drop 19V to 12V

[timtheenchanteruk]

following on from my other post for a DMX LED driver.

 

Wondered if there is an easy way to drop 19V to 12?

 

The LED strip I will be using has a total length current draw of approx 1200mA

 

Im not always using a full length though, so I presume I cant just whack a resistor inline, working out the value for each length would be a PIA, although not impossible, I can work out the section draw from the 1200/No of sections.

 

potential divider, would this work, am I right in thinking the resistors would need to be able to dissipate the full 1200ma? (about 15W if im right)

 

I cant reduce the Power supply voltage, the rest of the LEDs in the set have been designed round the 19V, and they are done (all 400 of them), the LED strip was added onto my remit yesterday.

 

Im very rusty on this stuff, as you can probably tell!

[adam2]

Buy a stepdown power supply unit such as would be used to power 12 volt equipment in a truck or bus that has a 24 volt electrical system, all the ones that I have seen work fine with a less than 24 volt supply.

 

Or use a 12 volt battery

 

Or, presuming that the 19 volts is derived from the mains, use a seperate power supply with mains input and 12 volt output.

 

A dropper resistance will work PROVIDED THAT the LED tape uses internal dropper resistors for each group of 3 LEDs. If the LED tape uses a transistor regulating circuit for each group of 3 LEDs then an external dropper wont work reliably.

 

 

[timtheenchanteruk]

LED tape does have resistors, not transistors (just had a look)

 

seperate PSU wont work, its Mains>PSU>Led DMX Driver>LEDs I need to keep the DMX driver there for control. if it were that simple, I wouldnt be on here, got standard LED drivers at work, but need the DMX dimming/control

[patrick_keys]

OK, just so I understand what you're trying to do:

 

- You've got some tape which runs at 12V and draws about 1.2A

 

- You've already got a mains -> 19V PSU

 

- You've got a DMX driver (which one?) that will drive the LEDs at 19V from a 19V PSU. I assume this is a constant voltage driver?

 

Before working out how to drop the voltage, it might be useful to understand how an LED driver works for LED tape. It is known as a constant-voltage dimmer, and essentially switches on and off the supply very quickly - usually in the kHz range. The ratio of on time to off time dictates how bright the LEDs get - e.g. 50% on, 50% off should give you somewhere in the region of 50% brightness. This kind of dimming is very different to constant current dimming required for Luxeon LEDs.

 

This means that you can't use a linear regulator (or similar) on the output to lower the voltage, as you'll likely end up with pure DC (constant brightness). You may be able to add a simple resistor to the output - 6ohm, 8W might get you close, but it'll get very hot and isn't particularly nice.

 

The real question is: have you got the right driver, and does it accept a low-voltage input or mains input? Personally, I've never seen a 19V LED anything in the constant voltage world - LED tape etc. usually runs at either 12 or 24V. If your driver accepts a low-voltage input, it should be able to work down to 12V, which means you can use a simple DC-DC converter or linear regulator to lower the voltage prior to the driver. Most of them work by switching the ground line anyway, so you can usually vary the input voltage quite a bit.

[timtheenchanteruk]

this is my driver the driver basically spits out whatever voltage is put in in the 12-24V range.

 

that's pretty much it,

my problem is I need to change the OUTPUT voltage on the driver down to 12V, there are other LED bits of the set, namely a ruddy great sign with 400+ LEDs in it, the LED arrays in this were designed for a 19V supply, I don't have time to change the rest of the set to take a 12V supply.

 

Had I been doing these extra bits in the first place, Id have used a 12V supply, I chose 19V as its easy to get a decent rated PSU to supply it.

[patrick_keys]

this is my driver the driver basically spits out whatever voltage is put in in the 12-24V range.

 

that's pretty much it,

my problem is I need to change the OUTPUT voltage on the driver down to 12V, there are other LED bits of the set, namely a ruddy great sign with 400+ LEDs in it, the LED arrays in this were designed for a 19V supply, I don't have time to change the rest of the set to take a 12V supply.

 

Had I been doing these extra bits in the first place, Id have used a 12V supply, I chose 19V as its easy to get a decent rated PSU to supply it.

 

Ok - that makes sense (I think). I'll slightly over-explain this for the benefit of anyone else reading the thread...

 

You're using the multi-channel driver to drive some 19V stuff and you also need to drive some 12V tape. The driver is powered from the 19V PSU.

 

Now, the 19V connected to the driver input is used to power the DMX receiver electronics, but is passed through unmodified to provide the positive rail for the LEDs, i.e. all of the +V terminals will be at 19V. When it dims, it is actually switching the ground line rather than the 19V rail - you should be able to prove this with a meter - measure the +V terminal relative the ground on your supply with a channel at 0% - it will still read 19V.

 

This fact makes it much more useful to us, because it can be used to switch any LED voltage (within reason). Connect up as follows:

 

- 19V supply to driver input as normal

 

- +V and channel terminals connected to existing LEDs

 

- +12V connected directly to tape (we'll worry about how to make this in a minute)

 

- negative side of tape connected to channel terminals on driver (ignore the +V terminals for those channels)

 

- ground on 12V supply and 19V supply connected together

 

Now, the 12V problem is much simpler - it is a straight power supply, rather than anything complicated. If I was making this, I'd add a separate mains to 12V PSU, however you may be able to build something with a linear regulator. A simple 7812 should do the job, but be prepared to add a heatsink.

 

Hopefully I've explained that well enough - I can add a sketch to make it clearer if you like...

[timtheenchanteruk]

that makes perfect sense, and im not sure why I didn't think of it before, except in a "circuit" kind of mind the LED tape needs to be fed and controlled form the same supply, except it doesnt does it, as long as the LED tape has a +ve and -ve from the same mains all is well.

 

so essentially

for the 19V stuff its mains>19Vpsu>driver>LEDs>driver

for the 12V its mains>12Vpsu>Led>Driver

 

thankyou very much, 12V stable supply it easy, already got a couple of those in the form of ATX supplies ive used a few times for LEd power.

[patrick_keys]

that makes perfect sense, and im not sure why I didn't think of it before, except in a "circuit" kind of mind the LED tape needs to be fed and controlled form the same supply, except it doesnt does it, as long as the LED tape has a +ve and -ve from the same mains all is well.

 

so essentially

for the 19V stuff its mains>19Vpsu>driver>LEDs>driver

for the 12V its mains>12Vpsu>Led>Driver

 

thankyou very much, 12V stable supply it easy, already got a couple of those in the form of ATX supplies ive used a few times for LEd power.

 

That's pretty much it - as soon as you understand that the driver is switching the ground line (and that's the same regardless of the supply voltage) all is good. Just remember not to connect the +19V and +12V together!

[adam2]

LED tape does have resistors, not transistors (just had a look)

 

seperate PSU wont work, its Mains>PSU>Led DMX Driver>LEDs I need to keep the DMX driver there for control. if it were that simple, I wouldnt be on here, got standard LED drivers at work, but need the DMX dimming/control

 

 

I have no doubt that you are correct regarding the type that you have examined, but this not true of all makes.

 

I have some that definatly uses a transistorised or IC regulator for each group of 3 LEDs, the extra components are visable between the LEDs.

This adds to costs but gives a constant light output over a range of voltages from typicly 10.5 up to 14 volts. I selected this type for architectural use so as to ensure absolutely constant light over long runs.

It comes in 6M lengths, and with the simple resistored type, the far end was detectably dimmer than the supply end.

[DrV]

that makes perfect sense, and im not sure why I didn't think of it before, except in a "circuit" kind of mind the LED tape needs to be fed and controlled form the same supply, except it doesnt does it, as long as the LED tape has a +ve and -ve from the same mains all is well.

 

so essentially

for the 19V stuff its mains>19Vpsu>driver>LEDs>driver

for the 12V its mains>12Vpsu>Led>Driver

 

thankyou very much, 12V stable supply it easy, already got a couple of those in the form of ATX supplies ive used a few times for LEd power.

 

That's pretty much it - as soon as you understand that the driver is switching the ground line (and that's the same regardless of the supply voltage) all is good. Just remember not to connect the +19V and +12V together!

 

One extra component - LEDs are not very good with reverse voltage so it would be prudent to put a diode in series with the -ve end of the LED tape so that you are not reverese biassing the 12v tape when the driver is switched off. It would only be 7v reverse bias max but better to be safe than sorry.

Dave

[mrboo]

I would just find a 2nd led driver for the 12v items and just give them same address so they are paired as that is the most simple route to take and it dont cost a lot on ebay for them. just make sure drivers use same channel config like DRGB or RGBD

[cedd]

The show opened a couple fo days ago and Tim's got them all working nicely. The sign looks pretty impressive! It's certainly bright! They're all dimming out so I guess whatever he did, worked! Looks good.

[timtheenchanteruk]

Cheers, been really busy,so not been able to get back on here much this week.

 

Hopefully will have a couple of pics of it all in action too, we all like a good piccie!!