Researching Alternatives to Dim Bulb Circuit to Control AC Current While Testing - Page 1
Forum section: electronics / beginners

author: t1d
author_rank: Super Contributor
timestamp: July 06, 2020, 01:15:52 am
content: I need to chase the voltages in a failed PSU, to find the fault. The current in the circuit will have to be limited, to prevent further damage, while I am testing it. I am familiar with the Dim Bulb approach, but, while so very simple, that device ends up being large, creates a lot of heat and the bulbs are fragile and not so cheap, anymore.This device is a common voltage regulator, in a variable current regulator configuration, nested in a bridge rectifier. But, there is not much discussion of it.http://www.seekic.com/circuit_diagram/Electrical_Equipment_Circuit/LOW_RFI_AC_LIMITER.htmlIf there are no other real concerns with it, then it would be very easy and cheap to assemble. I already have all the components in stock.I am also aware of capacitive AC voltage droppers. Is there anyway to configure that to control current variably? I know caps for AC are a special type.I just learned of Current Regulating Diodes, but they do not carry very much current.I know that voltage can be controlled by PWM. Any way to do that with current?I have an extra AC-AC SSR, on hand. All the usual DC parts; MOSFETs, Transistors, Op Amps, etc.The DUT PSU is a 0-30V/0-3A Linear DC supply. It is powered from a 24VAC transformer, which is rectified with a Full Bridge. Regulation is by Op Amps, MOSFETs and transistors.Dream design goals might be 120VAC/0-3A/60Hz(USA). That would let me deal with most any AC device. Cheap and simple; a Variac is not in the budget.I am just not so familiar with AC. I need some direction and suggestions... Devices and circuit names to research... Please and thank you!

author: bob91343
author_rank: Super Contributor
timestamp: July 06, 2020, 03:04:04 am
content: Of course you could borrow a variac.Having said that, why do you need to have a variable current limited source for this failed device? It would seem to me that ordinary troubleshooting methods would be appropriate. Test the transformer with no load, with just the rectifiers as a load, with the filter added, etc. Or just measure parts to find what's open or shorted. That's how most of us do it.

author: oPossum
author_rank: Super Contributor
timestamp: July 06, 2020, 03:57:12 am
content: An "electronic fuse" will react quickly to overcurrent.

author: t1d
author_rank: Super Contributor
timestamp: July 06, 2020, 04:33:02 am
content: Quote from: bob91343 on July 06, 2020, 03:04:04 amThank you so much, to everyone that responded. I greatly appreciate your generosity in sharing your knowledge.The reason for wanting such a device is that the power supply portion of the circuit is built directly on the PCB that holds the regulating components. There is not a wiring harness with which to decouple it. Re-powering the board with the transformer risks the full current of the transformer coming to bear at the failure point. Uncontrolled (as with a dead-short,) damage could cascade throughout the device.I could separate the portions, by removing components, and power the regulator portion with my bench supply. But, I have done enough rework to know that unexpected damage is all but certain. Lifting a pad, or trace. Heat stressing a component. Etc.I just thought that using such a device was the more conservative approach. Safer for the equipment and me, too. However, I am just a hobbyist and I have not done much repair work. Maybe there are methods to work around this issue.The electronic fuse is an interesting idea. The issue with it is that I need voltage to be present on the circuit to find where it stops at the failure point. Even though the trip point may be adjustable, a fuse just opens, when an over current condition happens. But, it does not limit the current, so that the circuit may be tested while (somewhat) live.If the circuit failed open, then I could power it with some modicum of safety. But, I won't know that, until after I find the problem. I rather think it shorted, because a big power transistor was toasted.Thank you, again.

author: bdunham7
author_rank: Super Contributor
timestamp: July 06, 2020, 04:57:29 am
content: If you can disconnect just the transformer, you can use your DC bench supply to power it there. If it miraculously works, reverse the polarity and make sure it works both ways.Does the transformer on this have just one secondary winding? If so, it seems a pretty simple supply. Do you have make/model/schematic you could share?

author: bob91343
author_rank: Super Contributor
timestamp: July 06, 2020, 05:22:47 am
content: You can measure many components in circuit without using a soldering iron. A shorted part will show up on an Ohmmeter. And open part also will. Measure transistor junctions, diodes, capacitors, and so on, and look for readings that don't make sense. Once you find a suspect part, then you can disconnect it and see if it's good.A 100 Ohm resistor will never measure more than 100 Ohms unless it's open. A diode will show a big difference when you switch meter polarity. Collector to emitter on a transistor will show short if the transistor has broken down.

author: Berni
author_rank: Super Contributor
timestamp: July 06, 2020, 05:24:29 am
content: Yeah there are other ways to do it, but none are as simple as a just lightbulb (that actually works really well)The reason that they work so well is that the filament in a lightbulb has much lower resistance when cold. So because the lamp tries to draw a lot more current when still starting up this passes all the current trough the load under test without a huge voltage drop. But once you get the lamp enough current to warm up its filament the resistance goes up and this makes the voltage across the lamp go up, making it burn even more power in the filament, making it get hotter etc... until the lamp is visibly glowing and burning most of the energy on itself rather than letting it reach the test load. Halogen lamps work especially well for this as they are designed to run hotter, so have a larger cold to hot difference in resistance.Yes a lamp is not adjustable at all (Apart from perhaps having 3 different power level lamps and switches) but in most cases it does not have to be. You typically use the lamp current limiter just to power it on for the first time. Its mostly to avoid a badly faulty device causing a short across mains, making a loud bang and tripping the circuit breakers. Once the device works the lamp is typically just bypassed completely. The lamp will not save you from getting shocked or causing damage to the circuit, it just prevents huge bangs and showers of sparks leaving a big black charred mess if something goes really really wrong.My typical way of working on mains powered stuff is using a 1:1 isolation transformer. Helps save me from shock since any single point in circuit can be touched (But just using this as a safety net, not relying on this), makes using grounded instruments safer (ie osciloscope) and the transformers own capability limits the short circuit current to a reasonable amount (~15A for my 250VA one) so that a short circuit is more of a crackle and pop rather than a loud bang.

author: wizard69
author_rank: Super Contributor
timestamp: July 06, 2020, 06:37:47 am
content: Actually I'm interested in a solution to this problem also. The fact is light bulbs of the old incandescent style are becoming very hard to come by. At work we have many different types of microscopes, light sources & etc; which have light bulbs that are just about impossible to fine. I suspect household bulbs will be likewise in tight supply soon.

author: Gyro
author_rank: Super Contributor
timestamp: July 06, 2020, 08:43:16 am
content: You need to think a bit wider - oven lamps are compact, rugged and cheap. I can't see them being replaced by LEDs any time soon. Remember that you can parallel them up if necessary (a useful feature for a variable current rig).

author: David Hess
author_rank: Super Contributor
timestamp: July 06, 2020, 11:41:32 am
content: A linear or switching based AC power supply with current limiting is feasible and they do exist as test equipment but they are neither trivial to design nor inexpensive to buy.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: July 07, 2020, 06:28:11 am
content: It's hard to beat a Variac and an inline ammeter for this task. I usually just remove the fuse and clip the ammeter across the fuse holder, and slowly bring up the Variac until the indicated current approaches but never exceeds the fused value. Then look for hot spots in the DUT, using a q-tip and isopropyl alcohol. Or a thermal imaging camera, whatever you have on hand.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 07, 2020, 06:44:24 am
content: Quote from: alsetalokin4017 on July 07, 2020, 06:28:11 amUnfortunately, some switching power supplies throw a temper tantrum when you feed them a low voltage from a variac. At some voltage, some parts of the circuit starts to wake up, but other parts haven't started so things get locked into the wrong state, etc. I've heard that this can fry a perfectly good power supply. YMMV. But to be on the safe side, when I'm working on a unit with a SMPS, I never use a variac - only a dim bulb tester.Ed

author: tautech
author_rank: Super Contributor
timestamp: July 07, 2020, 07:59:09 am
content: What could possibly be easier to make from junk on hand ?Incandescent bulbs will be around for many years yet in specialty shops.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 07, 2020, 05:42:28 pm
content: Quote from: tautech on July 07, 2020, 07:59:09 amI completely agree! I've always felt that the 'best' solution is the one that solves the problem while using the lowest level of technology. The light bulb is the perfect solution. The bulb(s) will last for decades unless you drop it!Ed

author: David Hess
author_rank: Super Contributor
timestamp: July 08, 2020, 12:29:14 am
content: The brute force method is feasible also. Instead of using a full current limited AC power supply, a power AC current limiter can be made with power transistors operating in their linear mode.Quote from: edpalmer42 on July 07, 2020, 06:44:24 amWhat usually happens is at low voltages, the input current is high and repeatedly trying to start at high input current overheats or over stresses the power switch.

author: t1d
author_rank: Super Contributor
timestamp: July 08, 2020, 03:56:38 am
content: Wow, what a great conversation! Thank you, everyone.Quote from: David Hess on July 08, 2020, 12:29:14 am@David - Do you have a particular schematic for this critter? Or, a name to search by? My skill level is still too low to design it from scratch, but I am working on the learning curve.I was thinking of breadboarding this circuit to test it. I will go with an adjustable LT1084 for a greater current rating. What do you think of it? There are only a couple of lines that discuss it.Can I just swap the v reg for the LT1084 and still use the same pot and resistor values? Here's the Data Sheet:https://www.mouser.com/datasheet/2/609/108345fh-1270486.pdfI certainly will study out that point, for myself. But, you have way more expertise and may see a problem that I would not.I know that some things must be de-rated, when used in AC verses DC configurations. Is that of concern, with this circuit?http://www.seekic.com/circuit_diagram/Electrical_Equipment_Circuit/LOW_RFI_AC_LIMITER.htmlThanks for the help!

author: Gyro
author_rank: Super Contributor
timestamp: July 08, 2020, 09:21:48 am
content: Watch the voltage rating of the TL783, it is only rated to 125V input-output differential (good for a 3 terminal reg though) so it would probably blow on the voltage peak, even on US mains, if driving into a shorted target.I'm not sure how it would work with an LT1084, because that's only rated for 30V input-output differential. The regulator inside the diode bridge will see rectified full mains peak voltage when running into a shorted target.You don't want an electronic circuit breaker either for this type of application - you want something that provides a safe limited current so that you can probe for the fault. The lightbulb takes some beating for this.

author: Berni
author_rank: Super Contributor
timestamp: July 08, 2020, 09:34:30 am
content: There are some depletion N FET transistors out there that handle 400V and come in a large heat dissipating package. They act a lot like a JFET where they can work as a current source. A divider can be added to the gate in order to adjust its operating point to the desired current.

author: t1d
author_rank: Super Contributor
timestamp: July 08, 2020, 01:40:32 pm
content: Thank you for your help, Gyro.Quote from: Gyro on July 08, 2020, 09:21:48 amExcellent point. I am sure I would have missed it, because it is so basic.Quote from: Gyro on July 08, 2020, 09:21:48 amAgreed.

author: t1d
author_rank: Super Contributor
timestamp: July 08, 2020, 01:44:57 pm
content: Quote from: Berni on July 08, 2020, 09:34:30 amHi, Berni. This sounds like a good idea. But, as I mentioned earlier, I don't have the skills to design something like this from scratch. So, can you give me the specifics, or tell me the name of such a circuit, so that I can google it?

author: t1d
author_rank: Super Contributor
timestamp: July 08, 2020, 02:03:46 pm
content: Quote from: Berni on July 08, 2020, 09:34:30 amYour suggestion seems to be very much like the equivalent circuit diagram of the TL783. If I can build the circuit out of individual components (to be able to get the needed voltage and current ratings), I might be able to scratch a design together. I'll give that some thought.Warning... Server Image Error. This is not the correct image.

author: Unixon
author_rank: Frequent Contributor
timestamp: July 08, 2020, 02:11:36 pm
content: I would go for 1:1 transformer + polyfuse. A transformer must be small enough so that it couldn't blow anything even if it wished to but big enough to support standby PSU operation. A polyfuse isn't fast enough to save transistors, but still helpful, with a polyfuse you can limit the current even further when something is very wrong. Having isolation transformer also allows for safe probing of a live PSU, not that safe, but at least you won't blow your scope.

author: David Hess
author_rank: Super Contributor
timestamp: July 08, 2020, 02:16:07 pm
content: Quote from: t1d on July 08, 2020, 03:56:38 amI have seen schematics in the past but do not have one handy. You have the right idea.The problem is that under short circuit conditions, the power pass element has to sustain the maximum current at the maximum voltage which requires considerable power capability. Further, 170 volts (peak of 120 volts AC) is well above the secondary breakdown point of a bipolar transistors or thermal instability limit of a power MOSFET so further derating is required. So it is not a very practical concept although it is feasible with a heroic effort.One solution to the above problems is to implement foldback current limiting so that the current limit decreases as the voltage across the power pass element increases but this has undesirable side effects as far as how the current limiter behaves into a difficult load.The more common solution is to implement some type of electric circuit breaker despite the inconvenience.

author: t1d
author_rank: Super Contributor
timestamp: July 08, 2020, 03:18:31 pm
content: Something has gone very wrong with the attachments/drawings/images that I am posting. When attach a new image to a new post, the server is changing the attachment on an old post to the new image. Strange.I am sorry for the confusion that this will cause. I will report it to the moderators.

author: Seekonk
author_rank: Super Contributor
timestamp: July 08, 2020, 07:19:45 pm
content: Quote from: tautech on July 07, 2020, 07:59:09 amI have to ask, is that a doorbell at the fan location? Evidently this is some new technology I am not up on.

author: Jwillis
author_rank: Super Contributor
timestamp: July 08, 2020, 08:13:42 pm
content: Quote from: Seekonk on July 08, 2020, 07:19:45 pmI think its an old style ceramic light switch .

author: Berni
author_rank: Super Contributor
timestamp: July 08, 2020, 08:45:32 pm
content: Quote from: t1d on July 08, 2020, 01:44:57 pmA little googling actually results in finding this exact idea using a N depletion FET:https://electronics.stackexchange.com/questions/218976/ac-constant-current-source-designNow that blueskull mentioned it i also see you can get actual JFETs with high power handling capability such as this one (But depletion FETs are more still more common):https://www.digikey.com/product-detail/en/unitedsic/UJ3N120070K3S/2312-UJ3N120070K3S-ND/12083301Tho with the load being a dead short the transistor will indeed burn a lot of power on itself (Just like the light bulb), so the current must be set for something in the 100s of mA range or less in order to make sure the transistor can keep itself from overheating in all cases. Tho i suppose if you wanted a 1kW capable model you could still just put something like 20 of these current sources in parallel on a massive heatsink along with a bimetal thermal switch for protection.

author: tautech
author_rank: Super Contributor
timestamp: July 08, 2020, 09:06:11 pm
content: Quote from: Seekonk on July 08, 2020, 07:19:45 pmOld plastic surface mount mains switch. They were used on switchboards and wooden wall blocks on top of the end of steel conduit runs.Just a simple sprung leaf switch and available in 2 way versions too.More modern version of much older porcelain and Bakelite types.Just junk I had around when I built that dim bulb tester more than a decade ago.

author: t1d
author_rank: Super Contributor
timestamp: July 09, 2020, 05:33:42 pm
content: Quote from: Berni on July 08, 2020, 08:45:32 pmGreat find. I am sure it was no problem for you to find it. But, you have the advantage of knowing the proper terms to search by. That's why I always ask, specifically, for the proper terms... So, I can do the search myself. I hate to put the burden on someone else to do my "homework." It is totally reasonable for those that help to expect that those in need have done all that they can... first. I did do an extensive search, before posting. Thanks for the link.Interestingly, as I was researching FETs, this is the exact series that I found best for the purpose. You beat me to proposing it. lolQuote from: Berni on July 08, 2020, 08:45:32 pmYes, multiples FETs might be required. I did catch in the Data Sheet that the current rating may be wholly sufficient, but the watts rating is not. Tricky, tricky...I surely do appreciate all the great help! Thank you.

author: Berni
author_rank: Super Contributor
timestamp: July 09, 2020, 06:20:56 pm
content: No problem. These special types of FETs are actually not al that well known in electronics. Mostly because the common enhancement mode FETs are vastly more useful in most cases where you need a transistor, so they have become far more popular and so are the kind of FET everyone knows about. But this is one of the rare applications where a depletion FET is very handy. To be honest i didn't expect to find a write up on this online. I was just looking for a picture of a JFET current source to use as an example of how to do it but came across a schematic for the exact thing by accident (Too lazy to actually draw it)In terms of power dissipation, yes you will run into this problem with all semiconductor based designs. The tiny silicon dies inside can only work up to about 150°C, so a lot of cooling is needed to remove this much heat at high powers. The reason lightbulbs are so much better at dissipating this excess power is that the filament inside can operate at 2500°C. This massive temperature rise gives entropy a ton of motivation to spread the heat out into the room temperature environment so a lot of heat energy is dissipated despite the small size.But to be honest i don't really see any reason to build this circuit for supplying more than 50W. Such an amount of power is enough to start up pretty much any mains power supply. Once the PSU of the device being repaired is up and running then there is no reason to continue using the current limited supply. It will not protect anything downstream of the PSU since the PSU itself typically has a good bit of power stored in its capacitors, so despite a limited input power it is still capable of a plenty strong enough output surge to blow something else up. The only job of the current limited supply is to safely power up the mains side of the PSU in a way that doesn't cause a huge bang with sparks flying due to your wall outlet capable of supplying >300A for a short moment before the breakers trip. After the PSU it is the PSU deciding how much current it wants to provide.

author: t1d
author_rank: Super Contributor
timestamp: July 09, 2020, 08:42:03 pm
content: Quote from: Berni on July 09, 2020, 06:20:56 pmAfter looking at the link, a bit, I realized that I had come across it. However, I did not consider it in depth, because it appeared to me that the remainder of the text indicated that it was not a good solution. Far more complicated circuits were then proposed.Quote from: Berni on July 09, 2020, 06:20:56 pmFrom the very beginning of joining EEVBlog, I knew I had found a wellspring of wisdom and this proves it. I had not even thought about the capacitors building up energy. That's a great point!The more we talk about this, the more I am convinced that the safest way to deal with the onboard PSU is to disconnect it from the rest of the circuit and, then, inject a controlled voltage/current source with a bench PSU. The effort to separate the two and the risk of rework damage is just part of the game, but well worth it. Of course, the onboard PSU would then be test-able, itself. I have also learned that I need to add a break-point, either jumpers or cables, between the PSU and the circuit, on all my PCB designs.Do we need to consider this further, or should I mark the title as "Solved"?

author: alsetalokin4017
author_rank: Super Contributor
timestamp: July 10, 2020, 06:29:05 am
content: Quote from: edpalmer42 on July 07, 2020, 06:44:24 amYes, you are right. I almost never work on things powered by switching power supplies so I didn't think of it, sorry.