I'm building a three-channel, 0-28V, 0-2 amp (both variable) power supply. I took the schematic straight from the L200C datasheet and just triplicated it (common input with an added 12 V reg for a fan, besides that just three duplicated strings). Here's the design, identical to the app circuit in the datasheet (the right hand schematic):
Well, I can't seem to insert the schematic here, but anyone familiar with this chip will probably know, it's straight from the L200C datasheet, variable voltage with variable current limit. I'll keep trying; this is my first post and I've probably broken some rule. Meantime, here's a link to the picture, and it's the schematic on the right:
http://www.eeweb.com/blog/circuit_projects/l200-based-voltage-regulators
When I hook it to a 20VDC supply (multimeter reads about 20.5V), no load, I can turn the voltage control pot (labeled TR2, above) about half way up, and all looks good as the voltage climbs to about 12.7V, then the voltage stop climbing as I turn the pot the rest of the way and stays constant. The pot controlling the current seems to work fine (tested with a 10 ohm resistive load). Any adjustment to the current pot has no effect on the output voltage (which is as it should be, I think).
The voltage should climb to Vin (~20V) minus about 2-2.5V. When I measure the voltage on pin 4, Vref, it is about 2.77 (almost perfect from the spec sheet) up until the pot hits that 12.7 Vout mark, then Vref starts decreasing and Vout stays constant as I further turn the pot.
All three strings exhibit identical behavior.
The way I understand this chip to work is that it uses Vref as a (what else?) reference voltage, and holds it constant, causing Vout to rise as TR2, and therefore the ratio TR2/R4, increases. This is not happening, or is only happening to a point. Can anyone see why?
I'm going to see if I can further parameterize the problem this afternoon, figure out exactly where TR2 resistance is at this stop-point and measure anything else that seems relevant, but thought I'd get this out there in case it's already been seen. Also, I can accommodate any response that has additional measurement points.
TIA,
Allen
Well, I can't seem to insert the schematic here, but anyone familiar with this chip will probably know, it's straight from the L200C datasheet, variable voltage with variable current limit. I'll keep trying; this is my first post and I've probably broken some rule. Meantime, here's a link to the picture, and it's the schematic on the right:
http://www.eeweb.com/blog/circuit_projects/l200-based-voltage-regulators
When I hook it to a 20VDC supply (multimeter reads about 20.5V), no load, I can turn the voltage control pot (labeled TR2, above) about half way up, and all looks good as the voltage climbs to about 12.7V, then the voltage stop climbing as I turn the pot the rest of the way and stays constant. The pot controlling the current seems to work fine (tested with a 10 ohm resistive load). Any adjustment to the current pot has no effect on the output voltage (which is as it should be, I think).
The voltage should climb to Vin (~20V) minus about 2-2.5V. When I measure the voltage on pin 4, Vref, it is about 2.77 (almost perfect from the spec sheet) up until the pot hits that 12.7 Vout mark, then Vref starts decreasing and Vout stays constant as I further turn the pot.
All three strings exhibit identical behavior.
The way I understand this chip to work is that it uses Vref as a (what else?) reference voltage, and holds it constant, causing Vout to rise as TR2, and therefore the ratio TR2/R4, increases. This is not happening, or is only happening to a point. Can anyone see why?
I'm going to see if I can further parameterize the problem this afternoon, figure out exactly where TR2 resistance is at this stop-point and measure anything else that seems relevant, but thought I'd get this out there in case it's already been seen. Also, I can accommodate any response that has additional measurement points.
TIA,
Allen