Advice needed on adjustable lab power supply design - Page 1
Forum section: electronics / projects

author: Entropia
author_rank: Regular Contributor
timestamp: August 17, 2011, 05:45:36 pm
content: Hi guys! I have designed this small lab power supply around MIC29152. I need some advice if it's going to work as expected or if there are some major flaws in it. Improvement suggestions are also welcomed.The schematic is here in addition to the attachment:http://entropia.kapsi.fi/pics/labpsu_sketch.jpg(Link instead of image because it's rather big.)Thanks!

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 17, 2011, 06:02:28 pm
content: Other than moving the F2 fuse to the out of the MIC29152 , I do not see anything worrying.I do not have a clue about the MIC29152 pin alignment and wattage, but this was your own responsibility.But in any case you could always drive an 2N3055 or a Motorola MJ15024 and get some amperes from it.

author: Entropia
author_rank: Regular Contributor
timestamp: August 17, 2011, 06:27:27 pm
content: Okay, good point about the F2. Thank you for the suggestion. I will move it. MIC29152 is rated for 1.5 amps, but there is even 5.0 amp version (MIC29502) available so I can get more current that way if needed. The transformer T1 should be able to output around 2.2 amps. Power dissipation maybe be a problem if I was to draw maximum current (1.5A) at minimum voltage (2.5V). The regulator would have to dump over 30 watts of heat and that's too much.Is there any concerns about having a huge load on the output while switching the regulator on? I expect the reg to limit the inrush current, is there some precautions I should take against this?

author: Unknown
author_rank: Unknown
timestamp: August 17, 2011, 09:12:15 pm
content: Quote from: Entropia on August 17, 2011, 06:27:27 pm2.2A DC or 2.2A AC? In the latter case, expect about 1.3A DC.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 18, 2011, 12:31:42 am
content: Quote from: Entropia on August 17, 2011, 06:27:27 pmWhat do you like to tell you ? , that this simplified design is protected against amperes as the PSU with adjustable Volts and Amperes ?No its not.This is why you have to use with it, devices with known specifications (or needs) in amperes.Example a Computer fan, or something with given specifications.The true bench type PSU , it must be from 5-10A and adjustable output in both volts and amperes.And one note about the transformer, I do not like it to be 2X22V , I would suggest an single output and strong enough.

author: amspire
author_rank: Super Contributor
timestamp: August 18, 2011, 12:35:56 am
content: I don't see the point of the enable switch - just leave the regulator permanently enabled.What you really want instead is a switch on the output, so you can isolate the load. It also means that if you attach a voltmeter panel to the regulator, you can set the output voltage before you switch it to the load.In general though, you could make a useful power supply this way, but if you are going to build the box, bridge rectifier, capacitors, fuses, etc, it just seems a pity to limit yourself with such a limited regulator.The supply doesn't go down to 0 volts, has no current limit control and only regulates the output to about 1%. Once you add a current limit control to a power supply, you are actually getting two devices in one - a constant voltage supply and a constant current source and the current source mode is extremely useful.Also when you are debugging or repairing boards, I can't tell you how useful it is to be able to slowly wind the voltage up from 0V, or to be able to set precise current limits. If there is a fault or miswired component on the test circuit, your supply will go to the regulator's 1.5A or 5A limit which is more then enough to seriously blow things up. A good lab supply is designed to protect the load, whereas this regulator IC is designed more to protect itself.The 2.48V output minimum is a real limitation. I can think of hundreds of times that I have needed less then this out of a power supply. You can't even use this supply to replace alkaline or NiMH battery, and you cannot gradually wind the voltage up to see how a circuit behaves as the power is rising on start-up.The thing is once you have the big components like the transformer, capacitors, heatsink, box, the actual price of a 5 ppm/degrees C stable voltage reference, two low drift op-amps, a power transistor and a couple dollars worth of resistors and capacitors is not much at all, and you end up with a device with far more performance in every way. Regulation of the output may be 1000 times better, current is precisely controlled, you can add current meters without the meter affecting the output regulation, you can have dual tracking power supplies, you can tune the supply for no overshoot on a load change (the regulator you are using can go 400mV over the set voltage on a step current out change), and so on.Then again, if I needed a secondary power supply and your one was handy, would I use it? Definitely.It would be useful, but you just aren't getting as much out of the supply hardware as you could.

author: Entropia
author_rank: Regular Contributor
timestamp: August 18, 2011, 05:04:04 am
content: Very interesting points, I have to research some more then! Constant current in addition to constant voltage would definitely be a plus. Can a regulator like the MIC29152 even be used for such a design? Can its output go below its reference voltage (of 1.24 volts)? The datasheet for it isn't as newbie-friendly as National's that have plenty of application examples.And yes, that 2.2 amps from transformer was AC. Maybe get a beefier transformer? I was going to do this design in double since the transformer I was looking at had dual outputs. Another possibility would be two connect the two secondaries together, though.

author: amspire
author_rank: Super Contributor
timestamp: August 18, 2011, 06:36:40 am
content: There are a whole lot of reasons why it is hard to turn power supply chips which have no external voltage and current sense pins into a great lab power supply.Instead, here is a fairly traditional lab power supply design.http://www.electronics-lab.com/projects/power/001/index.htmlThere are changes I would do.U1, R4, R5, R6 and D8 is used to provide a reference voltage of around 11V across the voltage control P1. I would replace this with a 10V reference IC, and this would give much better stability.U2 is the voltage regulator IC and U3 is the current limit control IC. Both are TL081 which is not a bad IC but it does need a negative supply rail as the inputs cannot go to the negative supply rail on that IC.You could probably get better performance by using a modern opamp that can have the inputs go to the negative rail (so you can power the op-amps of the 0V eliminating D5, D6, C3, R3 and D7) and an op-amp that has a much lower offset voltage and drift. That would get rid of R10 and RV1 that are there to adjust the opamps offset voltage.However, I just have to warn you that the moment you start changing the Opamps, you may get stability problems that need to be debugged. The compensation in this circuit is tailored for the TL081 (C6, C9, C5). If you feel like a challenge, find one of the newer opamps theoretically can give superior performance, but the TL081 is not a bad opamp at all.You could change Q4 for a MOSFET transistor which might mean a simpler drive circuit, but again, this will risk the need for reworking the compensation circuit. The circuit uses a 2N3055 transistors - which people have been using to make lab power supplies for over 40 years - you will be in good company. You wouldn't actually use the old 2N3055, but probably one of the TO247 packaged versions like the TIP3055.I suspect that Q2 is under rated. I would probably look for something I can mount on the heatsink with a current rating of at least 0.5A.R7 is the current sense resistor, and you would put a panel meter across that to measure the supply current.One huge advantage this circuit has over your IC power supply chip is that the voltage regulator connections connect to the actual supply output, so the effects all the other voltage drops in the circuit are eliminated by the regulator. The IC you were using senses the voltage on the chip, so all the voltage drops from the chip to the circuit board, drops across an ammeter, wiring drops, etc are not regulated. This is why the chip only does 1% regulation.

author: Zero999
author_rank: Super Contributor
timestamp: August 18, 2011, 05:48:00 pm
content: The rectifier is short circuit because the DC 0V is connected to one side of the transformer. Remove the connection between C1 and the rectifier.Why are you only using one winding? If the transformer has two secondariness with the same voltage they should normally be connected in parallel, unless you want to use the other for something else. Using only one secondary will halve the power rating of the transformer, increase the heating and give poorer regulation on the secondary.amspire,The PSU you found on Electronics Lab contains a few errors.The thread linked below discusses them and the corrected schematic and parts list are attached.http://www.electronics-lab.com/forum/index.php?topic=7317.1610

author: amspire
author_rank: Super Contributor
timestamp: August 19, 2011, 04:39:47 am
content: Quote from: Hero999 on August 18, 2011, 05:48:00 pmThey have fixed many of the issues I mentioned, so that is great. The latest schematic looks like a pretty good circuit now.I would still replace U1 and the zener with a 10V reference IC like the TI REF102APG4 - 5ppm/deg C for under $7 from Mouser.I was also wondering at first why the second transformer winding wasn't used, until I realized that Entropia intend run 2 identical isolated power supplies from the transformer. That is why it is a 160VA transformer.

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2011, 04:47:19 pm
content: You're right and even the TL431 would be better than a zener.

author: BrAtKo
author_rank: Newbie
timestamp: March 16, 2013, 04:14:48 pm
content: Hello all, I'd like to kindly ask you to help me with values of the resistors. I have read through datasheet and the minimal current load should be 5mA. If the pot is at 0 value, than minimal possible R2 value should be 1.24V/0.005A am I right ? I have 5k/N potenciometer in my shack, maybe I should buy another one, but Im thinking about add another one to serial for fine tuning. Thank you for any advice.

author: Thor-Arne
author_rank: Unknown
timestamp: March 16, 2013, 05:19:11 pm
content: Quote from: Entropia on August 18, 2011, 05:04:04 amMake sure you read the transformer datasheet properly.I.e. RS specifies the voltage at full load for their transformers, at least some of them.If you order a transformer from the asumption that it's amperage is specified at no load and it's not, you'll end up with a lot higher unloaded voltage.