Voltage through controllers

BundyRoy

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I've noticed that when powering a controller circuit board it seems to be either 5V or 7-35V. Sometimes you even have to change a dip switch or pin connectors when using the 5V. Why is this. One setting seems to handle 5V only and the other setting handles a large range of volts, ie. anything greater than 5V within the range that we commonly use. I guess that maybe the 5V setting handles less than 5V but there are no lights that operate on less than 5V.

I figured there must be a reason and there was only one way to find to find out. Thanks.
 

Devo

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The reason for this is because most voltage regulators/converters only regulate properly at a certain threshold (also called dropout) voltage above the rated voltage output. For example, most 5v regulators have a dropout voltage of 1.5-2v, meaning that you have to input at least 6.5-7v before it will regulate the output voltage and current properly. So that's why on most controllers you have to either input above that threshold voltage (ie ~7-30v) or just run the board straight off a 5v input directly without any onboard regulation.
 

Kaden

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The board requires 5 volts to run its own circuitry.


The board also has a regulator on it which will take varied voltages and bring it down to 5 volts.


So you can power the board directly with 5 volts or connect the regulator (via a jumper) and the regulator will sort out the voltage.


Does that make sense?
 

algerdes

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I do believe, and hope to be corrected if I have been misled, that most circuit boards use 5v for the control circuits. If you are feeding the board a properly regulated 5 volts for output to the lights, then you can set the board's control circuits to use that 5v directly. If you are using more than 5 volts to power the lights, the control board must switch in a voltage regulating circuit to bring down to 5 volts for use with the onboard control chips.


Note that most items I have read say to use a "well regulated 5v supply".
 

BundyRoy

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Thanks. I figured it would have something to do with ratings on regulators etc. Didn't realise the board circuitry ran on 5V though.

So a regulator can transform say 7.5V back to 5V and the same regulator can transform 35V back to 5V.
 

BundyRoy

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Also why was 5V chosen as the voltage to run the board. Cheaper, easier for parts, just because that's the way it's always been done?
 

Devo

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Most logic circuitry runs off either 5v or 3.3v. 5v is still a very common running voltage for most logic ICs, and this stems back to the early days. Today however a lot of new logic runs off lower voltages such as 3.3v or even down to as low as 1.8v (like your PC CPU.) The lower the voltage the more efficient it is in terms of power draw.
 

algerdes

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On the computer side, and therefore on most electronic based systems, the lower the voltage you can use, the less heat generated during operation. When pushing so many changes through a transistor, it tends to get warm anyway. We don't need a double whammy.


Heat is NOT your friend.
 

BundyRoy

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Was thinking about the extra voltage required for the regulator to kick in. Wouldn't this mean you could do it with one circuit. You put in 5V, the regulator doesn't kick in, yous still have 5V. Put in 12V, regulator kicks in, you have 5V. Or is it the case that if the regulator doesn't kick in you have no circuit and 0V.
 

AAH

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With the exclusion of my mega60 board which can have an input anywhere in the range of about 4 to 35V (5-35 is specified but at least 1 user has dropped below 5V with no issues) all the boards that I know of use a stepdown regulator. These regulators require between 1.5 and 3V above the 5V to maintain regulation. This means the providing they have that required couple volt headroom they will output 5V across the full range of input voltage. Once the voltage drops below the threshold (call it 6.5V) then the regulator doesn't have sufficient voltage for its own electronics to regulate to 5V. This means that the voltage that the microprocessor etc are fed is something like the input voltage minus 1.5V but it is actually fairly current dependent. In the case of the boards that require a jumper for running off 5V what they are doing is disabling the regulator and applying the input 5V to the boards electronics.
The simple regulators used on most of the pixel and DC boards are either a single component regulator or use up to 3 or 4. The switchmode regulator I used on the mega60 uses about a dozen components so it can handle voltages above and below 5V whilst still outputting 5V. This simplifies things for the user but adds about $5 or $6 just in that 1 feature.
As far as why 5V is used for the boards rather than 3.3V or 1 of the common low voltage regulator voltages there are a few reasons. For pixel boards the data and clock lines are 5V lines so you need to have 5V available on the board somewhere. A few of the pixel boards have 3.3V and 5V as fast micros are often 3.3V but at some point you need to get the clock and data lines back up to 5V. When it comes to DC dimmer boards there's a pretty good reason for using 5V. The output mosfets require a voltage to turn them on and they turn on harder which provides a lower voltage drop across them when the voltage is higher. Most of the common mosfets used on the dimmer boards will start turning on at about 2.5V but aren't fully turned on unless the gate voltage (the voltage that turns them on) is above 4V. What this means is the the power loss in the mosfets would be way higher if you attempted to drive them off 3.3V logic.
 

BundyRoy

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Thanks Alan. The logic about the 5V makes sense to me. I'm not sure I understand the bit about the power loss in the mosfet. Hell, I don't even know what a mosfet is or does really. It is one of those things I have to put in the I don't understand but will take for granted that it's required and that's how it works basket.
 
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