Variable Bipolar Power Supply

[audioguru]

I recalculated to 240 Ohm R and 2k2 potentiometer so the output voltage will be 1.25V - 12.7V.
Now I don't have to worry about the minimum load current?

240 ohms is used with the more expensive LM117.
120 ohms is used with the LM317.

You might not find a 1.1k pot so use a 100 ohm resistor and a 1k pot for an output voltage of 1.25V to 13.75V if your input voltage is as high as at least 16.25V.

 

[mrx23]

I found a better: 100 Ohm + 10 Ohm (series) and 1k pot: 1.25V - 12.61V.

What difference makes 120 Ohm instead of 240 Ohm in an LM317 circuit?
There are a lot of LM317 sample circuits in the datasheet with 240 Ohm resistor.
Thanks!

 

[Roff]

I found a better: 100 Ohm + 10 Ohm (series) and 1k pot: 1.25V - 12.61V.

What difference makes 120 Ohm instead of 240 Ohm in an LM317 circuit?
There are a lot of LM317 sample circuits in the datasheet with 240 Ohm resistor.
Thanks!

Whose datasheet are you looking at? The National datasheet has LM117 in the examples. LM117 has a minimum load current of 5mA. LM317 has a minimum load current of 10mA.

 

[audioguru]

What difference makes 120 Ohm instead of 240 Ohm in an LM317 circuit?

With a 240 ohm resistor and an LM317 that has a max minimum load current then the output voltage will rise without a load. The voltage regulation will be useless.

 

[mrx23]

I had a National 96' pdf, I downloaded the newest 07' one.
Sorry for that, I will use 120 Ohm.
Thanx!

 

[picbits]

I did a simulation with the LM317 and a TL074 opamp driving the adj pin with a 120R resistor between the output and adj. I got very good results (simulation wise) and a constant 10.5ma flow from the ADJ through the TL074 over a 0-24v regulation.

Does this 10.5ma count as a load current ? If I omit the 120R resistor then the circuit still simulates without a problem but the current flow through the ADJ pin is almost non-existent.

 

[Roff]

I did a simulation with the LM317 and a TL074 opamp driving the adj pin with a 120R resistor between the output and adj. I got very good results (simulation wise) and a constant 10.5ma flow from the ADJ through the TL074 over a 0-24v regulation.

Does this 10.5ma count as a load current ? If I omit the 120R resistor then the circuit still simulates without a problem but the current flow through the ADJ pin is almost non-existent.

The 317 is not smart enough to know (or care) where the output current goes, so yes, it counts.
I ran a sim with the LM317 model I have, and swept the value of Rref (the resistor from out to adj). The reference voltage started to change drastically when Rref was greater than about 1k. With an op amp in the feedback loop, this probably won't cause a problem unless the op amp saturates at the negative rail.

 

[Krumlink]

I have never checked out this project till now, and I would have to say that it looks really good hero999! The Dual rails is pretty cool.

 

[DanM]

I am looking for a schematic for an independent/tracking dual power supply 0 to 30 Volts 0 to 3 Amps. Can anybody help me. please

 

[picbits]

What has this power supply thread got to do with stepper motors ?

 

[HellTriX]

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.

 

[Hero999]

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.

 

[tswan]

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.

 

[Hero999]

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.
**broken link removed**
0 to 13.8V LM317 Power Supply

 

[tswan]

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

 

[Hero999]

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.

 

[tswan]

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

 

[MrAl]

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?

 

[Hero999]

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.

 

[mcbmcb0]

just checking...

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!