Need Help regaring a 0-10V Variable Voltage Regulator

Thread Starter

buddyengineers

Joined Mar 19, 2005
26
I just designed a Variable Voltage Regulator for 0 to 10V dc.
For tht I used a LM7812 with about 17V as the input and I get 12V at output.

Now, to regulate the 12V o/p dc, I connected a Voltage Divider ckt with a Potentiometer.
The pot is of 600Kohms and so I connnected a 120kOhms resistor in series to get the voltage divider output to 10v.

Now at no load, everytime I move my pot, the output varies from 0v to 10v. but immediately as I connect a load (which is a standard Dimmable Balast for tubelights which takes 1-10v as input), the output voltage goes to 11.5v and on turning the pot, I dont get a linear response.

The output remains at 11.5v for a long time and only after the resistance of the pot has decreased to less than 300kohms that the output starts reducing, but its not gradual and shows a exponential format...

Am very confused on y is this happening??? Load rating of the dimmable ballast is 1-10v, less than 0.6mA.

I know that this is not the ideal way to create a voltage regulator... but still am confused as to wat is the problem...
Also suggest me a better way to make a 0-10v regulator please.

Thanks
 

beenthere

Joined Apr 20, 2004
15,819
That large resistance is most of your problem. Use a 2K fixed resistor and a 10K pot instead of the 720 Kohm string. The divider you are using will permit only 16.7 microamps current. That is a lot less than the 600 microamps you need.

If you want a regulator for 0 - 10 volts, use an LM317. Tying the ground pin to -1.25 volts will allow adjustment to 0 volts.
 

eblc1388

Joined Nov 28, 2008
1,542
Also suggest me a better way to make a 0-10v regulator please.
With your present setup, the easiest way is to add a NPN transistor as shown, forming an emitter follower. Just about any small signal NPN will do nicely. I2 represent the 0.6mA loading.

Due to the external loading, even with a 10K POT as suggested above, the voltage change is not linear when the variable resistor is near the bottom of the range.

 

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Thread Starter

buddyengineers

Joined Mar 19, 2005
26
That large resistance is most of your problem. Use a 2K fixed resistor and a 10K pot instead of the 720 Kohm string. The divider you are using will permit only 16.7 microamps current. That is a lot less than the 600 microamps you need.

If you want a regulator for 0 - 10 volts, use an LM317. Tying the ground pin to -1.25 volts will allow adjustment to 0 volts.

I tried using LM317.. but to no avail... I connected a 100k resistor as R1 and the 600k pot as R2. According to the formula, I should be getting about 8.75v... but my multimeter showed 13v... <snip> confused now!!!
 
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Thread Starter

buddyengineers

Joined Mar 19, 2005
26
With your present setup, the easiest way is to add a NPN transistor as shown, forming an emitter follower. Just about any small signal NPN will do nicely. I2 represent the 0.6mA loading.

Due to the external loading, even with a 10K POT as suggested above, the voltage change is not linear when the variable resistor is near the bottom of the range.

Thanks... This worked...
 
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