Dimming a 24v AC Halogen Bulb - Retrofit old projector - Page 1
Forum section: electronics / projects

author: davivid
author_rank: Newbie
timestamp: March 15, 2023, 11:20:19 am
content: Hello!I want to control the brightness of an old projector that uses 24v AC 250w Halogen bulbs without any internal modification.It has an external control connector that exposes the 24v AC lamp supply.I have read Triac dimmer circuits are not suitable for low-voltage AC.I will be using an MCU to provide the control and other features.How could I accomplish dimming the 24v Ac bulb?Thanks

author: Terry Bites
author_rank: Super Contributor
timestamp: March 15, 2023, 11:48:48 am
content: A PWM controller? Look up PWM and how averaging (integrating the signal) works- your filament does the averaging via its thermal mass.Loads of these kind of things on the market. They don't mind that your load is resistive.eg 1400W for £13!www.ebay.co.uk/itm/225217279646?hash=item346ffef69e:g:07cAAOSwW9JjUhGn&amdata=enc%3AAQAHAAAA8A2tUT%2FNJ%2FBFdYzWkyVt9J4%2Fu5VcVlGHhXfeS5QMiSsbizVDiWpa6k7TIr9jxnv7MNSjT9KcuoxKr4Wg2SgWwSol8p2VorS9eSPPS%2BdHrrYdJQnOcuU3u%2BSGxWe5D7l5LNWe1%2BnB6QcLYBsaT6K7wCm3VSYS%2BPAAuKuQ2nfiw0P%2BXW%2BHIdxBtXiT%2BF6PWjkZ3gY2CkkdgmQbtxX%2BzGPMb08box3bAPqQgOM7ixl5N2wybsOtui9EioANsVxI4J%2FIGHZ0n2rGQCx89Dr9eEJ6nj5FCpAcwXB9UfqPC%2Bph3EmSLuoO75P1HpQ8CaokM0uy8g%3D%3D%7Ctkp%3ABFBMmoSg0dxhhttps://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313&_nkw=pwm+motor+controller+24v&_sacat=0

author: wraper
author_rank: Unknown
timestamp: March 15, 2023, 11:52:06 am
content: Triac will work just fine though it's not efficient as it will drop about 1.5V-2V which means dissipating about 6-8% of total power.

author: davivid
author_rank: Newbie
timestamp: March 15, 2023, 12:40:37 pm
content: Quote from: Terry Bites on March 15, 2023, 11:48:48 amOh great - that's simple enough! I assumed I would have to implement a zero-crossing circuit.(I'm also aware these bulbs shouldn't really be dimmed, but it will just be the occasional fade)

author: wraper
author_rank: Unknown
timestamp: March 15, 2023, 02:02:12 pm
content: Maybe just buy a cheap lab-ish SMPS based PSU like Gophert or hack a feedback loop on secondary side of industrial SMPS like made by Meanwell to adjust voltage. In this way it will be totally flicker free. While DC motor controller will allow you to regulate brightness it still needs DC power source. And in case of simply rectifying AC from transformer, loses will be similar to using a triac. You can also do dimming with two MOSFETs connected back-to-back which will not need any rectification.

author: davivid
author_rank: Newbie
timestamp: March 15, 2023, 03:08:20 pm
content: To clarify - the control circuit will need to be in-line on the high-side of the lamp.24VAC+-> Control circuit <-Lamp <- 24VAC-(24VAC- is available on another pin)Quote from: wraper on March 15, 2023, 02:02:12 pmThis sounds like a good option. Would it look something like this:https://forum.allaboutcircuits.com/threads/problem-with-dual-channel-back-to-back-mosfet-ac-dimmer.132455/

author: wraper
author_rank: Unknown
timestamp: March 15, 2023, 03:18:57 pm
content: Quote from: davivid on March 15, 2023, 03:08:20 pmHigh side of AC? It AC, there is no + or -.

author: davivid
author_rank: Newbie
timestamp: March 15, 2023, 03:24:02 pm
content: Haha true. I guess more in the functional block diagram I have in my head

author: themadhippy
author_rank: Super Contributor
timestamp: March 15, 2023, 03:51:45 pm
content: Quotealternative option is a black out arm in front of the lens,old cd drives were a common diy option.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 15, 2023, 05:08:16 pm
content: Yeah. Dimming messes with the halogen cycle, it messes with your colour temperature as well.You'll get poor life for sure. Aperture reduction can reflect the heat back where you dont want it.Neutral density filters also back scatter IR and they might melt.Upgrade to an LED source and save yourself some grief. Sadly you would probably have to tinker with the innards of your nice projector.(https://en.wikipedia.org/wiki/Halogen_lamp)

author: themadhippy
author_rank: Super Contributor
timestamp: March 15, 2023, 05:33:02 pm
content: QuoteAnd open up a new box of grief trying to get the optics and light source play nicely together,if it was that simple 1000's of theatre lanterns would have been switched over to a led source instead of being replaced.

author: james_s
author_rank: Super Contributor
timestamp: March 15, 2023, 06:42:10 pm
content: Dimming halogen lamps is perfectly fine, you just don't want to operate them at low brightness for prolonged periods.

author: IanB
author_rank: Super Contributor
timestamp: March 15, 2023, 06:59:35 pm
content: As mentioned by others, it should be possible to implement phase angle dimming on a 24 V AC supply, subject to efficiency losses due to voltage drop.It would be a matter of finding a suitable circuit design to use.One challenge is that 24 V 250 W is over 10 A RMS, with a peak current higher than that. So the TRIAC, thyristors, MOSFETs or whatever would need suitably high current rating and heatsinking.(You would need to know whether the projector is using a conventional iron core transformer or a more modern electronic transformer.)

author: wraper
author_rank: Unknown
timestamp: March 15, 2023, 07:41:40 pm
content: Quote from: IanB on March 15, 2023, 06:59:35 pmMOSFETs with low enough RDS(on)can do this without a heatsink easily. As this does not require fast switching, you can use extremely beefy ones without concern how to drive them fast enough.

author: Zero999
author_rank: Super Contributor
timestamp: March 15, 2023, 09:38:57 pm
content: Presumably the 24VAC is coming from a transformer? I would've thought it would be easier to put the dimmer on the primary of the transformer. Yes, it's true, dimmers don't like transformers, but the transformer's primary won't be that inductive, as it's fully loaded by the lamp. Use a dimmer designed for at least 500W and add a snubber network to the transformer.There are also electronic halogen transformers, which are designed to work with a dimmer. I've not seen one designed for 24V, only 12V, but it should be fairly to re-wind for 24V, or add a high frequency transformer to the output, to increase the voltage to 24V.