120V 0.7A Constant current source
120V 0.7A Constant current source
(OP)
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,
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,





RE: 120V 0.7A Constant current source
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!
RE: 120V 0.7A Constant current source
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?
RE: 120V 0.7A Constant current source
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 120V 0.7A Constant current source
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.
RE: 120V 0.7A Constant current source
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
RE: 120V 0.7A Constant current source
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..
RE: 120V 0.7A Constant current source
Second thing I found googling ""LED current control"".
Sounds like your situation.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 120V 0.7A Constant current source
(I'll have to search to see if there are similar premade solutions... I'd like to get this up and running soon)
RE: 120V 0.7A Constant current source
RE: 120V 0.7A Constant current source
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
RE: 120V 0.7A Constant current source
RE: 120V 0.7A Constant current source
How do I use two strings of LED's at half the voltage (~50V) with one buck converter? Suggestions?