“the number of LED’s (actually 3-LED modules) in parallel may vary. (Long story.) So, I don’t want to use a typical constant current LED supply.”
No, you DO want to use a constant current output. Then it doesn’t matter how many LED modules you string together, the brightness will remain constant.
The only thing you have to worry about is if the output voltage from the regulator is high enough.
Measure the voltage drop across one module at the brightness you want, then multiply that by the number of modules you want to string in series. That will tell you how much headroom you need.
Ding!
The only thing you have to worry about is if the output voltage from the regulator is high enough.
Measure the voltage drop across one module at the brightness you want, then multiply that by the number of modules you want to string in series. That will tell you how much headroom you need.
This.
Uh, I can’t put the all modules in series and retain the switching scheme. Or, at least I can’t series all without fancier switches (or relays) and a great deal of new wiring run...
Notably, if I put 8 of these small LED modules in series that’s 8 x 13v, taking me out of a “low voltage system”.
There’s also separate user-controlled dimming for 2 separate pairs of 2 modules. The modules in each of the dimmed pairs can be in series, but not constant current.*
I “could” use multiple constant current LED drivers, after the switches or relays, to the sets of modules that don’t sometimes get dimmed. But that’d get a bit pricey.
*I’ve also not even looked into what happens to a constant current supply when it’s faced with an input voltage sag.
Really, all I want to do here is prevent over-voltages (and therefor over-currents) to the modules, but when the source supply to the over-voltage protection falls to around 12.5-13v, the voltage to the LEDS should stay around the source voltage level, not 2-3 volts less. If the overvoltage device does not generate much heat in operation, that’s a bonus, as it means less current in total is drawn from the vehicle’s electrical system. However, KISS may apply, as the simplest solution is a big honking 13V zener diode to ground following a chunky power resistor in series. ;-)
(The big [ 25 watt? ] power zener and resistor would cost more than a 3 amp PWM supply on Amazon, eBay, etc., but, a few $$ instead of a couple $ is not a problem, unless I have to buy significant heat sinking for the zener... Getting rid of heat in some nook in a vehicle or trailer can get quite interesting on a clear hot summer day.)
In all this, if the source voltage falls under 12 volts, I have bigger problems than the LEDs!