Conversion of resistance based Adjustable Switching Power Supply to PWM based  - Page 1
Forum section: electronics / projects

author: gnvrvikram
author_rank: Newbie
timestamp: January 19, 2019, 02:45:45 pm
content: I have a switching mode power supply and it can be adjusted from 0 to 12 V DC by adjustable pot. Now I want to control using PWM signals from Arduino. Please let me know how to do that. What are the necessary changes that I need to do for the switching mode power supply for this?The link for the product is:https://www.amazon.in/gp/product/B07MPPTMLY/ref=ox_sc_act_title_3?smid=A1Z1J4ZDNUBVP8&psc=1

author: Zero999
author_rank: Super Contributor
timestamp: January 19, 2019, 03:50:30 pm
content: My advice is don't. This power supply isn't suitable for controlling with PWM or an external voltage signal. If you can get hold of a schematic for it or reverse engineer it, then you stand a change, but I suspect if you could do that, you wouldn't be asking this question, in this manner.Get a power supply which a digital or analogue remote control capability. There's one where I work, but I don't have the make and model number to hand. If you reply to this thread, I'll have a look on Monday.

author: gnvrvikram
author_rank: Newbie
timestamp: January 19, 2019, 06:46:12 pm
content: Please let me know if you can get the details.My thought is that pot there is serving the purpose of the voltage divider, if not wrong. If not PWM, if I can get the maximum voltage between the terminals of the POT, I think I can use the analogue voltage output of the arduino. Why I am asking the question is that I do not want to damage this power supply. If I want to buy a new one that can be controlled with PWM, in this current range, the cost is going to be too high about 6-7 fold increase. Please suggest any alternate low cost solution.

author: iMo
author_rank: Super Contributor
timestamp: January 19, 2019, 07:29:43 pm
content: The first step in your effort would be to get the schematics of your power source unit..

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 19, 2019, 07:44:03 pm
content: Assuming it is a voltage divider referenced to output or to Vref, it should be a simple matter of setting it to a mid-range value (this depends on which type was used, and whether there are other resistors around it setting the range), and tying a resistor from the wiper or error amp input, to a DAC's output, which is programmed by Arduino. The DAC can be a filtered PWM channel.Again, this requires knowing what the circuit is. If it's completely different, this may not be possible.Tim

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 19, 2019, 08:00:18 pm
content: Without the schematic it's kinda pointless as said above. You could always try and reverse-engineer it, but anyway, depending on how this PS is designed, it could even be a safety hazard. Be careful.My suggestion: use this PS (or any other one) as the input power source of a DC-DC buck converter which could be easily controlled with an external voltage. Then you have a safer solution and one that can be reused in many other settings. You can start here for an idea how to do this:https://www.fischl.de/dcdccontrol/

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 19, 2019, 10:59:47 pm
content: Did anyone notice the big dents in the casing of that Amazon power supply brick?I am somewat surprised you can buy these with 0-12V adjustment.I've seen some schematics of these, and there tend to be multiple switched mode power supplies in these and there is no intuitive "0".The Fischl tutorial is a nice introduction of externally influencing the voltage of a (small) smps, and it is also a safe way to experiment with.With a bit of filtering, and maybe an extra opamp as buffer (LM358) you can also use PWM.A long time ago Tuxgraphics had a nice idea.He used a 4 or 5 bit ladder R-2R DAC and added PWM to the least significant bit.This has a bunch of advantages:1). PWM has inherent linearity.2). R-2R network already attenuates the PWM ripple.3). With a few capacitors in the R-2R network you can add additional filtering. (Bit weird he did not do that).

author: ArthurDent
author_rank: Super Contributor
timestamp: January 20, 2019, 12:40:31 am
content: Doctorandus_P has a right to be surprised to see a supply like this be 0-12 volts. If you look closely at the photos and check the specs which say there is a fan and LCD, you'll see the rating label doesn't say it is a zero to 12 volt supply but a 12 volt supply that probably has a +/-5% adjustment range. The "LCD" mentioned is a paper label that looks something like a 3-digit LCD meter but is just taped on the cover. There also is no fan. This looks like a scam.

author: gnvrvikram
author_rank: Newbie
timestamp: January 20, 2019, 09:45:35 am
content: I am need of a variable DC power supply to control the voltage in the range of 0-25 V and 0-20 A. I am looking for a low cost solution.I also got another thought.... Could you please let me know if I put these DC-DC buck converters in parallel, would they be able to deliver me 20 A of current?(https://www.amazon.in/Robodo-MO12-Lm2596-Converter-Module/dp/B073Q2HY3T/ref=pd_sbs_328_4?_encoding=UTF8&pd_rd_i=B073Q2HY3T&pd_rd_r=f96efd71-1c9b-11e9-bf71-8556915635fa&pd_rd_w=ATwix&pd_rd_wg=ncchp&pf_rd_p=9fc668a0-2aac-4fb6-970f-606919bc0185&pf_rd_r=VDRV4SBB72N09EJ0398H&psc=1&refRID=VDRV4SBB72N09EJ0398H?

author: Zero999
author_rank: Super Contributor
timestamp: January 20, 2019, 06:45:50 pm
content: Quote from: gnvrvikram on January 20, 2019, 09:45:35 amRegulators tend not to parallel very well.The schematic is probably very similar to that on page 26, figure 35 on the data sheet, but with the potentiometer replacing R1 and R2: how are you going to control many of them simultaneously?http://www.ti.com/lit/ds/symlink/lm2596.pdfWhat's your budget?

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 20, 2019, 06:54:44 pm
content: Quote from: gnvrvikram on January 20, 2019, 09:45:35 amDo you need the full current range over the full voltage range? 500W?I would start with more humble specs if I were you. You probably don't want to burn your house down or something.The linked module can be found everywhere on eBay, it's almost a "classic". This would be good to start a project such as what we suggested above, it's very easy to hack, but it's rated for 3A max. And you should definitely add some heatsink if you're using it at full power. But it won't get you anything near what you required above.Paralleling DC-DC converters is usually not a good idea.You can find 200W DC-DC converters on eBay but they are usually much more complicated than this one and will be a lot more difficult to hack especially if you don't have any schematic.

author: gnvrvikram
author_rank: Newbie
timestamp: January 21, 2019, 03:48:44 am
content: I am looking for a microcontroller controlled Variable DC power supply that can supply me voltages in the range of 0-25 V and 0-20 A. I will not be using at full power continuously, may be for a minute or two. I need to sweep voltages from 0-25 volts repeatedly.

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2019, 09:35:33 am
content: Lots of bench top power supplies have these sorts of features, but aren't cheap. You need to understand that this sort of thing will cost money.Here are a couple of examples I found on RS Components.https://uk.rs-online.com/web/p/bench-power-supplies/1752542/https://docs-emea.rs-online.com/webdocs/1653/0900766b81653c32.pdfThis one is cheaper but has a USB interface and is controlled using proprietary PC software.https://uk.rs-online.com/web/p/bench-power-supplies/7568821/https://docs-emea.rs-online.com/webdocs/138c/0900766b8138cf36.pdfShop around, no doubt they can be found for less elsewhere.

author: gnvrvikram
author_rank: Newbie
timestamp: January 21, 2019, 12:15:49 pm
content: Thanks for letting me know of the available supplies. I am from India, and I could not find one that is suiting my needs or they are costly. I am looking for some solution under INR 10K.Now I am trying with the solution with the Buck converters with low current output. Meanwhile I will try to get high current Buck converters. I shall post back once I find the solution.Thanks and regards.

author: IconicPCB
author_rank: Super Contributor
timestamp: January 21, 2019, 01:04:08 pm
content: I am not sure just what INR10K buys in India but...Meanwell have voltage controlled switchmode supplies which will readily respond toa floating control voltage of 0 to 5Vto produce 20% to 110% variation in output.They also have powersupplies which respond to resistance value.The trick is to use PWM outptu from Arduino and drive an LED LDR coupler and use variation in LDR value to controlthe power supply.LDR =light dependent resistor.The other option is to use a PWM signal through an optocoupler and then filter to obtain DC to control the powersupply.

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2019, 02:05:32 pm
content: The trouble is he wants to go down to 0V.One of those power supplies plus a linear regulator would do, but will probably be over-budget.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 21, 2019, 04:24:56 pm
content: Quote from: gnvrvikram on January 21, 2019, 03:48:44 amHow fast?

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 21, 2019, 04:48:40 pm
content: should be good for your INR10K...https://www.ebay.com/itm/Adjustable-KXN-3020D-30V20A-DC-power-supply-LED-High-Power-Switching-Variable/132692230825?hash=item1ee512c2a9:g:ThEAAOSwo4pYVMYP:rk:1:pf:0if you still think its expensive, rethink of your spec again and reduce to 10A... around INR4Khttps://www.ebay.com/itm/30V-10A-220V-Digital-Switching-DC-Power-Supply-Adjustable-Variable-Precision-Lab/132613751949?epid=2086349239&hash=item1ee065448d:g:s0YAAOSw5-hcB3X0:rk:8:pf:0

author: gnvrvikram
author_rank: Newbie
timestamp: January 21, 2019, 05:04:49 pm
content: Quote from: Mechatrommer on January 21, 2019, 04:48:40 pmI have seen them. I need them to have a computer control input, which is generally not available. I need to automate the heating.

author: gnvrvikram
author_rank: Newbie
timestamp: January 21, 2019, 05:10:34 pm
content: Quote from: SiliconWizard on January 21, 2019, 04:24:56 pmMay be about 1 V per minute or even lesser.

author: james_s
author_rank: Super Contributor
timestamp: January 21, 2019, 06:19:54 pm
content: Well something you could try if you really want to modify an existing pot controlled power supply is to replace the pot with a LDR stuck to an LED. There are commercially made analog opto isolators used for audio applications but it's not hard to make one yourself. The LED can easily be controlled by PWM and this will vary the resistance of the LDR. It may not be very linear but that can be corrected in software.