Triple Digital power supply Voltage  display on one LCD display - Page 1
Forum section: electronics / projects

author: Arrow
author_rank: Contributor
timestamp: January 23, 2013, 03:40:01 pm
content: Hello all,I'm building a triple digital power supply. The outputs are all isolated, but I like to use one big 40x4 LCD display to displays everything of these 3 supplies. As they are isolated I can not use a common ground. I was wondering how they do this in real life. I was thinking of optic communication. Do anyone have an idea?greets.

author: ve7xen
author_rank: Super Contributor
timestamp: January 23, 2013, 07:27:27 pm
content: It really depends on the topology of your power supply what the best way to handle this will be. I2C is a bit tricky to isolate because it's bidirectional on all ports. I'd probably do this by using a 2 channel SPI DAC + 3x optos (CLK, MOSI and CS) for each channel and running it at relatively low speed. But there are lots of other ways that will work.

author: c4757p
author_rank: Super Contributor
timestamp: January 23, 2013, 08:52:44 pm
content: I2C is a huge pain in the ass if you're doing anything different from a single one-voltage, non-isolated bus. Out of all the various projects I've done, I think I had one occasion where I found I2C useful without some jury-rig. (Not counting the times I've used a single I2C chip, because it's not really a bus then, is it? Much easier in that case.) What I would personally do depends on whether I'm looking to save BOM cost or development time. If development time, I'd isolate SPI with optos. If BOM cost, I'd use a unidirectional slow software UART (just one opto per channel, a slow software version so I don't need separate UART transceivers per channel). Bidirectional if you need central control as well as display, and you still save three optos per unit. But it would take a good bit more time to write the code for that.

author: M. András
author_rank: Super Contributor
timestamp: January 23, 2013, 09:09:21 pm
content: http://www.digikey.com/scripts/DkSearch/dksus.dll?site=us&lang=en&v=336&c=57&WT.z_supplier_id=336&WT.z_page_type=SP&WT.z_page_sub_type=SS&WT.z_oss_type=Category&chp=0what about these? not exactly for bidirectional fairly fast/cheap

author: c4757p
author_rank: Super Contributor
timestamp: January 23, 2013, 09:16:32 pm
content: Those are nice. In small quantities, a two channel digital isolator barely costs more than an opto, is much faster, and much easier to use. I amend my suggestion to recommend digital isolators unless you are going to be making a bunch of these (optos get cheaper in quantity). I've used them before, but I still don't always think of them.

author: Arrow
author_rank: Contributor
timestamp: January 23, 2013, 10:12:39 pm
content: Quote from: c4757p on January 23, 2013, 08:52:44 pmWhy do I have to use slow UART? You mean the opto's are slow?

author: Arrow
author_rank: Contributor
timestamp: January 23, 2013, 10:13:59 pm
content: Quote from: M. András on January 23, 2013, 09:09:21 pmThese look great!!! and I did not know of this RF isolators.

author: c4757p
author_rank: Super Contributor
timestamp: January 23, 2013, 10:20:56 pm
content: If you do it slowly it's easier to do it entirely in software using GPIO pins. If you're using a microcontroller with three UARTs you're paying too much for a microcontroller.

author: M. András
author_rank: Super Contributor
timestamp: January 23, 2013, 10:39:37 pm
content: Quote from: Arrow on January 23, 2013, 10:13:59 pmfound these while looking for isolating spi/parallel adc -s from the main controller while able to keep it at max speed to solve my own ideasbtw im courious about what the professionals say about these chips

author: Arrow
author_rank: Contributor
timestamp: January 23, 2013, 11:56:55 pm
content: Quote from: c4757p on January 23, 2013, 10:20:56 pmLike I said before it will be a triple power supply. Each supply will have their own uC. So in total it will be 4 uC. This last uC will be the master that will read the inputs and also control the display. I'm planning to use 1 single UART for all and use multiprocessor communication recognition code. So the master will broadcast the code for all, but only the assigned uC will talk back.BTW the power supply will be 3x 0-50VDC /6A over the entire range. And it will be a mix supply. I used a switching power supply in the first stage. This SMPS supply delivered power to the second stage linear power supply. The linear power supply controls the switching power supply and controls in this way the Vdrop for the linear which will be a constant 5V. In this way I could decrease my series pass transistors to only 2 for the entire range. I preferred the linear PS for the noise, and I use the switching ps for the efficiency.@M. Andrásyes, they look great, I have already put them in my digi-key cart ready to order.thanks

author: MikeK
author_rank: Super Contributor
timestamp: January 24, 2013, 12:46:37 am
content: Why can't you run the outputs through scaling op-amps?

author: ve7xen
author_rank: Super Contributor
timestamp: January 24, 2013, 12:49:14 am
content: Quote from: Arrow on January 23, 2013, 11:56:55 pmThis can work, you'll still need two optocouplers (or isolators) per isolated supply. It's a bit trickier to do right though.QuoteSounds interesting. Posted it anywhere?QuoteThe three outputs are supposed to be isolated from one another (and mains earth).

author: Harvs
author_rank: Super Contributor
timestamp: January 24, 2013, 05:50:46 am
content: QuoteJust remember that if you're planning on joining all the reception lines into a bus it needs to be an open-drain type arrangement with common pull up (or something like that). I.e. the digital isolator referred to I'm pretty sure drives in both high and low direction, so you can't just join the outputs together.So you're building a switching pre-regulator. Are you building a triple off-line isolated switching supply with independent isolated channels? That's not exactly straight forward!

author: ptricks
author_rank: Frequent Contributor
timestamp: January 24, 2013, 11:34:08 am
content: It seems you might have a use for something that I came up with a while back. I needed to do isolated communications in a small area where I needed to send from one board to the others and then have them send data back when it was their turn. I used IR LED and those IR detectors used to read things like tv remote controls. The sensors have all the circuitry needed and are cheap and reliable. Each board has a detector. The main control board pulses out the data using IR LED, the detectors are so sensitive that in a space, like inside a PSU case, you don't need to worry about line of sight to send /get data. After the sub board is triggered it can process the data and send it back using another IR LED. It is a very flexible system and low cost.Essentially this is what opto-isolator do but this makes it so that you can broadcast data from a main board using light to broadcast to multiple targets instead of having to wire up each sub board to the main board, a lot easier to design on a pcb as well since you can put the send/receive parts anywhere.

author: jeroen74
author_rank: Frequent Contributor
timestamp: January 24, 2013, 11:42:19 am
content: QuoteYou could use a regular and cheap optocoupler for this. I don't think this whole system requires that much speed, something like 300 to 1200 baud should be sufficient. I would use the UART with the RX and TX combined into one open-drain line.

author: Arrow
author_rank: Contributor
timestamp: January 24, 2013, 12:46:24 pm
content: Quote from: Harvs on January 24, 2013, 05:50:46 am

author: Arrow
author_rank: Contributor
timestamp: January 24, 2013, 01:24:19 pm
content: Quote from: AcHmed99 on January 24, 2013, 12:57:21 pmWhat topology are you using? I'm using a switching pre-regulator as well 300W x2. I 'm in the process of testing a scaled down version 30W. I elected to use a flyback topology 3 converters interleaved 100W per phase.There are three separated switching power supplies of 300W, connected to three separated linear PS. Am I missing something here?

author: Arrow
author_rank: Contributor
timestamp: January 24, 2013, 01:33:33 pm
content: Quote from: jeroen74 on January 24, 2013, 11:42:19 amYou are right, but the testing of the digital isolators RF modulator are also nicer.When the times arrives I will evaluate the cost the space the package (smt or through hole) and the signal edge.

author: jeroen74
author_rank: Frequent Contributor
timestamp: January 24, 2013, 01:42:45 pm
content: Something like a TLP281 is quite small.

author: Arrow
author_rank: Contributor
timestamp: January 24, 2013, 02:19:04 pm
content: Quote from: AcHmed99 on January 24, 2013, 01:30:15 pmSo , you are building a PS aswell, and you are using interleaved converters. great!!!I'm using the forward conv. But the switching part is already done. I'm not worry about that. I am busy with the design for the linear part. Finding op amp that can handle rail to rail with single 60V supplies and 0V offset ect wish me luck !!!

author: jeroen74
author_rank: Frequent Contributor
timestamp: January 24, 2013, 02:21:15 pm
content: Why no circumvent the RR problem with a negative rail and higher rail voltage?

author: digsys
author_rank: Unknown
timestamp: January 24, 2013, 02:32:37 pm
content: If you don't need super high accuracy, I've used this Linear opto configuration many times.Saves having to put a micro on every PS stage. There are a few types of optos around, just check specs.

author: Arrow
author_rank: Contributor
timestamp: January 24, 2013, 02:36:31 pm
content: Quote from: jeroen74 on January 24, 2013, 02:21:15 pmYes, I do that, but then I have to connect the DAC en uC also to the negative supply and then I will end in an endless virtual circle. That's why I think 0V offset is best. I gonna check if I can send a schematic, it will be easier to understand. The schematic can change anytime it is still in the design stage. But let me give some specs. I'm using the simple LM317HVT regulator. First I was planning to use the TL783 it has a better Line regulation ect. but the datasheet tells me that it can not draw so much current unless it has a higher Vdrop ( I didn't like that) when the LM317HVT can draw higher current at lower Vdrop.

author: jeroen74
author_rank: Frequent Contributor
timestamp: January 24, 2013, 02:40:26 pm
content: I once emulated that kind of circuit, by using an extra optocoupler of the same type as a temperature compensation element inside the feedback path. Without it, the output pulse shape was very temperature dependant (not good if you use PWM and filter this to get a DC voltage); with the new circuit the pulse shape was almost completely insensitive to temperature.If you google for 'optocoupler temperature compensation' you should find the simple circuit.

author: Arrow
author_rank: Contributor
timestamp: January 24, 2013, 02:53:23 pm
content: What do you think of this?http://www.ti.com/lit/ds/symlink/opa547.pdfOne more thing, once I get all the opto's signal, I have to do a constant scan on the ADC to refresh the output voltage and the current. But I think it can be done.

author: Arrow
author_rank: Contributor
timestamp: January 24, 2013, 07:41:52 pm
content: Quote from: digsys on January 24, 2013, 02:32:37 pmGreat schematic, I will take a look!! Thanks!!

author: Harvs
author_rank: Super Contributor
timestamp: January 25, 2013, 06:01:04 am
content: Quote from: Harvs on January 24, 2013, 05:50:46 amMaybe you design off-line switchers for a living or something, I don't. Having read a few of Keith Billings' texts on switching supplies while making some car amp psu's, I just understand there's a whole pile of stuff to consider with respect to safety isolation and how the transformers need to be wound etc so you don't kill anyone. Hey maybe I'm over thinking it, maybe there's some straightforward recipes for designing off-line switchers, I've just never tried it.If you were willing to post some schematics, PCB layouts and transformer design info that'd be really nice to look at.

author: Arrow
author_rank: Contributor
timestamp: January 25, 2013, 12:46:35 pm
content: You are completely right. But in the past I has design some small switching power supply with the uc3842 /TL431 and other controllers. But the switching PS ( 1st stage) I am working on right now is not my design, I bought them some years ago. They gave fixed 50V. But I tear them apart like Dave said and hack the schematic so now I can control the voltage from 8 to 58V, just what I needed. It is the linear part that I'm going to design. I'm doing this just for fun and to test my electronic skills with DAC and ADC and constant voltage on regulator controller ect. I already have a power supply the BK9130. works good or better very good!!