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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 14th May 2024, 5:19 pm   #21
mediaseller
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Default Re: Dim Bulb Tester Alternative

Thanks to all who replied
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Old 14th May 2024, 11:14 pm   #22
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Default Re: Dim Bulb Tester Alternative

Quote:
Most forum members will have a box of old incandescents if anybody is desperate for one.
Who's been peeking in my loft!

AFAIK there is no ban on lamps for showbiz & theatrical purposes. Good choice of high powers there. Maybe check theatre/film industry suppliers.
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Old 15th May 2024, 4:48 am   #23
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Default Re: Dim Bulb Tester Alternative

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Originally Posted by Station X View Post
Search online for "Rough Service" lamps. These are still available.
In Oz, for some unfathomable reason, those were marketed as "Rough Construction" lamps!
I had one in service in my car port for years, until my son killed it with a basketball.
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Old 15th May 2024, 5:15 am   #24
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Default Re: Dim Bulb Tester Alternative

12v car headlamp bulb on the 12v secondary of a mains transformer, use the primary winding as the dim bulb substitute.

Transformers work TWO ways, they can also transform the impedance presented by a load

It needs to be a big transformer, though, to handle the primary current when the bulb is dim. I think the watage of transformer will need to be bigger than the wattage of the set.

There are mains voltage PTC thermistors disguised as ceramic heaters, hair tongs and all sorts of things, but these will be too slow acting to be good substitutes.

David
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Old 15th May 2024, 8:31 am   #25
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Default Re: Dim Bulb Tester Alternative

Not sure I understand this, David.

If the bulb is dim then the current through it is low, and the currents in both the transformer primary and secondary will also be low.

The VA rating of the transformer needs to match the wattage of the bulb at full brightness, i.e. when to load has gone short-circuit, and that wattage needs to be significantly greater than the load, i.e. the set under test. Headlamp bulbs are around 55W but you could always use 2 bulbs in parallel, or both filaments of an H4 bulb, (with a 110VA transformer) for higher power sets.

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Old 15th May 2024, 9:21 am   #26
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Default Re: Dim Bulb Tester Alternative

The bulb has a fierce PTC and can pass more current than it takes at normal operating voltage and brightness. The low voltage bulb has a thick and heavy filament with a longer time-constant than 240v bulbs. You can build flip-flops and oscillators with bulbs as the active devices!

This is the cause of large surge currents on turn-on, especially un-funny in string heater sets where the little 6AL5/EB92 flashes brightly between the heater pins and entering the cathode tubes. You don't want the transformer to saturate because that puts full mains on the set during turn-on surge time.

A 240v bulb is simpler and simpler is good.

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Old 15th May 2024, 10:10 am   #27
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Default Re: Dim Bulb Tester Alternative

I agree. 240V bulbs are not that difficult to source, and you avoid the side effects, not to mention the size, weight etc of the transformer.

Save the headlamp bulb limiter for 12V supplies.

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Old 15th May 2024, 4:53 pm   #28
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Default Re: Dim Bulb Tester Alternative

There are now two similar threads running side by side - Dim Bulbs/limiters/headlamp Bulbs - whatever.
Seems a lot of fannying about to me. What is wrong(or unattractive) about using a decent wee variac & an ammeter to slowly & safely power up a radio whilst observing the AC current slowly rise? To this end - cant beat an AVO8, on it's AC 1A range, & it's super reliable "cut-out" facility. Fluctuations can be quickly observed.
I'm not that much into domestic table-top valve radios, but over the years have observed that most, if not all,( if all is well, circuitry-wise) commence signal reception round about 190 - 200V on the Variac. Initially drawing about 90 to 120mA ac, obviously depending on the size of the set, then resolving on 160 to 200mA when 230/240V is reached, and the volume is turned up on a decent station. This mains voltage once it's rectified supplies the HT, & also Heater circuitry via a t/f secondary winding or dropper. Connecting another AVO8, on its DC 1A current range, in the HT line, and a DMM monitoring the HT voltage is also worth doing.
Maybe, this lamp limiting/dimiting carry-on is for folk who just have a "suck it & see" approach to initially testing a newly acquired set - just plugging it in to a mains socket & seeing what happens!
Lamp limiting, as part of any fault-finding/testing techniques, was never used in any military training or maintenance establishment I've been aware of. Nor was it used, or even mentioned in any civilian FE establishment I attended for ONC/HNC studies.
Modest sized Isolation Transformers & Variacs, and AVO8's and their siblings, often come up for sale in BVWS/VMARS/ARS sales/auctions/etc. UK-wise. I've seen prices range between a tenner & thirty quid for these items.

Regards, David

Last edited by David Simpson; 15th May 2024 at 4:58 pm. Reason: Add'l info
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Old 15th May 2024, 6:20 pm   #29
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Default Re: Dim Bulb Tester Alternative

Talk to your local electrician, they will come across dozens of them in the course of house rewires, when the old fittings (complete with bulbs) are pulled out of houses being refurbished.

They are probably pleased to get rid of them rather than have to recycle them.
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Old 16th May 2024, 5:00 am   #30
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Default Re: Dim Bulb Tester Alternative

I feel that a Variac and a series light bulb are different devices with different uses.

A Variac will not help you much with a device that suddently breaks down (or starts up and draws a lot more current) at a particular input voltage. It certainly won't help, and may cause damage, with many SMPSUs.

A series light bulb is almost essential when repairing SMPSUs. If you choose the bulb size appropriately, the power supply will start up and run into a dummy load (typically I use a bulb of at least 10 times the wattage expected in said dummy load) without damage. And if something goes seriously wrong on the primary side, no the series bulb will probably not save the chopper transistor. It will prevent components being blasted off the board and PCB traces melting.

It's also useful as a quick check for shorted turns in mains-frequency power transformers. And to check you've connected the 2 115V primary windings in series with the right phase.

If you plug an old radio into the mains, with or without a series bulb, you are asking for trouble. If you do other tests first, then a series bulb can be a useful device. I would not want to be without mine.
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Old 16th May 2024, 10:52 am   #31
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Default Re: Dim Bulb Tester Alternative

Hi Tony. AVO8's were & are still renowned for their ultra-quick cut-outs. One of them, plus the smallest possible rated fuse in a 13A socket is just the ticket for protecting sets. Some makes of "LockBlocks" are able to accept 0.5A or less fuses. No radio or most items of test equipment leave my workshop with more than a 1A fuse in a modern BS 3 pin plug.
Jesus, the number of sets I've come across with 5,10 or heffing 13A fuses in the plugs - ridiculous! Not to mention table lamps, tranny radios & HiFi stuff, etc. All taking just a few hundred mA at most.
If I was into switched mode stuff a lot, me thinks I would build a wee test set-up, protected by anti-surge fuses. Domestic ES or BC bulbs, here & there on a bench - prone to being bonked, I reckon.
I'm sure I've recently seen one or two decent, low uA fsd, 6" panel meters for sale. Suitably shunted & multiplied etc., great for homebrew dedicated PSU's or Test Panels. In my book, nothing beats a decent analogue meter display - for detecting a pending fault by visually seeing a steady needle start to "twitch". The sub-unit random lcd wandering display on a DMM is often confusing for some folk.

Regards, David
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Old 16th May 2024, 1:20 pm   #32
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Default Re: Dim Bulb Tester Alternative

Hi David and Tony - I find myself sitting on the fence between your two basically opposing views on this subject, or perhaps rather enjoying the best of both worlds. After many years of picking up bits of test equipment here and there at very reasonable prices, I have an isolation transformer feeding a variac feeding a lamp limiter via a dedicated Avo 7 (1A or 0.1A range) that feeds my "unit on test".

A few days ago I had the SMPS module from a faulty Philips VCR on the bench and after fitting replacements for the 1A A/S blown fuse and a couple of burnt resistors, then checking electrolytics I powered it up to a rather spectacular illumination via this arrangement. No blown fuses, no tripped Avo cut-out, no harm done. Turned out to be the power MOSFET which was shorted when subsequently tested.

What I learned from this experience is that in all that belt and braces configuration it was the cheapest and easiest to provide element that saved the day. On reflection, I had acquired another Avometer (ex-MOD based on the Model 9) from an expert Forum Member that came with his calibration sheet of some 200 tests. The cut-out check box showed "x8", which I assumed meant that it was satisfactorily tested 8 times, but in our later discussion he clarified that it actually meant the trip current was 8 times the FSD, as per spec.

So, in practice if we are relying on an Avometer cut-out for protection, on the 1A range then it's likely to allow up to 8A of fault current to be passed before operating. I think I'd rather use that as my back-up to a lamp limiter preventing the situation arising in the first place, while still enjoying the benefit of watching the AvoMeter needle showing the dynamic changes in operating current that David eloquently describes in Post#28.

Cheers
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Old 16th May 2024, 1:34 pm   #33
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I'm not a huge fan of the AVO cut-out I'm afraid. They only trip if the needle hits the end stop fairly hard (10x FSD?), and if you apply AC to a DC or resistance range with no rectifier in circuit, you get no needle deflection, so no cut-out.

I prefer the Unigor type cut-out, these trip at around 3xFSD, or 2V AC or DC on resistance ranges, and if there is any defelction, the cut-out trips long before the needle gets to FSD. Also, Unigor current ranges are in 3.16:1 steps (sqrt10, or 10dB), so you can always be above 30% FSD.

Unigor cut-out uses a relay with a slightly magnetised armature which is normally stuck to the core, and the usual spring to try to pull the armature away from the core. An overload of any polarity DC or AC energises the core in the direction to repel the armature magnet, the spring then pulls the armature away from the core and releases the cut-out.

I do like an analogue meter to monitor suppy currents as you turn up the voltage. Very useful for transistor power amplifiers when you've missed a faulty component which will take out all your newly replaced transistors if you went straight to full power.

My isolated Variac has moving iron meters on its output, true RMS, unlike some "true RMS" digital meters which I believe ignore a DC component on an AC voltage.

I haven't done much with SMPS, but I note the various warnings on this forum about slowly turning up the mains (particularly on older designs?), so maybe a lamp limiter is best for these, and to provide protection while soak testing on full mains.

So yes, you need a Variac and a lamp limiter.

Stuart
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Old 16th May 2024, 7:19 pm   #34
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Default Re: Dim Bulb Tester Alternative

"Rough service" bulbs and oven lamps have already been suggested as alternatives.
Another possibility would be incandescent lamps intended for use in traffic signals. These are available in 240 volt 65 watt and 240 volt 105 watt versions.

Linear halogen floodlight lamps are not banned and are yet another possibility, the common wattages are a bit big for current limiting, but they can be found in a 100 watt version.
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Old 16th May 2024, 7:59 pm   #35
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Default Re: Dim Bulb Tester Alternative

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"Rough service" bulbs and oven lamps have already been suggested as alternatives.
Another possibility would be incandescent lamps intended for use in traffic signals. These are available in 240 volt 65 watt and 240 volt 105 watt versions.

Linear halogen floodlight lamps are not banned and are yet another possibility, the common wattages are a bit big for current limiting, but they can be found in a 100 watt version.
Wow might be on the railways (although I doubt it) but Traffic signals in the UK moved away ES incandescent to Bi Pin 12V 50W halogen in 1970 (ish) (Mellor heads), by the mid 1970's apart from large city's the tin lantern with 65W or 100W ES lamps was becoming thing of the past. Now it's a struggle to get long life Halogen since the move to LED started in the 90's.

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Old 16th May 2024, 8:30 pm   #36
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Default Re: Dim Bulb Tester Alternative

Linear halogens are rather prone to blasting their filaments to bits, particularly when starting up in low ambients. Quoted 2000hr life on some of them is not properly indicative of the real world.

It's interesting to read about alternative systems in this thread, seems to be horses for courses. One side issue that occurs to me is portability- there's a lot of weight to a pair of AVO8s and a Variac compared to a lamp on a bit of wood.

Dave
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Old 16th May 2024, 8:53 pm   #37
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Default Re: Dim Bulb Tester Alternative

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Linear halogens are rather prone to blasting their filaments to bits, particularly when starting up in low ambients. Quoted 2000hr life on some of them is not properly indicative of the real world.

Dave
That's not been my experience with 12V 50W HQ lamps we found that 5,000 hr lamps in the 80's and 90's would do more like 10,000hrs and they were cold starting over 1,000 times a day all year around outside and they were intentionally under run at night (typically 6 to 9V) . Failures between annual bulk lamp changes were typically around 5% One contractor didn't do the bulk changes for 3 years and we barely noticed the increased fail rate. These were quality lamps by Osram or GE.

Getting back on topic the Lamp limiter is a very convenient way to test domestic radios etc. It's a different world when your working on industrial equipment or communications transceivers.
Each tool has its uses, the lamp limiter no fuses to blow the worst that can happen is the lamp comes on full brightness.

Everyone to their own.

Cheers

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Old 16th May 2024, 10:21 pm   #38
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Default Re: Dim Bulb Tester Alternative

The lamp limiter not only comes on at full brightness from its dim state, but the positive temperature coefficient also gives it a current foldback effect. Auto transformers and AVOs cannot replicate this. It's quite handy for the given purpose.

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