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Old 6th February 2013, 09:53 PM   #11
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Hi, just quickly put a basic schematic together. I haven't put any values of components on, it's just an idea:

Click the image to open in full size.

Now I know that this will give me two separate PSUs, which will be handy for when I want 2 different voltages to test with, but could I use a switch that can connect +V2 and 0V1 together to give me a +v - 0 -ve supply? If so, I think it would be good to have a 3 pos switch, 1 position would link +V2 and 0V1, the middle position would leave everything as it is and the last position would link 0V1 and 0V2 for a common 0 on both supplies. Any thoughts?

I also thought I would use a PIC microprocessor so I could show volts and amps of each output. If I were to put a high wattage resistor in series with each output and take a wire from each side of the resistor to the + and - of an OpAmp, I could then use the output of this through a voltage divider into the PIC and calculate the current. The problem is, if I use a single PIC for this, when I have the 2 supplies in series (+V2 connected to 0V1) I would have a problem with connecting the PIC ground pin. Maybe I would need a separate PIC for each PSU.
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Last edited by portreathbeach; 6th February 2013 at 10:03 PM.
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Old 6th February 2013, 10:21 PM   #12
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Quote:
could I use a switch that can connect +V2 and 0V1 together
Yes, that could be done. Make sure that the winding polarities are correct for that connected rail. The switch would in effect create an Isolated/Not Isolated front panel option. That connection would also create a common 0 for both supplies, so a third position isn't needed. I'm not sure what purpose linking 0V1 and 0V2 would serve, but that could be done on the board under test if needed - the +V outputs could not be linked also without modification of the regulator circuits.

addendum: Regarding the meter displays, yes, you'd probably need those to be separate also since the supplies are isolated. I don't know if a single PIC can be programmed to accept both inputs. It doesn't seem that unreasonable. And you want a low value resistor, not a high wattage one. Consider that with 2 amps output, a 0.1 resistor won't dissipate even a half watt.
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Last edited by sofaspud; 6th February 2013 at 10:30 PM.
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Old 6th February 2013, 10:39 PM   #13
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Thanks sofaspud. The reason I though about having a 3 position switch so that the two 0 volts could be connected is for when I want 2 separate voltage supplies for something I'm making, maybe a PIC project (running at 5v) controlling relays (which may be 12v). Having a common ground at the supply may be advantageous, but I suppose they would be commoned on whatever it is that I am making anyway.

As for heat sinking the LM317s..... If I were to use a a 12 volt secondary transformer, that would be 16.8v DC. If I were to set one of the PSUs to 5v at it was drawing 1A, the LM317 would be dissipating 11.8W, what sort of size heat sink would be required? I'm not too good with heat sink sizing.
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Old 6th February 2013, 10:55 PM   #14
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Am I right in thinking that;

- If I want to dissipate 12 watts of heat
- The ambient temperature is 25C
- I don't want the heatsink to get hotter than 60C

Then I am looking for a heatsink that will dissipate 35C / 12w = 2.9C/W

so something like this:

10DN-01000-A-200 - H S MARSTON - HEAT SINK, 2C/W | Farnell United Kingdom

would easily do the job, as it is 2C/W ?
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Old 6th February 2013, 10:55 PM   #15
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I will try to draw you a schematic. I haven't got one as all of my work is in my head.
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Old 6th February 2013, 10:58 PM   #16
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Quote:
Originally Posted by portreathbeach View Post
Hi, just quickly put a basic schematic together. I haven't put any values of components on, it's just an idea:

Click the image to open in full size.

Now I know that this will give me two separate PSUs, which will be handy for when I want 2 different voltages to test with, but could I use a switch that can connect +V2 and 0V1 together to give me a +v - 0 -ve supply? If so, I think it would be good to have a 3 pos switch, 1 position would link +V2 and 0V1, the middle position would leave everything as it is and the last position would link 0V1 and 0V2 for a common 0 on both supplies. Any thoughts?

I also thought I would use a PIC microprocessor so I could show volts and amps of each output. If I were to put a high wattage resistor in series with each output and take a wire from each side of the resistor to the + and - of an OpAmp, I could then use the output of this through a voltage divider into the PIC and calculate the current. The problem is, if I use a single PIC for this, when I have the 2 supplies in series (+V2 connected to 0V1) I would have a problem with connecting the PIC ground pin. Maybe I would need a separate PIC for each PSU.
Why not use a Volt Meter and Ammeter on each output?????
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Old 6th February 2013, 11:01 PM   #17
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Don't forget to add a 1N4003 diode reversed bias across pins 3 and 2 of the LM317. It will stop it from being killed!
To aquire more current, feed the +V1 and +V2 outputs directly to the base of a high power NPN transistor. And again protect it with a diode.

Last edited by JonSnell Electronic; 6th February 2013 at 11:03 PM. Reason: Extra info
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Old 6th February 2013, 11:04 PM   #18
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Yes, I suppose that would be a much simpler way. I was actually just looking of eBay and saw these:

DC0-10A Ampere LED Amp Panel Meter 3Bit Display No Need Shunt Digital Ammeter | eBay

and

0.56" Digital Voltmeter Green LED Panel 3/2-Wires DC 0-100V 4-30V Power Monitor | eBay

That would work nicely I think, and they are pretty cheap too!
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Old 6th February 2013, 11:18 PM   #19
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Thanks Harleyjon. Most power supplies have a current limiting potentiometer, I notice that the LM317 can be wired for a constant current source, so would it be possible to use one to adjust the voltage (as in the schematic I posted) and another in series with the output in the 'constant current' configuration to give me what I want. If so, how do you calculate the power it has to dissipate? Also, if I can use an LM317 in my PSU in constant current configuration, could I use a potentiometer (of a high enough wattage) so I could use it as a current limiter? Sorry for all the questions
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Last edited by portreathbeach; 6th February 2013 at 11:47 PM.
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Old 7th February 2013, 01:27 AM   #20
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Quote:
configuration to give me what I want
You should probably make a list of exactly what you want from your bench supply. It will make things easier. For example, if you want the LM317 circuit to adjust down to 0V you'll need to bias the adjust pin with a negative voltage.
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