably sure of getting its output as close as possible to 1A. In addition, the accuracy (as contrasted with the resolution) was bad enough to introduce a significant grey area to my calibration.

I have laying about a bunch of 1% resistors I bought to act as current sense resistors for various projects and I went about testing them. Given the schematic from last time, the readings tended to be off (and high) by about 2.5%. For the 1Ω 1% resistors this netted a reading of 1.022-1.025 on my multimeter. In the ball park but most certainly out of spec. Having ten of these resistors, and finding them all out by the same amount, it’s easy to come to the conclusion that my current source is off calibration, rather the resistors being out of spec.

Adopting a standard

Lacking any better equipment, I decided to take these 1Ω 1% resistors as my standard and set about fixing my current source so they read within 1% of 1Ω and call it a day (as that is as good as I am going to get without better test gear).

After a bunch of fidding, I decided to remove the 15Ω resistor out of the parallel arrangement to slightly drop the current output by about 50mA. I then adjusted the pot until one of the 1Ω resistors read exactly 1.000V (i.e. 1.000Ω). This puts it (theoretically) within 1% of reality.

I then tested the other nine resistors and trimmed the pot slightly so the distribution of values all hovered within 1% of 1Ω. This means a reading beween 0.990 and 1.010V on my multimeter. I found that I was able to do this and have the results repeatable. Job done, right?

Always some fly in the lube

Though I was able to get it to repeatedly read these ten resistors within 1% of their stated values, I did notice something different when I checked a bunch of 0.1Ω 1% resistors I also had laying about. Their readings tended to be high by as much as 30%! So, for a resistor that should show up as 0.100V on my multimeter (well, okay, between 0.099 and 0.101) I was getting values around 0.120 and 0.130. Frustrating!

I checked the current output and it was (in)exactly the same as when I was testing the 1Ω resistors so it doesn’t appear as if the current regulation of the LM317 is going into crazy non-linearity. I’m a bit stumped by this one. How could it get 1Ω so right and 0.1Ω so wrong?

I did pick up the 0.1Ω resistors from a junk place, so i’m not at all confident they are what they should be, still sort of unlikely. Other culprits could be that the thing is heating up and affecting it’s performance, or the construction lends itself to be in the “noise floor” and subject to all sorts of random perturbations.

Acceptance

Given that I do not own my much coveted Fluke 87-V yet which does have the accuracy and resolution for me to tune the current output, I really can’t go any farther with this one. I could try feeding it 100mA instead of 1A and seeing if the results are more linear (indeed, this would have been in line with the original specs of the schematic I adapted this from) and if they are to replace the set resistors with the appropriate values for 100mA. This does give me the annoying problem that the volts display on my multimeter would then read in 10s of ohms which I had hoped to avoid. I could then, conceivably, add in an opamp with a gain of 10 to amplify the result, which would then doubtless introduce more error into the design.

I will also have to check this heating issue and seeing if the drift is due to the LM317 overheating. An easy fix for that would be to give it a 5V supply (instead of 12+V) to drop the voltage differential that the LM317 has to deal with and lower it’s power dissipation. I may try that now actually.

Nope, just tried it and the same result. I fear this will have to be “good enough” for now. Testing a couple of 0.75Ω resistors looked to be okay, 1Ω is okay, 0.1Ω is quite a bit off. I might try the 100mA option but otherwise I’ll just finish assembly and call it a day. I wish I had some other 0.1Ω resistors to check…

A word of warning

My design (perhaps foolishly) dumps a constant 1A into anything you connect to it. The test resistors I used were all 2W+ power resistors and this circuit will definitely (proven by experiment) melt 1/4W carbon film resistors so please do not do that. Think of that math: a 1Ω resistor with one amp running through it will drop 1V. 1V at 1A = 1W or 4x the rating of the little 1/4W resistor. So do not use this on something that can’t take it. As always, I am not responsible if you set yourself on fire. The 100mA option is looking more attractive by the minute…

An update

Because I simply love beating dead horses, and really do not like nagging problems, I chose to investigate further. I modified the circuit to produce an output of 100mA or thereabouts thinking that perhaps asking the LM317 to dump an amp might be skewing the results a tad. This also allows me to use the higher resolution 200mA DC current range on my multimeter for a clearer picture. I bypassed the parallel resistor array and whacked in a 15Ω resistor. The results were… interesting:

Resistor Current Voltage measurement
83.3mA 86.6mV
0.1Ω 83.3mA 11.9mV

Note something interesting? Well I notice a few things. According to Ohm’s law, 100mA into 1Ω should net 100mV. Likewise, 100mA into 0.1Ω should net 10mV. Since the actual current delivered is 83.3mA, then the readings measured should be 83.3mV and 8.33mV respectively. The 1Ω resistor is pretty close at an actual reading of 86.6mV, off by 3.3mV or 3.96%. The 0.1Ω resistor however is reading 11.9mA when it should (ideally) read 8.33, a difference of 3.57mV or 42.9%! Even accounting for nominal irregularities, that’s a huge difference in accuracy.

What’s most interesting about this result is – the measured current of the resistor in both cases was the same. Making the variance in results even more surprising.

Now let’s go back to the 1A current output, and this time i’ll trim it to exactly (as I can) 1A.

Resistor Current Voltage measurement
1.00A 1.066V
0.1Ω 1.00A 0.155V

Now this is very telling. Again, using Ohm’s law, 1A should equal 1V in this case (making our math much less messier than before). The actual reading in both cases for the current is 1.00A (limitation of the 10A range on my multimeter). The 1Ω resistor again acquitted itself well by being off by 6.6mV or 6.6%. The 0.1Ω resistor however is having serious trouble reading 0.155V, or 55% over it’s expected value! What on earth is wrong here?

It’s obvious since part of the trouble in both cases is that the results are shifted in a positive direction. This could easily be attributable to losses and random craziness in the project design itself and the foibles of the components that make it up. Fortunately, we can trim that. If we take our 1Ω 1% resistor as standard (and why not, it’s proven by math to at least be close to its stated value), we can trim the current so it reads exactly 1.000V and compare. Here are the results:

Resistor Current Voltage measurement
0.94A 1.000V
0.1Ω 0.94A 0.143V

Now this is much better, and you can see why I used the 1Ω for calibration. The loss of 60mA (to God knows where) is sort of incidental. The important thing is each resistor is getting identical (or within half a bee’s dick) current. The 1Ω resistor shows 1.000V exactly as it should. Look, however, at the 0.1Ω resistor. It should be reading 0.100V (or within 1% of that) now that we have trimmed out any errors in the system. Instead, it’s showing 0.143V or 43% off of what it should.

Why exactly this is happening I am unable to determine. I do not have other 0.1Ω resistors to test but it could very well mean that the batch of 0.1Ω 1% resistors I received from the junk shop are horribly off tolerance. Or it could mean that my multimeter is not reading well into the mV range (even though it has a stated accuracy of 0.5%). Since it seems that the LM317 is dutifully dumping the exact same amount of current (be it 100mA or 1A) it would follow that the results would be reasonable regardless of which resistor I used.

Conclusions

It has to be good enough for now since I lack the equipment to test further. Either the 0.1Ω resistors are way off, or my multimeter is. Either is likely but hard to say which. The easiest test would be if I grab some other 0.1Ω 1% resistors that inspire more confidence. The best test would be for me to use a good, properly calibrated multimeter (when I can afford that). The stated 0.5% accuracy of my current heap of shit meter would make it allowable for the readings to be off by a millivolt, but not 43mV! This is of course assuming it was ever calibrated of which I am doubtful.

Though exactitude is again proving elusive, I can at least measure low value resistors more accurately, at least in the range of about 0.7Ω-10Ω which is a bonus compared to what I had before.

Caveman Dummy Load

Okay, so I thought I’d start with the fun post :) . For the truly nerdly, electronics is always entertaining, even if you can’t directly see what’s going on. For everyone, however, the fun bit comes in with a bit of mad science. We want fireworks, smoke, sparks, and flames. So long as we don’t get hurt our burn our house down, we can enjoy a bit of drama and cackle evilly as we dump 3A through something and feel the power coursing through it.

I mentioned in the last post about the need for an immediate, low-tech dummy load that doesn’t involve me sourcing parts, puzzling over schematics, waiting for online deliveries, or rushing to the shops to grab that one part I’m missing (I’m always missing one!). With that in mind I had a problem with my power supply project. I’m busy designing away the various modules, simulating what would happen and punching the math to make sure it doesn’t blow up on me. I need to test the damn thing as I build and although I can simply check the output voltage of each stage I prototype up, it doesn’t tell me anything of what I will do when i put a load on it. Will it be stable? Will it melt into a pool of toxic goo? The only way to know for sure is to find out. I take a calculated risk as everyone does quite literally by simulating my circuits but it doesn’t take into account the real world of component tolerance and the million weird and wonderful little variables that are assumed not to exist in mathematical simulations. Besides, it’s more fun to build stuff up and see it in action :) .

My inquisitive searches popped up a number of interesting ideas that don’t involve me making yet another complex project like the electronic dummy load I want to build. I needed something simpler and nothing can be simpler in electronics than a resistor. I mentioned in my last status post that the logical dummy load is a resistor, but of course finding one accurate enough that can dissipate the required power without melting has been challenging. Most power resistors have really wide tolerances, are bloody expensive, and not so easy to find. Keeping a stock of all the required values would then be a challenge even if I could find the right ones. So, like any enterprising maker, I squared my shoulders and proclaimed “I shall make my own!”.

What is a resistor anyway

First, I needed to examine what a resistor is before I could actually build one. Put simply, it is nothing more than a length of wire trimmed to a known resistance. All wires have resistance although it’s usually assumed to be negligible and with good reason – it usually is. This is because the highly conductive copper we usually use for wires are designed to have very low resistance so we get our signals through and don’t waste so much power in our projects. A wire’s resistance is a function of it’s material’s resistivity (a constant that is different for every conductive metal, copper is 1.7 x 10^-8 Ω m by the way) multiplied by it’s length, and all divided by the cross-sectional area of the conductor in question. So it’s easy to see from that statement that the length of the conductor increases the resistance, and the cross-sectional area decreases it. So the thicker the conductor, the lower the resistance, the longer the the conductor the higher.

It seems simple now, doesn’t it? just make it thin and cut it to the desired resistance, done. One problem – the current it’s resisting has to go somewhere, the laws of physics prevent energy from just ceasing to exist. As expected, it’s dissipated as heat. With enough current flowing through it that could be disastrous. Not only would it heat anything touching it, but could melt itself making is a very dangerous thing from both an electrical and “burn down your house” standpoint.

Copper, having such a low resistivity, is hardly ideal to use for a resistor. I would need many kilometres of the stuff to get what I want and/or have it so fine a gauge that it would melt from the current I’m intending to dump through it. As I contemplate this, I toot on my ecig and it hits me – the kanthal wire in the coil could work perfectly. I check a packet of the coil wire I have and sure enough it says clearly 18Ω/m. Perfect.

I laugh to think – all a resistor is a heater.

The build

Seems simple enough, I need a precise 10Ω resistor, I have a meter of kanthal which I know is 18Ω/m, and I know it will take a few amps dumped through it since I do this many times a day with my ecig. All I have to do is cut it and test, cut it and test until I hit 10Ω. Then I can mount it on… what to mount it on… oh jeez. Here I have a length of wire, that’s going to get really hot if I dump more than an amp through it (I intend to dump three) and I have to have some way of holding it down. A live wire with enough current to kill me and enough heat to burn me (and anything around it) very badly is never a great combination. I need something non-conductive yet heat resistant. I look about and grab a pencil, maybe this could work?

IMG_0551
IMG_0552
IMG_0553

As you can see above it did work after a fashion. It measured 10Ω, I was able to put lower currents (<1A) through it without a problem. The above happened when I tried 1.33A from my little power supply and the pencil predictably started to burn. Yeah, that was a dumb idea. Also, my house now reeks of burning pencil.

I rummaged about for anything else I could find that might hold this hot potato and came up with nothing. I was hoping some sort of ceramic something might be floating about somewhere but no dice. In rummaging around I found a part that a friend gave me ages ago. It was a piece of a heater from an old car which is nothing more than a piece of plastic, some sort of heat resistant material and some brass strips. The nichrome heating coils were still on it so I cut them off and fitted on my coiled kanthal wire. Bingo, I had a solution.

In action

With my homemade resistor somewhat safely mounted in something that probably wouldn’t fly apart and melt, it was time for the real deal. Time to use it as my test load. I chose 10Ω as my intention was to test my newly prototyped pre-regulator with it and powers of ten make calculations really easy to do in my head. 30V over 10Ω is 3A or my target max current for my power supply. I will detail the results of the pre-regulator test in a following post about it specifically but the short of it is IT WORKS. Gwahahaha

Pretty isn’t it? that’s 3A of current running through it thereabouts and 30V across it. It was a lovely glow and did indeed give off a lot of heat. My chilly basement lab was quite cozy :) . The pre-regulator played nice too and gave me more or less expected results

IMG_0564

Conclusion

It just goes to show that even complex problems can still have caveman DIY solutions in this modern age. I built me a dummy load with no controls, no readouts, no fuss, and no real expense. I will still build my electronic dummy load of course, since I do need a constant current sink that is finely adjustable for more accurate measurements and to calibrate my power supply.

Return, re-education, progress

So, after a long hiatus I am back in the lab and back to try and finish at least one damn project before I die. Started innocently enough, was bored on an evening and not knowing what to do with myself I thought I’d catch up on some of Dave Jone’s excellent video blog entries. Naturally, his energy and charisma stoked the flames of interest and had me missing the heady smell of flux and the hum of an energized transformer. I looked over what I had on the go and found with delight that I actually remembered a lot and was able to get back to where I was intuitively in no time at all (read: a couple of weeks of review).

Staus

Here I’ll summarize the projects I have on the go before I get to the good stuff – the experiments.

Nixie Clock

Almost finished. Kinda. Sorta. The schematic I reviewed for any errors and made sure I got everything right – no problem. I gave the half-finished board a good look-over and found it just fine, needing only to be populated. I know for certain I will be needing a second board on top of it, which is fine, to house the ridiculous number of high voltage transistors as well as the remainder of the 4017 counter ICs. I mapped out a plan of how i’m going to do my interconnects as well and puzzled over the problem of having something like 30 connections going from one board to the other. This is what happens when you use data that isn’t multiplexed and no microcontroller, you basically end up with a lot of wiring. For the power connectors and a few other things I don’t mind using molex style PCB connectors but I was faced with a challenge of how to route the 16 connector outputs from the bottom board 4017s to the top. I briefly flirted with the idea of keeping them all on the same board but then I would end up with 28 odd connections I would have to take to the second instead of 16. So with that in mind it dawned on me that the old IDC connector and the ribbon cable are ideal for this application! I’ll pick some up at the shops when I finally make it down there. As always, I’m s a few parts short on every project so it’s a worthwhile journey. All this one needs apart from this is a few switches to set the time some way to mount the nixie tubes securely and safely and of course a box to put it all in.

Power Supply

The power supply is, and always was, a beast of a project. Some do their first power supply simply but I wanted something flexible, cool, powerful, and more or less something I will want to use years down the road. The inevitable revisions take forever and the whole project is quite complex by this point. As I’ve mentioned before I’ve broken it into modules to make it easier on myself: AC Power, Pre-regulator, Current Limiter, Voltage Regulator, Control and display.

The AC Power board, as shown from previous posts, is complete and overall I’m quite pleased with it. It does have a rather high DC voltage output of ±42.6V which necessitates the inclusion of the pre-regulator block (mentioned previously) but otherwise performs just fine and will take more than I could possibly throw at it before it dies. Subsequent video watching and research has made me want to add the rather important addition of MOVs (Metal-Oxide Varistors) to add some over-voltage protection to the AC input of the supply though this is a rather trivial addition and simply have to add one each between the hot-ground, neutral-ground, and hot-neutral. I will probably add them to the rectification board or may have them on a separate board or hanging off the terminal block, I have not yet decided. Over-current protection is already present in the 5A hot fuse in the terminal block, as well as redundant case fuses I will employ in the final build. There is also the question of the ground lift. Research has shown me the wisdom of not tying the centre tap of the transformer (the 0V rail) to mains earth so with that in mind I will keep it floating by default with an aircraft switch on the front to enable mains earth referencing should I need it. Easy.

The pre-regulator has seen much progress since the last time I took a poke at it. I had previously included it, discarded it, then included it again in a much more workable form. After much fiddling in circuit lab, I settled on using a zener diode/darlington transistor regulator and crunched all the numbers into a workable solution. I did build it up and test it but will expound on my results in a subsequent blog post. The upshot is I can knock that crazy 42.6V down to much more usable 28-29V and have it work over a variety of loads which is nice. There still is the question of stability and whether or not it will play nice with the rest of the circuit.

The other modules are untouched from last check. It will be a long time before a completed product. I’m still fuzzy on a bunch of things and I’m expecting pitfalls along the way which could be both frustrating and highly amusing. Of course, that is why i’m doing this in the first place – I’m learning, and that is its own reward.

Dummy Load

Of course this ties in closely with the power supply project as I need some practical way to test the thing under working conditions not to mention calibrate it. It has taken a back seat to other projects yet I will have to build it to build my power supply. Projects always lead to more projects. It all started, as mentioned previously, with Dave Jone’s excellent example, but I’ve been further spurred on by the discovery of Martin Lorton’s excellent version which will probably be much more suited to my needs. Naturally, I will add my own modifications to both make it my own and to suit my needs. Martin’s has a 2A cap (I believe he reduced it to 1500mA by the end though) and I need mine to sink 3A to properly test my power supply. It should be a simple matter of selecting the right mosfet and/or using mosfets in parallel.

Given that this project is likely to take me as long as the others and I still have not been able to leave my house to grab the appropriate parts, I went back to why I need this damn thing in the first place. The easiest solution for a dummy load is to of course whack the right value resistor that can handle the power you want to dump through it. This has proven most frustrating since not only do I not have a collection of power resistors on hand, but finding ones with the appropriate tolerance and power dissipation capability has proven to be difficult.

I decided for a more low-tech approach and see if I could make my own power resistor to act as a static dummy load just for now. That is a subject of the next blog post which I will marry with the power supply pre-regulator test. Quick answer: I did, and it not only worked but I’m still alive and my house is still standing.

New Projects

Always something new an shinier on the horizon, isn’t there? This is why I never get anything done.

Milliohm meter

Keeping with the theme that projects beget other projects, the power supply needs a dummy load, the dummy load needs a precise high power low-ohm resistor, I need a way to measure low resistances. It’s commonly known that most DMMs do a woefully awful job of measuring low resistances. One has to dump enough power into it to see a measurable result, and things like the leads now have a nontrivial resistance. So I started building an adaptor for my multimeter that fixes these problems. A post will be written on this also. No, I didn’t finish this either.

eGo Charger

This is merely an idea and a helpful schematic posted by someone on a forum somewhere. One of my eGo chargers for my ecig is malfunctioning and not doing it’s job and I sit here rather nervously waiting for my one functioning one to die and deny me my fix. I’m merely thinking about this one for now, if I get it wrong i’ll have explody batteries on my hands so you can bet I’m going about this one carefully.