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Power Supply - behaviour

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tresca

Member
Ive had this 30V 3A regulated power supply for quite some time. It can either be a voltage source or current source. When its being used as a voltage source theres a green led, and when its current, its a red led.

I've come across behaviour which I have not seen and am just looking for some insight.

I recently built a circuit with a GLCD + PIC on a breadboard. I set the power supply to 5V. However, when I turned it on, the green led disappeared and the red led came on (telling me current source...ie. voltage shorted). I checked the connections, and everything checks out ok.

The circuit works even though I'm not measuring any noticable voltage. So I started to adjust the voltage knob slowly and voila, green led came back on (voltage source). I measured the voltage and it was hovering at about 4.1V. However, if I go higher than that, it seems I go into current source.

I've tried to look for documentation on my power supply but its pretty old (analog meter).

Any ideas why when I increase voltage, my source goes from voltage to current ?
Aren't pics voltage devices ? I have no voltage, yet my circuit works ?

Could it be that I'm trying to draw to much ? I can set the limit of my current source, to whatever I want, and if I increase my current value, I don't get this problem. But my display goes SUPER bright like when you are about to fry an LED bright.
 

ericgibbs

Well-Known Member
Most Helpful Member
hi,
I have a like bench psu, but the current source limit feature can be set to limit the current at the required value.
It also has the standard voltage adjustment control.

So you can set the Vout to say, 5V and the Current limit to say 150mA.

If the 150mA limit is exceeded a RED led is lit.

I suspect your psu works in the same way.??:)
 

tresca

Member
Thats what I was thinking too, but I'm using an ICD2 clone right now, which from what I remember, limits it to about 170mA. My PSU was set to 1.5A.

Side question: Is it normal for a GLCD to get hot ? When I put my fingers on the back of the pcb, its pretty hot (not enough to burn but enough for me to ask)
 

ericgibbs

Well-Known Member
Most Helpful Member
Thats what I was thinking too, but I'm using an ICD2 clone right now, which from what I remember, limits it to about 170mA. My PSU was set to 1.5A.

Side question: Is it normal for a GLCD to get hot ? When I put my fingers on the back of the pcb, its pretty hot (not enough to burn but enough for me to ask)

hi,
The GLCD [led] backlight requires a low value series resistor, have you got the back light led connected directly to the psu.???
 

tresca

Member
Ya...I put a 100ohm resistor in series with the power for backlighting, I get no backlight, or severly dim. I even tried a 47ohm. Nada.
 

ericgibbs

Well-Known Member
Most Helpful Member
Ya...I put a 100ohm resistor in series with the power for backlighting, I get no backlight, or severly dim. I even tried a 47ohm. Nada.

hi,
As a check, disconnect the GLCD led back light leads to the psu, see if the psu works OK and the GLCD dosnt get hot.
Lets know what you find.:)

The series resistor is normally quite low about 4.7R thru 10R.
 

tresca

Member
Well well well, it IS my backlight thats causin the problems.

I disconnected the backlighting, and connected the psu back up, and slowly increased voltage, im at 5V now, and green light. I dont really have anything smaller than 2x47ohm, so really about 23.5ohms is as low as I can get.
 

ericgibbs

Well-Known Member
Most Helpful Member
Well well well, it IS my backlight thats causin the problems.

I disconnected the backlighting, and connected the psu back up, and slowly increased voltage, im at 5V now, and green light. I dont really have anything smaller than 2x47ohm, so really about 23.5ohms is as low as I can get.

hi,
At a guess, when you had the backlight on, you said 4.1V ok.?

So approx 5v - 4v = 1V/0.1A = 10R for a starting value...:)
 

ericgibbs

Well-Known Member
Most Helpful Member
Managed to find a 10ohm resistor, its a bit brighter, but not by much.

sigh.

I did say they are usually very low,, Ive used 4.7R on some backlights.

The GLCD datasheet should tell you the led Vdrop at what current.

At least its not frying anymore...:rolleyes:
 
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