multi polar power supply tracking IC? how about circuits? - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: May 27, 2024, 10:22:43 pm
content: Is there a IC for different polarity PSU tracking?I see circuit ideas with op-amps, but there is also some nice ICs. But they seem to have a single polarity in the reference diagrams, its unclear how to rig it to control different polarities.I want to control +/- LDOs to track voltage output till maximum. Right now it looks like a op-amp circuit needs to be tested, but I thought there might be a IC to look at.for instance, this, but it only seems to control positivehttps://www.analog.com/en/products/ltc2923.htmlAnd for circuits, it seems that there is alot of options, if you don't have a IC. I see you can isolate the feedback with a opamp and split the output with two voltage dividers. Of course that seems like something weird might happen during startup, because what is the OPA being powered from lol.In another case, I see you can isolate one of the LDO, and keep the other one 'hard wired' to the proper feedback.What is the best place to start for least grief (even if it takes more parts?)

author: coppercone2
author_rank: Super Contributor
timestamp: May 31, 2024, 05:49:59 am
content: Ima try to wire it up with a LT1001

author: dietert1
author_rank: Super Contributor
timestamp: June 01, 2024, 11:28:13 am
content: The second schematic has some easy to correct mistakes (typical web find).The transformer secondary needs a center tap connected to Gnd.The divider 407K with the 10K potentiometer can never become 1:1.The positive voltage regulator needs an output cap. A capacitor across R3 might also help.A capacitor is missing across D1.And both voltage regulators need reverse protection diodes, between rectifier and output and between output and Gnd (two more diodes).The LT1001 is just another opamp, isn't it?

author: coppercone2
author_rank: Super Contributor
timestamp: June 01, 2024, 05:50:08 pm
content: oh yeah just seem to have success making circuits with that one.. the low offset seems like a good idea for handling a virtual groundI am trying the first circuit. The second circuit has the benefit that one regulator has a real ground

author: David Hess
author_rank: Super Contributor
timestamp: June 02, 2024, 12:09:54 am
content: Most tracking circuits involve an operational amplifier to control one side, however a tracking regulator is possible as shown below.

author: coppercone2
author_rank: Super Contributor
timestamp: June 02, 2024, 12:40:38 am
content: that one is the best because there is no active thing. But you need matched parts I think (337 and 317 are similar).

author: coppercone2
author_rank: Super Contributor
timestamp: June 02, 2024, 08:50:42 am
content: damn OK it works but its VERY noisy. Like 100mV rms. When its grounded on the breadboard normally, its the normal <1mV measured. I wonder if there is something fundamentally wrong or if it just the breadboard not being friendly to sensitive virtual ground circuits on pretty high Z set regulators with long wires.I thought it might not work at all. But it is a giant feedback loop. I wonder if I should go to the PCB. I will try a few different types of op amp and see if anything changes, and play with decoupling/response. But I have a feeling it needs to have tight feedback loops

author: dietert1
author_rank: Super Contributor
timestamp: June 02, 2024, 10:58:27 am
content: Why are you calling it "virtual ground"? I think all of the above circuits require the input to be bipolar with a real ground (i.e. center tap of transformer). A LM317 circuit works as a supply but not as a load.

author: coromonadalix
author_rank: Super Contributor
timestamp: June 02, 2024, 12:02:33 pm
content: if you check the 1st image it is a virtual ground, witch is a principle i would not use or avoid, same for the second.always use an xformer with an center tap who'll become the ground point for positive and negative voltage, unless you need fully isolated positive or negative rails, and they become common if you connect a positive and negative to make it a center tap, but you're not floating anymorethe problem is often the imbalance in current consumption between the positive and negative rails, the regulators may have drifting issues since the ground may shift in one direction or the othermany smps use this trick with resistors in the 1st power stage partsor you have this self centering projecthttps://www.diyaudio.com/community/threads/problem-with-dual-tracking-power-supply.167213/

author: dietert1
author_rank: Super Contributor
timestamp: June 02, 2024, 12:17:04 pm
content: Quote from: coromonadalix on June 02, 2024, 12:02:33 pmI doubt that. In order to establish a virtual ground you need something with strong enough push/pull capability (like audio amplifier IC). Otherwise even a minor DC imbalance at the output side will disrupt all regulation.

author: David Hess
author_rank: Super Contributor
timestamp: June 02, 2024, 12:53:24 pm
content: Electronic control by itself will not be enough. The welder *must* have the inductance provided by the magnet shunt to work properly. If you do go the electronic route, then a transformer sized inductor will have to be added in series with the output.

author: coppercone2
author_rank: Super Contributor
timestamp: June 02, 2024, 01:21:44 pm
content: Quote from: dietert1 on June 02, 2024, 12:17:04 pmI think its accurate to call it that. You can call it low power virtual ground. Who decides the current level for something to be called a ground? It is approximating the voltage of the reference with a math trickSignal virtual ground. The op amp can maybe do between 5-100mA. For high impedance loads.However what it seems to be is 'noisy ground'.I dunno how much I can squash 100mV lol

author: coppercone2
author_rank: Super Contributor
timestamp: June 02, 2024, 01:34:10 pm
content: and I did this morning squash the noise. When I put in a LT1128 (g=1 stable) op amp in place of the LT1001, the noise went down to sub mV levels.I wonder if that op amp is damaged or if its just not the right thing there. Its still out of spec (400uV rms) but maybe I can do something about thatdeiter, in opamp literature, the node between the feedback and input resistor is called a virtual earth BTW. barely anything flows in there. i.e. the node tied to opamp + . If they can call a node sandwiched between two 10 meg resistors and a gigaohm input a virtual earth, I think im OK with my naming convention.

author: dietert1
author_rank: Super Contributor
timestamp: June 02, 2024, 01:53:30 pm
content: Quote from: coppercone2 on June 02, 2024, 01:21:44 pmOK, but if we are talking about a "multipolar tracking power supply" your usage of the term is misleading at best. Necessary current capability is defined by the built-in current limit of the LM317. And unless Gnd can absorb that current, the supply can go off rails unexpedtedly. None of the circuits shown includes provision to avoid that. Better use a bipolar input.

author: coppercone2
author_rank: Super Contributor
timestamp: June 02, 2024, 02:19:02 pm
content: how on earth is it going to get through 65k (or way more for modern regulators) resistance to require the op amp to sink anything close to the output current of the regulator>? anything you connect it to has a ton of resistance limiting current to nothing

author: coppercone2
author_rank: Super Contributor
timestamp: June 02, 2024, 04:50:19 pm
content: well I guess there was something wrong some where because I went through like 8 different op amps (cross range of spec and 4x lt1001) and it works on all of them pretty similar. Maybe a probe lead got loose or something. So it works okish with that.I thought basically anything should work there, and it seems to.I think the key is offset voltage and noise... maybe a chopper amplifier even might be a good idea to try. I think the LT1001 is pretty good choice, with the 100k divider. maybe lt1056?I think the feedback impedance network is too high a value for the LT1028 to be a great choice.

author: David Hess
author_rank: Super Contributor
timestamp: June 02, 2024, 06:05:42 pm
content: Noise from the reference in the regulator is higher than almost any operational amplifier, and bandwidth of the regulator is limited, so practically any operation amplifier will work, and there is little to no advantage for anything faster or lower noise than an LT1001. Even a 741 is sufficient.If you want a design with less noise and better drift, then a different circuit using an external low noise zener reference is required.

author: coppercone2
author_rank: Super Contributor
timestamp: June 02, 2024, 06:45:50 pm
content: How about chopper stabilized amplifiers ? For less drift contribution from the op amp.I have not however acutally checked what the opamp drift is compared to the LT3083. I suppose it might not be a big contribution.The reference by itself is something like 500uV/C if you ignore the resistor. So that op amp is fineeven a good 741 is 15uV/C, which is still small compared to the reference of the 3083Yeah this is just a power supply, I don't need it more precise then it is. I thought it might be more stable then it is.

author: coppercone2
author_rank: Super Contributor
timestamp: June 03, 2024, 03:17:01 am
content: I made a mistake, I had jumpers setup so I can connect the LDO to ground or the op amp, I got confused and thought it was working OK with the opamp. It acutally turns out that all the measurements are bad when I inspected by setup, basically noise with every op amp. I noticed the LT3083 does not like the breadboard, i.e. even if ground connected, adding a decoupling capacitor from set to ground increases noise by alot, in the standard configuration, meaning that its not happy, it never seemed to cause a problem on a PCBMaybe I will just see how startup looks like with unequal loads to get an idea of how that function performs. I this circuit just won't breadboard well. I don't have the patience to solder it together or make a PCB.. I think I will stick to the schottky diode to ensure the power rail does not fry power opamps in my circuit. it seems to be the cheapest and cleanest solution.I think a simpler experiment would be to just see how it does on a more simple circuit, where its grounded to a voltage follower op amp connected to ground for input, rather then any feedback circuit on a single rail.I think this virtual ground is like replacing a high quality toilet with a rickety portapotty, like setup at a construction sight up over a flooded hole on some wooden planks standing right next to a crane. the electrons just can't go there

author: coppercone2
author_rank: Super Contributor
timestamp: June 03, 2024, 04:10:29 am
content: hmm and while the tracking seems to improve the transient response, I would not call it great. There is some over shoot on the positive voltage near the end, and they dont really have the same rate of rise, but they do stabilize at close to the same time. Without it, there is no over shoot, but they still have different rise times and way longer before the rails settle. I think the LT3080 is causing problems because its a weirdo op amp with a single resistor set point. that one gives everyone loads of problems lol.. how many threads have I seen on that one? it damn near drove some engineers mad on the forum.I am almost curious enough to make a PCB with the same design to see how much it changes. But it would need to be really perfect to convince me to remove the diode from my apex power amplifier. and there might be other problems im not seeing yet, just trying to handle this on the PSU end.I think the diodes will stay! Trying to implement tracking for that circuit seems like some real psycho shit to get rid of a elegant low cost low parts count solution to power supply timing problems....... I think I have better uses for my time.