Author Topic: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)  (Read 358853 times)

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Offline Tazz

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #675 on: April 05, 2017, 11:38:53 am »
Simply bigger res with same TCR will have lower "to ambiant" thermal resistance.
Two for four bigger sized res with big coper area for R65 could minimise the effect.
Otherwise, the best approach is to have a T sensor near the res (below, through a pcb hole ?) and calibration (adc correction to apply vs temp)
 
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Offline prasimix

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #676 on: April 05, 2017, 01:00:48 pm »
Adding at least one more resistor (R65A) shouldn't be a problem. There is enough space on existing PCB layout. It should looks like this:


Offline prasimix

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #677 on: April 05, 2017, 01:32:37 pm »
... and even third (R65B) can be placed :)



Now, one can add third and fourth current range (50 mA, 5 mA) and we have a new power board revision  :phew:.

Offline texaspyro

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #678 on: April 05, 2017, 03:30:37 pm »
You don't really need a temperature sensor.  You can probably do some software based compensation.  You know the voltage across the resistor and can calculate the power it is dissipating.  From that you should be able to come up with an approximate temperature and compensation factor.  It's far from perfect, but should offer an improvement over an uncompensated value.
 
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Offline Tazz

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #679 on: April 05, 2017, 04:07:31 pm »
With acceptable power dissipation as now with multiple R in // that is exactly what I have in mind.
But you still need to have an in case ambient temp measurement (which seems to be the case) at which you will apply the deltaT
 

Offline prasimix

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #680 on: April 05, 2017, 04:08:18 pm »
Firmware based compensation has to be pretty smart. For example let's say that we enters CC mode with output current set to 5 A. At the beginning it could be 5.018 A and it will gradually decreased to 5.000 A (± 1 mA depending of how accurate calibration was), a -0.36 % change. But if output current is set to new value e.g. 1 A, initial current will be first lower e.g. 0.991 A and will start to increase (as resistor is cooling down) to 1.000 A. That is +0.91 % change that has to be taken into account. 

Offline prasimix

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Temperature compensated current measurement
« Reply #681 on: April 09, 2017, 07:14:18 am »
I didn't have a lot of time to search around to find out how current measurement could be temperature compensated directly in analog circuit, and wondering if you have something in mind. Possibly such kind of question is more suitable for metrology section of the forum, but just to be in line with what is recently discussed I'm asking it here.
My first guess is that NTC could be introduced in parallel with either R60 or R70 (see sheet #3) to affect (decrease) IC7 gain as R65 is start to heat up. Mentioned NTC could be in fact consists of two resistors connected in series one ordinary and another NTC to easier find a proper ratio.
Another question is where to place NTC: on top layer near (side by side) the R65 or on the bottom layer beneath R65?
Does it represent too simplistic approach, or something that could be deployed for level of accuracy that we are talking about here (i.e. to have up to ±1 mA accuracy on the full scale of 5 A)?

Offline voltsandjolts

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #682 on: April 09, 2017, 01:36:06 pm »
You could add a bunch of thermal vias right inside the sense resistor pads to pull some heat through to a copper area on the underside of the board. I would mount the SMT thermistor directly under the sense resistor(s), on other side of PCB.

You could go for off-board sense resistor to allow for much better heatsinking but that adds assembly costs I guess.

Edit: 1mA accuracy on 5A FSO == 0.02% would be hard, do you mean 1mA accuracy on your low amp range setting?
« Last Edit: April 09, 2017, 01:41:28 pm by voltsandjolts »
 

Offline prasimix

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #683 on: April 09, 2017, 02:52:46 pm »
Edit: 1mA accuracy on 5A FSO == 0.02% would be hard, do you mean 1mA accuracy on your low amp range setting?

You're right that is 0.02% but that was just my guess. Currently I have 18 mA on 5 A (-0.36%). If we can cut that by 10, down to about 2 mA, that will be good achievement.

Thanks for pictures for that power resistors. Such resistor alone properly cooled could be sufficient to decrease temperature drift significantly, but I'm afraid that is something in range of 20 EUR/USD in qty of one?

Offline voltsandjolts

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #684 on: April 09, 2017, 03:06:12 pm »
To get that accuracy you will need to do some software linear calibration procedure for current measurement so you can just use the 5% accuracy types (circa 2 GBP) or if you want 1% they are a little more.

https://www.digikey.co.uk/products/en/resistors/through-hole-resistors/53?k=Current+Sense+Resistors&k=to220&pkeyword=Current+Sense+Resistors&FV=ffe00035&mnonly=0&newproducts=0&ColumnSort=1000011&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25
 

Offline prasimix

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #685 on: April 09, 2017, 03:13:01 pm »
Resistor tolerance here is not an issue. It could be anything since software calibration will rectify that to the great extent. I have an issue with TCR, and resistor what I'm currently use has 75 ppm/K and for example suggested PWR221T-30-R020J has "massive" 600 ppm/K but that could be misleading since with its TO220 package I can easily mount it on the pretty big heatsink. Hopefully that can give us effective TCR below 10 ppm/K.

Offline voltsandjolts

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #686 on: April 09, 2017, 03:42:03 pm »
Yes, good point that 600ppm/K is a poor TC but as you say TO220 heatsinking might make it an improvement over SMT 75ppm/K

How about these 15ppm SMT:

https://www.digikey.co.uk/product-detail/en/stackpole-electronics-inc/CSSH2728FT20L0/CSSH2728FT20L0TR-ND/1923231

Edit: Sorry that was 1K quantity pricing, unit price is 1 GBP:
https://www.digikey.co.uk/product-detail/en/stackpole-electronics-inc/CSSH2728FT20L0/CSSH2728FT20L0CT-ND/1923243
« Last Edit: April 09, 2017, 03:43:54 pm by voltsandjolts »
 
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Offline prasimix

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #687 on: April 09, 2017, 03:50:17 pm »
Wow, 5x better TCR for bargain price. Sounds fantastic! I wish I have some of them before.

Offline ebclr

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Offline prasimix

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #689 on: April 12, 2017, 06:12:36 am »
A little expensive, but better

https://world.tmall.com/item/543882232437.htm

This one is still 15 ppm (link), and cost a fortune, possibly due to exposed 4-wire terminals.

Anyway, it seems that affordable low TCR resistors exists, remaining question is what about adding NTC for temperature compensation? In that case we'll have more freedom in selecting current sense resistor with theoretically and TCR.

Offline Tazz

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #690 on: April 12, 2017, 08:50:38 am »
Passive compensation with a NTC would be very elegant but need a lots of tests and validations to select the good parts.
Big affordable low TCR resistor will already give excellent results and necessitate a this stage no more development (apart small routing mods).
If as a later option you want to be able to squeeze the last precision/compensation from the measurement, just expose an ADC input somewhere and a way to mount (a hole) a little PCB with a spacer on top of the resistor(s). It would give the possibility to add a fast thermocouple junction and it conditioning circuit.
Yes, this is a not very elegant big hammer, but with max reproducibility and result.
 

Offline Teemo

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #691 on: April 12, 2017, 10:03:25 am »
Hello all. I was just stopping by and see a nice discussion here. I was beginning to think maybe there are some more alternative ideas how to compensate in real time for the shunt temperature drift.
 The one idea that comes to my mind for example is periodic real time recalibration with cool(unheated) shunt. Lets say in about every 1s. The extra shunt would be turned on for only a small amount (few ms) of time to prevent it heating, idle times it be bypassed with MOSFET(those are dirt cheap these days).
 
 

Offline Liv

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #692 on: April 12, 2017, 10:32:58 am »
If you look at the best branded PSU, there is nothing done to compensate for the drift of the shunt resistance. Use good resistors, and most importantly, with a large margin of power.
PSU PSL-3604 Pulse gen. PG-872 Freq. cnt. FC-510
 
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Offline Electronix

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #693 on: April 23, 2017, 06:57:35 pm »
Hello prasimix,

i can confirm what liv is saying.
Please have a look at the screenshot from my power supply.
I enter for example 3,444 amps and you can see on my keysight it
Is very stable also for a longer time period.


Best regards

Sven
« Last Edit: April 23, 2017, 07:02:00 pm by Electronix »
 

Offline prasimix

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #694 on: April 24, 2017, 07:36:58 am »
Thanks, I got the point. What type and how many resistors you have in your design?

Offline Electronix

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #695 on: April 24, 2017, 06:43:21 pm »
Hi prasimix,

I used one of the following resistors :

https://www.conrad.de/de/messwiderstand-002-10-w-l-x-b-x-h-22-x-4-x-17-mm-isabellenhuette-pbv-002-1-st-447374.html

For shunt measure I used an op27.

Cheers

Sven
 

Offline prasimix

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Re: DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005)
« Reply #696 on: April 25, 2017, 06:47:51 am »
Ok, that's a real stuff both by price and capacity (10W!). It cannot be simply used in my design.

Offline Rerouter

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I'm guessing now that your campaign has ended there is no way to order one until the next batch?
 

Offline prasimix

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I'm guessing now that your campaign has ended there is no way to order one until the next batch?

You're right (if next batch happen).

Offline Helix70

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Any chance of an update? I can't wait to get mine!
 


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