Great project Hero. Was nice to read a few years worth of discussion on it. I even learned a bit on these voltage regs I've been using.
Was getting some nasty ringing and instability when driving a PWM mosfet with an LM7812A. I will need to build a fairly stable 12-15v 1-2A supply for my drive circuitry soon. After I address the Mosfet output stage setup for my PWM motor controller.

Anyway, thank you guys for this good discussion.

 

Here's a picture of the inside of the bipolar PSU.

I suppose it could be neater but you don't normally see the inside.

Attached Thumbnails

 

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Hello Hero,

You guys are far more advanced than myself, so please forgive 2 ignorant questions:

1. Am I correct that this supply positive and negative voltages track?
2. What purpose does the diode parallel to the 220 ohm resistor near the LM217 serve?

I have been goofing around with some simple designs and was wondering how difficult it is to build a bipolar supply that permits zero crossings. If anyone wants to educate me on what's required to build a supply that permits continuous voltage adjustment from negative to positive I would be appreciative.

Thanks.

 

1) Yes the negative voltage is always equal to the positive voltage setting.

2) The diode protects the regulator by discharging the capacitor if the regulator is short circuited.

What do you mean by zero crossings?

This supply enables adjustment down to 1.25V.

It's possible to build an LM317 regulator which works down to 0V, here's a design, but it's positive only. It's possible to incorporate this into the bipolar design so it works down to 0V. In which case you could use a dual op-amp such as the LM1458 rather than two µA741s.

0 to 13.8V LM317 Power Supply

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Hello and thanks for the answers.

By zero crossings I mean a supply that can force negative and positive voltages on the same channel. This is nothing I really need, I was just thing about it...

The thought I had was to create one supply that generates a constant voltage of negative 15 volts and another supply (completely isolated from the first by a 2nd transformer) that is variable from 0 to 30 volts. By connecting the 2 supplies together you can vary the output from -15 to 15 on a single channel with one pot. I breadboarded this and it seemed to work, but since these voltage regulators are not like simple batteries I'm not really sure if stacking them like this is permissible.

As I said, I'm just experimenting with this. If anyone has any input or thinks this is a foolish idea please let me know. I just thought it would be convenient to have a dual channel supply that could force positive or negative voltages from either channel.

Thanks

 

It's possible to create a supply that can sweep from +15V to -15V but the way you've described won't work for loads above 1mA at the most.

The trouble is that these regulators can only supply current, they can't accept it. In other words an LM317 set to +10V can push current into a load connected to 0V but it can't accept current if the load is connected to +15V.

The correct way to do this is to build a push-pull amplifier (e.g. an audio amplifier) and supply the input with a DC reference voltage ranging from +15V to -15V. Providing the amplifier is all DC coupled there shouldn't be any problems.

How do you get the reference voltage?

Easy, use a centre tapped transformer with the usual bridge rectifier and connect a 15V zener, in series with a suitable resistor, from the positive and negative to 0V.

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Excellent. Thank you for pointing me in the right direction.

 

Hi,


For what it is worth, i was able to simulate the original dual tracking circuit and
found that the controlling op amp needed some additional compensation.
A 0.01uf cap seemed to work and provided a clean output free from noise.

Just wondering though, did anyone actually build up this circuit yet and test it
out with a scope? If so, what tests were done? Was any comp installed?

 

I built it.

I tested it with a MOSFET and resistor connected to a signal generator to give a load transient. I viewed the waveform on a scope.

The transient response wasn't perfect but it was just as bad on the negative side as it was on the positive side.

No compensation capacitor was used, simulations don't always provide an accurate representation of reality. There again my test with one LM217 and an LM337 doesn't prove it's totally stable either as components vary in their characteristics.

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Hi Hero999
i'm building a similar thing and have eagerly read the whole thread. glad its still alive. my knowledge is basic and rusty. Can i check:
1. is the verison you actually built from the schematic on the first page of this thread? as far as i can tell nothing changed except for the debate about compenstaion (=100nf or 10nf from the op amp's out to inverting input, that you didn't need to add anyway) and the 120ohm (out to adjust) for lm317.
2. any thoughts about adding adjustable current limiting to this? I've previously tried (years ago) with sensing the voltage across a 0.1ohm resistor on an NPN emmitter follower (on the +ve channel) and that eventaully driving a simple transistor across lm317's adjust to ground. as I recall it i had the risk of osciliation and also no longer have a scope to check. also i never got the -ve rail current regulated successfully. any sage advice here or links to other schematics??
3. eng1 made the suggestion of a reference voltage driving a +ve and -ve op amps to drive each regulator at the adjust pin. that may be easier to current limit. but the suggestion fell off the thread. any quick thoughts on its merits?

thanks!

 

Hi again,

Yes, i find that simulations dont always equal real life, but what else
i find is that the simulations often tell me a lot about the theory of
the circuit, in that once it is working in theory there is often a way to
get it working in real life with some minor modification unless the
circuit is too far from real life to begin with.
For my simulation i used op amps that had good models that model
their frequency characteristics quite well. I did have to simulate
the two ICs though, with op amps and transistors.

Just in case you are interested, there might be a way to replace the
op amp with transistors and that may greatly improve the performance
in the real life circuit.

 

Try it without the capacitor first and add it if you have any problems.

If you change to 120R, you'll need to adjust the value of the pot or it'll give 27.4V out.

For 15V the pot will need to be about 1k3 so try finding a pot in this value.

Change the 120R to 82R and use a 1k pot, this will give 16.5V out or use 91R instead of 82R for exactly 15V.

For adjustable current limiting use a a separate transistor current limiter in series with each regulator. Of course the current limiting for each side will be separate so use a dual ganged pot if this is a problem.

Apart from building your own op-amp I can't think of anything so post your idea.

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Hi again Hero,


Ok, i'll post something soon.

In the mean time, isnt it true that that 10uf cap screws up the transient
response? In other words, did you try the real life circuit without that cap
or rather with that cap significantly reduced to say 1uf instead?
Yes, the ripple isnt as good, but that's life :-) use more filter caps.

ADDED LATER:
I found out that the two transistor op amp replacement works for tracking
with a single set point (like plus and minus 10v for example) but as the
voltage is adjusted the two voltages (plus and minus) go out of sync
a little. Thus, reducing that 10uf cap and using a high speed op amp might
be the best way to go, or possibly even simply lowering that 10uf cap.

 

I don't see how removing C3 would improve the transient response; how could it slow down the speed of U1's output transistor?

I'm pretty certain that a high speed op-amp won't make any difference because the voltage at U2's ADJ pin should remain constant during transients. The op-amp's output only needs to change when the power supply is adjusted which happens very slowly.

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Hello again,


No, C3 only slows the turn on transient.

When do you consider the (steady state) transient to be bad?
Did you try it with one LM317 alone and compare to the dual circuit?