Recapping 3D80 What’s involved?
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caution
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Question - does having a large disparity between a regulator's input and output run hotter, even under no load? I'm wondering if using a regulator that's just over the normal turn-on voltage for an LED, like 5V, would help provide a stable output for a larger range. An LM7805 works down to 7V. But, if it runs hot even when not doing anything I'm not sure that would work.Superduper said:
I need to re-wire the LEDs illuminating my C100-F needles, they're all in series and expect full system voltage at the moment, so any blip makes them dim. I'm curious if I hung a 7805 off of each LED, it would run cooler than running all of them off of one 7805.
Hey Eric. Good question. I'll try to answer as thoroughly as possible as I think that a number of other members might either have a question about this, or may in the future.
First with respect to regulators, the important spec is heat dissipation. In order to figure out whether this will work for you, you first need to determine how much current your circuit is going to consume. You can use the general figure of ~20mA per LED series string. Then you are going to need to know how much voltage needs to be dropped. If you have a 15v source, and you want to drop to 5v, then the regulator will drop 10 volts. Multiply that by current consumed and that will tell you the watts dissipated. I didn't look up the datasheet of the regulator you had in mind so I'll leave that up to you. Once you figure out the amount of dissipation required, then you might need to figure out whether or not a heatsink is required. As you can see, the higher the disparity between source and regulated voltage, the higher the dissipation. As for how much work it will be doing... it depends how much current is being drawn. In other words, if the LED's are off, then even though the regulated rail is active, the amount of current consumed at that point is so low that dissipation at idle is negligible.
However, before you decide to go that solution, you may find the following suggestions more useful and practical for your problem. The reason you are having such sensitive LEDs is because you have too many of them strung in series. I'm sure that you did this to minimize the amount of resistors required since not only does this make for a simpler circuit, it also requires smaller resistors since less current will be drawn and less dropping is required. However with little or no resistance in that LED subcircuit, it becomes very voltage sensitive. A better solution is to break up the LED string so that you have, maybe 2 strings instead of 1. This will require you to increase the dropping resistor for each string but the result is that the LEDs will become less sensitive to voltage transients. There are literally tons of led resistor calculator websites out there so I won't waste any keystrokes addressing that... just google and then punch in the figures and get the resistance/watts value you need.
The other thing that you can do is add a single stiffening cap in the led subcircuit. This shouldn't require much capacitance but you might want to start with maybe 47 or 100 and see how that works out for you. If the source voltage is regulated, then you'd want to be careful about using too much capacitance since the initial draw can be high but if the source is unregulated, you can go crazy. However, I don't recommend that you stuff 10,000uf because although that will work dandy, those LED's will probably remain lit for a few minutes after you shut down the boombox until the stored potential is drawn down.
Ideally, you will use a combination of splitting the LED strings, add a proper resistor AND then adding a stiffening cap. This allows you to have a solution with only passive components which in the long run, is more reliable and simpler solution than using active components path. I would suggest you give it a try and see if the results is satisfactory first before going the regulator route. By if you do decide to go the regulator route, then why not use an LM7812 instead of LM7805? With the 12, the regulator will only need to drop 3 volts instead of 10. The rest of the dropping can be handled by the dropping resistor. The whole point is to have a stable voltage to work from. Dropping the voltage all the way down to 5 from 15 (or 18+ on AC) means that you'll need to split the LED strings which means more current consumption from the parallel strings. Remember the first paragraph above about regulator heat dissipation calculations? Well now you have higher dropping and higher current requirements... a double whammy.
First with respect to regulators, the important spec is heat dissipation. In order to figure out whether this will work for you, you first need to determine how much current your circuit is going to consume. You can use the general figure of ~20mA per LED series string. Then you are going to need to know how much voltage needs to be dropped. If you have a 15v source, and you want to drop to 5v, then the regulator will drop 10 volts. Multiply that by current consumed and that will tell you the watts dissipated. I didn't look up the datasheet of the regulator you had in mind so I'll leave that up to you. Once you figure out the amount of dissipation required, then you might need to figure out whether or not a heatsink is required. As you can see, the higher the disparity between source and regulated voltage, the higher the dissipation. As for how much work it will be doing... it depends how much current is being drawn. In other words, if the LED's are off, then even though the regulated rail is active, the amount of current consumed at that point is so low that dissipation at idle is negligible.
However, before you decide to go that solution, you may find the following suggestions more useful and practical for your problem. The reason you are having such sensitive LEDs is because you have too many of them strung in series. I'm sure that you did this to minimize the amount of resistors required since not only does this make for a simpler circuit, it also requires smaller resistors since less current will be drawn and less dropping is required. However with little or no resistance in that LED subcircuit, it becomes very voltage sensitive. A better solution is to break up the LED string so that you have, maybe 2 strings instead of 1. This will require you to increase the dropping resistor for each string but the result is that the LEDs will become less sensitive to voltage transients. There are literally tons of led resistor calculator websites out there so I won't waste any keystrokes addressing that... just google and then punch in the figures and get the resistance/watts value you need.
The other thing that you can do is add a single stiffening cap in the led subcircuit. This shouldn't require much capacitance but you might want to start with maybe 47 or 100 and see how that works out for you. If the source voltage is regulated, then you'd want to be careful about using too much capacitance since the initial draw can be high but if the source is unregulated, you can go crazy. However, I don't recommend that you stuff 10,000uf because although that will work dandy, those LED's will probably remain lit for a few minutes after you shut down the boombox until the stored potential is drawn down.
Ideally, you will use a combination of splitting the LED strings, add a proper resistor AND then adding a stiffening cap. This allows you to have a solution with only passive components which in the long run, is more reliable and simpler solution than using active components path. I would suggest you give it a try and see if the results is satisfactory first before going the regulator route. By if you do decide to go the regulator route, then why not use an LM7812 instead of LM7805? With the 12, the regulator will only need to drop 3 volts instead of 10. The rest of the dropping can be handled by the dropping resistor. The whole point is to have a stable voltage to work from. Dropping the voltage all the way down to 5 from 15 (or 18+ on AC) means that you'll need to split the LED strings which means more current consumption from the parallel strings. Remember the first paragraph above about regulator heat dissipation calculations? Well now you have higher dropping and higher current requirements... a double whammy.
caution said:
caution
Member (SA)
I think you lost the point of my question. I want to ensure I can have stable light under the highest dips in power as possible, and using a regulator that needs, at minimum, the system voltage of 15V, which is the minimum input level for a 7812, I have very little leeway before it starts to falter. I'm not sure a stiffening cap with a reasonable drain time, in conjunction with a 7812, would be adequate.
If I use 7805s, yeah sure I can only power a couple LEDs per regulator without producing too much heat, and it won't be as efficient, but they won't be affected until the system rail falls all the way down to 7V, the minimum input for a 7805.
The other option would be to install a 6V wall wart inside if there's room and tie all the LEDs to that. Cheap, easy to install and 100% stable.
If I use 7805s, yeah sure I can only power a couple LEDs per regulator without producing too much heat, and it won't be as efficient, but they won't be affected until the system rail falls all the way down to 7V, the minimum input for a 7805.
The other option would be to install a 6V wall wart inside if there's room and tie all the LEDs to that. Cheap, easy to install and 100% stable.
I see. So the message that I'm trying to convey is that the higher the voltage you need dropped, the greater the power sinked by the device (mA x voltage dropped from source to regulated). For that reason, it would serve you better to regulate at a higher voltage rather than at a lower voltage. This keeps heat and wasted energy down and reliability up. If the unit is powered on AC, it would be highly unlikely that you will see any significant dimming of the LED's, especially if you also include a stiffening cap and series no more than 3 per string. This is because the voltage on AC is usually well in excess of 15V, probably closer to 20. I'm mentioning the AC thing because you stated the option to use an internal wall wart which suggests the LED's being powered on AC is the primary goal. You might want to simply do a voltage check with the system on AC and see if you can get the voltage to sag below 15. But if you go with an LM7805 and the unit is powered on AC, your regulator will need to sink up to possibly 15volts x the current consumed. That's why I suggested an LM7812 but if you feel that wouldn't provide enough headroom, you can go with a LM7809. I think fairchild makes one. As for using multiple regulators, that shouldn't be necessary for a few LED's. We are talking mA here and those regulators should all be capable of 1A.
Or... if you want to go old school and use a zener and a power transistor, either something like a TIP35 alone or driven via a smaller 2N3904, or something like that, this should provide you all the regulation you need and the ability to handle 25a continuous and 125w sinking... This should be reliable and needs minimal headroom compared to the LM78xx series.
Out of memory, when I did a LED mod for a gentleman on his M9998 (he has a youtube video of it), we came across a problem which is that he had a 220v box and didn't even know it. Frankly, it's hard to believe he didn't even notice the degraded performance when on AC but he said he didn't. In any event, I told him if we were to swap the transformer, it would be impossible to find another dual windings that would match his old one... a main winding and a second one for minimal current lamp only use. We settled for a single secondary 12.6v main transformer and another small transformer for LED use when on AC, got on eBay for very cheap and about the size of a finger. It worked fine.
Or... if you want to go old school and use a zener and a power transistor, either something like a TIP35 alone or driven via a smaller 2N3904, or something like that, this should provide you all the regulation you need and the ability to handle 25a continuous and 125w sinking... This should be reliable and needs minimal headroom compared to the LM78xx series.
Out of memory, when I did a LED mod for a gentleman on his M9998 (he has a youtube video of it), we came across a problem which is that he had a 220v box and didn't even know it. Frankly, it's hard to believe he didn't even notice the degraded performance when on AC but he said he didn't. In any event, I told him if we were to swap the transformer, it would be impossible to find another dual windings that would match his old one... a main winding and a second one for minimal current lamp only use. We settled for a single secondary 12.6v main transformer and another small transformer for LED use when on AC, got on eBay for very cheap and about the size of a finger. It worked fine.
caution
Member (SA)
Yeah, my C-100F measures ~18.5V on AC. I put four LEDs in series so I could get away with a small resistor (below is what I used - for anyone interested, here's the link for the calculator) but that small resistor is killing my stability. Obviously I had to design it to run at the highest level, which is on AC, but with four LEDs in series that have a turn-on voltage of ~3V, that means anything under 12V is very bad. So yeah, I'm currently very sensitive to rail dips as you can imagine.
Yeah what you did with the tiny transformer for the man's M9998 was what I was getting at with a wall wart. I know it works well for the C-100F and M9998, so it could be a possible way to get super-stable LEDs on AC without adding a regulator. In those cases it was an actual bulb, so the high power draw may have made that a more cost effective solution than a transistor/zener combo (good idea by the way, yes, lower overhead than 78xx) or perhaps powering the bulb would be impossible with a regulator in those models, because at least in the C-100F, the bulb is incredibly bright compared to like an M70 or TRK-9150, presumably in order to achieve acceptable brightness with edge lighting.
But yeah, I forgot about batteries when I suggested it. When I went bulb-->LED on my C-100F I also moved to system DC, but I've LED-ized a couple C-100F dial light boards for members here where it stayed on bulb AC power, and I simply built a rectifier onto the back. It'll work without the rectifier/smoothing cap but the LEDs are all dim and flickery from the 50% duty cycle.
And yes, you can get 7809, most integer-based values are available. I'd have to pull the scope out and see how low I can get the rail to dip on a loud heavy beats, that would be interesting to see on fresh (or nearly fresh) batteries.
Yeah what you did with the tiny transformer for the man's M9998 was what I was getting at with a wall wart. I know it works well for the C-100F and M9998, so it could be a possible way to get super-stable LEDs on AC without adding a regulator. In those cases it was an actual bulb, so the high power draw may have made that a more cost effective solution than a transistor/zener combo (good idea by the way, yes, lower overhead than 78xx) or perhaps powering the bulb would be impossible with a regulator in those models, because at least in the C-100F, the bulb is incredibly bright compared to like an M70 or TRK-9150, presumably in order to achieve acceptable brightness with edge lighting.
But yeah, I forgot about batteries when I suggested it. When I went bulb-->LED on my C-100F I also moved to system DC, but I've LED-ized a couple C-100F dial light boards for members here where it stayed on bulb AC power, and I simply built a rectifier onto the back. It'll work without the rectifier/smoothing cap but the LEDs are all dim and flickery from the 50% duty cycle.
And yes, you can get 7809, most integer-based values are available. I'd have to pull the scope out and see how low I can get the rail to dip on a loud heavy beats, that would be interesting to see on fresh (or nearly fresh) batteries.
Well sounds like you get the idea. Going from 12 to 5 was a big, certainly bigger than necessary or ideal drop. Just check and see how much headroom you actually need and adjust accordingly. 9 might be a good compromise to minimize unnecessary sinking and wasted energy.
Zener/transistor regulators have been around for a long time and sized right, are pretty reliable. In fact they are present, sometimes multiple regulators, in virtually all legacy boomboxes. If they can handle deck solenoid, motors, tuners and audio circuits, they certainly can handle the simple and undemanding chore of powering some LEDs, right?
As for the separate transformer mod in the M9998, I went that route to retain the always on during AC and switch activation route during DC use features, which would have required some circuit changes if the LEDs were instead powered using common power source instead of original dual source.
Zener/transistor regulators have been around for a long time and sized right, are pretty reliable. In fact they are present, sometimes multiple regulators, in virtually all legacy boomboxes. If they can handle deck solenoid, motors, tuners and audio circuits, they certainly can handle the simple and undemanding chore of powering some LEDs, right?
As for the separate transformer mod in the M9998, I went that route to retain the always on during AC and switch activation route during DC use features, which would have required some circuit changes if the LEDs were instead powered using common power source instead of original dual source.
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