Using BJT as a LED lamp simulator for test purposes is OK?

[treez]

Can i use this BJT circuit for simulating a 33V LED lamp?


https://i47.tinypic.com/ormofr.jpg

.....its easier than buying a led lamp.........for the time being


...i need to test the led driver, and dont want tthe hassle of making up a leds pcb.

 

[kripacharya]

your circuit might not work properly.

here is a better one. You might need to adjust the R1 for better current management and power ratings of the zener and itself.

 

[treez]

Thanks but i need a resistor in series with the base so it limits base current.

Why is R2 there....?...the idea is its a led simulator, so i dont want much resistance in the power path, becasue then the voltage will depend too much on he current.

 

[kripacharya]

Only the part inside dotted line is the LED emulator, rest is for doing spice modeling and checking.
i don't think you need resistor in base path, but add it if you want to

 

[treez]

Why is a resistor needed across the base emitter?

 

[betwixt]

It probably isn't necessary but it's there to make sure leakage current in the Zener and transistor are carried to ground and to ensure enough Zener current flows to stabilize it's operating point.

Brian.

 

[treez]

thanks, but i am not bothered about "leakage current in the Zener and transistor are carried to ground "...........this is a led simulator................as long as it gives me something like 33V then thats fine.......i am simulating a 33v led string................or rather , i cant be bothered to may out a 33v led pcb so im just doing it like this.

 

[betwixt]

In that case, just use a 33V Zener diode. Make sure it's power rating is 33 x I Watts where I is the LED current you are simulating in Amps.

Brian.

 

[treez]

Thanks, though it would need to be 33V and rated 5W, i havent seen any zener diodes like this.......the biggest i see is one Watt.

......i am certain that high power zeners dont exist as i remember wanting to use one as a flyback transformer reset........but you cant get them at high enough power

 

[betwixt]

They make them up to 75W in standard voltages.

For a 33V 5W type, look at the 1N5364 from various manufacturers.

Brian.

 

[treez]

Sorry betwixt but i am afraid i have bad news for both of us.......at least for me anyway.......................The 1N5364 comes in a terrible DO201 package, and cannot dissipate 5W continuously under realstic conditions..........

You can look at the ratings part of the datasheet if you wish........

1N5364:
http://www.datasheetcatalog.org/datasheet/microsemi/1n5333b-88b.pdf


..they guiltily confess the 85 degc per watt thermal resistance, and thats with a huge amount of PCB heatsink copper area, and at 25degC ambient.

The DO201 package is not a viable proposition for dissipating 5W for any length of time.


I know this because i wasted many hours about 12 months ago, desperately seeking a ~100V zener or TVS in a TO220 package (so i could put a heatsink on it).....they just dont exist in these thermally "conducive" packages.

The reason i was looking then was because i was looking into reducing the clamp dissipation at light load in a 150 Watt flayback...............so i wanted an RCDZ clamp...hence my seeking (and not finding) a TO220 zener or TVS........they dont exist.................. why?.........i have no clue.....................i asked this on many forums at the time, but found no knowledge on it.

Sorry but DO201 packages are for occasional "jolts" of high power only.

 

[betwixt]

Now you see why the 'Amplified Zener' was suggested, you can use a power transistor with heatsink and a low power Zener. You can't get around the power dissipation, it will be the same as your real LED load so you will have to remove heat by some method to keep it cool. Incidentally, the ON-Semi version of the 1N5364 has a fatter body and is rated for continuous 75 C operation and a maximum of 200 C.

Other types you could consider: 1N3325B and NTE5269A both in production and easily available but rather expensive. They have DO5 stud mounts and are rated at 50W.

Brian.

 

[BradtheRad]

Six 5.6V zeners rated 1W each, hooked up as a string, will do the job of a single 33.6V zener rated 6W.

Change one to a 5.1 V zener and you'll have 33.1 V.

(Give or take the typical 5% tolerance for zener diodes.)

 

[treez]

Thanks BradtheRad, but somE of the LED loads are 5V, 5W.

By "Amplified zener" i take it that Betwixt kindly means as in the 1st post?

.....do you think its ok?....it looks odd to me...............kind of like the BJT is wanting to saturate but can't.

 

[betwixt]

That's exactly right. At Zener voltage + Vbe, the transistor conducts and pulls it's own supply down until it reaches equilibrium. The advantage is you can use a lower power rated Zener because it only carries base current and the transistor packaging can be one that is already designed for good heat transfer. It's a fairly standard and widely used configuration.

Brian,