Andrew Morris designed a dimmable LED driver circuit that is simple and energy efficient. He then installed the circuit into a portable fluorescent lamp.
Use the circuit in figure 2 as a reference. The strings are essentially tied in parallel. R8, D9, D10, etc is the primary string and R10, D46, D47, etc are the secondary string. Both strings connect to the source of the MOSFET, Q2. Therefore, the voltage applied to all strings would be the same and if the strings are identical, the current through them will also be the same, even though only the current through the primary string is sensed (by the voltage across R8). D9 is being used as the voltage reference to control the voltage across R8. All added strings would be connected in the identical way as the string containing R10.
The voltage on R8 is sensed by Q1 through R2 and R3 while Q2 is switched on. The switching threshold is set by the voltage on Q1's emitter.
I didn't follow up on your last comment, about adding a complement to R8 and D9 to paralleled strings. This would just be a series LED and resistor, not tied into the FET drive, right? To balance the voltage applied to the strings?
You can take 2 polarized caps of double the desired value and connect them in series such that both -Ve terminals are connected together. The resulting equivalent capacitor can be used as a Non polar capacitor of desired value. I would use rated voltage on each to be safe and also I have always connected only -ve terminals togther although it may work with both +ve terminals connected as well.
If you want to drive just a few LEDs, here is the link to a 20mA LED ballast circuit that I created many years ago and still use nightly to drive three white LEDs. It's even simpler than the featured circuit, but it's not suitable for large strings of LEDs. Please put a 100 ohm, ¼ watt carbon composition resistor in series with it to limit in-rush current and to act as a fuse in case of a short. Mine has one, but I don't know why it's not in the schematic. The website has not been responsive to changes or new submittals. Put a low-value pot in series with R2 and R3 if you want dimming. Like the featured circuit, this circuit is not isolated from the power line.
The SCR can be deleted if the LEDs are permanently wired and you don't need dimming. In my application, the LEDs were switched off after the ballast circuit, requiring the SCR voltage limiter to keep C2 from exploding. You can replace the SCR with a zener diode, but then the circuit would consume even more power when the LEDs are turned off. The LED current will be approximately 20mA per microfarad of C1. The LED current will go down, requiring a larger value for C1 if a significant number of LEDs is added. Also, the peak SCR current will increase as more LEDs are added.
Hi, John, You don't seem to realize that Radio Shack is not "making money." It's losing money and its stock has fallen from $13 to $2 a share in the last year. It's fighting for its life! It's business model needs to change fast. People can buy cell phones in lots of places. I want Radio Shack to *be there* in two years as a brick and mortar store and that's why I offered suggestions. They need to mix Sparkfun, Adafruit and brick and mortar into a lively model.
Keep up the good work on your electronic designs. I'm working on something new myself.
Andrew Morris designed a circuit that could detect a stroke victim's groan and convert the sound into a signal so caregivers would know when help was needed.
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