Overdriving/Overvolting with PWM

[Databyter]

I'm interested in what the limitations would be in using a PWM circuit to drive an incandescent with an over voltage power supply with no regulation.

For instance AW's Soft start - multi-level switch http://www.candlepowerforums.com/vb/showthread.php?t=177316

We all overdrive bulbs in the voltage range, and we all rest packs, and or use soft starts to increase bulb life and decrease the possibility of flash.

But my question is specifically, using a partial duty scenario such as 30% or 60% on an incandescent could you not use a higher voltage pack to drive your bulb as long as you didn't crank it up to full power?

I can see several advantages to this, but also some limitations.

One advantage would be that you could use say a 12 AA pack (instead of 9AA) without dummies to drive an 1185 type bulb. Theoreticly this would seem to work on paper but would be very much over-driving the bulb for small fractions of a second.

One of the critical aspects here would be the frequency of the PWM. It would have to be high enough to prevent flash in the duty fraction of time. It's not as much the percentage of duty time as the frequency of modulation combined with the overall duty percentage. If the chunks of duty are too big this type of overdriving would lower bulb life significantly I would think.
At higher frequencies I think it might be great. How quickly does a bulb flash or reach over-temperature reducing life in a high voltage scenario??

The advantage if this worked would be more capacity (run-time), and as the pack went down you could even crank it up a notch (although in my mind this is where you start to run into serious limitations, I think the usable voltage even in a mostly drained over volted pack could flash a bulb even with very little capacity left if you aren't careful.

The advantage is that using an over-volted pack for a given solution at a lower level, if it worked would "rest" the pack equally allowing you to crank up duty as the voltage dropped.

The idea of a bulb filament is to get it hot. Voltage ranges, and destructive testing done on these forums for over-driving assume 100% duty.

Has anyone had success in over-driving bulbs by using higher voltage packs in partial duty?

Another advantage would be that you could use several packs interchangeably for different bulb builds.

I realize it is not only the voltage that would change but the current, and that the characteristics of bulb life might go to hell in a hand-basket, but, I think that research into this area might be interesting.

It would be convenient to not have to use dummy cells for larger holders and use lower duty as well (just thought of that one). There are a lot of possibilities to change out bulbs etc, and to "power rest" packs hot off the charger etc..

Thoughts?

(off to work I go, il read up in the A.M.)

EDIT: Of course I realize that the regulated circuits such as the Alan B project are superior, I'm just wondering how the math and physics of this work, and what I can get away with with the circuitry and packs I already have. I'm sure I'll do some of my own tests once I finish getting some parts.

 

[leukos]

Databyter,

Your basic question is that you want to know if you can significantly overdrive bulbs with AW's 30% or 60% settings whereas they might flash at the 100% setting?

I think the simple answer is no. TMK, the AW switch is not 30% the voltage, but the pulsing voltage is slowed down enough at that setting to give 30% of the output. The lamp is still seeing the same voltage at 30% as it does at 100%, it just doesn't have enough time to warm all the way up before the voltage cuts out again. This cycle repeats itself over and over to create the dimmer output. So, if I am not misunderstanding how the circuit works, then if it will flash at full power, it might flash at 30% as well, or at least degrade quickly.

 

[Databyter]

Databyter,

Your basic question is that you want to know if you can significantly overdrive bulbs with AW's 30% or 60% settings whereas they might flash at the 100% setting?

I think the simple answer is no. TMK, the AW switch is not 30% the voltage, but the pulsing voltage is slowed down enough at that setting to give 30% of the output. The lamp is still seeing the same voltage at 30% as it does at 100%, it just doesn't have enough time to warm all the way up before the voltage cuts out again. This cycle repeats itself over and over to create the dimmer output. So, if I am not misunderstanding how the circuit works, then if it will flash at full power, it might flash at 30% as well, or at least degrade quickly.

I understand what you said, and I understand how a PWM works.

But lets look at what makes a bulb flash. in a word, heat.

Driving a bulb with higher voltages causes more electro-motive force which creates more current and more heat. At a certain point the filiment which is designed to be thin to get hot enough to glow under current will because of it's thinness become too hot and just fall apart.

Partial duty reduces the heat to a certain extent although probably not very efficiently because it takes time for the components to cool down. That's also why for purposes of over-volting a bulb a higher PWM frequency would be better. Less under load time = less heat to disperse during the off duty time, and also better bulb life. Too slow of a frequency would give you too long of a duty time, and even though you would also get more of an off duty time, it would be too late at earlier voltages because the bulb would have already flashed. With an incandescent all you really need to do unless I'm missing something, is control heat.

So, in a nutshell, even though of course you are still getting too many volts for 100% duty cycle, you can offset over-voltage heat by decreasing the duty time with the PWM.

I know this is true, I just want to know to what extent it works in the real world. I expect that due to inefficiencies of cooling in the off duty time you would have to be in the lowest level (30% in the AW switch) for it to work even a few volts extra, (well past the flash point still), but for my purposes this would be enough.

I'm just wondering if anyone has already experimented with this and posted results.

 

[LED_astray]

I'm interested in what the limitations would be in using a PWM circuit to drive an incandescent with an over voltage power supply with no regulation.

...

EDIT: Of course I realize that the regulated circuits such as the Alan B project are superior, I'm just wondering how the math and physics of this work, and what I can get away with with the circuitry and packs I already have. I'm sure I'll do some of my own tests once I finish getting some parts.

Yeah, it's a long (series of) thread(s), but all the math is worked out Alan B, JimmyM, wgiles, & others for the Hotwire Regulator Design:

http://www.candlepowerforums.com/vb/showthread.php?t=220475

Basically, (& over simplified, I'm sure,) the bulb cares about RMS power and the PWM adjusts that. Their project adjusts the period to reduce the power, measures the battery voltage to sense the current voltage, and then does a calculation and adjusts the PWM to provide regulation.

I don't see any reason you couldn't do the calculations manually and accurately predict the voltages for 30% or 60% PWM with the max battery voltage. Of course, without the feedback adjusting the PWM, the Vbulb will sag as your Vbatt sags.

If you intend to do measurements, you should also note their explanations of RMS versus DC average voltage measurement. (It took me a lot of time doing examples to get my head around it and I still look up their equations when I need to do the calc.)

Added:
I found one spot where Alan B & JimmyM are explaining the Vrms measurement and calculation:

http://www.candlepowerforums.com/vb/showthread.php?p=2959810#post2959810

and an old post where I worked out a couple examples for someone else with questions:

http://www.candlepowerforums.com/vb/showthread.php?p=3092469#post3092469

 

[Databyter]

Yeah, it's a long (series of) thread(s), but all the math is worked out Alan B, JimmyM, wgiles, & others for the Hotwire Regulator Design:

http://www.candlepowerforums.com/vb/showthread.php?t=220475

Basically, (& over simplified, I'm sure,) the bulb cares about RMS power and the PWM adjusts that. Their project adjusts the period to reduce the power, measures the battery voltage to sense the current voltage, and then does a calculation and adjusts the PWM to provide regulation.

I don't see any reason you couldn't do the calculations manually and accurately predict the voltages for 30% or 60% PWM with the max battery voltage. Of course, without the feedback adjusting the PWM, the Vbulb will sag as your Vbatt sags.

If you intend to do measurements, you should also note their explanations of RMS versus DC average voltage measurement. (It took me a lot of time doing examples to get my head around it and I still look up their equations when I need to do the calc.)

Added:
I found one spot where Alan B & JimmyM are explaining the Vrms measurement and calculation:

http://www.candlepowerforums.com/vb/showthread.php?p=2959810#post2959810

and an old post where I worked out a couple examples for someone else with questions:

http://www.candlepowerforums.com/vb/showthread.php?p=3092469#post3092469

Thanks man!:twothumbs

This is exactly what I needed to look at.

I didn't realize the Alan B project was a feedback based PWM design using RMS values, I thought it was true voltage/current regulation. Or maybe there is more than one project, at any rate thanks for the links, I'm sure I'll learn all about it in there. I just have to resist buying one of thier regulator sleds, because I know Iwould get all the programming junk for it too, and I'm going BROKE

I already have a word document made with all my settings for thier controller heh.

EDIT Just wanted to say thanks again. This will save me time and money, thanks for taking the time.

 

[LED_astray]

You're welcome. I had most of that bookmarked because it had caught my interest (and I had some trouble figuring it out when it caught my attention.)

You have my sympathies on the time & money thing, I literally have boxes of cool parts I haven't gotten to yet and there are even more cool things I didn't buy at all. Have fun!

 

[Popsiclestix]

I found an interesting link on Google for 14V automotive incandescent driven with a 42V source.

That being said, I've used AW's switch as a cheap regulator for my 9AA WA1185 hot off the charger. So it appears for small difference between Vpeak and Vmaxbulb, PWM suffices.

Theoretically PWM regulated voltage should have the same effect as DC current in steady-state conditions, but it seems that a linear ramp-up effect is not sufficient enough to protect the bulb during start-up.

I too would be curious as to the relationships between Vpeak and Vrms and vaporizing the filament.

 

[Databyter]

I found an interesting link on Google for 14V automotive incandescent driven with a 42V source.

That being said, I've used AW's switch as a cheap regulator for my 9AA WA1185 hot off the charger. So it appears for small difference between Vpeak and Vmaxbulb, PWM suffices.

Theoretically PWM regulated voltage should have the same effect as DC current in steady-state conditions, but it seems that a linear ramp-up effect is not sufficient enough to protect the bulb during start-up.

I too would be curious as to the relationships between Vpeak and Vrms and vaporizing the filament.

I'll check it out, I've only been using the AW switch for a few weeks, I first noticed the PWM sound it makes when I had the tailcap open to do a tail-cap amp test (I gained.3 amps current over my partially resistance fixed Mag switch!!),
At first I thought I shirted out the battery pack because I just wasn't expecting to hear any sound, then I quickly deduced it was the switch modulating, not noticed previously because the tube damps it out quite a bit.

Today I got some stuff from Five Mega including a 1909 Bulb I wanted to test in my Mag 85 setup (same Vrange more Amps higher flash, higher lumens/Volt) Now I notice the PWM sound alot more with 5 amps than with the lower current draw, I suppose this is normal.

It's not really loud it's just that now that I know it's there I suppose I listen for it and to it, and it seems more prounounced under higher load, or maybe it's just because I'm in my quiet bedroom.

Anyway I suppose it's normal and not a sign of any issues.

EDIT: BTW this 1909 bulb is a lot hotter temp wise than the 1185. The focused spot is very noticably warm from a foot away on my hand, at the LOW setting. It's not dark yet here so I'll have to see how it lights up the stuff I usually light up. The spot focuses down nicely with an FM deep, but when you open focus you can see there is a little bit more artifacts with this bulb than with the 1185 due to the bulb being sealed on the top leaving a glass dippy doo on the top, the 1185 is a round top and it opens from focus with a perfect little donut when centered properly.. When focused though both bulbs make a fairly tight spot and nice spill and I'm sure based on the heat alone that the 1909 will prove to be much brighter tonight. I'm going to use eneloops just this once even though my usual rule is nothing over 4Amps.

 

[Popsiclestix]

All 3 of my switches do that. The sound you describe is perfectly normal, I think it's PWM whine + resonance of the tube I think.

I don't believe it is inductor whine (well actually, you can think of the whole circuit as a single coil inductor resisting current flow many times a second from being switched on and off) so I can't think of any other electrical phenomenon that would generate such noise. The whine is also much more pronounced when you use higher powered bulbs, commensurate with the higher amounts of current switching on and off.

 

[^^Nova^^]

It is more likely to be the bulb filament vibrating. Does the sound change with the different levels? The PWM pulses will cause the filament to heat and cool slightly as it turns on/off real quickly which is how it delivers the lower voltage to the bulb.

Cheers,
Nova

 

[JimmyM]

Thanks man!:twothumbs

This is exactly what I needed to look at.

I didn't realize the Alan B project was a feedback based PWM design using RMS values, I thought it was true voltage/current regulation. Or maybe there is more than one project, at any rate thanks for the links, I'm sure I'll learn all about it in there. I just have to resist buying one of thier regulator sleds, because I know Iwould get all the programming junk for it too, and I'm going BROKE

I already have a word document made with all my settings for thier controller heh.

EDIT Just wanted to say thanks again. This will save me time and money, thanks for taking the time.

Look at my Sig. None of our designs is a feedback regulator. They use a microcontroller to measure battery pack voltage and compute the duty cycle required to deliver the desired RMS voltage. I've driven a 5761 (6V bulb) with a 40V input. No problems.
Mine are $40 and come pre-set to your specs.
Have a look.

 

[JimmyM]

All 3 of my switches do that. The sound you describe is perfectly normal, I think it's PWM whine + resonance of the tube I think.

I don't believe it is inductor whine (well actually, you can think of the whole circuit as a single coil inductor resisting current flow many times a second from being switched on and off) so I can't think of any other electrical phenomenon that would generate such noise. The whine is also much more pronounced when you use higher powered bulbs, commensurate with the higher amounts of current switching on and off.

The sound is called magnetostriction. It's why transformers hum at 60Hz. Mine and Alan's operate at ~240Hz. It's more pronounced at low duty cycles.

 

[Linger]

Yes, I've been using this idea with my soft-start.
Just a paste from another post I wrote:
"real advantage of the soft-start is that you can use a bulb to run a set of batteries down until you're ready to open it up to full and give the bulb %100.
For example, I put 3xIMR26500's (charged 4.17v) in a 2D m@g with a Hikari 5607 (destructive test reports 4.11v /cell is max). Using the soft-start I clicked to level 1 (%30) and bulb was pretty bright, very white. After a bit I upped it to level 2 (%60) and ran the combination no trouble. Level three (%100) came after.

So if you have a set-up that crosses the edge of the bulb's range, it *is* possible to load the configuration and 'condition' the cells by running the bulb with them. Just be sure not to cycle the light and only move up to levels two, and then three (full) when when the vbatt has dropped into desired voltage."

 

[Popsiclestix]

The sound is called magnetostriction. It's why transformers hum at 60Hz. Mine and Alan's operate at ~240Hz. It's more pronounced at low duty cycles.

Ahh, thank you. Guess I slept through that EMag lecture