Researching Alternatives to Dim Bulb Circuit to Control AC Current While Testing

[t1d]

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.html
If 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!

[bob91343]

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.

[oPossum]

An "electronic fuse" will react quickly to overcurrent.

[t1d]

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.

Thank 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.

[bdunham7]

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?

[bob91343]

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.

[Berni]

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.

[wizard69]

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.

[Gyro]

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).

[David Hess]

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.

[alsetalokin4017]

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.

[edpalmer42]

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.

Unfortunately, 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

[tautech]

What could possibly be easier to make from junk on hand ?



Incandescent bulbs will be around for many years yet in specialty shops.

[edpalmer42]

What could possibly be easier to make from junk on hand ?



Incandescent bulbs will be around for many years yet in specialty shops.

I 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

[David Hess]

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.

Unfortunately, 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.

What 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.

[t1d]

Wow, what a great conversation! Thank you, everyone.

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.

@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.pdf I 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.html

Thanks for the help!

[Gyro]

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.

[Berni]

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.

[t1d]

Thank you for your help, Gyro.

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.

Excellent point. I am sure I would have missed it, because it is so basic.

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.

Agreed.

[t1d]

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.

Hi, 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?

[t1d]

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.

Your 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.

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[Unixon]

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.

[David Hess]

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.

@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 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.

[t1d]

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.

[Seekonk]

What could possibly be easier to make from junk on hand ?



Incandescent bulbs will be around for many years yet in specialty shops.

I have to ask, is that a doorbell at the fan location?  Evidently this is some new technology I am not up on.