Posted on 02/05/2024 9:14:32 PM PST by Paul R.
I am modestly(!) familiar with 4 types of DC voltage output power supplies that offer some degree of regulation to a load.
Those are:
Zener regulated supply: This has a resistor in series with the incoming / source unregulated DC voltage rail, and a zener diode to ground that fixes the output voltage at the zener voltage. Usually a capacitor to ground at the output is added to help stabilize the voltage, esp. if the load tends to vary a bit.
Zener-pnp transistor regulated supply: This adds a transistor and a couple more resistors for more current capability and better regulation. Darlington configurations are also available.
Linear regulator supply using, say, a LM317 IC, or any one of a number of fixed voltage regulator IC's. This can drive an output transistor(s) for higher current.
The above configurations are not particularly efficient, somewhat dependent on current drawn, etc.
PWM (Pulse Width Modulation) power supply. This is a type of "switching supply". These are very efficient and a quite physically small supply can cork out a few amps without generating much heat. Such supplies sometimes cause problems with RF radiation / interference that needs suppression.
MY application (a personal / hobbyist project, BTW, of no commercial intent or value) is to power a LED(s) load with a constant 13 volts, BUT, 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. The source voltage is a 12v nominal(!) automobile battery system, which system can on occasion reach 14.5 or 14.6 volts with the IC engine running.
The LED modules appear to have minimal current limiting if any "onboard". Brightness without excessive heat is good at 12-13 volts. (Very good at 13 volts.) As one approaches 14 volts the current goes up quickly, and the modules get quite hot. Brightness begins to fail to increase comparably to current drawn and heat. (It may actually decrease if the module has time gets hot enough for that -- I've not tested this as I didn't want to blow up a module!)
The LED module consists of 3 LEDs and (I suspect) a resistor, all (I suspect) in series. The resistor provides modest, but not a lot, of current limiting. The vendor basically just tells buyers to hook up to a "12v" source in an automobile. But, some buyers report overheating and burnouts, even though most do not. Ergo, I'm looking to limit the voltage in this lighting circuit supply to ~13 volts.
Best I can tell, PWM supplies need to be supplied with a voltage source 2.5 to 3 volts higher than the output. Since the car's TYPICAL battery voltage is 14 volts or less (esp. if the engine is not running), if I try to set the LED supply at ~ 13 volts, I'm guessing the PWM supply's output will "drop out" with an input of 13-14 volts?
I believe the linear regulator needs a differential of around 1.5 volts?
The zener "regulator" output voltage will basically track the supply voltage as it falls below the zener voltage. Thus the LED's will dim but not go out until the voltage the LED module sees is well under 12 volts.
The Zener-transistor circuit will also "gradually fade out" as source supply voltage decreases below 13v (given a 12v zener diode -- I think???
Could be. But it’d probably seem “pretty dumb” on such a site. This is pretty basic stuff - hoping a hobbyist on FR sees it...
OTOH, I am a member on “All About Circuits”. Might try over there if anybody would deign to answer. (eye roll)
There ARE “boost-buck” PWM supplies that can work with a source that varies both above and below the desired output voltage to the load. Those get a bit more complex / pricey, though.
I have parts coming to add to some I already have here that should make either of the 1st 2 options run. Not too worried about heat dissipation / sinking for another couple months... :-)
I’ve found there are no dumb questions on “All About Circuits” and Stack Exchange. Just lots of friendly hobbyists willing to help out.
I’m not much good at electronics — just dabbled a bit with Arduino circuits.
I’m a big fan of 7812 and associated regulators. They are NASA reliable, cheap, and usually stupid simple to wire up and use. Except! There have been counterfeit ones sneaking into the market.
All linear regulators require an input voltage higher than their output voltage. A datasheet somewhere might claim 1.5 volts, I have found that’s not enough. More like 3 (or more) volts if you never want to experience dropout.
Sounds to me like you would want an ordinary 7812 and just insert a diode into the ground leg so that your 12 volts output becomes 12.7 volts. In other words, the ground of the 7812 references .7 volts instead of zero volts. Or two series diodes if you want a 13.4 volt output.
Buy this, when you are done and they work I will answer your question
https://www.amazon.com/Soldering-Practice-DZRCOXI-Training-Soldering-All/dp/B08KDLR6P6/ref=sr_1_10?
“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!
Bump for later!
A diode is a current device - so look at the LM317 and other methods to make a constant current rather than voltage. Some simple ones are transistor only, there are a few LM317 configurations.
and there is an advantage on premature failure. Basically if one led fails, the constant current is maintained through the other LEDS (so each does not need a resistor). in a voltage circuit, the current is usually set to a value dependent on other devices in the circuit.
I am sure an electronics guy/gal will make what i am saying clearer... but a bit of looking will show you a ton of stuff
forum.digikey.com/t/constant-current-led-lighting-premature-failure/2686
Rather than a voltage regulator have you considered an adjustable Buck Convertor? Both voltage and current are adjustable and remain as a stable output even when the the source input is unstable. We use these on Solar panels that are rated much more in voltage max than what the charge controller is rated to receive.
“Buck Convert-e-r”
I suggest you look at the Inland DC-DC Voltage Step Down switching converter module. Input is 3 - 40 VDC, output current up to 3A, precision Vout adjustment via multi-turn pot. Micro Center has these for near $7, their designation is K98R.
I think these may go into continuous conduction so voltage drop can be minimal (module is TI LM2596 switcher based).
I guess it depends on what your load is going to be and how much room you have for the unit. We have been using these for general purpose especially for higher current applications such as solar panel input. They will let you use 40 volt, 150 watt, 7 amp (x2) panels input. They are adjustable down to 1.2 volts or up to 36 volts at up to 20 amp 300 watt output. Of course they convert the extra voltage input not needed to additional current output.
Both voltage and current output are adjustable and they are 95% efficient especially if you don’t need to throttle down the output current for any real reason. And if room is not a problem they are only 2”x 2”x 1”. These we have tested extensively and they work well for about everything. Custom LED displays on cars is the most common use. $20 for two.
Technical Specifications:
Input Voltage: 6V to 40V DC (10V to 40V is suggested)
Output Voltage: 1.2V to 36V DC
Output Current: 20A (max.), 15A (suggested)
Efficiency: 95% (24V to 12V, 20A)
Output Ripple: less than 50mV
Wiring Method: Terminal
Short Circuit Protection: Self-recovery (cannot short circuit for long time)
Size:60×53×27mm / 2.36×2.08×1.06 inch(L * W * H)
Pins Description:
VO+: Output positive
VO-: Output negative
+IN: Input positive
-IN: Input negative
CV: Output voltage adjustment
CC: Output current adjustment
When input and output is common grounded, no-load CV output constant voltage,CC output constant current
Look at Texas Instruments’ LM5177 buckboost datasheet. It’s more than uou need, but will work and is auto-qualifird.
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!
Well, I know there are PWM buck-boost converters that can supply a constant voltage from a supply that may swing either above or below the target output (in this case, 13 volts.) They run significantly more money, of course, as they have to handle high current if Vin is low, and high voltage if Vin is high. However, I’m fairly sure these are made in big quantities for a limited input range, say, 8-18 volts (cutting out beyond either way) for automotive use. ??
Yes, the 7812 was the 1st thing, well, 2nd thing I thought of, as I’ve used the 7815 and 7915 (and 7818 & 7918) in split power supplies for audio op amps, many moons ago.
However, the dropout behavior is my question: If using a 7812 and the input to it drops to, say, 12.7 volts, or even 12.0v, does the output turn off? Drop to who knows what lower voltage, possibly very unstable?
I’m thinking a zener-transistor circuit might perform better in this regard. (?)
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