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Old 14th January 2011, 06:47 PM   #121
AndrewT is offline AndrewT  Scotland
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Vf is the voltage drop across the LED when forward conducting at the current specified by the manufacturer.

7.5V must surely be the voltage across the 1k0 resistor.
That tells you that the test current is 7.5mA (+-resistor tolerance +-meter tolerance +-temperature effects) and the corresponding Vf that still needs to be measured.

Do not rely on an answer that is the difference between two similar value measurements, unless there is no better way to get a result.

I have red LEDs which vary in Vf from 1.55V to 1.95V at the same current. Changing the current can expand that range from 1.5V to 2V

It depends on the LED manufacturing process and the tolerance intrinsic to that process.
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Last edited by AndrewT; 14th January 2011 at 06:51 PM.
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Old 14th January 2011, 08:01 PM   #122
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Hi all.

I ve been reading the threads on how to build/and hotrod a board for some time now and i want to be sure i got it right.

So bear with me once again and confirm or correct me if i am wrong.

Building a Mesmerize board using the values suggested on board gives a standard version running on 60mA.
For the standard version a 12v or a 15v / 50-100 VA transformer can be used, 15v being better if one wants to hotrod.

In order to hotrod, 3*68 Ohm resistors can be placed resulting to somewhat 90mAmps. In order to go to 200 mA, one needs to install 1*10Ohm/5 W resistor or 3* 30Ohm ( or value close to that )/ 2-3 W resistors. Paralleled resistors give the advantage of better dissipation.
Lowering the resistors value below 10Ohms results to more Icss.
Up to 600 mA the use of MUR120 is ok ( why not 1A which is the diode limit? ) but would be better if replaced with 840 or 860. Irfps need to be sinked good.
Replacing the PScaps from the standard value to something bigger is good but not mandatory for hotroding. If memory serves my right the 4700μF caps can do good for up to 600 mA.

One thing i dont get for sure though is from up to what value do the MUR's ( 840/60 ) need sinking? Or dont they need?

To sum up:
Populating the board with the values suggested on it, gives a standard version running at 60mA
Populating the board for having, let's say 200 mA ICCS, things that need to be done are -
1. Replace the PS resistors with 1*10R/5W or 3*30R/2-3W
2. Better sinking for the irfps
3. Not necessarily, replace the MUR120 with 840/60.
4. Not necessarily, replace the PS caps with something bigger.

Do i forget something here? Have i got anything wrong?

Regards

Last edited by CaliCanjaros; 14th January 2011 at 08:11 PM.
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Old 14th January 2011, 08:11 PM   #123
Horio is offline Horio  United States
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I read somewhere on the DCB1 Blue Hypno thread, that the MUR860's were good up to about 300mA without heatsinking. I don't have the link, but go do a search on that thread, and you should find it.

I'm in the process if building my Blue Mez with 2 x 20ohm 2W resistors in place of the 68 ohm ones (per Tea-Bag's recommendation). This gets you in the range of the "standard" hot rod from what I understand.
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Old 14th January 2011, 10:10 PM   #124
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Quote:
Originally Posted by AndrewT View Post
Vf is the voltage drop across the LED when forward conducting at the current specified by the manufacturer.

7.5V must surely be the voltage across the 1k0 resistor.
That tells you that the test current is 7.5mA (+-resistor tolerance +-meter tolerance +-temperature effects) and the corresponding Vf that still needs to be measured.

Do not rely on an answer that is the difference between two similar value measurements, unless there is no better way to get a result.

I have red LEDs which vary in Vf from 1.55V to 1.95V at the same current. Changing the current can expand that range from 1.5V to 2V

It depends on the LED manufacturing process and the tolerance intrinsic to that process.
Yes, the voltage reading is across the resistor as stipulated by various build threads that show this method.

However what you are saying is that the current I have is not the same as the current in the actual application, hence the higher voltage value. That makes sense.

I need to go through the original DCB1 thread as I know there was reference to measuring the LED's at a current value similar to the application.

Thanks
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Old 14th January 2011, 10:36 PM   #125
ramallo is offline ramallo  Europe
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Quote:
Originally Posted by Salas View Post
90mA per rail then. How is you output DC offset?
L=0,1mV
R=-0,8mV

I did a noise test of the power supply

DCB1
Click the image to open in full size.
7812
Click the image to open in full size.

Last edited by ramallo; 14th January 2011 at 10:42 PM.
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Old 15th January 2011, 03:03 AM   #126
nezbleu is offline nezbleu  Canada
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Quote:
Originally Posted by Salas View Post
Here is such a thread. Merged.
Thanks! I think I read this thread before, then couldn't find it the other day, which I attribute entirely to my mind slipping. Don't get old...
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Old 15th January 2011, 03:18 AM   #127
nezbleu is offline nezbleu  Canada
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Quote:
Originally Posted by Bill_P View Post
Are the switches mechanically ganged to keep from enabling more than one input at a time?
Well, first of all, there no actual switches at all, I'm just spit-balling.

But you have identified one definite bug in that design (or maybe it's a "feature"). If 2 switches are closed simultaneously (within a few nanoseconds) it should turn on 2 inputs. I don't see that as huge problem, because (1) it is unlikely, I think it would actually be difficult to close 2 switches within the right time window, and (2) a press of a single button would fix it. If I implement the indicator LED part, then there would be a visual indicator if 2 inputs were selected.

I don't like physically ganged switches, and in this circuit (as I imagine it) the switches are momentary-contact, not radio-buttons (for those of us who remember radios with mechanical presets, as opposed to something in a UI design). There should be a logical circuit which would prevent selecting more than 1 input at a time, perhaps leveraging the "clear" pin. IIRC the clear pin sets all outputs low regardless of the state of inputs and clock. So what logic circuit would say "if more than 1 output is selected then assert the clear pin"? A six-way NAND?

For that matter I have also not addressed what happens at power-on. Should all relays be open until an input is selected? Should there be a default input selected? I can't conceive of a way to preserve state across power cycles.

Thanks for making me think, please let me know if you have other concerns or ideas.
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Old 15th January 2011, 04:10 AM   #128
nezbleu is offline nezbleu  Canada
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Hi Andrew, thanks for the detailed reply. I intend to use a 12V regulator on the Mez board and 12V relays (already have them). My earlier comments about power referred to powering the logic chip and its inputs. Either way I will steal some output from the 12V reg on the Mez board for the switch board, but I can either power the chip at 12V or add a 5V regulator on the switch board to power the chip and provide input to the switches. Probably a TO92 regulator would be fine.

Quote:
Originally Posted by AndrewT View Post
I love the idea of "press the selector button" and the LED confirms the selection. It would be even better if the selected button lit up, (a push to make button with a LED inside).
Yeah, I like that idea too, haven't seen the right switches yet. If I can't find a light-up switch then an LED adjacent to the switch would be OK.

Quote:
Originally Posted by AndrewT View Post
The LED could replace the emitter resistors R12 to R18, but then the supply voltage would need to be raised by ~2V to either 7V or 14V. This series LED would pass the same current as the relay coil. It will be very bright. Adding a resistor bypass across the LED will adjust the brightness.
So how much current does an average LED need for average brightness? The rated coil current for the relay is 11.7mA, which I would not have thought excessive for some LED's, but then I really don't know. As for the voltage, while you are absolutely correct, I think the relays I have (ATQ203) will operate at 9V which gives me a couple of volts in-hand to play with; I think I should be able to find LED's with Vf about 1.6-1.7V. So that might work without messing with the regulator on the Mez board. OTOH, replacing the 7812 with an LM317 and a couple of resistors should be trivial.

Quote:
Originally Posted by AndrewT View Post
If instead the LED is placed in parallel, then it must be in parallel with the relay. That involves 6 extra wires tapping into the PCB.
Must it?Couldn't all 6 LED's share a common power source but be individually switched on by their associated transistor? That is, the coil circuit and an LED-resistor circuit each connecting to the collector of the transistor? Of course then the transistor needs to pass the coil current (12mA) plus the LED current (??mA), but that shouldn't be an issue.

Quote:
Originally Posted by AndrewT View Post
Those emitter resistors are not required.
Excellent, thanks. This was very much a back-of-the-envelope sketch, I included emitter resistors as a matter of form thinking I might need to limit current somehow. Of course, since a mechanical switch would connect the relay directly to ground, that seems unlikely to be needed!

Thanks again for the comments, much appreciated.
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Old 15th January 2011, 09:00 AM   #129
Salas is offline Salas  Greece
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Quote:
Originally Posted by ramallo View Post
L=0,1mV
R=-0,8mV

I did a noise test of the power supply
Nice results. Your build seems fine.
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Old 15th January 2011, 09:05 AM   #130
Salas is offline Salas  Greece
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Quote:
Originally Posted by westers151 View Post
I need to go through the original DCB1 thread as I know there was reference to measuring the LED's at a current value similar to the application.

Thanks
Hook up a 2SK170 Drain side to battery + (even better use the one destined to feed each leds row on final board). Connect its G,S together. From G,S connect a led's anode, connect its cathode to battery -. That way you can measure Vf across each led on actuall current it will see in circuit. Note them down and assemble reasonably matching string pairs.
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