120V 0.7A Constant current source

[gijim]

Here's the problem.... I have a load that varies from 76 to 120VDC, but always needs a constant 0.7A. I figure "okay, no problem, just need to rig up a constant current source for 120Vac use".... In the interests of keeping costs and parts counts low, I decided an NPN current source might be the way to go. Lots of calculations later, I see huge amounts of power being dissipated by the resistors (12W) and the transistor (up to 30W @ 76V min output).... Not good!

Any suggestions on what I should use to keep parts count low, but heat dissipated even lower? I don't mind wasting 6 or 12 watts, but 42W is ridiculous. Thanks in advance,

 

[jimkirk]

Does your load varies from 76 to 120 volts, or does it draw 0.7 amps no matter what supply voltage you power it with, over the range 76 to 120 volts DC?

If that's the case, I'd say get a full wave bridge and capacitor (select values for voltage, current and acceptable ripple), power it with an appropriate transformer so that the output voltage is within 76 to 120 volts over your mains variations, and they you go. With that kind of range, you shouldn't need any other sort of regulation.

And be careful!

 

[Comcokid]

No matter how you look at this, a constant current source will dissipate a minimum wattage of (120-76)times 0.7 or 31 Watts. In a pratical circuit, you will need to dissipate more.

Typical circuit will consist of a voltage reference buffered by a opamp which drives several NPN transistors or N-channel MOSFETS with a sense resistor of 0.1 to 1 Ohm.

A quick look at a book ("Current Sources & Voltage References" by Linden Harrison, Newnes Press) showed current sink circuits for high power which used power MOSFETS. These circuits could be rearranged for as a current source.

Your original question indicated "VDC" and then "Vac" Do you need a DC or AC current source?

 

[itsmoked]

If your device draws a constant current, feeding it a constant current is going to cause a constant problem....

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[gijim]

Sorry, I always tend to muck descriptions up....

It's a diode array that will be powered by mains (120V) voltage. Problem is there is a wide range of voltage tolerance on these devices... Nearly 100%. So these devices will need "somewhere" between 76V-120VDC..... But 0.7A is their maximum current, which the power supply must provide at all times.

The "easy" solution would be to insert some resistor once the voltage of the array is known, but that's not a one size fit all solution..... And it may cause problems if mains voltage fluctuates.

 

[itsmoked]

Okay! So what you need is a 700mA constant current source with a voltage compliance of 120V.

You should use a switching supply that has current control.

Remember too, that LEDs can take big currents briefly! So as long as the average and the maximum peak currents are not violated you're good to go.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[gijim]

Well if you wanted to get concise, yes!

Can you suggest a switching supply with current control off the top of your head? The only ones I knew existed were straight forward fixed voltage..

 

[itsmoked]

http://www.greensupplyline.com/products/174403397

Second thing I found googling ""LED current control"".

Sounds like your situation.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[gijim]

Wow, the actual design of a switching power supply is a bit over my head, but they have a really good app note that walks you through it. Thanks!!

(I'll have to search to see if there are similar premade solutions... I'd like to get this up and running soon)

 

[gijim]

Well the HV9910 will let me get by with about 10 parts. I have a question about ESR though... The ap note wants a 12uF low ESR (tantalum or ceramic) cap on the output. Since my output is >130V, that's a costly proposition. They don't make them, so I'd have to build it out of about $50 worth of caps. What benefit is there to that vs a standard $2 electrolytic? This isn't a precision power supply by any means.......

 

[itsmoked]

Yep, that is a bit expensive..
But why are you looking at the app note on boost supplying? I thought you were at a high voltage and trying to drop things. The data sheet never mentions any caps much bigger than a few nano-farads.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[gijim]

Because according to another app note, you can only use the buck converter if your Vin > 2*Vout. Obviously in my case Vin~Vout, so the buck-boost converter has to be used.

 

[gijim]

Okay, buck boost is bad....... I don't want it blowing up if the load went open circuit.

How do I use two strings of LED's at half the voltage (~50V) with one buck converter? Suggestions?