Transistor use to control lower voltage - Page 1
Forum section: electronics / general

author: kolbep
author_rank: Frequent Contributor
timestamp: September 27, 2013, 06:56:56 pm
content: Hi All.So here is the story.I have an RGB LED Parcan Light Fitting that is DMX Controlled (The Fitting is unused at the moment).I also have 2 x5m Lengths of RGB LED Striplights.I need to control the RGB LED Striplights with DMX, but due to time restrictions (Meaning I need to get this done this weekend), I cannot order a DMX to RGB LED Pwm Interface.So what I was thinking is as follows :The LED Parcan gives out a +24v Common for all it's LEDS,and then it PWM's the 0v for the R G and B Sections seperatelyThe Striplights need a +12v Common, anda PWM'd 0v for the R G and B Sections seperately.What I was planning on doing is isolating the Parcan's LEDS, and using the Parcans Output to drive transistors, which can then run my LED Striplights from a 12v Supply.I am fine when it comes to hooking the transistors up in Common Emitter, but because I need the +ve Common, and that I do not want to invert the signal, I Presume I need PNP transistors.Please look at the attached schematic of what I am Scheming.At the left is the existing Parcan outputs, which drive it's LED's. I want to tap that off with resistors (in the center) to Transistors that are to run my Striplights.Does this look right what I am trying to do, I am not too good with Transistor theory, and I always used to avoid PNP because they seemed like a strange concept.In other words, will this work, or have I made a bugger up.Thanks for taking the time.P

author: David_AVD
author_rank: Super Contributor
timestamp: September 27, 2013, 08:18:06 pm
content: Is the PAR can output man enough to drive the strip directly? If so, just power the strip from a separate 12V rail.

author: SeanB
author_rank: Super Contributor
timestamp: September 28, 2013, 04:44:54 am
content: You will need 3 diodes to make sure the 24V side will not overdrive the 12V side transistors in reverse, as otherwise they will become very unhappy very fast. Base emitter resistors to make sure the transistors turn off will not harm either when you add the diodes, use 1k, for the base side will probably be around 220R as you really do not have much headroom there, as this is an emitter follower, not a saturated switch.

author: tom66
author_rank: Super Contributor
timestamp: September 28, 2013, 12:47:47 pm
content: Your transistors appear to be the wrong way around. In the majority of circuits, the emitter is more negative than the collector.

author: SeanB
author_rank: Super Contributor
timestamp: September 28, 2013, 01:24:07 pm
content: For NPN yes, for PNP the emitter will be more positive.

author: kolbep
author_rank: Frequent Contributor
timestamp: September 28, 2013, 01:58:58 pm
content: Thanks for the tips.When I pulled the LEDPar apart, I see the transformer has 2 seperate windings, one at about 8 volts, and the other 24 volts.The 8 volts is used for the logic sideand the 24 volts is for the LED's.I disconnected the 24v side, and connected a 12v (or thereabouts) transformer.I was getting about 12v out on the LED side now, so I hooked up my white LED Striplights, and they work great.But that is just one strip of RGB lights. I want to run 3 strips off this one ParCan.And I don't have the tricolor striplight here with me, so I cannot measure what current/channel it draws. If I did, then I could multiply by 3 to get the current required.Normally these striplights come with a remote control receiver, that has a 'transistor' of sort for each channel (It is an SMD device, with 3055 60188MC) Printed on it. And the controller is marked only to be used for 1 strip, so I assume that these transistors are already maxed out on the current side, which means that if I can find the datasheet, I should be able to see what current they allow for per strip.The LED Parcan has TIP122's on the outputs, without heatsinks.The Datasheet for the TIP122 has DC Collector current of 5A, with 8A Peak.Does anybody know where I can get the specs on the 3055 60188MC, so that I can see its maximum current rating, and work out if the TIP122's will work fine.P

author: SeanB
author_rank: Super Contributor
timestamp: September 28, 2013, 02:49:14 pm
content: If they look like these:http://www.digikey.com/product-detail/en/MJD3055TF/MJD3055TFCT-ND/1473931then they will do 10A. The strips typically are around 200mA per colour per section of 300mm from what I have measured.

author: David_AVD
author_rank: Super Contributor
timestamp: September 28, 2013, 10:22:02 pm
content: Typical 12V RGB strip (30 LEDs per metre) that you buy from eBay use about 200mA per colour per meter.

author: kolbep
author_rank: Frequent Contributor
timestamp: September 28, 2013, 10:46:50 pm
content: Ok, lets take the 200ma/channel/meterFor 3 x lengths, that is 15mSo current/channel is 3A.the transistors in the par can should handle that. They are rated at 5A, with peak of 8A.so I just need to make sure my transformer is about 120VA to handle the 9A of all 3 channels.but I will not run all 3 strips in series, cos that may lead to volt drops and overheating, and uneven brightness. I will star connect the strips to the par.thanks all.

author: David_AVD
author_rank: Super Contributor
timestamp: September 28, 2013, 11:33:10 pm
content: Since the PAR can uses low-side switching, as long as you disconnect the original LEDs, you should be fine driving those strips.And, as you say, don't chain those strips together. As it is there is some voltage drop at the end of one strip.

author: kolbep
author_rank: Frequent Contributor
timestamp: September 30, 2013, 10:45:31 pm
content: So, it is many hours later.I was feeding a 12v DC in to where they normally put the 24VAC (More like 30v), and I was only getting 9v out (Damn Rectifier Diodes, Damn LM317, Damn Other things there)Messed up on a few things (Picture LM317 blowing up, and Diodes melting themselves off the board....Well, Fixed those things up, so now the Parcan is working like it was before I touched it (Phew)Then I had an idea,Why Bother with PNP's and trying to mess around connecting after their transistors, or changing the supply for their circuit,All I do is tap off between the PWM controller and their existing Base resistors, and take it out to my own base resistors, which then control my NPN Power Transistors for the 12v Side.Oh, and also take a computer powersupply, and cut off all wires except for 12v, Ground, And PS On, and use that to power my transistors.And, As Dave would say : Bobs your uncle.Now to take it to site, and see if it blows up when it runs the strip lights....

author: David_AVD
author_rank: Super Contributor
timestamp: September 30, 2013, 11:33:30 pm
content: It sounds like you're doing this the hard way.Are the PAR can outputs ground switching?Do you have a separate 12V supply available?

author: kolbep
author_rank: Frequent Contributor
timestamp: October 01, 2013, 05:16:48 am
content: Yes and yes...

author: David_AVD
author_rank: Super Contributor
timestamp: October 01, 2013, 05:30:06 am
content: Disconnect existing LEDs from PAR can drivers.Connect PAR can drivers to -ve lines of RGB strip(s).Connect 12V PSU V+ to RGB strip(s) common V+ wire.Connect 12V PSU V- to PAR can 0V point.Make sense?

author: kolbep
author_rank: Frequent Contributor
timestamp: October 01, 2013, 05:47:20 am
content: Thanks for that.I did see a similar comment (I think by you) Previously on this thread,but in my Overworked, Sleep Deprived state, when I got back to the bench, I totally forgot that.Lemme do that, at least it is one less point of failure

author: David_AVD
author_rank: Super Contributor
timestamp: October 01, 2013, 06:23:51 am
content: That's the beauty of "open drain" (or "open collector") drive systems. Very easy to mix and match voltages with a common 0V.