linear power supply - Page 1
Forum section: electronics / testgear

author: sdouble
author_rank: Frequent Contributor
timestamp: November 25, 2017, 10:45:03 am
content: I needed a linear power supply, moderate current (0.5A) , fixed voltage (9V)... reasonable finish.. and cheapI went to my friend Ali.. and buy that device.It will operate 24 hours , 7 days.I'm a bit puzzled by the lack of ground connection.Any comment (even nasty ones) are very much welcome !

author: sdouble
author_rank: Frequent Contributor
timestamp: November 25, 2017, 10:47:31 am
content: and now, what's in the beast

author: sdouble
author_rank: Frequent Contributor
timestamp: November 25, 2017, 10:49:51 am
content: more of those nice pics

author: sdouble
author_rank: Frequent Contributor
timestamp: November 25, 2017, 10:51:04 am
content: the last one.If needed, I can take more pics, no big deal.I'll measure the noise level on monday at the lab.

author: bd139
author_rank: Super Contributor
timestamp: November 25, 2017, 12:13:15 pm
content: Looks pretty reasonable. Get that earth sorted though! Metal case + no earth =Not sure why it's as complicated as it is though. a LM317 would do the job just as well.

author: sdouble
author_rank: Frequent Contributor
timestamp: November 25, 2017, 12:27:27 pm
content: love the GIF.Part of the reasons for which I posted here is that I don't get why the design is so complex.The bank of output cap is ok.The 4 diodes for the rectifier bridge are fine.the output regulator too...

author: capt bullshot
author_rank: Super Contributor
timestamp: November 25, 2017, 01:00:43 pm
content: Neat, it's an "discrete" linear regulator built with an OP-Amp and a pass transistor - there's nothing wrong with that.Check if the output "-" is connected to the enclosure, if so, you don't want to connect earth to enclosure. If everything is done right, one would consider this "double insulated" and no earth connection is required. The heat shrink at the power connector points towards this direction.

author: sdouble
author_rank: Frequent Contributor
timestamp: November 25, 2017, 02:00:03 pm
content: a proper "safe way" would not be to connect the housing to the ground and keep the output isolated from the housing ?

author: nctnico
author_rank: Super Contributor
timestamp: November 25, 2017, 04:17:14 pm
content: Quote from: capt bullshot on November 25, 2017, 01:00:43 pmExcept for the adjustment pot which means the output voltage will probably drift a bit over time. A solution using a 7809 regulator would have been much easier and simpler.

author: capt bullshot
author_rank: Super Contributor
timestamp: November 25, 2017, 05:03:15 pm
content: Quote from: sdouble on November 25, 2017, 02:00:03 pmDepends ...The "safe" way would be to connect the housing to earth. You can connect the ouput "-" to earth or not, as your application requires.Connecting output to earth easily can give you nasty ground loops, depending what other equipment is connected, so I warned about connecting the housing to earth in case the output is connected to earth.Usually you don't leave a metallic housing floating, it's either earthed or connected to your circuit ground reference. Not connecting the metallic housing to earth can be as safe as earthing it, as long as the construction is OK (double insulated).For low noise stuff, it's always preferable not to have an circuit ground to earth connection and a line frequency transformer - that is what this supply looks like. So I suppose to leave it unchanged.

author: sdouble
author_rank: Frequent Contributor
timestamp: November 25, 2017, 09:38:41 pm
content: Quote from: nctnico on November 25, 2017, 04:17:14 pmwould you know how much it could drift ?

author: bd139
author_rank: Super Contributor
timestamp: November 25, 2017, 10:15:45 pm
content: It won’t drift much. A few mV usually.7809 will give you a 2.5% tolerance on voltage. Tends to be better to use an adjustable regulator and a good quality (Bourns!) trimmer to set the voltage.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 26, 2017, 04:50:25 am
content: Seems a very decent supply for me if you stay within its specs for the output current, with an op amp, a reference voltage etc ... and disspipating on the bottom case ...For the complexity of the circuit, i dont see a shunt resistor for a current limitation, the 0.1 ohms location is shorted with a wire, but maybe implemented differently ?Maybe lacking some fuse protectionI hate the ac socket somewhat glued / maintained on the case

author: sdouble
author_rank: Frequent Contributor
timestamp: November 26, 2017, 07:03:45 am
content: I have foreseen some margin in teerm of power.The PSU is rated 1.5 Amp as it will run below 300mA most of the time. I hope to get a satisfactory noise level

author: sdouble
author_rank: Frequent Contributor
timestamp: November 29, 2017, 07:58:53 pm
content: Hi guys, this is a follow up.I bought a second power supply unit for another need.THis time, it's a 2 channel unit with individual transformers.Same transformer as per my order.I forgot to take pics before I dismounted the device.However, your cans the 2 LED displays for the 2 voltages.

author: sdouble
author_rank: Frequent Contributor
timestamp: November 29, 2017, 08:01:41 pm
content: a couple of additional pics

author: sdouble
author_rank: Frequent Contributor
timestamp: November 29, 2017, 08:05:28 pm
content: and the last one.The design is much simpler.a Linear regulator is onboard (actually below the board).A rather big cap replace the bank on nice smaller ones from the single channel one.The power connector is not glued anylonger, it's screwed, which is better i guess.The noise level is satisfactory with both unit at 0 load.Test at 500mA load will come next week.

author: anotherlin
author_rank: Frequent Contributor
timestamp: November 29, 2017, 11:24:03 pm
content: Quote from: sdouble on November 25, 2017, 10:49:51 amBy looking at the photos, this unit seems to be pretty well made.The components seem of quality, the small electro capacitors seem to be Nichicon (big ones too), and there is a big WIMA capacitor.The metal film resistors are much like the Yageo or Koa Speer you can find on Mouser.The TL072 is even socketed!It seems hand soldered, that's not a bad thing per se.It looks like the job of an EE student building and selling some PSU for making some money.The design using "discrete" components seem classic (see HP linear power supply for example).However, the voltage is fixed, why not just use a 7509?It's overly complex for the job, whereas any of the standard off the shelf vreg IC would do the job.The second power supply using LT1084 is much more cost effective and reliable.

author: anotherlin
author_rank: Frequent Contributor
timestamp: November 29, 2017, 11:27:10 pm
content: Quote from: coromonadalix on November 26, 2017, 04:50:25 amGuess the wire is a good substitute for .1 ohm resistor

author: sdouble
author_rank: Frequent Contributor
timestamp: November 30, 2017, 06:20:31 am
content: my bad :the second picture of reply 16 is from the 1st LPS (single channel)