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Light Bulb Limiter

Started by J M Fahey, March 17, 2011, 12:23:04 AM

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Quoteswitch mode power supplies

Not a good idea. Those things are often self regulating so they kinda hate everything that reduces the mains input voltage... The results might be catastrophic or the internal protection circuit might just disable switching due to low voltage condition or something.

Light bublb limiter might work with some SMPS but probably the odds are against it most of the times. Therefore I don't really recommend trying.


Thanks for the replies guys... OK just thought of something else. Would like to run it by you. Obviously there are amps out there with SMPS... I already came across a faulty Sub-Bass unit, but one where the SMPS was the problem, and managed to fix it. It was dead. I found some (rather thin) traces open circuit - maybe 'fused'..? I jumpered them and the SMPS sprang into life... sparks everywhere, coming from under the large 16 legged transformer on there. I ended up removing the T/X and found the arcing path... blackened and conducting. I cleaned everything up, replaced the T/X (added a few more jumpers) and everything was working fine. (I have attached a photo....) anyways, this leads me to my question/scenario...

If that SMPS had been OK and the actual amplifier needed attention, obviously I could not use the bulb-limiter on the mains input side.... BUT, I noticed that the output(s) from the SMPS were two really thick wires carrying DC (cannot remember the value) and also a small muti connector. Now then, would it be feasible to say that you could connect your 'bulb'... in line with the main HT out from the SMPS going into the amp section? It was all very obvious and modular, using spade connectors and this could have been easily done.

Just a thought for future reference....

Quote from: J M Fahey on April 30, 2012, 11:30:31 AM
It depends on the PS design.
Most of them do not like anything blocking current.


Re safety on supply rails;

2/3rds down read *Powering up* 22 ohm in  place of fuse but this might need to be a different value for other voltage/current requirements.

I'm sure other here will know more. :tu:


Thanks for the link Phil - but I am now confused. The 2nd paragraph says to "use a bench supply and increase voltage slowly"... but didn't we say that using a Variac on SMPS was a no-no...?


Quote from: phatt on May 01, 2012, 10:43:01 AM

Re safety on supply rails;

2/3rds down read *Powering up* 22 ohm in  place of fuse but this might need to be a different value for other voltage/current requirements.

I'm sure other here will know more. :tu:

J M Fahey

Dear noddys, as we said before, all SMPS designs are different.
Some will stand being fed from a Variac, some will not.
So trying to write an "universal truth" answer here is difficult.
Rather than asking different people what they think, which is a guide but of course depends on the particular SMPS they are working with, you'll have to test it by yourself, with the actual smps you have on your bench.


Hi @noddyspuncture; Rod's amp doesn't use a SMPS, and the particular par starts;

"If you do have a suitable bench supply - This is much easier! Slowly advance the voltage ..."

He's talking about having a variable DC bench power supply, not a Variac, and bringing the DC on the newly built amp up slowly.  As you will see just below there he shows a very conventional split rail supply.

To go with my light bulb limiter I have a collection of globes of various wattages ranging from a 10 watt "pygmy" pilot light up to 100 watter, but even testing valve/tube amps I rarely use anything higher than the 40 watt - when you need limiting at all it is generally at a pretty low level.

I also have a couple of blown 3AG fuses with a 150 ohm 10 watt resistor soldered across each, and I normally use these when servicing solid-state amps, particularly rack amps, as they often have exposed fuse clips for each supply rail and I can just plug them in.

The general line of attack with an amp that used a SMPS would be to first isolate the SMPS from the amp and see if it is functioning correctly on its own ('tho it may need a small load to fire it up).

If so, then a power resistor or two can be used between the SMPS and the amp, but if you can't get something reasonable out of the SMPS then you have to start by fixing that first (and SMPS repair is a whole topic on its own).

Variacs are certainly handy, but over a lifetime of servicing I've never really felt the need to get one.  Since they are actually a transformer they are still a pretty "stiff" supply and a limiting lamp is a much more effective way of restricting the power input, which is what you really want to do to avoid frying anything.
If you say theory and practice don't agree you haven't applied enough theory.


Many thanks Roly - that was very educational sir!

Quote from: Roly on May 02, 2012, 10:13:29 AM
Hi @noddyspuncture; Rod's amp doesn't use a SMPS, and the particular par starts;

Kaz Kylheku

The FAQ maintainer for the sci.electronics.repair newsgroup also maintains a SMPS repair FAQ.

This repair FAQ has a whole extensive section on lightbulb limiters.


He claims that "[M]ost of the time you will get away with putting [the light bulb] in series with the AC line".

But it is cautioned that "[SMPSs] can still supply bursts of full (or excessive) current even if there is a light bulb in series with the AC line" and "at some range of line voltage, the output regulation may not work properly and the output(s) may go much higher than expected".

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Quote from: Casey4s on March 24, 2011, 09:37:53 PM
This is just my second post here, but I couldn't help notice this thread.   I have used a ligt bulb limiter several times, it's a quick easy way to safely control startups and testing.

Here's my take on the limiter...

what software do u use to make that- i really need a good schematic designing program

J M Fahey

Chiming in uncalled  :loco

Don't know what he used , but for a very similar look I downloaded Express PCB and use their ExpressSch module.

Very easy and clean.
And you can draw from scratch any special symbol you need and add it to the library.

Just as an example:


yes in tube radio world it is known as a Dim Bulb Tester.  high current is dropped across the bulb protecting your equipment instead of high current across expensive delicate components such as transformers.  you can gage the bulb size by device power consumption.  100W power consumption = 100W bulb.  not to be confused with power output which is not close to power consumption :duh


transformers transistors

Start low, work up.  Particularly with a solid-state amp that has just had an output transistor transplant I always start on 40 watts or lower.  If you start too high you defeat the idea.  It only needs a trickle of power to bring an un-driven amp up; with S.S. amps the idle power is typically quite low, only a few watts.
If you say theory and practice don't agree you haven't applied enough theory.


just a question about the function/purpose of the light bulb limiter (finally made one after a couple stupid mishaps):

does it function as BOTH an

1) indicator - brightness giving you some info about current draw in amp
2) safety measure - if amp IS drawing too much current, will the bulb actually dissipate it - as power/heat in the filament - and thus prevent dangerous currents from getting into the amp itself? in this case its working as a sort of fuse that doesnt blow?



I can state this much... 40watt bulb with 120volt AC line will limit the current to 333ma. 60w at 120vAC will limit current to 500ma. 100w at 120vAC will limiter current to 833ma. When connecting any one of those those rated light bulbs in series through the light-bulb limiter that is the limit of current through the amp when the bulb is glowing fully. So if you have a 1 amp fuse in the amp, it won't blow. The fact that the bulb is glowing bright indicates a short in the device and now is drawing excessive current, if it was not limited it would cause further damage to the amp. The light bulb stands in the way of the draw of current and limits it just in the way I just described. Well that is just my way of explaining it and I am sure other's here will explain it better.

"A person who never made a mistake never tried anything new." -Albert Einstein


(1) yes, but the indicator function is just a bit of a bonus really.

(2) is spot on.  You can consider it to be a sort of self resetting "fuse".

The important detail here is that the resistance of a filament lamp changes quite radically from cold to operating temperature.  Its resistance at rated voltage is easy to work out from its wattage rating, but the cold resistance you measure with an ohm meter will be somewhere around one-tenth (depending on the rated wattage).  Generally the higher the wattage the greater the difference.

Aside: in big theater globes (e.g. 500W - 2kW) the cold inrush is such a problem that lighting desks often have a "keep alive" minimum setting that holds the filament at a (very) dull red heat to reduce the inrush if they are just banged on.  Taking such a theater light and plugging it directly in to an ordinary outlet is a good way to blow the globe.  This is one reason they often have oddball plugs that are incompatible with normal outlets.  {another is that, in 240V countries at least, you encounter half-voltage lamps that are intended to be connected in series pairs.}
If you say theory and practice don't agree you haven't applied enough theory.