Convert Old Halogen System to LED - Page 1
Forum section: electronics / beginners

author: RyanG
author_rank: Regular Contributor
timestamp: July 08, 2017, 09:47:23 pm
content: I have an old American DJ light fixture that originally used 82v 250w halogen bulbs.The bulb:https://www.amazon.com/AMERICAN-DJ-250w-halogen-light/dp/B0002GXW4II want to change the circuit over to LED. I was wondering what you guys thought the best way to do that would be and maybe what wattage LED would be a decent replacement. The actual output to the bulb is around 60v. The light goes through a metal plate with colored designs on it then through a lens to increase the size of the spot. I was thinking of rectifying the AC then using a simple switcher to step the voltage down, but I figured someone here knows the best way. There is also a 12v circuit inside the fixture also that can put out up to ~8.5A, so I was thinking I could probably just use that and completely bypass the AC. There are a couple servo motors inside to spin the metal disc with the colored filters and to move a mirror. And, a 12v 0.16A fan for cooling. I'm thinking there are enough amps left over for a COB LED. Let me know your ideas.

author: tooki
author_rank: Super Contributor
timestamp: July 09, 2017, 08:41:06 am
content: What about just using a bunch of off-the-shelf LEDs in series?The COB LED filaments are designed for about 70V IIRC and will light readily in 60V.Five 12V COB LED bars are also 60V, as are twenty 3V LEDs.

author: Zero999
author_rank: Super Contributor
timestamp: July 09, 2017, 08:45:17 am
content: I did a quick Google and MR13 is a standard fitting for halogen light bulbs. Unfortunately I couldn't find any direct retrofit LED replacement lamps, so you're stuck with making one.

author: RyanG
author_rank: Regular Contributor
timestamp: July 10, 2017, 08:15:55 pm
content: Alright, I think I am going to put a 10W LED in there. I will have to provide a driver for it. Should I just buy one or make one? Obviously making one is probably better for learning.LED specs:Forward voltage: 9-11vForward current: 900-1000maIt will be running off the 12VDC line in the light fixture.

author: RyanG
author_rank: Regular Contributor
timestamp: July 11, 2017, 09:18:09 pm
content: I am trying to build the constant current source for the LED. I don't have the LED yet so I am testing it on a regular 5mm LED. The circuit is in the picture below. The problem I have with it that doesn't match that simulation is that my sense resistor is dropping about 9v instead of the 0.7v it should be. My current is also around 18ma instead of the 4ma. What am I doing wrong?

author: bitseeker
author_rank: Super Contributor
timestamp: July 11, 2017, 11:36:07 pm
content: It looks as though your attachment didn't get attached, so I can't see what you've attempted there.Thanks for reposting the pic. Taking a look.

author: RyanG
author_rank: Regular Contributor
timestamp: July 12, 2017, 12:10:49 am
content: Attachment that didn't attach

author: bitseeker
author_rank: Super Contributor
timestamp: July 12, 2017, 12:15:51 am
content: So, you're saying that your actual circuit doesn't match the simulation you attached? What does your actual circuit look like? Is it exactly the same as the one in the simulation? For example, what transistors are you actually using vs. how close are the simulations to the real thing?

author: RyanG
author_rank: Regular Contributor
timestamp: July 12, 2017, 01:03:46 am
content: BJT: 2n3904FET: IRF630B (have others -- first one I found)According to the simulation I should have voltage drop across my sense resistor equal to the drop in the BJT, but the voltage drop on my resistor is around 9v, so my sense resistor gets very hot very quickly. And, since that doesn't work correctly I am getting about 4x the current in my LED over the simulation current. The layout of the circuit is the same on my breadboard as the picture.

author: bitseeker
author_rank: Super Contributor
timestamp: July 12, 2017, 02:24:45 am
content: Maybe I'm not reading this correctly. The circuit appears to be using a P-channel FET, but the IRF630B is an N-channel with a minimum VGSof 2 volts. I can only get the simulation to work with the N-channel.

author: RyanG
author_rank: Regular Contributor
timestamp: July 12, 2017, 02:38:48 am
content: The sim uses an N channel. It works now. The circuit acted like it was working correctly, but apparently my BJT was bad.

author: bitseeker
author_rank: Super Contributor
timestamp: July 12, 2017, 02:50:55 am
content: Ah, OK. It ran fine on my end, too, but the diagram looked like it was a p-channel. Glad you found the culprit. Other than the misunderstanding, there wasn't anything conceptually wrong. So, the BJT was running wild.

author: Zero999
author_rank: Super Contributor
timestamp: July 12, 2017, 08:12:14 am
content: Quote from: RyanG on July 12, 2017, 01:03:46 amWhy not use a mains powered constant current power supply, rather than adding a power wasting linear regulator to a 12V supply?http://uk.rs-online.com/web/p/led-drivers/7717770/http://uk.rs-online.com/web/p/led-drivers/6600296/

author: RyanG
author_rank: Regular Contributor
timestamp: July 12, 2017, 07:13:24 pm
content: The whole thing has to go inside of this fixture that has very limited space. Also, what do you mean with the linear regulator?

author: Zero999
author_rank: Super Contributor
timestamp: July 12, 2017, 10:38:31 pm
content: Quote from: RyanG on July 12, 2017, 07:13:24 pmI don't see how using a mains powered constant current source uses any more space, than a 12V power supply. If anything, it will use less space as you don't need another circuit to regulate the current.QuoteA circuit which reduces the voltage or regulates the current by converting the energy to heat.

author: RyanG
author_rank: Regular Contributor
timestamp: July 13, 2017, 02:43:16 am
content: The 12V circuit is already inside the light to control some stepper motors and a cooling fan. I'm not adding that to the light. I was just going to pull off of that circuit.

author: Zero999
author_rank: Super Contributor
timestamp: July 13, 2017, 08:15:46 am
content: Quote from: RyanG on July 13, 2017, 02:43:16 amOh, that makes more sense now.What about using a switching constant current current LED driver with 12V input? It will be more efficient than a linear design.https://www.digikey.co.uk/products/en/power-supplies-board-mount/led-drivers/924?k=&pkeyword=&pv1120=203&pv1120=95&FV=ffe0039c&mnonly=0&ColumnSort=1000011&page=1&quantity=0&ptm=0&fid=0&pageSize=25https://www.digikey.co.uk/product-detail/en/xp-power/LDU1416S1000/1470-2154-5-ND/4488533https://www.digikey.co.uk/product-detail/en/leddynamics-inc/3021-D-N-1000/788-1073-ND/3114419https://www.digikey.co.uk/product-detail/en/recom-power/RCD-24-1.00-PL-A-R/945-1652-2-ND/2767999It's also possible to make your own. I'll post a schematic later.

author: Kjelt
author_rank: Super Contributor
timestamp: July 13, 2017, 02:35:07 pm
content: I have tried something like this for a friend and failed, hope you have more success but to sum up some of my learnings:Normal halogen bulbs have around 16-20Lm/W and a life expectancy of a couple of thousand hours.The bulbs for stagelighting and effects you have are designed to operate beyond normal operation so they render more light but only live around 100-200 hours.Expect 1,5x the flux so expect around 25Lm/W.For this bulb with 250W I would expect around 4000-6000 Lumens !This is all from a small spot and light is concentrated this could be important for the light effect that the new light also is concentrated in a small area (shadows).For a led which has 100Lm/W you would need around 40-60W leds to match this halogen bulb, that is not a trivial thing since leds need a lot of cooling and concentrate many leds on a small area would need a huge active (water?) cooler.Good luck.

author: Zero999
author_rank: Super Contributor
timestamp: July 13, 2017, 10:03:28 pm
content: Here's the schematic I was referring to in my previous post. It's a simple buck regulator, featuring high efficiency and positive output current with 0V return. Vref is just a 300mV voltage reference. If accuracy isn't important, it can be a silicon diode with a pull-up resistor and potential divider. For greater accuracy, replace the diode with a 1.2V band gap reference and adjust the potential divider accordingly.