# What incandescent lamp wattage to buy for PSU troubleshooting ?

#### R1S8k

Jul 28, 2018
23
You didn’t buy incandescent. You bought LED!
wow I'm really surprised that it looks like an AC lamp, as there's no obvious DC circuit which converts the 220V AC to the proper DC voltage that the LED requires.

This looked to me as an AC one and I was happy I found it so easily

#### R1S8k

Jul 28, 2018
23
If you use an incandescent light bulb not an "LED light bulb" on the output of a power supply for a load then you must realize the low cold resistance results in an inrush current spike and can trigger over current protection circuits in the power supply unit under test.

Please explain more, I didn't get what you mean exactly.

Do you mean, if I; for example, used this PSU output pins which provide DC voltages 3.3, 5 and 12 volts, there will be inrush current, which will pull a high current and then blow the PSU fuse ?

#### R1S8k

Jul 28, 2018
23
OK, can anyone help me finding an incandescent lamp in aliexpress ?

#### Delta Prime

Jul 29, 2020
1,969
Please explain more, I didn't get what you mean exactly.

Do you mean, if I; for example, used this PSU output pins which provide DC voltages 3.3, 5 and 12 volts, there will be inrush current, which will pull a high current and then blow the PSU fuse ?
Yes. If there's a short circuit on the output the light bulb is not going to help you. For example.
Okay the power supply Max output voltage is 12 volts at 275 watts.
Maximum output current is Watts divided by volts.275÷12=22.916Amps.
Now you just bought a 12 volt 275 watt light bulb because you're going to use it as a load on your output just in case there is a short circuit the output will see a load? maybe?,that 22.916 amps is what the light bulb will draw when it's hot with a resistance of
0.524 Ohms (rounded up)when it's hot!
Rule of thumb is 10 times...so in this case it is 10 times less resistance.
0 0524 Ohms when the light bulb is cold. 12 Volts ÷ 0.0524 Ohms =229amps . That is your in-Rush current the cold light bulb is going to draw(That will never happen). That is overcurrent and that will trigger overcurrent protection circuitry whatever that may be. My point is if you're going to use a incandescent light bulb as an output load resistance keep in mind the difference between the light bulbs cold resistance and hot resistance of the element because it takes several milliseconds for the bulb to heat up and if there's a short circuit on your output the short circuit current could be 10 times greater than the rating of the light bulb. Whatever light bulb you get make sure when you "ohm it out" (measuring the cold resistance), it's at least 0. 524 ohms and you'll be okay. I have used incandescent lamps as a load and still do...

#### Bluejets

Oct 5, 2014
6,928
Yes. If there's a short circuit on the output the light bulb is not going to help you.
Rubbish...series lamp will light full on any short.

#### Delta Prime

Jul 29, 2020
1,969
Yes. If there's a short circuit on the output the light bulb is not going to help you. For example
Hope I didn't confuse you.
Like you I'm here to learn as well for me it's how to convey information so hears a polished version of what I trying to convey.Problem that every power-supply subsystem must expect: short circuits and resultant excessive current flow.
The problem and opportunity when using a bulb as current limiter is that you have to select a bulb with the appropriate voltage and current ratings to make this setup work. Also, you’re putting a resistance in series with the power-supply output, which is always a tricky thing to do since it affects regulation, dissipates power and reduces the available voltage to the load. It’s further complicated since the “alien” series resistance is not linear, but varies with the applied current

#### R1S8k

Jul 28, 2018
23
If there's a short circuit on the output

Wait a minute !!

You might enlightened me to another approach in troubleshooting techniques and I'm will happy if I learned something new. You're talking about finding a short of the output rails.

Actually my whole thread was about connecting a lamp in series with mains input lines to measure short of HOT side of the PSU.

And now I've learned that there is also another way to test shorts on output on PSU launch by connecting a lamp on the output. But from my experience, if there's a short on the output rails, then that output line won't have any voltage on it. And my way of measuring shorts on output rails by measuring continuity or resistance between GND and OUTPUT line without connecting the PSU to power. If there's a short then that output line is broken.

So I don't know what is the point of connecting a lamp on output lines ? I'm using a lamp on mains input is just to protect me and also to protect the PSU from getting more damage during troubleshooting process.

#### Bluejets

Oct 5, 2014
6,928
Series lamp goes on the input mains.
Any fault which would normally draw excessive current and cause the mains fuse to blow, now will have current/voltage limit input.
Best not to get confused and overthink the issue.

#### ramussons

Jun 10, 2014
464
One person talks about the lamp in Series with the Mains Input, another about using the lamp as a Dummy Load.
A 60 Watt Filament Bulb at the mains input acts as a Current Limiter ( with a visual indication of the current drawn) so that there is no damage to any component in case of a fault.
Commenting on Cold Resistance vs Hot Resistance of a filament bulb is irrelevant in this case and only adds to confusion.

The reason for a working PSU to flicker On and OFF could be because of a Minimum Load requirement of the PSU not being met. Some PSU's will switch OFF under no load. You can put a dummy load to draw about 1 Amp on the 5 Volt line and try again.

#### Bluejets

Oct 5, 2014
6,928
One person talks about the lamp in Series with the Mains Input, another about using the lamp as a Dummy Load.
A 60 Watt Filament Bulb at the mains input acts as a Current Limiter ( with a visual indication of the current drawn) so that there is no damage to any component in case of a fault.
100w series on input (mains) will, in many instances, protest against further fuse blowing and other damage and have the ability to run the whole unit to check outputs. Anything smaller quite possibly will not do both.
Pretty certain the Op has the idea by now.

#### R1S8k

Jul 28, 2018
23
100w series on input (mains) will, in many instances, protest against further fuse blowing and other damage and have the ability to run the whole unit to check outputs. Anything smaller quite possibly will not do both.
Pretty certain the Op has the idea by now.

Yep, I bought the 100W heating lamp, and added it to the test bench:

#### Delta Prime

Jul 29, 2020
1,969
Where did you get that crazy idea from?

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