The recommendation to use a bulb limiter is a very good one, but many don't know what one is. I thought I'd just put up a diagram to make it clear. Use a standard electrical box used for lighting fixtures with appropriate wire clamps and a plain old bulb base with a regular incandescent bulb. The wiring can simply be a short extension cord cut into two for the purpose.
Hope more folks new to the hobby build this first.
Hope more folks new to the hobby build this first.
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Yes. Extremely useful item for anyone building or repairing mains-powered electronics.
I included an output switch, and a lamp bypass switch. Mounted the whole thing on a piece of wood.
The big snag is that incandescent bulbs are getting harder to find, and those which are available are clear (i.e. unfrosted) so it is harder to judge the degree of illumination. I have a small stock which I hope will last longer than I do!
I included an output switch, and a lamp bypass switch. Mounted the whole thing on a piece of wood.
The big snag is that incandescent bulbs are getting harder to find, and those which are available are clear (i.e. unfrosted) so it is harder to judge the degree of illumination. I have a small stock which I hope will last longer than I do!
I did the same with mine, mounted up on a board and with a main on/off and limiter bypass switches. I just wanted to show the simplest approach so folks can really see what we are talking about.
Also see this thread and it's other links:
http://www.diyaudio.com/forums/power-supplies/167579-light-bulb-tester.html
http://www.diyaudio.com/forums/power-supplies/167579-light-bulb-tester.html
I have one I built 30 years ago. It also has a pair of sockets for measuring currant across the bulb. The bulb has to be removed when there is a meter connected or else the reading will be a waste of time.
I usually tell people to google "light bulb limiter" and they will find an endless stream of various descriptions and techniques.
My father build one 70 years ago ... it is an old trick ... a good one .... my fathers one had a fuse in the bypass switch .... the first electric experiments I did were done with this tester ..... 45 years ago ....
The switch he used had 3 positions .... OFF .... power through the lightbulb.... power throught the fuse ...............
The switch he used had 3 positions .... OFF .... power through the lightbulb.... power throught the fuse ...............
Thanks guys,
For some reason I thought that the bulb was used as the load on the amp. So it is just like a large resistor on the power line before the amp or whatever circuit you are using. I thought that some used the bulb with a variac?
For some reason I thought that the bulb was used as the load on the amp. So it is just like a large resistor on the power line before the amp or whatever circuit you are using. I thought that some used the bulb with a variac?
You can use a bulb and variac too. The bulb is in series with your unit. If the unit has a major short circuit, it acts like a low resistance, so the bulb lights brightly. If the unit is OK, then it acts like a high resistance, and the bulb stays dim.
Actually it's the other way around, and here's why:
Contrary to a resistor, a bulb has a very high temperature coefficient. A normal resistor has only a small variation in a given temperature range, but a bulb has a huge one.
When cold, a bulb has a very low resistance (a couple of ohms, depending on its voltage and wattage ratings). But as the current increases it heats up and the resistance rises sharply.
A 230 V / 60 W bulb measures only a few ohms when cold. But in operation, Ohm's Law calculates its resistance as almost 882 Ohms.
In case of a no fault condition, there's a relatively small current and the bulb tester won't really do much (low voltage drop across it). In case of a fault, the high current brings the bulb quickly to it's operating temperature and resistance and this limits the current to no more than what the bulb is rated for.
This behaviour is the reason why a bulb is so suitable for this purpose.
In the OP diagram it states the lower the bulb wattage the lower the current. If you have different wattage bulbs available, how do you decide what to use?
For instance if I'm testing a chip amp should I use a high wattage bulb as it's going to draw more current than say testing a DAC where a lower wattage bulb would be better?
I remember using a bulb tester on a chip amp project a while back and the bulb was reasonably bright - I wasn't sure at the time if there was a fault or it was bright because the bulb was low wattage and the amp was trying to draw too much current for the bulb used.
For instance if I'm testing a chip amp should I use a high wattage bulb as it's going to draw more current than say testing a DAC where a lower wattage bulb would be better?
I remember using a bulb tester on a chip amp project a while back and the bulb was reasonably bright - I wasn't sure at the time if there was a fault or it was bright because the bulb was low wattage and the amp was trying to draw too much current for the bulb used.
Yes, that's indeed the challenge with a bulb tester, getting the wattage of the bulb right for the DUT.
Perhaps someone with hands on experience can chime in here? Are there rules of thumb?
Perhaps someone with hands on experience can chime in here? Are there rules of thumb?
Well it's called a Dim Bulb tester for a reason. If the bulb is bright, either it's too low a wattage bulb or the component under test is drawing a lot of current.
If the component under test is using 25 Watts then a 25 Watt bulb will be almost bright.
It's best to start with low wattage bulbs and work your way up.
A Class 'A' power amplifier will need a real big bulb.
If the component under test is using 25 Watts then a 25 Watt bulb will be almost bright.
It's best to start with low wattage bulbs and work your way up.
A Class 'A' power amplifier will need a real big bulb.
Use a bulb which would have similar power to the item under test, or a slightly more powerful bulb. When sharing the mains voltage the bulb should stay cooler and so will have much lower resistance and so will consume much less power - hence the item under test will get most of the mains voltage unless it is faulty.
I have three switches, 40W, 60W and 500W. I start with the 40 watt. If it lights up full brightness I stop there. That is usually a sign of a dead short. If it glows a good bit the I go to just the 60W. If there is more than a glow then I turn the 40W and 60W both on. The 500w is used when I think everything is working before going directly to the line.
If you are cheap just look at the fuse! Set the lamp size so it won't blow.
If you are cheap just look at the fuse! Set the lamp size so it won't blow.
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Started clearing out Mum's flat yesterday.
I inherited the 5 incandescent bulbs in her stock.
linear Halogen bulbs are available from around 80W to 1000W.
300W and 500W are common easy to find bulbs.
You will need a holder from an old corroded floodlamp that you didn't throw out because it was useless!
Don't try to start up a "biased" amplifier using the Bulb Tester.
It is for identifying and protecting from damage an incorrectly wired amplifier.
Turn the bias adjuster right down to ZERO (measure the bias resistor for correct direction of rotation) and then start up via the Bulb Tester.
A no. 2 photo flood is a 500 watt lamp that fits a standard base.
I often adjust the bias based on the lamp glow. Turn down the bias and the light dims. Turn it up until the lamp just starts to brighten a bit. You should be very close if not dead on by final meadurement done while plugged straight into the mains.
A nicely constructed unit will last a lifetime or two. Built mine 40 years ago. Soon I will have gotten my money's worth out of it.
I often adjust the bias based on the lamp glow. Turn down the bias and the light dims. Turn it up until the lamp just starts to brighten a bit. You should be very close if not dead on by final meadurement done while plugged straight into the mains.
A nicely constructed unit will last a lifetime or two. Built mine 40 years ago. Soon I will have gotten my money's worth out of it.
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