Negative volts and grounding in bench power supply

[falcott]

I'm adding a variable current regulator to my bench supply (0-30v dc) following a schematic in National's LM350 data sheet. I need approx -5v to -10v for the regulator. I know that I can use a 7909 regulator for this, which has 3 pins: input, output and ground. Where does the ground pin go to? Assuming that I take negative from the bridge to the input pin, do I ground the regulator via an electro cap to the positive rail?

Any help appreciated!

[janneman]

It is not clear exactly what yuo want to do. Assuming you have a bridge to get the 0-30VDC for the main supply, you want to make neg voltage from the same bridge? I don't think you can do that.

Or do you want to make a neg voltage with respect to the +30V point?

Jan Didden

[falcott]

To clarify, look at National Semi's LM350 data sheet (for example here ), "Adjustable Current Regulator" on p.10.

Thanks for your response, Jan.

[jackinnj]

you can derive a "negative" voltage from a single positive supply by using an inverting switching regulator -- National Semiconductor has several examples on their site. Just use a linear regulator following the switcher to clean up some (not all) of the switch transients.

[falcott]

Something like the LM2576?

If I use the 7909 on the output to clean up the transients, that still leaves me with the question as to where the ground pin is connected.

Or, using an adjustable LM2576 set at the desired voltage, can I simply put, say, 2 large caps on the output, instead of the 7909?

Thanks for the leg-up!

[sek]

When you have the switcher 'create' a negative output voltage, it's reference would be the old supply's ground, because this is where you want it to be negative from.

Then, this also becomes your ground for the 7909, because it's still where you want it to be negative from.

This should then be a +30V <-> 0V <-> -9V supply at this stage, right?

Also, you want caps and the 7909 at the switcher's output, because it really creates noise and spikes.

[moamps]

Hi,

the simplest (and maybe the cheapest) way to get negative source is by using an additional small transformer 9V/2-3VA, small bridge and electrolytic capacitor without any stabilisation (load is CS).

Regards,
Milan

[jackinnj]

Originally posted by moamps
Hi,

the simplest (and maybe the cheapest) way to get negative source is by using an additional small transformer 9V/2-3VA, small bridge and electrolytic capacitor without any stabilisation (load is CS).

Regards,
Milan

nah, a 3524 or even a 555 is cheaper than a transformer and takes up a fraction of the space. you just have to invest a fraction of intellectual capital to design the thing.

[falcott]

Originally posted by sek
When you have the switcher 'create' a negative output voltage, it's reference would be the old supply's ground, because this is where you want it to be negative from....

Thanks for the explanation, sek. Just the sort of logic I was looking for

Originally posted by moamps
the simplest (and maybe the cheapest) way to get negative source is by using an additional small transformer 9V/2-3VA, small bridge and electrolytic capacitor without any stabilisation (load is CS).

Thanks Milan. I agree, and yet it sounds a bit too easy. I do like a little challenge occasionally (well, it is a big one for me at present ). I will resort to that scheme if I have to, but I would like to work out a way of doing this with the original transformer and bridge if I can

Originally posted by jackinnj
nah, a 3524 or even a 555 is cheaper than a transformer and takes up a fraction of the space. you just have to invest a fraction of intellectual capital to design the thing.

See above. Thanks for your vote of confidence, jackinnj . I'm still not sure that I know what I am doing, however...

I'm looking at the possibility of using the MC34063 chip (ON Semi data sheet ). Will it be a quieter/smoother option, compared to the LM2576? Maybe the 7909 won't be so vital??

Thanks all!

[janneman]

Originally posted by falcott
[snip]Thanks Milan. I agree, and yet it sounds a bit too easy. I do like a little challenge occasionally (well, it is a big one for me at present ). I will resort to that scheme if I have to, but I would like to work out a way of doing this with the original transformer and bridge if I can [snip]Thanks all!

Falcott,

Please don't think too fast it is 'too easy'. Remember the question you started this thread with? That would be considered 'too easy' a problem by many. Don't want to discourage you, just trying to increase your chances of succes.

Jan Didden