Light Bulb Current Limiter Questions

[Cathode Ray]

I read the thread on this, and have the PDF schematic and am going to construct one this weekend.

I have used my variac in the past to start-up a new build, but I recently learned that the variac only limits voltage - not current.

So, if I have something mis-wired yet only run the variac at 12V, I could still destroy my new amp build

Can someone explain how the Light Bulb Current Limiter differs from a variac to start-up a new amp, and how does it work with CFL's

You can't even find incandescent bulbs anymore.


Thanks

[xtian]

Yeah, CFL and LED bulbs won't work.

Use Ohm's Law to calculate max current thru a given bulb. For example, a 40w bulb will allow 333mA current.

[Cathode Ray]

So I checked online and my local Home Depot does have incandescent bulbs (who knew?)

If I understand the PDF schematic, I need to get a selection from 25W to 100W, and this is how you control the current (?)

[Stevem]

Even food stores household sections have incandescent builds still!

[sluckey]

So I checked online and my local Home Depot does have incandescent bulbs (who knew?)

Sure they do. Look at special purpose bulbs such as flood lights, décor lights, etc.

[R.G.]

I have used my variac in the past to start-up a new build, but I recently learned that the variac only limits voltage - not current.

So, if I have something mis-wired yet only run the variac at 12V, I could still destroy my new amp build

Well, kind of. They don't directly limit current, but by making the voltage lower, any current that goes through is lower as a consequence of nearly everything pulling less current at a lower voltage. So it's a back-handed current limit. Also, things like tube rectifiers and tubes themselves won't conduct much at lower voltage, so without the higher voltages to "push" electrons through, they don't conduct as much current.

The thing that makes a HUGE difference is to use your variac with a current meter. As you turn the voltage up from zero, you watch the current meter and when it jumps suddenly, you spin that variac knob down. I am lucky that my variac is inside a metal box and has both a voltmeter and a current meter on the box as well. That's worth concocting a case for your variac and putting a voltmeter and current on the case so it's always there and doesn't divert your DMM for that use.

Can someone explain how the Light Bulb Current Limiter differs from a variac to start-up a new amp, and how does it work with CFL's

Incandescent bulbs have a hidden special talent. An incandescent bulb has a cold resistance about 1/10 of its full-temperature resistance. That's how they stabilize at full voltage. The resistance increases until it limits the bulb's current at a certain voltage.

What that means is that with an incandescent bulb in series, the current can never get larger than the bulb's design current. This is a bit less than 1A for a 100W bulb, proportionately higher/lower with other watt ratings. If less current goes through it, as with a functioning amp limiting its own current, the bulb gets dimmer with the lowered current and the amp limits the current, not the bulb. The bulb also gives that visual indication of how much current is going into the amp.

It's a kind of squishy limit.

I like a variac AND a light bulb limiter for first power up, then remove the LBL when things look like they won't melt down.

Built in meters on a variac? Priceless.

[Cathode Ray]

I have used my variac in the past to start-up a new build, but I recently learned that the variac only limits voltage - not current.

So, if I have something mis-wired yet only run the variac at 12V, I could still destroy my new amp build

Well, kind of. They don't directly limit current, but by making the voltage lower, any current that goes through is lower as a consequence of nearly everything pulling less current at a lower voltage. So it's a back-handed current limit. Also, things like tube rectifiers and tubes themselves won't conduct much at lower voltage, so without the higher voltages to "push" electrons through, they don't conduct as much current.

The thing that makes a HUGE difference is to use your variac with a current meter. As you turn the voltage up from zero, you watch the current meter and when it jumps suddenly, you spin that variac knob down. I am lucky that my variac is inside a metal box and has both a voltmeter and a current meter on the box as well. That's worth concocting a case for your variac and putting a voltmeter and current on the case so it's always there and doesn't divert your DMM for that use.

Can someone explain how the Light Bulb Current Limiter differs from a variac to start-up a new amp, and how does it work with CFL's

Incandescent bulbs have a hidden special talent. An incandescent bulb has a cold resistance about 1/10 of its full-temperature resistance. That's how they stabilize at full voltage. The resistance increases until it limits the bulb's current at a certain voltage.

What that means is that with an incandescent bulb in series, the current can never get larger than the bulb's design current. This is a bit less than 1A for a 100W bulb, proportionately higher/lower with other watt ratings. If less current goes through it, as with a functioning amp limiting its own current, the bulb gets dimmer with the lowered current and the amp limits the current, not the bulb. The bulb also gives that visual indication of how much current is going into the amp.

It's a kind of squishy limit.

I like a variac AND a light bulb limiter for first power up, then remove the LBL when things look like they won't melt down.

Built in meters on a variac? Priceless.

Awesome

I will read thru your answer several times till it sinks in.


Thanks

[Cathode Ray]

I have used my variac in the past to start-up a new build, but I recently learned that the variac only limits voltage - not current.

So, if I have something mis-wired yet only run the variac at 12V, I could still destroy my new amp build

Well, kind of. They don't directly limit current, but by making the voltage lower, any current that goes through is lower as a consequence of nearly everything pulling less current at a lower voltage. So it's a back-handed current limit. Also, things like tube rectifiers and tubes themselves won't conduct much at lower voltage, so without the higher voltages to "push" electrons through, they don't conduct as much current.

The thing that makes a HUGE difference is to use your variac with a current meter. As you turn the voltage up from zero, you watch the current meter and when it jumps suddenly, you spin that variac knob down. I am lucky that my variac is inside a metal box and has both a voltmeter and a current meter on the box as well. That's worth concocting a case for your variac and putting a voltmeter and current on the case so it's always there and doesn't divert your DMM for that use.

Can someone explain how the Light Bulb Current Limiter differs from a variac to start-up a new amp, and how does it work with CFL's

Incandescent bulbs have a hidden special talent. An incandescent bulb has a cold resistance about 1/10 of its full-temperature resistance. That's how they stabilize at full voltage. The resistance increases until it limits the bulb's current at a certain voltage.

What that means is that with an incandescent bulb in series, the current can never get larger than the bulb's design current. This is a bit less than 1A for a 100W bulb, proportionately higher/lower with other watt ratings. If less current goes through it, as with a functioning amp limiting its own current, the bulb gets dimmer with the lowered current and the amp limits the current, not the bulb. The bulb also gives that visual indication of how much current is going into the amp.

It's a kind of squishy limit.

I like a variac AND a light bulb limiter for first power up, then remove the LBL when things look like they won't melt down.

Built in meters on a variac? Priceless.

Please forgive my naivete here, but does it go wall outlet -> variac -> light bulb current limiter -> amp...

or wall outlet -> light bulb current limiter -> variac -> amp



Thanks guys

[R.G.]

Please forgive my naivete here, but does it go wall outlet -> variac -> light bulb current limiter -> amp...

or wall outlet -> light bulb current limiter -> variac -> amp

First, good thinking that you realize those are different!

They differ in that if the order is LBL first, any current "eaten" by the variac shows up in the light bulb brightness as well as any current "eaten" by the amp. If the order is variac first, then LBL, the LBL only shows current taken by the amp.

So the question is then - does that matter?

I had a giant variac available to me in the power supply lab I worked in once. It was about 25kG/55lb. Using it, the AC wall power breakers would pop about every fourth time I turned it on. I thought it was defective, but upon measuring the current it took on an oscilloscope, I found that the " charge-up" current when it was first switched on was as much as 200A peak. The magnetizing current to charge up the magnetic field was huge, even though the variac was working perfectly. After the first "charge up" pulses, the current to the variac only settled down to about 100ma of AC wall current. Big transformers take big "start up" pulses, but then settle back down.

So putting the LBL before the variac will also show and limit the current to the variac - a little. The only problem with this setup is that the light bulb is low resistance when it's cold, and lets through about 10x the "hot" current when the bulb is cold. But still, it limits the variac inrush a little. After that, the variac current settles down to being very small.

That may or may not be critical or useful to you. I tend to turn off the power to the amp being tested by rotating the variac knob, not switching off the variac. So for my uses the variac startup current isn't too important. In my setups, I put the LBL after the variac.

[Cathode Ray]

Please forgive my naivete here, but does it go wall outlet -> variac -> light bulb current limiter -> amp...

or wall outlet -> light bulb current limiter -> variac -> amp

First, good thinking that you realize those are different!

They differ in that if the order is LBL first, any current "eaten" by the variac shows up in the light bulb brightness as well as any current "eaten" by the amp. If the order is variac first, then LBL, the LBL only shows current taken by the amp.

So the question is then - does that matter?

I had a giant variac available to me in the power supply lab I worked in once. It was about 25kG/55lb. Using it, the AC wall power breakers would pop about every fourth time I turned it on. I thought it was defective, but upon measuring the current it took on an oscilloscope, I found that the " charge-up" current when it was first switched on was as much as 200A peak. The magnetizing current to charge up the magnetic field was huge, even though the variac was working perfectly. After the first "charge up" pulses, the current to the variac only settled down to about 100ma of AC wall current. Big transformers take big "start up" pulses, but then settle back down.

So putting the LBL before the variac will also show and limit the current to the variac - a little. The only problem with this setup is that the light bulb is low resistance when it's cold, and lets through about 10x the "hot" current when the bulb is cold. But still, it limits the variac inrush a little. After that, the variac current settles down to being very small.

That may or may not be critical or useful to you. I tend to turn off the power to the amp being tested by rotating the variac knob, not switching off the variac. So for my uses the variac startup current isn't too important. In my setups, I put the LBL after the variac.

My variac is a Tenma #72-110



So, if I understand.. it's best practice to allow your start-up rig to warm up before switching on the amp ?

[R.G.]

The "start up" time on a variac is only about a dozen AC wall power cycles, so it's a fraction of a second.

So switch on variac; by the time you can switch on the amp, the variac and LBL are stable.

[Cathode Ray]

The "start up" time on a variac is only about a dozen AC wall power cycles, so it's a fraction of a second.

So switch on variac; by the time you can switch on the amp, the variac and LBL are stable.

Got it.

Thanks, RG

[Cathode Ray]

Completed the LBCL.

Finally understand the operation of this thing once I got to building it and realized you remove those little tabs between the outlets.

I've done a bunch of household wiring in my day and never even knew those were there.

Makes perfects sense now as the current passes thru the bulb before it can get to the amp's outlet on the LBCL.
<smacks head>