12v Globe Dimmer - Page 1
Forum section: electronics / projects

author: Buddy Hy
author_rank: Contributor
timestamp: July 26, 2023, 06:35:35 am
content: Hi All,I have a design / circuit from circuits-diy to dim a 12 volt 21 watt globe, but I don't know what components to use. I've attached their schematic (I'll be using only one globe).The transistor specified seems a bit excessive, and the pot under value.Could anyone provide more reasonable alternatives.Thanks in advance, Buddy.

author: srb1954
author_rank: Super Contributor
timestamp: July 26, 2023, 12:19:28 pm
content: Quote from: Buddy Hy on July 26, 2023, 06:35:35 amA 2N3055 would be a more economical and more readily available device for this application. It seems odd that they specified a 2N3773, which is rated for 140V VCEO, in a circuit running off 12V. You will still need a high current transistor (15A) though as the lamp resistance will be quite low when cold. The initial turn-on surge current could easily exceed 10A even though the normal running current will be under 2A for a 21W 12V globe. Whichever transistor you use you will probably need to mount it on a small heatsink as it could dissipate more power under some conditions than a bare TO-3 transistor package can handle.The transistor base current will also be quite high (>20mA) with this circuit so you will need a substantial potentiometer that can reliably carry that current through its slider long term. I would recommend at least a 1W wirewound pot for this circuit and perhaps lower the value to 1K to ensure that sufficient base current can be supplied.A cheaper option might be to add another Darlington connected transistor to boost the current gain and reduce the base current so you can use a much cheaper, low power, pot with a higher value, say 10k.

author: ajb
author_rank: Super Contributor
timestamp: July 26, 2023, 06:20:37 pm
content: A linear dimmer like this will be extremely inefficient. At 50% power, you'd be consuming ~18W with only ~9 of that going to the lamps. Efficacy will be even worse, because as power drops, so does filament temperature, and more of the power consumed is emitted as infrared instead of visible light. A PWM dimmer would be much more efficient, although it doesn't help the efficacy problem.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 27, 2023, 01:19:53 pm
content: Just get a generic PWM driver. They're often sold as DC motor controllers.

author: Ian.M
author_rank: Super Contributor
timestamp: July 27, 2023, 01:26:23 pm
content: Correction to Ajb's post:Quote from: ajb on July 26, 2023, 06:20:37 pm@srb1954,Poor component choices from circuits-diy and other sites that don't accept comments so don't benefit from peer review never surprises me. Rather than a Darlington pair, as you suggest, adding a PNP to make ahttps://en.wikipedia.org/wiki/Sziklai_pair]Sziklai pairwould be a better choice, to avoid the additional 0.7V Vbe drop the NPN Darlington driver would introduce limiting the max brightness.

author: srb1954
author_rank: Super Contributor
timestamp: July 27, 2023, 08:52:06 pm
content: Quote from: Ian.M on July 27, 2023, 01:26:23 pmI agree that a Sziklai pair would be a better option but I didn't want to confuse the OP too much by suggesting more complex circuit changes.Adding a PNP predriver would make the Sziklai pair appear like a composite PNP transistor and that would require inverting the sense of the rest of the circuitry. To directly substitute the single NPN transistor in the original circuit with a Sziklai pair would require an NPN predriver and a PNP output driver transistor. Sourcing a suitable PNP output transistor for such a configuration is likely to be a little more difficult that sourcing the ubiquitous 2N3055.

author: Buddy Hy
author_rank: Contributor
timestamp: July 30, 2023, 09:47:41 am
content: Quote from: srb1954 on July 27, 2023, 08:52:06 pmI was thinking a darlington pair would be better, but a sziklai pair does look to be even better.Why would sourcing a suitable PNP be difficult? Which NPN/PNP pair should I be looking for? What size pot should I be using to control the circuit?So Many Questions

author: Zero999
author_rank: Super Contributor
timestamp: July 30, 2023, 10:11:12 am
content: Use a 555 timer with a MOSFET. It'll probably even work out cheaper, as no heatsink is required.That 21W incandescent lamp can also be replaced with some LEDs, to achieve the same brightness, at a fraction of the power consumption. Put three or four white LEDs in series and use a variable constant current sink.

author: Buddy Hy
author_rank: Contributor
timestamp: July 30, 2023, 11:20:28 am
content: Something like this, Zero999?

author: Buddy Hy
author_rank: Contributor
timestamp: July 30, 2023, 11:25:17 am
content: I want to put whatever circuit in an existing camper light - I haven't been able to find a LED replacement for the globe yet

author: Zero999
author_rank: Super Contributor
timestamp: July 30, 2023, 09:34:40 pm
content: Quote from: Buddy Hy on July 30, 2023, 11:20:28 amI prefer to vary the reference, rather than sense resistor. I'd use a higher voltage reference, even the L78L05.Quote from: Buddy Hy on July 30, 2023, 11:25:17 amCan't you make your own?What sort of bulb fitting does it use?

author: Buddy Hy
author_rank: Contributor
timestamp: August 01, 2023, 03:02:33 am
content: Hi All,Here's the current light I have. I think the globe is a standard car indicator globe.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 01, 2023, 03:35:48 am
content: Are you trying to save power or do you just want it not as bright?To save power, you can buy plug in replacement 12V LED lights. I liked the "paddle" style so I bought these from Amazon and glued them into old bulb sockets while paying attention to physical alignment.For not as bright, just buy a lower wattage bulb.

author: Buddy Hy
author_rank: Contributor
timestamp: August 01, 2023, 03:51:54 am
content: We don't use the light that often, Kim, to worry about the power drain - It's more that sometimes a soft light when going to bed is nice to have.

author: Buddy Hy
author_rank: Contributor
timestamp: August 01, 2023, 03:55:27 am
content: Quote from: Zero999 on July 30, 2023, 10:11:12 amThis circuit is for 9v, I'll be running 12v - would that make a difference to the resistor values?

author: Ian.M
author_rank: Super Contributor
timestamp: August 01, 2023, 04:39:08 am
content: No, the 555 is fairly insensitive to slow supply voltage changes, and higher voltage is actually beneficial in this circuit as it reduces the influence of the steering diodes' Vf drop on the duty cycle.However I believe your application is automotive? If so the nom. 12V supply is anything but, rising to close to 15V with the engine running with lots of high energy spikes. A NE555 wont like that much, as its max operating voltage is only 16V, and its likely to be damaged if the supply ever spikes above 18V. One way to deal with this would be to use a Zener diode shunt regulator circuit to limit the supply to the 555 to about 9V and remove the spikes, but feed the bulb from the 'raw' 12V.

author: Buddy Hy
author_rank: Contributor
timestamp: August 01, 2023, 05:08:26 am
content: Quote from: Ian.M on August 01, 2023, 04:39:08 amNo, its not automotive. Just running off a solar battery - though the voltage of system can reach to just over 14v.I'm unsure what these two diodes do in the circuit?

author: Ian.M
author_rank: Super Contributor
timestamp: August 01, 2023, 05:35:33 am
content: OK, a solar system (not driving large DC motors etc.) shouldn't have the spikes an automotive 12V system has, so you'll almost certainly be OK powering the 555 direct from it.The diodes steer the charging/discharging current of the timing capacitor through opposite sides of the pot track. That way, as you move the wiper, one increases and the other decreases, and visa versa when moved the other way. This gives you adjustable duty cycle PWM to drive the MOSFET.

author: Buddy Hy
author_rank: Contributor
timestamp: August 01, 2023, 06:32:20 am
content: @Ian.M,Will a sziklai pair and a PWM produce the same results?I have a feeling that a sziklai pair will produce a more orange light (due to the lower power) than a PWM (which will run at full power) - Is that correct?

author: Zero999
author_rank: Super Contributor
timestamp: August 01, 2023, 07:22:38 am
content: Quote from: Buddy Hy on August 01, 2023, 06:32:20 amPWM is a means to vary the power, whist minimising power loss in the transistor. It works because the light bulb has a large thermal time constant, in other words it takes awhile for the filament to warm up and cool down. A Sziklai or Darlington pair can be used with the 555 circuit, rather than a MOSFET, but more power will be lost in the transitor and the lamp will never be able to run near full power.

author: Buddy Hy
author_rank: Contributor
timestamp: August 01, 2023, 07:55:27 am
content: Ahhhh.Is there a way to delay the 555 timer a second to get the filament up to working temperature before the PWM kicks in?

author: Ian.M
author_rank: Super Contributor
timestamp: August 01, 2023, 09:02:56 am
content: Not without considerable extra complexity, and it usually isn't necessary.

author: Buddy Hy
author_rank: Contributor
timestamp: August 03, 2023, 12:54:16 am
content: Went out and got the components - they're not cheap any more are they? Can't get single resistors and diodes like I used to.Is an IRF540N the same/similar to the IRL540? I just got 2x 1N4004 diodes too.I assumed 0.5W resistors would be sufficient.Just laying out the PCB now...

author: Ian.M
author_rank: Super Contributor
timestamp: August 03, 2023, 01:27:36 am
content: AnIRF540Nis not the same as anIRL540. If you compare key parameters from their datasheets, you'll see the IRF540N has a max Vgs of +/-20V and needs 10V Vgs to turn fully on. The IRL540 has a max Vgs of +/-10V and only needs 5V Vgs to turn fully on. Other parameters are 'in the same ballpark'. The low max Vgs makes the IRL540 unsuitable for the above circuit, as the 555's output is likely to significantly exceed that when the supply voltage is above 12V. You'd have to modify the circuit to limit the max. gate drive voltage. The IRF540N will drop right in running at >12V with no circuit changes due to its greater max. Vgs.Where a low Vgs threshold N-MOSFET like the IRL540 shines is when you are driving it from 5V logic or MCUs.

author: Someone
author_rank: Super Contributor
timestamp: August 03, 2023, 01:34:58 am
content: Quote from: Buddy Hy on August 03, 2023, 12:54:16 amIf there is any product already close to what you desire, it's almost certainly cheaper than building from scratch. New designs with tungsten/halogen lamps are expensive as their time has past and now something with LEDs would likely be a fraction of the price.

author: themadhippy
author_rank: Super Contributor
timestamp: August 03, 2023, 02:27:46 am
content: Id just chuck in some 12v warm white led tape in the fitting,if its too glary stick a bit of tissue paper to the plastic cover

author: DarkMode
author_rank: Contributor
timestamp: August 06, 2023, 12:12:15 am
content: Sadly the most cost effective way of making your bulb soft dimmable, is to go the non electronic route. Yeah components are expensive for your project.You can diffuse the bulbs output physically, by adding a top layer of frosted plastic (choose the colour you want). If you want controlled dimmable, well there are ways that are going to be over kill. I had wired up some new LED lights for my old ute last year, the brake lights didn't work correctly, so I had to add a large resistor (5 Watt style from the hobby store). It worked.Unfortunately, in electronics efficiency comes with a cost, the most efficient way to make this work would be to use a Buck converter, which a step down transformer to the level you wanted the brightness to be. So you are looking at $10 - $30 for a board depending on your Countries location etcThis would be the most efficient, at the cost of Cost and practicality.The light would be used periodically, throw a large resistor in there.

author: DarkMode
author_rank: Contributor
timestamp: August 06, 2023, 12:15:36 am
content: Also remember that soft switches (electronic switches) will have some leakage, unlike the physical switch, so If you leave the circuit openly connected to your battery, it will drain while doing nothing. Sometimes a physical switch is better

author: Buddy Hy
author_rank: Contributor
timestamp: August 06, 2023, 09:50:00 am
content: I finally drew the component layout and wiring route- I hope I got it right..

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 06, 2023, 02:51:59 pm
content: Quote from: Buddy Hy on August 06, 2023, 09:50:00 amThere's no way for us to tell. Image contrast is terrible (Black on black). Not sure which schematic you ended up using...

author: Buddy Hy
author_rank: Contributor
timestamp: August 06, 2023, 09:45:47 pm
content: Sorry about that, it was a transparent background - but it must have defaulted to black while uploadingLet's try again..

author: Ian.M
author_rank: Super Contributor
timestamp: August 06, 2023, 10:23:46 pm
content: Hmm. Life is too*short to check that without a netlist to tick off the connections against. If you don't have a netlist, and cant generate one, its possible to work through the schematic and layout side by side with a highliter pen going over each interconnect as you check it.However I prefer to use tools that detect layout errors for me e.g. VeeCad which can work from a netlist generated by a schematic editor, and can list incorrect connections and ones that haven't been routed yet. See my post:https://www.eevblog.com/forum/beginners/comparator-circuit-seems-to-be-oscillating-(well-pulsing-really)-solutions/msg4320124/#msg4320124*Exceptions: I check my own layouts and ones I am incentivised to check!

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 06, 2023, 11:10:27 pm
content: A bit of a messy layout, but it looks correct.Mirrored + Schematic:

author: Buddy Hy
author_rank: Contributor
timestamp: August 07, 2023, 02:10:47 am
content: @KimThanks for doing that. I'm sure it could be layed out better, but I need to get this built and installed by Wednesday.

author: Ian.M
author_rank: Super Contributor
timestamp: August 07, 2023, 03:45:11 am
content: I'd increase the 10uF decoupling capacitor to 100uF, and place it far closer to the 555 pins. Bipolar technology 555s (e.g. the original NE555 and clones) are notorious for drawing a very large supply current spike when the output switches (known asshoot-through current), and 100uF decoupling is commonly used to get low enough ESR when ordinary electrolytics are used. Alternatively, keep the 10uF where it is and add 0.1uF disc ceramic directly across the chip's Vcc and Gnd pins.If the min. resistance of the pot at the track ends is close to zero (because the wiper runs off the carbon track onto a metallised contact) you may need to add a resistor in series with the pot wiper to limit the peak current into the 555's discharge pin. 100 ohms would keep the peak current under 100mA, and have negligible effect on the duty cycle limits.

author: Buddy Hy
author_rank: Contributor
timestamp: August 07, 2023, 11:46:58 am
content: Quote from: Ian.M on August 07, 2023, 03:45:11 amI moved the components around to so the caps are closer to the 555.Quote from: Ian.M on August 07, 2023, 03:45:11 amYeah, I thought the same originally, but then stuck with the circuit diagram. I'm going to use a 1K resistor as I have a few those and shouldn't affect the circuit too much (I hope).I've updated the circuit diagram and layout.Thanks for every ones help so far

author: Ian.M
author_rank: Super Contributor
timestamp: August 07, 2023, 02:20:18 pm
content: Putting 1K in series with the 555's combined Threshold and Trigger inputs does nothing useful whatsoever. The extra resistor needs to be in series with the pot wiper, as I stated earlier so its in the discharge current path.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 07, 2023, 03:10:26 pm
content: Or you can put it in series with the "discharge" diode for a slightly larger PWM range:

author: Zero999
author_rank: Super Contributor
timestamp: August 07, 2023, 08:09:24 pm
content: A current limiting resistor in series with the discharge pin is unnecessary. None of the schematics on the data sheet show one. The discharge transistor's current is limited by the fact it only has a finite hFEand base drive. It won't be damaged as long as the power dissipation is below the rating of the IC, as is the case here. All it will do is limit the range of duty cycles achievable and is one extra part, with no advantage.

author: Buddy Hy
author_rank: Contributor
timestamp: August 07, 2023, 09:55:05 pm
content: Quote from: Ian.M on August 07, 2023, 02:20:18 pmOf course!Late night, not thinking straight

author: Buddy Hy
author_rank: Contributor
timestamp: August 07, 2023, 10:11:44 pm
content: Quote from: Zero999 on August 07, 2023, 08:09:24 pmIs this the resistor you are referring too?

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2023, 06:09:13 pm
content: Quote from: Buddy Hy on August 07, 2023, 10:11:44 pmNo, that resistor is required. I meant Ian.M is incorrect in his assertion another resistor is required in series with pin 7, as there isn't one in any of the schematics on the555 datasheet. Thecircuit I postedis fine as is.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 08, 2023, 06:45:59 pm
content: I think Ian was worried about the surge currentthrough the POT wiperwhen it was set at maximum duty cycle. (Bulb fully bright)I don't think the concern was regarding the 555 itself. ie: pin 7 is designed to discharge the timing capacitor, which can be quite large, when connected directly in monostable mode.

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2023, 10:28:07 pm
content: Quote from: Kim Christensen on August 08, 2023, 06:45:59 pmI don't see how that could be a problem. The 555 should already limit the current and the wiper will have a relatively large thermal mass and low resistance. Most of the energy will be dissipated by the 555, not the potentiometer, which will dissipate a tiny amount.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 08, 2023, 11:03:55 pm
content: Some POTs have a maximum current spec for the wiper. Page 3 ofthis Bourns application notehas this line:QuoteNot sure if they mean average or peak current here. I would assume average, which shouldn't be a problem then.

author: Buddy Hy
author_rank: Contributor
timestamp: February 16, 2025, 01:05:00 am
content: Hello Everyone,After nearly a year and a half I finally got an opportunity to make this project.Unfortunately it didn't work.Given I have limited equipment, any advice on how to diagnose the problem?

author: Langdon
author_rank: Frequent Contributor
timestamp: February 16, 2025, 01:15:56 am
content: Bro!You can buy a fully operational switching voltage regulator/dimmer for 89¢!https://www.taydaelectronics.com/breakout-boards/dc-dc-adjustable-step-down-xl4005-module.htmlThis looks cool too (though it is very expensive):https://www.taydaelectronics.com/breakout-boards/dps5005-with-usb-dc-dc-voltage-step-down-power-supply-module-50v-5a.html

author: Buddy Hy
author_rank: Contributor
timestamp: February 17, 2025, 02:29:58 am
content: Quote from: Langdon on February 16, 2025, 01:15:56 amThanks for showing me this, Langdon, but I need full voltage across the globe. That's why I chose the PWM circuit.

author: Buddy Hy
author_rank: Contributor
timestamp: February 17, 2025, 02:35:26 am
content: OK. Checked my work, and I realised I wired up the globe incorrectlyHowever, it will only dim an incandescent globe while the LED globe stays full on.Any reasons why this is, as I'd prefer to use the LED's as there's less power draw?

author: Buddy Hy
author_rank: Contributor
timestamp: February 17, 2025, 06:59:37 am
content: Before installing the light fitting I tested it thoroughly and it worked perfectly with an incandescent globe.So I put it all together, and mounted the light fitting to the wall.When I turned it on, the globe lit up fully.But when I twiddled the POT thething didn't dim.What could I have possibly done to make it stop working?

author: Zero999
author_rank: Super Contributor
timestamp: February 17, 2025, 09:34:54 am
content: There's obviously an issue with one of the connections. A poor solder joint is normally the case, but it's also possible the potentiometer is bad. Closely inspect the solder connections and reflow with solder if any look suspicious. Is it a new potentiometer or an old one? Did you buy it from a distributor or eBay?If it isn't dimming the LED properly, then it might be due to the leakage in the transistor keeping it on, which can be fixed by adding a resistor across the LED.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: February 17, 2025, 07:38:36 pm
content: Quote from: Buddy Hy on February 17, 2025, 02:35:26 amIs the LED bulb designed for PWM dimming?If it has a diode + cap inside the bulb driver, it will peak rectify your PWM, and not dim at all.Another simple sanity check is to wire the LED and incandescent globe in parallel.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 17, 2025, 09:29:55 pm
content: Quote from: Buddy Hy on February 17, 2025, 02:29:58 amJust remove the filter cap on the output of the switching regulator.If you wanna be hard core, replace the inductor with a piece of hookup wire and put the inductor on the voltage input (positive side).Now, it will supply PWM.You can't use PWM for a bulb with an internal cap.Use individual LEDs and resistors for good pwm.Note: If the flickering PWM light annoys you (as it does me), add a small cap (I recommend ceramic, about 0.1µF) across the LED to smooth the PWM and make it more constant.

author: Buddy Hy
author_rank: Contributor
timestamp: February 19, 2025, 09:10:35 am
content: Hey Zero999, I bought everything from Jaycar.It's odd that it would work at all if there's a connection issue. The only thing I can think of is that, when I crammed the board into the light fitting, it shorted somethin' out and fried it.I found my old component box, so I have enough bits to make another, better one. But would still like to know what went wrong.

author: Buddy Hy
author_rank: Contributor
timestamp: February 19, 2025, 09:19:55 am
content: Quote from: PCB.Wiz on February 17, 2025, 07:38:36 pmHi PCB.Wiz, I suspected that there might be a diode/capacitor in the LED globe, just wasn't sure and hoped it hadn't. Buuut, obviously it has.