Slow fade-in circuit...

[rhor...@gmail.com]

On Tuesday, September 29, 2020 at 10:10:00 AM UTC-5, bitrex wrote:

On 9/29/2020 11:05 AM, rhor...@gmail.com wrote:
On Tuesday, September 29, 2020 at 9:45:42 AM UTC-5, John S wrote:
For want of a micro controller the kingdom was lost.
I thought about using a PIC. They take time to boot, though. I could supply power full time, but I am sort of avoiding that.

Yea they \"boot\" in like half a microsecond.

That has not been my experience. I haven\'t done a huge amount of work with PICs, and I have never created a design with one, but the ones I have used (DMX and WS3811 Pixelnet controllers) take several secomds.

 

[rhor...@gmail.com]

On Tuesday, September 29, 2020 at 3:34:30 PM UTC-5, John S wrote:

On 9/29/2020 10:09 AM, bitrex wrote:
On 9/29/2020 11:05 AM, rhor...@gmail.com wrote:
On Tuesday, September 29, 2020 at 9:45:42 AM UTC-5, John S wrote:
For want of a micro controller the kingdom was lost.
I thought about using a PIC. They take time to boot, though. I could
supply power full time, but I am sort of avoiding that.


Yea they \"boot\" in like half a microsecond.
And how long does the buck regulator take to come up?

I have set the startup delays for about 50 milliseconds. According to the circuit emulator I am using, that is enough for the start-up transients to settle and the control capacitors to charge to their working values. If I set the holdoffs lower than that, I get some instability which might cause some noticeable flicker when the circuit is first energized.

 

[Tabby]

On Tuesday, 29 September 2020 08:07:05 UTC+1, bitrex wrote:

On 9/29/2020 3:05 AM, bitrex wrote:

The \"best\" way to do it if a switcher is going to be used is to have
high-voltage LEDs that have a string integrated into a common substrate
and use a boost rather than a buck or buck-boost, and boost the
rectified mains up to say 400 from 270.

Boosts are just simpler and more efficient in this context than bucks or
buck-boosts.


And ideally have no electrolytic filter caps in the circuit, or no caps
at all, even. There are research papers to that effect idk if it\'s done
in any commercial products, though.

There are 2 ways that can go:
1. Little to no flicker & low pf
2. Good pf & heavy flicker


NT

 

[Tabby]

On Tuesday, 29 September 2020 15:34:09 UTC+1, John S wrote:

On 9/29/2020 8:37 AM, rhor...@gmail.com wrote:

*IF* it turns out LED light bulbs can work on 170VDC, then I have cobbled together a circuit I think might work. Please take a look at the link below and see if any of you can find any issues with this.

http://http://siliconventures.net/images/Fader1.PNG

All that in lieu of one component!

 

[piglet]

On 29/09/2020 3:32 pm, rhor...@gmail.com wrote:

*IF* it turns out LED light bulbs can work on 170VDC, then I have cobbled together a circuit I think might work. Please take a look at the link below and see if any of you can find any issues with this.

http://siliconventures.net/images/Fader1.PNG >

The 12VDC supply is provided by an MP9488GS-P Buck regulator. U1 integrates the 0V differential signal provided by R1 - R4 with a time constant determined by R5 and C1. C2 provides a delay, keeping the output of U1 high until C2 is charged to 6V by R4 (~2ms). Once C2 is charged, the output of U1 slowly decreases from ~10V to ~2V over a period of 5 seconds. U2 inverts the slope of the signal from U1 and produces a necessary offset. Its output changes from 4.5V to 12V over the same 5 second time period. The 555 timer X1 operates as a short pulse generator, creating pulses a few microseconds wide approximately every 8 milliseconds. Transistor Q1 prevents X1 from generating any pulses until C2 is charged to ~1.4V by R8, allowing time for the 12V buck converter to settle before X1 begins operation. X2 is configured as a variable pulse width monostable timer triggered by Q2 from the short pulses produced by X1. The output of X2 is held high by the internal flip flop until the voltage across C6 rises to be greater than Control voltage, which is the output of U2. Initially, the output of U2 is within 1/2 volt of the Trigger voltage, so the voltage across C6 reaches this value very quickly. As time passes, the Control voltage increases, causing the output of X2 to remain high longer and longer, increasing the duty cycle to near 100%. The output of X2 drives Q3, causing M1 to turn on whenever X2 is high.

Thanks for sharing. I figure the exact PWM frequency does not matter so
it can then be simplified to a self oscillating comparator pwm. By
limiting maximum light to just below 100% duty then the modulator supply
voltage can be derived from across the fet. By using micro-power devices
then supply current can be kept in the tens of microamps and dissipation
in the dropper resistor kept low.

<https://www.dropbox.com/s/b66wb6tgh37ref4/pwm_fade_up.pdf?dl=0>

Play with R1, R2, R3, R4 ratios to change the fade ramp curve from
exponential thru linear to log. The values I show give a exp curve with
a slow start then faster rise that I guessed is more eye friendly. Not
shown in the schematic but I think R4 should be shunted by a small
22-47pF capacitor to keep U2 from trying to oscillate.

The three ICs are available in SOT-23 packs and whole circuit could be
built very small.

piglet

 

[George Herold]

On Wednesday, September 30, 2020 at 3:55:46 AM UTC-4, piglet wrote:

On 29/09/2020 3:32 pm, rhor...@gmail.com wrote:
*IF* it turns out LED light bulbs can work on 170VDC, then I have cobbled together a circuit I think might work. Please take a look at the link below and see if any of you can find any issues with this.

http://siliconventures.net/images/Fader1.PNG

The 12VDC supply is provided by an MP9488GS-P Buck regulator. U1 integrates the 0V differential signal provided by R1 - R4 with a time constant determined by R5 and C1. C2 provides a delay, keeping the output of U1 high until C2 is charged to 6V by R4 (~2ms). Once C2 is charged, the output of U1 slowly decreases from ~10V to ~2V over a period of 5 seconds. U2 inverts the slope of the signal from U1 and produces a necessary offset. Its output changes from 4.5V to 12V over the same 5 second time period. The 555 timer X1 operates as a short pulse generator, creating pulses a few microseconds wide approximately every 8 milliseconds. Transistor Q1 prevents X1 from generating any pulses until C2 is charged to ~1.4V by R8, allowing time for the 12V buck converter to settle before X1 begins operation. X2 is configured as a variable pulse width monostable timer triggered by Q2 from the short pulses produced by X1. The output of X2 is held high by the internal flip flop until the voltage across C6 rises to be greater than Control voltage, which is the output of U2. Initially, the output of U2 is within 1/2 volt of the Trigger voltage, so the voltage across C6 reaches this value very quickly. As time passes, the Control voltage increases, causing the output of X2 to remain high longer and longer, increasing the duty cycle to near 100%. The output of X2 drives Q3, causing M1 to turn on whenever X2 is high.


Thanks for sharing. I figure the exact PWM frequency does not matter so
it can then be simplified to a self oscillating comparator pwm. By
limiting maximum light to just below 100% duty then the modulator supply
voltage can be derived from across the fet. By using micro-power devices
then supply current can be kept in the tens of microamps and dissipation
in the dropper resistor kept low.

https://www.dropbox.com/s/b66wb6tgh37ref4/pwm_fade_up.pdf?dl=0
Nice. Thx. PWMing the fet keeps it cool.

George H.

Play with R1, R2, R3, R4 ratios to change the fade ramp curve from
exponential thru linear to log. The values I show give a exp curve with
a slow start then faster rise that I guessed is more eye friendly. Not
shown in the schematic but I think R4 should be shunted by a small
22-47pF capacitor to keep U2 from trying to oscillate.

The three ICs are available in SOT-23 packs and whole circuit could be
built very small.

piglet

 

[bitrex]

On 9/29/2020 8:41 PM, Tabby wrote:

On Tuesday, 29 September 2020 08:07:05 UTC+1, bitrex wrote:
On 9/29/2020 3:05 AM, bitrex wrote:

The \"best\" way to do it if a switcher is going to be used is to have
high-voltage LEDs that have a string integrated into a common substrate
and use a boost rather than a buck or buck-boost, and boost the
rectified mains up to say 400 from 270.

Boosts are just simpler and more efficient in this context than bucks or
buck-boosts.


And ideally have no electrolytic filter caps in the circuit, or no caps
at all, even. There are research papers to that effect idk if it\'s done
in any commercial products, though.

There are 2 ways that can go:
1. Little to no flicker & low pf
2. Good pf & heavy flicker


NT

Well..

<https://www.jstage.jst.go.jp/article/ieejjia/8/2/8_379/_pdf>

If you look at the graph on page 3 of LED string current I think through
a clever switching arrangement they\'ve got good efficiency and power
factor while keeping flicker that would be objectionable (noticeable to
the human eye) to a minimum

 

[piglet]

On 30/09/2020 3:51 pm, George Herold wrote:

On Wednesday, September 30, 2020 at 3:55:46 AM UTC-4, piglet wrote:
Thanks for sharing. I figure the exact PWM frequency does not matter so
it can then be simplified to a self oscillating comparator pwm. By
limiting maximum light to just below 100% duty then the modulator supply
voltage can be derived from across the fet. By using micro-power devices
then supply current can be kept in the tens of microamps and dissipation
in the dropper resistor kept low.

https://www.dropbox.com/s/b66wb6tgh37ref4/pwm_fade_up.pdf?dl=0
Nice. Thx. PWMing the fet keeps it cool.

George H.

Yes, but kind of depends ... the FET has to be chosen to enhance
sufficiently with 6V gate drive. HV fets seem to have higher Vgs
requirements in general. It is a design choice battle between Vgs and
the max Vdd for the chips - from 5.5V to 7V depending how close to abs
max one feels comfortable

piglet

 

[George Herold]

On Wednesday, September 30, 2020 at 2:51:37 PM UTC-4, piglet wrote:

On 30/09/2020 3:51 pm, George Herold wrote:
On Wednesday, September 30, 2020 at 3:55:46 AM UTC-4, piglet wrote:
Thanks for sharing. I figure the exact PWM frequency does not matter so
it can then be simplified to a self oscillating comparator pwm. By
limiting maximum light to just below 100% duty then the modulator supply
voltage can be derived from across the fet. By using micro-power devices
then supply current can be kept in the tens of microamps and dissipation
in the dropper resistor kept low.

https://www.dropbox.com/s/b66wb6tgh37ref4/pwm_fade_up.pdf?dl=0
Nice. Thx. PWMing the fet keeps it cool.

George H.

Yes, but kind of depends ... the FET has to be chosen to enhance
sufficiently with 6V gate drive. HV fets seem to have higher Vgs
requirements in general. It is a design choice battle between Vgs and
the max Vdd for the chips - from 5.5V to 7V depending how close to abs
max one feels comfortable

piglet

OK, why not run it at higher voltage? Burn more power Scotty!
Would a lnd150 current source feeding the zener make any sense?

George H.

 

[rhor...@gmail.com]

On Wednesday, September 30, 2020 at 2:55:46 AM UTC-5, piglet wrote:

Thanks for sharing. I figure the exact PWM frequency does not matter so
it can then be simplified to a self oscillating comparator pwm. By
limiting maximum light to just below 100% duty then the modulator supply
voltage can be derived from across the fet. By using micro-power devices
then supply current can be kept in the tens of microamps and dissipation
in the dropper resistor kept low.

https://www.dropbox.com/s/b66wb6tgh37ref4/pwm_fade_up.pdf?dl=0

The three ICs are available in SOT-23 packs and whole circuit could be
built very small.

piglet

 

[rhor...@gmail.com]

On Wednesday, September 30, 2020 at 2:55:46 AM UTC-5, piglet wrote:

Thanks for sharing. I figure the exact PWM frequency does not matter so
it can then be simplified to a self oscillating comparator pwm. By
limiting maximum light to just below 100% duty then the modulator supply
voltage can be derived from across the fet. By using micro-power devices
then supply current can be kept in the tens of microamps and dissipation
in the dropper resistor kept low.

https://www.dropbox.com/s/b66wb6tgh37ref4/pwm_fade_up.pdf?dl=0

That is *VERY* clever!

 

[rhor...@gmail.com]

On Wednesday, September 30, 2020 at 2:55:46 AM UTC-5, piglet wrote:

On 29/09/2020 3:32 pm, rhor...@gmail.com wrote:
*IF* it turns out LED light bulbs can work on 170VDC, then I have cobbled together a circuit I think might work. Please take a look at the link below and see if any of you can find any issues with this.

Why did you choose a mylar capacitor for C1? Why not ceramic?

 

[rhor...@gmail.com]

I put the circuit into a simulator, and according to it, you are very correct about needing a cap around R4 (no big surprise, there). It rang like a bell. A 47pf cap quiets it nicely. The value of C1 you gave seems way too low, though. According to the sim, the driver reaches essentially 100% duty cycle in considerably less than 2 seconds. Going with C1 = 2.2uF and R1 = 100M, the profile looks pretty nice. I\'m not sure how far I can push R1. The datasheet on the MCP6231 series isn\'t all that thorough. Do you know what the input impedance of the MCP6231 op amps is? Are they JFET inputs? Newark has the MCP6231UT-E/LT in stock for $0.20 each. Those should fit the bill.

<http://siliconventures.net/images/Ramp.PNG>
<http://siliconventures.net/images/Trace%20Analysis.PNG>

 

[piglet]

On 30/09/2020 20:30, George Herold wrote:

On Wednesday, September 30, 2020 at 2:51:37 PM UTC-4, piglet wrote:
On 30/09/2020 3:51 pm, George Herold wrote:
On Wednesday, September 30, 2020 at 3:55:46 AM UTC-4, piglet wrote:
Thanks for sharing. I figure the exact PWM frequency does not matter so
it can then be simplified to a self oscillating comparator pwm. By
limiting maximum light to just below 100% duty then the modulator supply
voltage can be derived from across the fet. By using micro-power devices
then supply current can be kept in the tens of microamps and dissipation
in the dropper resistor kept low.

https://www.dropbox.com/s/b66wb6tgh37ref4/pwm_fade_up.pdf?dl=0
Nice. Thx. PWMing the fet keeps it cool.

George H.

Yes, but kind of depends ... the FET has to be chosen to enhance
sufficiently with 6V gate drive. HV fets seem to have higher Vgs
requirements in general. It is a design choice battle between Vgs and
the max Vdd for the chips - from 5.5V to 7V depending how close to abs
max one feels comfortable

piglet

OK, why not run it at higher voltage? Burn more power Scotty!
Would a lnd150 current source feeding the zener make any sense?

George H.

The power on reset chip, op amp and comparator I picked have 5.5V
recommended and 7V absolute max rating. There must be higher voltage
micro=power devices around but I did not spend time looking for them.

Since the lamps are low power a FET with very low Rds-on is not needed
and so getting by with 5.5-6V gate drive seemed OK.

You are right that using a LND140 or BSS126 as dropper is better
electrically - but a resistor is cheaper and probably good enough.

piglet

 

[piglet]

On 30/09/2020 22:14, rhor...@gmail.com wrote:

On Wednesday, September 30, 2020 at 2:55:46 AM UTC-5, piglet wrote:
On 29/09/2020 3:32 pm, rhor...@gmail.com wrote:
*IF* it turns out LED light bulbs can work on 170VDC, then I have cobbled together a circuit I think might work. Please take a look at the link below and see if any of you can find any issues with this.

Why did you choose a mylar capacitor for C1? Why not ceramic?

Aluminum and tantalum are way too leaky, I would normally avoid larger
value ceramic in precision timing because of their voltage coefficient.
However you are right - this case does not need high precision and the
voltage coefficient of X7R and similar dielectrics actually work in
favor of an exponential fade ramp, although at these low voltages the
effect is not large. So please go ahead and use ceramic

piglet

 

[piglet]

On 01/10/2020 02:15, rhor...@gmail.com wrote:

I put the circuit into a simulator, and according to it, you are very correct about needing a cap around R4 (no big surprise, there). It rang like a bell. A 47pf cap quiets it nicely. The value of C1 you gave seems way too low, though. According to the sim, the driver reaches essentially 100% duty cycle in considerably less than 2 seconds. Going with C1 = 2.2uF and R1 = 100M, the profile looks pretty nice. I\'m not sure how far I can push R1. The datasheet on the MCP6231 series isn\'t all that thorough. Do you know what the input impedance of the MCP6231 op amps is? Are they JFET inputs? Newark has the MCP6231UT-E/LT in stock for $0.20 each. Those should fit the bill.

http://siliconventures.net/images/Ramp.PNG
http://siliconventures.net/images/Trace%20Analysis.PNG

Well done for simulating the circuit! Sorry I didn\'t optimize parts
values and the values are mostly estimates. You will need to check and
refine if actually building.

The MCP6231 is CMOS so input currents are pico-ampere level. Datasheet
typ 1pA room temp, 20pA at 85degC. PCB leakage will likely dominate. The
open drain on the MCP121 is likely to be a leakage limit too.

If you go to very large C1 like 10 or 22uF then I wonder if that is too
much strain on the MCP121 and a resistor in series with output of MCP121
may be good, perhaps 330 ohm?

piglet

 

[rhor...@gmail.com]

On Thursday, October 1, 2020 at 1:29:56 AM UTC-5, piglet wrote:

On 30/09/2020 22:14, rhor...@gmail.com wrote:
On Wednesday, September 30, 2020 at 2:55:46 AM UTC-5, piglet wrote:
On 29/09/2020 3:32 pm, rhor...@gmail.com wrote:
*IF* it turns out LED light bulbs can work on 170VDC, then I have cobbled together a circuit I think might work. Please take a look at the link below and see if any of you can find any issues with this.

Why did you choose a mylar capacitor for C1? Why not ceramic?

Aluminum and tantalum are way too leaky, I would normally avoid larger
value ceramic in precision timing because of their voltage coefficient.
However you are right - this case does not need high precision and the
voltage coefficient of X7R and similar dielectrics actually work in
favor of an exponential fade ramp, although at these low voltages the
effect is not large. So please go ahead and use ceramic

I had already bought the mylar cap. It\'s only $0.40. You are certainly correct Aluminum and Tantalum are way too leaky at these impedances.

 

[rhor...@gmail.com]

On Thursday, October 1, 2020 at 1:45:51 AM UTC-5, piglet wrote:

On 01/10/2020 02:15, rhor...@gmail.com wrote:
I put the circuit into a simulator, and according to it, you are very correct about needing a cap around R4 (no big surprise, there). It rang like a bell. A 47pf cap quiets it nicely. The value of C1 you gave seems way too low, though. According to the sim, the driver reaches essentially 100% duty cycle in considerably less than 2 seconds. Going with C1 = 2.2uF and R1 = 100M, the profile looks pretty nice. I\'m not sure how far I can push R1. The datasheet on the MCP6231 series isn\'t all that thorough. Do you know what the input impedance of the MCP6231 op amps is? Are they JFET inputs? Newark has the MCP6231UT-E/LT in stock for $0.20 each. Those should fit the bill.

http://siliconventures.net/images/Ramp.PNG
http://siliconventures.net/images/Trace%20Analysis.PNG

Well done for simulating the circuit!

Thanks!

Sorry I didn\'t optimize parts
values and the values are mostly estimates. You will need to check and
refine if actually building.

No worries. I just thought perhaps you were expecting the values you used to yield 5 - 10 seconds.

> The MCP6231 is CMOS so input currents are pico-ampere level. Datasheet

That should be good. One pA at 0.1Gohm is less than a millivolt error. That should not be significant in this case. I am happy with +/- 20% for this application.

typ 1pA room temp, 20pA at 85degC. PCB leakage will likely dominate. The
open drain on the MCP121 is likely to be a leakage limit too.

I used a 2N2222 to simulate the open drain of the MCP121 in the sim. It didn\'t seem to cause much of an issue. (My sim doesn\'t have a model for the MCP121.)

> If you go to very large C1 like 10 or 22uF then I wonder if that is too

I expect not. 2.2uF + 100M gets me very close in the sim. I don\'t expect to be an order of magnitude off, and with leakage on the order of a few pA, I should be able to increase R1 to nearly a Gigaohm.

much strain on the MCP121 and a resistor in series with output of MCP121
may be good, perhaps 330 ohm?

I suppose if it becomes necessary. I doubt it will.

 

[piglet]

On 01/10/2020 09:56, rhor...@gmail.com wrote:

On Thursday, October 1, 2020 at 1:45:51 AM UTC-5, piglet wrote:
On 01/10/2020 02:15, rhor...@gmail.com wrote:
I put the circuit into a simulator, and according to it, you are very correct about needing a cap around R4 (no big surprise, there). It rang like a bell. A 47pf cap quiets it nicely. The value of C1 you gave seems way too low, though. According to the sim, the driver reaches essentially 100% duty cycle in considerably less than 2 seconds. Going with C1 = 2.2uF and R1 = 100M, the profile looks pretty nice. I\'m not sure how far I can push R1. The datasheet on the MCP6231 series isn\'t all that thorough. Do you know what the input impedance of the MCP6231 op amps is? Are they JFET inputs? Newark has the MCP6231UT-E/LT in stock for $0.20 each. Those should fit the bill.

http://siliconventures.net/images/Ramp.PNG
http://siliconventures.net/images/Trace%20Analysis.PNG

Well done for simulating the circuit!

Thanks!

Sorry I didn\'t optimize parts
values and the values are mostly estimates. You will need to check and
refine if actually building.

No worries. I just thought perhaps you were expecting the values you used to yield 5 - 10 seconds.

The MCP6231 is CMOS so input currents are pico-ampere level. Datasheet

That should be good. One pA at 0.1Gohm is less than a millivolt error. That should not be significant in this case. I am happy with +/- 20% for this application.

typ 1pA room temp, 20pA at 85degC. PCB leakage will likely dominate. The
open drain on the MCP121 is likely to be a leakage limit too.

I used a 2N2222 to simulate the open drain of the MCP121 in the sim. It didn\'t seem to cause much of an issue. (My sim doesn\'t have a model for the MCP121.)

If you go to very large C1 like 10 or 22uF then I wonder if that is too

I expect not. 2.2uF + 100M gets me very close in the sim. I don\'t expect to be an order of magnitude off, and with leakage on the order of a few pA, I should be able to increase R1 to nearly a Gigaohm.

much strain on the MCP121 and a resistor in series with output of MCP121
may be good, perhaps 330 ohm?

I suppose if it becomes necessary. I doubt it will.

We haven\'t mentioned RFI yet but switching at a few hundred Hz into the
building wiring could be a potent source of radio interference. I don\'t
know what your site needs are but it may be prudent to allow for some
inductance and filter caps needed somewhere.

piglet

 

[Clifford Heath]

On 1/10/20 6:56 pm, rhor...@gmail.com wrote:

On Thursday, October 1, 2020 at 1:45:51 AM UTC-5, piglet wrote:
If you go to very large C1 like 10 or 22uF then I wonder if that is too

I expect not. 2.2uF + 100M gets me very close in the sim. I don\'t expect to be an order of magnitude off, and with leakage on the order of a few pA, I should be able to increase R1 to nearly a Gigaohm.

How are you going to protect the PCB from moisture and accumulated
conductive cruft? The leakage into the op-amp isn\'t likely to be your
limit, not for very long anyhow.

I\'d go with lower impedance and bigger capacitors.

CH