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Zero Cool 30th April 2008 08:28 PM

Using Bulb in Place of fuse for testing
I am working on a Crown K2 switching amp and there is a problem in the 15V regulator circuit. it is a very complicated Switching stage. I think i have found all the bad parts but i don't know if there are problems downstream. The circuit is protected by a 1 amp pico fuse.

I was wondering if i could use a light bulb in place of the fuse to save from blowing more fuses should there still be problems.

Its a 15v circuit so a 15 watt bulb would be 1 amp...would that be about right. a 15V 15 watt bulb? maybe i can use a 12V 12 watt automotive bulb to be close enough.

or i need to buy a gross of 1 amp fuses....until i find the problem....


Mooly 1st May 2008 05:38 PM

Hi, Cant really comment on the Crown circuit but switching circuits can exhibit unwanted behavior if the supply is "ramped" slowly. They can latch in posibly destructive mode. My experience is in mains powered switching circuits and some but certainly not all can do this. When you say a 15 volt circuit do you mean a mains powered SMPSU giving 15 volts, or are you talking about a low voltage DC/DC convertor.
Regards Karl

unclejed613 1st May 2008 11:14 PM

i would first disconnect everything downstream from the convertor and see if you get 15V. you might try a 4.7 ohm/5W resistor in place of the fuse. this way the voltage doesn't ramp slowly, but there's enough current limiting to shut the convertor down if too much current is being drawn.

gni 2nd May 2008 02:43 AM

The light bulb trick works best with linear power supplies. . . .

Zero Cool 2nd May 2008 03:27 AM

Thanks for the replies.

I managed to find the problem and fix the amp I was working on., But for discussions sake.

Lets say we have a linear 15V supply that feeds a circuit that has an unknown problem that blows a 1 amp fuse. and for discussions sake, lets say its a circuit that cant be brought up on a variac. your only choice is on or off.

What size bulb should be use in place of the 1 amp fuse to limit current to 1 amp or less to the circuit?

hitsware 2nd May 2008 04:34 AM

>What size bulb should be use in place
>of the 1 amp fuse to limit current to 1
>amp or less to the circuit?

~100 Watt
A 100 Watt lightbulb dissapates 100 Watts @ 120 Volts
Power = Current * Voltage
P = I * E
(with a short)
100 = I * 120
I = 100 / 120 Amp ....

gni 2nd May 2008 04:35 AM

A 40 watt 120VAC light bulb will draw about 1/3 of an amp. . .
That would equal about 360 ohms on the filament at full current.
The filament would be equal to less than 360 ohms when less
current is drawn through it. If more than 1/3 of an amp is
drawn through the light bulb. . .then it will get too hot and melt
the filament. The light bulb will be in series with the equipment

120VAC Light Bulbs:
15W = 1/8 amp and 960 ohms
25W = 0.21 A and 576 ohms
60W = 1/2 amp and 240 ohms
70W = 0.59 A and 206 ohms
90W = 3/4 amp and 160 ohms
100W = 0.83 A and 144 ohms
150W = 1.25 A and 96 ohms

15VAC at 1 A on the secondary is 15 watts. Then there is 15 watts
on the primary. . .thus about 1/8 of an amp on the primary.

At turn on the transformer will inrush and the bulb will light brightly.
Then it will settle down to a smaller overall current and the light
bulb might not even light up.

Start small. . .what will happen is the more current the transformer
wants to draw will cause the light bulb resistance to go up and
steel more voltage drop and the transformer will see less voltage.
It is a balance game between voltage and current. . . .

If the unit will not turn on. . .go to the next size bulb. . .it is fun
with a working piece of equipment to watch the light bulb glow
brightly at startup. . .calm down and then pulse with the beat. .
I think I used a 70W bulb with a 125 wpc amplifier. . .

Zero Cool 2nd May 2008 12:28 PM

Ok, But the fuse is located on the secondary side after the rectifier and caps. So a 12V 12w automotive bulb should be pretty close to the 15V 15W rating of the fuse.

I wasn't sure if the theory behind test bulbs was to size the Lit bulb current for the current rating of the device or if there was some method of 1/2 or double the current rating or some other system. etc etc.

gni 2nd May 2008 02:07 PM

The light bulb will act like a slow blow fuse. . . .

It really doesn't matter where you put the light bulb. . .just as
long as the wattage on either side of the transformer are similar. . .

I find it easier to insert a light bulb into the circuit before the
transformer. . .most of the time the circuit is tight and you don't
want to disturb it too much. . . Then when something blows, I don't
have my face in the circuit that is potentially going to explode in
my face. . .

Once the light bulb is in series with the mains then all current
will travel through it. The transformer will have to go to 0 ohms
for 120VAC to appear across the bulb. . . The light bulb will act as
a "soft start" circuit. . . .increasing in ohms as current increases and
decreases in ohms when current decreases. . .

unclejed613 2nd May 2008 11:09 PM

i work at a shop where we have light bulbs with alligator leads. you just remove the fuse and clip the bulb to the fuseholder clips. switching supplies sometimes have their own soft start devices, usually in the form of an NTC thermistor in series with it's input. using a bulb with such a supply is ok. if there is no thermistor, then the switching supply doesn't use a soft start, and might misbehave if you use a bulb.

most switching supplies, if they are working properly, don't need to be current limited if there's a short across their outputs, since they very quickly detect the overcurrent and go into "chirp" (shutdown) mode. if it's a multiple output supply, you can easily troubleshoot which rail is shorted by measuring the output voltage. the ones that aren't shorted might have 0.6 to 1 volt on them, and the one that is shorted almost no voltage. this is just a rule of thumb and i have seen exceptions to it.

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