Bridge Rectifiers and SSR's

[joshlisa]

I have been doing this blinky flashy thing seriously for almost two years now.
Last season I felt my way through and with what seemed like blind luck (and heaps of advice from the good people here) it worked.
Now I am looking at expanding for this year and during my planning I had a thought.
Last year I altered quite a few strings of LED lights that ran from the old 8 function controller to be able to be controlled by my system. I used components of the old 8 function controller to effectively create a Bridge rectifier.

Now my lack of knowledge shows.......I didn’t realize that this would turn the AC produced by the power pack into DC for the LED's.

The question I need answered is if the power going to the LED's was DC from the rectifier on, how did my AC SSR's controlled by the olsen's switch the DC Power?
Is this going to hurt my SSR's if I keep doing this?
Why do we need DC SSR's if the AC SSR's work?

Thanks in advance for the help all you learned people.

Josh

[ErnieHorning]

What you have is rectified AC, not DC. As long as the voltage goes back down to zero( it could even be a squarewave), the AC SSR’s will function.

DC (i.e. a battery) does not change voltage.

http://img193.imageshack.us/img193/8125/rectifiedwaves1.png

[joshlisa]

Thanks for the reply. It had me scratching my head as to how the SSR's worked with DC.

For this year I have purchased large transformers to replace the bank of small trannys I currently have. Can I run a larger bridge rectifier to multipul strings of LED's or do I have to have a smaller one on each string? Further how would I work out the size I would need.

Thanks again for the reply.
I love this time of year......I learn so much......
Josh

[ErnieHorning]

Yes you could use a single bridge rectifier to supply power to multiple strings. You could also use a single bridge rectifier to supply power to multiple SSR’s. Maybe that’s what you were actually asking. A general rule of thumb is to stay below 80% of the full rating.

Be careful when rewiring LED strings. Some of them won’t tolerate being powered backwards. There should be a diode in series with the LED’s that prevents reverse voltage. If you remove it, you’ll need to mark the polarity on the wires somehow.

In a way, you guys got it kind of nice with the 24 volts now that LED’s are available. Current requirements are low and voltage drop on the wires isn’t nearly the problem it used to be with the old incandescent lights. I know that a lot of people there use FET transistors but now with LED’s you could even use cheap bi-polar transistors too.

Once the LED strings here drop down to the incandescent level of cheap, I want to start cutting these up and run them on low voltage DC like they really should be and then multiplexing becomes easier.

[Goldie]

Hi Josh
l use these for my bridge rectifier, the power comes from a 24volt Toriadal

The led's have there own resistors as they are different colours, but you can put the resistor on these if you know the correct size.

[joshlisa]

In the interest of trying to simplify my setup I have purchased 2x 500w 24 volt transformers this year to replace the plug packs. Now I wanted to wire these to my SSR's than then plug into Cat 5 and out to the lights. To do this I need to run through bridge rectifiers for the led's especially. Are there rectifiers large enough to have just one or two on these transformers prior to the SSR's or would I be better served (cost and work wise) to just have a small one for each SSR ?
I am tying to keep the setup neat as I can and currently have the whole lot in Rack enclosures in a network cabinet in the shed. Hence the larger transformers and the wanting to keep rectifiers to a minimum.
Thanks for your help so far.

Josh.

[sjwilson122]

Hi Josh,

Those transformers would be putting out 20.8 amps at rated power. Bridge retifiers can be had at much higher ratings than this. A 25 or 30 amp bridge would work just fine.

Scott

[P. Short]

Hi Josh,

Those transformers would be putting out 20.8 amps at rated power. Bridge retifiers can be had at much higher ratings than this. A 25 or 30 amp bridge would work just fine.

Scott

But bear in mind that a bridge rectifier passing 20A of current is going to be dissipating about 40V. It is going to get hot, and you will need to pay attention to preventing it from self-destructing from the heat.

[joshlisa]

Thanks for the pointers guys. Sounds like the way to go.
Just want to make doubley sure that the constant changing load on the system, many strings of lights and LEDs feeding from one rectifier changing on and off at diffrent times will not harm the system.
Thanks for all the help.
Josh

[ErnieHorning]

Also be aware that because of series resistance in the transformer the voltage will vary with load. The LED’s will be the most sensitive and you may see a visible pulsing.

[oldcqr]

Once the LED strings here drop down to the incandescent level of cheap, I want to start cutting these up and run them on low voltage DC like they really should be and then multiplexing becomes easier.

You got that right. How nice would it be to run 16,32 or even 48 channels from a single muti-strand cable instead of 16,32,48 16 or 18 AWG runs?

[synnie]

With my Olsens and LED strings Josh, im running them direct from the Olsen through the ULN, no need for SSR's.Powered by a DC power supply with no need for the bridge rectifier.probably a bit late to throw my 2 bits in.
G

[joshlisa]

Synnie, Nice idea and hadent given that a thought. What is the pratical upper limit per ULN?????? I have some pretty longish led strands in my show (read 400-500 per strand). Thanks for the idea and will runs some figures.
Josh

[dirknerkle]

Synnie, Nice idea and hadent given that a thought. What is the pratical upper limit per ULN?????? I have some pretty longish led strands in my show (read 400-500 per strand). Thanks for the idea and will runs some figures.
Josh

A ULN2803A can source up to 500ma per channel, and multiple channels can be combined for more. It can handle up to 30v maximum.

While you can theoretically combine all 8 output channels into one, you probably wouldn't want to put that kind of current through CAT5 wires though... Be careful...

[synnie]

There is a limit, ie you cant have all 8 channels running 500mA at once,havent worked out what it is yet though.I hade 8 channels of 140mA max @24VDC with the DIO96 using the ULN's.This year ill be putting 40mA @ 45VDC per channel on 20 channels. I did test one channel at this before so it will be trial and error for the higher voltage.The emitter can handle 50V in the specs.Im using 4 strand alarm cable soldered direct to the 595.hopefully ill be able to get my icicles done soon (took a chunk out of a finger and its not good for putting the bulbs back in just yet)

[dirknerkle]

Synnie, I'm not sure about that... here's what the data sheet says...

The ULN2801A-ULN2805Aeach contain eight darlington
transistors with common emitters and integral
suppression diodes for inductive loads. Each
darlington features a peak load current rating of
600mA (500mA continuous) and can withstand at
least50V in the off state. Outputsmaybe paralleled
for higher current capability.

I experimented with an LED flood design this year and pumped 450ma through each one and I didn't get any smoke....

[budude]

I got pointed to the same data sheet by ErnieH when I was looking into the ULN2803. It has a maximum current at the output of 500mA (this depends a bit on the vendor also) but has a maximum current of only 150mA when all 8 outputs are full on. The tables don't show that - you have to look at the graphs. As dirknerkle suggested they can be combined so you could easily get to 300mA and still have four outputs.