Strictly Linear PSU's

Do you enjoy making your own equipment or antennas? Discuss construction and design in here.
User avatar
sureshot
Veteran
Veteran
Posts: 2554
Joined: 24 Jan 2012, 21:26
Call Sign: 26TM413
Location: South East Coast UK.

Strictly Linear PSU's

Postby sureshot » 20 Jan 2016, 10:18

A long way back now there was a controversial thread on a linear power supply circuit, a few might recall it. Any way most know I'm an smps fan.. That's not to say I've written off linear psu circuits and construction.

After some thought I decided to try this so called controversial circuit, first as a 12 volt 5Amp version, and then as a 12 Volt 10Amp version. " The big one is capable it claims of 30 Amps at 12 Volts.

So far the two versions I've tried follow the same circuit configurations as the big 30 Amp version. After testing both versions under load for 24 hours each they both perform as expected. Although at this stage its the regulation and current carrying capability as a model only stage. I said to myself if it stacks up, I might look at a serious attempt at the 12 Volt 30 Amp version.

Below is the schematic of the single version, and the high current version. And the models of a single transistor 12 Volt 5 Amp unit, and a two transistor 12 Volt 10 Amp version. I know there is no output protection in the schematics, but if you've read this far you'll know what a crowbar circuit is (Google it if your not familiar) with all the above in mind, and having consulted opinions on what I've got so far, and the plausible possibility of building the high current version at the fraction of the cost a new psu of this calibre would be, its a real possibility. Testing on the two versions built below where very stable loaded for 24 hours, both in output consistency and temperature.

When you take into account radio use of these built units would be an average 50% duty cycle then its looking very good. I know there are more advanced complex circuits for linear power supply's out there, yet the attraction here is in its simplicity. The foot print my two models are on is 127mm x 95mm this is very small indeed for a linear circuit that incorporates the heat sink as well, any final cased completed version could be the same, or I believe smaller than a production retail unit. If one person out there found me testing these schematics useful, then it was worth posting !
You do not have the required permissions to view the files attached to this post.
Last edited by sureshot on 20 Jan 2016, 10:26, edited 2 times in total.
CB call. Shipwreck.

User avatar
sureshot
Veteran
Veteran
Posts: 2554
Joined: 24 Jan 2012, 21:26
Call Sign: 26TM413
Location: South East Coast UK.

Re: Strictly Linear PSU's

Postby sureshot » 20 Jan 2016, 10:20

The models of the 12 volts units, both 5 Amp and 10 Amp versions.
You do not have the required permissions to view the files attached to this post.
CB call. Shipwreck.

User avatar
cjay
Top Poster
Top Poster
Posts: 1775
Joined: 06 Jun 2013, 21:21
Call Sign: cjay
Location: Manchester
Contact:

Re: Strictly Linear PSU's

Postby cjay » 20 Jan 2016, 12:54

I've repaired a few hundred of those, that's pretty much the standard circuit used for 24V to 12V droppers :)

Love the CPU heatsink and fan assemblies, they're excellent for all sorts of projects like that.

It works reasonably well for a quick and dirty PSU but there are good reasons why decent linear supplies are more complex.

One nice feature of using the 7812, if you fasten it to the same heatsink as the TIPs you gain the thermal overload feature of the chip for the whole pass regulator assembly, including transistors so it won't die of heat.

The snags I see with it:

1. There's no short circuit protection (despite the 7812 having it built in), if you short it you will be very lucky to get away with only a blown fuse, it's highly likely that one or more of the TIP transistors will fail short circuit and stuff the full rectified 24V into whatever you're powering with it.

2. Some brands of 78 series regulators can produce significant amounts of RF noise so you need to add some 100nF capacitors from input and output to ground, as close to the 7812 as possible. (Bizarre experience that caused me to discover this, an 18 inch length of wire had different voltages along it's length when measured with a DMM and the FM radio in the workshop was wiped out)

3. At the 30 amp level you may find regulation is a bit dodgy, it might take more current than the 7812 is capable of delivering to drive the pass transistors, especially on transisents.

4. It needs a diode adding to the 7812, anode to the output, cathode to the input. If the load has any stored charge it's possible to destroy the 7812 when the input voltage is disconnected/switched off because that charge will flow back through the 7812. The diode allows it to pass back safely.

For the small extra effort, I would ditch the 7812, change to TIP3055 and use the ancient but excellent LM723 with current limiting and use one of the emitter resistors for current sensing

User avatar
sureshot
Veteran
Veteran
Posts: 2554
Joined: 24 Jan 2012, 21:26
Call Sign: 26TM413
Location: South East Coast UK.

Re: Strictly Linear PSU's

Postby sureshot » 20 Jan 2016, 18:04

All the feed back taken onboard, I know the pitfalls and thrashed this out in two electronics forums, both in OZ and the US lol. Its was generally agreed with added protection the series pass element emitter follower will work fine. As yet I've not taken it to the 30 Amp version, but at 10 Amps it runs fine 1/3 of the big one. Given the additional protection as you mentioned against over voltage, short circuit, it looks fine, echoed by a few experts on the electronics forums. The last thing I would want is to stuff 24 volts into any radio gear o-: The protection circuit " A decent crowbar " and some current limiting hasn't been included in the schematic by the person that drew it, I know his origin, and web site, and he has a lot of circuit design on there.

Your last circuit mention is the typical linear retail unit, and a very good one. But as I said the attraction here is cost, components count, physical size, and as long as where regulating evenly and there is no huge ripple on the output, and protection is added this is a really viable option. For the 10 Amp unit, the voltage drop was 0.5 volts at 8.2 Amps drawn, this was steady for 24 hours. The regulator tempreture an 78S12cv was 52° C the two TIP2955's where at 71° C again for the full loaded 24 hours. To get round the small volts drop I've got the 78S15cv well a few of every voltage for that chip lol. With the regulator just mentioned and a single diode drop on the output 13.80 volts can be achieved. And yes it goes with out saying a denent SCR crowbar to protect against over voltage.
CB call. Shipwreck.

Mattylad
Top Poster
Top Poster
Posts: 1495
Joined: 03 May 2014, 20:09
Call Sign: RDX64
Location: Lancashire

Re: Strictly Linear PSU's

Postby Mattylad » 20 Jan 2016, 18:36

The circuit for the PW Marchwood is an excellent PSU for this if you can find it. (wish I could).

Also have a look at http://warc.org.uk/?page_id=404

User avatar
Crusader
Veteran
Veteran
Posts: 2541
Joined: 27 Nov 2008, 23:43

Re: Strictly Linear PSU's

Postby Crusader » 20 Jan 2016, 18:45

Hi guys,just here to ask some advice,i have a BNOS (thats the brand) 25amp linear PSU,it accidently got shorted out between the + & - terminal posts,there is now no output but the transformer itself is still working fine as per spec & is providing the correct voltage to the 2N3055 pass transistors which i replaced,i also replaced the other following componant's,MC3432,MC1432 & LM376 ,yet still no output voltage,any idea's as to what would now be preventing it from working?? cheers.

Nearly forgot there is a PDF file of the circuit diagram on this forum...

http://www.vintage-radio.net/forum/show ... p?t=120493

User avatar
sureshot
Veteran
Veteran
Posts: 2554
Joined: 24 Jan 2012, 21:26
Call Sign: 26TM413
Location: South East Coast UK.

Re: Strictly Linear PSU's

Postby sureshot » 20 Jan 2016, 18:58

Mattylad wrote:The circuit for the PW Marchwood is an excellent PSU for this if you can find it. (wish I could).

Also have a look at http://warc.org.uk/?page_id=404

To the best of my knowledge your link is the Marchwood project Matt.Its a super design, not beyond mine or many other hobbyists ability. The components count is much higher, but for complexity and quality its to be expected.

I really wished the designer of the 30 Amp circuit above had added a crowbar to his circuit, in the drawn circuit above it assumed that's it and your good to go... This is not the case, you must add a current limit and short circuit and overload protection to the final output.

Staying with in a simple and safe practical circuit, and the cost of your transformer, and case and hardware, you can put together a high current supply for very little indeed. My point being some of the simplist designs can be the most durable adaptable and effective ! Sure you can go bells whistles and more for a labour of love.. But not everything needs over engineering, when the simplest answer is just waiting to be taken up.
CB call. Shipwreck.

User avatar
sureshot
Veteran
Veteran
Posts: 2554
Joined: 24 Jan 2012, 21:26
Call Sign: 26TM413
Location: South East Coast UK.

Re: Strictly Linear PSU's

Postby sureshot » 20 Jan 2016, 19:02

Crusader wrote:Hi guys,just here to ask some advice,i have a BNOS (thats the brand) 25amp linear PSU,it accidently got shorted out between the + & - terminal posts,there is now no output but the transformer itself is still working fine as per spec & is providing the correct voltage to the 2N3055 pass transistors which i replaced,i also replaced the other following componant's,MC3432,MC1432 & LM376 ,yet still no output voltage,any idea's as to what would now be preventing it from working?? cheers.

Nearly forgot there is a PDF file of the circuit diagram on this forum...

http://www.vintage-radio.net/forum/show ... p?t=120493

Found the PDF link on the other forum, saying page not available. Can you post a link to the circuit here.
CB call. Shipwreck.

User avatar
Crusader
Veteran
Veteran
Posts: 2541
Joined: 27 Nov 2008, 23:43

Re: Strictly Linear PSU's

Postby Crusader » 20 Jan 2016, 19:05

Hi mate,it is here..
You do not have the required permissions to view the files attached to this post.

User avatar
Crusader
Veteran
Veteran
Posts: 2541
Joined: 27 Nov 2008, 23:43

Re: Strictly Linear PSU's

Postby Crusader » 20 Jan 2016, 19:11


User avatar
Crusader
Veteran
Veteran
Posts: 2541
Joined: 27 Nov 2008, 23:43

Re: Strictly Linear PSU's

Postby Crusader » 20 Jan 2016, 19:11

double post,sorry.

User avatar
2E1IIP
Top Poster
Top Poster
Posts: 1172
Joined: 10 Apr 2009, 11:55
Call Sign: 2E1IIP
Location: Newcastle Upon Tyne IO95DA

Re: Strictly Linear PSU's

Postby 2E1IIP » 20 Jan 2016, 19:24

Looking at the schematic, thyristor THY1 is designed to short the output of the power supply in the event of the output voltage rising above a pre-set level from IC2 which is set by the potential divider formed from R16 and R18.
So if you have accidentally shorted out the main output terminals I would hazard a guess you have popped a fuses or a fuse link wire that is not illustrated on the schematic?

Cheers

Rob.
Amateur Radio Callsign: 2E1IIP
Website: http://www.vhfdx.org.uk

User avatar
Crusader
Veteran
Veteran
Posts: 2541
Joined: 27 Nov 2008, 23:43

Re: Strictly Linear PSU's

Postby Crusader » 20 Jan 2016, 19:28

Hi Rob,there is only one fuse from what i can see and that is fine,and i already replaced Thyristor THY1 which is a 2N6504,I forgot to mention earlier that i replaced it, regards,Shaun :)

User avatar
2E1IIP
Top Poster
Top Poster
Posts: 1172
Joined: 10 Apr 2009, 11:55
Call Sign: 2E1IIP
Location: Newcastle Upon Tyne IO95DA

Re: Strictly Linear PSU's

Postby 2E1IIP » 20 Jan 2016, 19:39

Crusader wrote:Hi Rob,there is only one fuse from what i can see and that is fine,so do you think i should check Thyristor THY1?? regards,Shaun :)

Hi Shaun, I am surprised that they have only included a minas fuse in the design - there must be a sacrificial component or fuse wire somewhere.
Using your Multimeter check the output of the bridge rectifier, this should read about 24-30 VDC.
Also, check the cathode of D5 and you should have a similar voltage on there, trace this to pin 2 of IC1 - all being well it should read ~24V DC.
If the Thyristor was faulty it would be either short or open circuit, check behind the Meter in the PSU as it might have a fuse wire and shunt hidden out of view.
Amateur Radio Callsign: 2E1IIP
Website: http://www.vhfdx.org.uk

User avatar
Crusader
Veteran
Veteran
Posts: 2541
Joined: 27 Nov 2008, 23:43

Re: Strictly Linear PSU's

Postby Crusader » 20 Jan 2016, 19:51

Hi Rob,yep i already checked the bridge rectifier,24vdc as you said,i will check the other stuff you mention & get back to you,cheers for your help,Shaun :)


Return to “Homebrew Equipment Discussion”

Who is online

Users browsing this forum: sec1223 and 1 guest