Power Supply - Voltage drop under load

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Monday 29th Nov 2010, 17:13

Ok, I have been doing some research on this crowbar issue and found this info on another forum where someone was gonna build basically the crowbar that I built:
Tripping the crowbar at 15V when the nominal supply is 13.8V is cutting it too close IMHO - that is less than 10% overvoltage. Look at crowbar set points on commercial supplies - They are usually 20 or 25% above nominal output voltage. Running it too close may very well result in unwanted crowbar action with inductive/capacitive loads or load transients.
So that would probably explain why its tripping when I connect a load :doh: So maybe I should increase the diode voltage to 15.7 or 16.4 by adding one or two regular diodes (0.7v) in series with the 15v zenner or just replace the 15v zenner with a higher voltage one...I'll have to look up which voltages are availible and experiment! (I love experimenting :lol: )

UPDATE: This has been confirmed as the cause of the problem... Without changing anything I had the idea to reduce the power supply voltage to 12v to test it and that solved the problem... the Crowbar never tripped, even when I quickly connected and disconnected the load many times :dance: so I recon that if I set the crowbar to 16.4v by adding two normal 0.7v diodes (say 1n4007) in line with the 15v zenner that should do it. (I would imagine that most items would survive at 16.4v without too much damage)
Norm

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Friday 3rd Dec 2010, 5:45

Here is the full Circuit Diagram of the completed 12A PSU that I built:
NormsPSU3.gif
Norms PSU V3
(43.42 KiB) Downloaded 215 times
It is a very stable supply, adjustable from 11 to 15 volts and has over voltage and over current protection.

The "crowbar" over voltage protection is set at 16.4 (by the two 8.2v zenners) because when its set at 15v it is too near 13.8v and it will trip falsely when loads are connected.

If you are thinking about building or replacing the blown guts of a power supply, you should give this one a try...
Norm

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Re: Power Supply - Voltage drop under load

Post by lbcomms » Saturday 4th Dec 2010, 1:12

If you can (i.e. you are buying a new transformer, or using one with multiple secondary taps), drop the secondary voltage. The pass transistors have to convert the excess to heat to do their job.

25V AC x 1.4 = 35V at the collectors.
At 12A there is 1.2V across the 0.1 ohm resistors, therefore 15V at the emitters.
Volts across the pass = 20V (35 - 15).
Virtual resistance = 1.6667 ohms (20V / 12A)
Power dissipated by the pass = 240 watts (V squared over R, or alternately current squared multiplied by R).

That's about as much heat (you typical halogen is 50% efficient) as 14 automotive high beam bulbs !


Dropping the secondary to 15V gives you:
15V AC x 1.4 = 21V at the collectors.
At 12A there is 1.2V across the 0.1 ohm resistors, therefore 15V at the emitters.
Volts across the pass = 6V (21 - 15).
Virtual resistance = 0.5 ohms (20V / 12A)
Power dissipated by the pass = 70 watts (V squared over R, or alternately current squared multiplied by R).

70 watts is still pretty high, but it's 3.5 times lower than 240W.
Can't go too much lower though, you need at least 3 to 4 volts to maintain regulation.
Unless you go for a switchmode design, but that's another story :-)

With the pass dissipation lower, as well as saving power and unwanted heat, you'll be able to get away with a smaller heatsink . You'll also be able to use higher uF value electrolytics (say 10,000uF/25V at the rectifier, and 4700/16V at the output terminal) which improves transient performance of the supply, important for SSB use.


Apart from that minor point: - well done!.

Cheers

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Saturday 4th Dec 2010, 2:02

You Cannot use a 15v transformer for 13.8v supply as this only leaves 1.2v difference which is not enough for the regulator to work correctly :doh: ...you say yourself that the regulator needs at least 3-4v to maintain regulation... so 13.8 + 3 (mimimum) = 16.8 so the smallest transformer you could safely use for it to work correctly is 16.8 volts and not 15 volts... I only make this point as I dont want anyone reading this to build a supply using a 15v transformer and wonder why the regulator is screwed up.

Also in terms of the heat dissipation, thats why I've used 3 pass transistors instead of 2... to share the load and hopefully not get as hot... (plus there were 3 already mounted to the PSU case...lol)

And if all else fails and it still runs warm, then thats a bonus, coz its quite cold in my shed :lol: Thanks for all your help on this topic Sue, much appreciated!
Norm

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Re: Power Supply - Voltage drop under load

Post by lbcomms » Saturday 4th Dec 2010, 3:11

You Cannot use a 15v transformer for 13.8v supply as this only leaves 1.2v
A 15 volt transformer = 21.2 volts at the filter cap. Your 15 volts is AC, at the filter cap it's DC.
To convert between them, multiply be the square root of 2:
Square root of 2 = 1.4142
15 x 1.4142 = 21.213 volts.

The reason? Your AC is an "RMS" value - an average of the highs and lows, changing 50 times per second. DC is a constant value.
If you have 240 volt mains, there is actually 340 volts when rectified, your insulation (and caps in switchmode supplies that rectify the mains) need to be rated for this.
Have a look in your PC or VCR, you'll find 400 volt capacitors :)

If you want the full details (math warning!) have a look at http://en.wikipedia.org/wiki/Root_mean_square
3 pass transistors instead of 2... to share the load and hopefully not get as hot
There will be NO difference in the TOTAL heat generated. 3 will be a bit easier to manage, as it's spread over a wider area. The total heat dissipated will be the same, though.
coz its quite cold in my shed
Yes please - this was at 9:00 this morning in mine...
Image

and we are only 4 days into summer...

Cheers from Down Under

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Re: Power Supply - Voltage drop under load

Post by lbcomms » Saturday 4th Dec 2010, 3:31

There are 3 main (and several other) ways to state AC voltage.
If the units are not mentioned (i.e. "the UK has 240 volt household mains") then RMS is assumed.

This will illustrate the differences:
Image

When you full wave rectify, you get the "Peak" value, minus the 0.3 / 0.7 volt drop across the diode (the Vf drop is usually ignored when working out power / voltage ratings).

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Saturday 4th Dec 2010, 4:10

Ok so its official, I'm a Muppet! :) I had tested the voltage of the transformer at 27v ac... then connected it to the circuit and and measured 27v dc at the filter caps... Guess what I did wrong? ... (no i dont have a true peak meter lol) I tested the transformers output with no load on it! :doh: I have now re-tested the voltages into and out of the Bridge Rectifier and there is 21v ac in and 27v dc out
this was at 9:00 this morning in mine...
its currently about 3.0c in my shed at the moment...you're too hot and i'm too cold..lol...about 20c would suit me nicely...maybe I should bring my workbench indoors for the winter :lol:

I Know a little about true Peak and RMS ect through fixing hifi amps... I blame the cold for slowing my brain down and not thinking about that...lol

EDIT: I have started to gather parts to build startup delay circuit, but a am a little confused by this:
Collector to -ve of coil, and +ve output terminal goes to the other side of the coil.

I have drawn this diagram of what i think is correct... can you confirm this?
delay.gif
delay.gif (20.22 KiB) Viewed 11233 times
Norm

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Re: Power Supply - Voltage drop under load

Post by lbcomms » Monday 6th Dec 2010, 0:58

the transformers output with no load on it!
That will have no bearing on it. Both will drop under load, due to the reactance (AC resistance) of the windings.
A typical 16V transformer will have 19V AC / 25.25V DC with no load, 17V AC / 22.5V DC at 50% load, and 16V AC / 21V DC at full load (rounded to nearest 0.25V)
The DC (assuming silicon diodes and a 4-diode bridge) voltage should be: (AC volts x 1.4) - 1.4 volts.
true Peak and RMS ect through fixing hifi amps
The "other" AC measurements (and their test gear, like true peak meters) are only useful when dealing with non-sinusoidal waveforms, like those found in switchmode supplies and audio outputs.
For 50Hz sinewave stuff, RMS is all you need.
I have drawn this diagram of what i think is correct... can you confirm this?
Unfortunately, I can't :) There are two errors:
1) The emitter of the transistor needs to be grounded
2) The other side of the coil (on the right side of your drawing) goes to +12V, assuming you are using a 12V relay).

Drawing (not showing the base part which is correct):
Image
If you are using a 12V relay you'll need a dropper resistor. For 25V, this will be roughly equal to the coil resistance.
Use ohms law to calculate this and the power level needed.

Example for a 12V, 200 ohm relay operating at 24V:
200 ohms: nearest standard value down = 180 ohms.
Power is (V x V) divided by R.
12 x 12 = 144.
144 divided by 180 = 0.8 watts.

Therefore, use a 180 ohm 1 watt resistor.

One of the contacts goes to the output of the regulator.
The other contact goes to the output terminal.
If there are more than one set of contacts, wire them in parallel to minimise the resistance.

Enjoy...

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Monday 6th Dec 2010, 7:25

Unfortunately, I can't There are two errors:
I thought my diagram was incorrect as the emitter had no connetion (except to the diode). Thanks for clearing that up for me!
If you are using a 12V relay you'll need a dropper resistor
Again I re-confirm my muppet status!!! I have just ordered a new 12v relay... Had I thought, I could have ordered a 24v one :roll: nevermind!
If there are more than one set of contacts, wire them in parallel to minimise the resistance.
The Relay I ordered has only one set of contacts, but I made sure to order a heavy duty 30 amp one, so it won't get stressed if I pull a heavy load off the supply, and I plan to fit the relay as close as possible to the output terminals of the supply, so it should be fine
Norm

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Re: Power Supply - Voltage drop under load

Post by zodiac » Tuesday 7th Dec 2010, 5:39

I'd like to add my thanks to you both for the questions and the answers you've both given, it's been very enlightening and educational to the workings of the most asked about piece of equipment used in CB.
It will serve as a directory to anyone needing any information about their PSU in the future.
Please keep up the good work, I always enjoy reading your posts.
Dave. =D>
How far is it.
Twice it's length from halfway.

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Tuesday 7th Dec 2010, 6:16

Thanks for your comments Dave, When I have built and added this startup delay to my PSU I will make and upload a diagram of the whole supply. I recommend this regulator design to anyone and everyone... this power supply is more stable and better under load than even my 15A Zetagi supply!

I've learned a lot through building this supply and I'm happy that others get to benefit too. Learning from my mistakes and wrong turns, I have ended up with a fantastic power supply that should last me for years. :dance:

The only other thing that I am thinking about adding to the supply is a thermal cutout system for if the 2N3055's get too hot, but that will come in time... and when the time comes i'll be raiding old PC power supplys for the thermistors, but thats a future project as I say snooze
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Re: Power Supply - Voltage drop under load

Post by zodiac » Tuesday 7th Dec 2010, 12:15

Norm,
My philosophy is, if you don't ask you'll never find out and you've had a good tutor in Sue.
He's answered all your questions in layman’s terms that anyone can understand and follow.
I've been following the thread with interest because I've built many PSU's in the past and was curious to see if I'd missed out on anything that would improve on what I've done.
The advantage of building your own PSU is, that you know what everything is in there for and what it's function is, it makes it easier to trouble shoot if it develops a fault in the future, plus the satisfaction that you know it will give many years of service without the worry of it going over voltage and blowing all the transistors in it to bits like most of the cheaper PSU's do.
Dave.
How far is it.
Twice it's length from halfway.

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Tuesday 7th Dec 2010, 13:56

and you've had a good tutor in Sue.
I couldn't agree more with that! Thanks Sue =D> =D> =D>

I've built a few basic supplys in the past, but none in this league... many using 7812 regulator with diode/resistor on the ground leg to get 13.8v from it, feeding single 2N3055... they worked ok, but obviously they had no protection of any kind, other than a simple fuse..(which is not much good on its own i now learn)...
Norm

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Re: Power Supply - Voltage drop under load

Post by lbcomms » Thursday 9th Dec 2010, 0:59

When I have built and added this startup delay to my PSU I will make and upload a diagram of the whole supply
Glad to see it all worked out OK.
I'd like to add my thanks to you both for the questions and the answers you've both given, it's been very enlightening and educational
I've written a few course books / tutes for my old uni, all written assuming the reader knows nothing - it's safer that way.
If they can't understand what I've written, they shouldn't be studying electronic engineering :)

There was one big mouthed type back when I was studying there - I think you call them "chavs" in the UK, we call them "bogans" or "rednecks" here in Oz.
He claimed to understand what was written, but I had my doubts... he'd get aggro when asked to do something basic, like rearrange a formula.
Sometimes he would turn up for classes half drunk and abuse females and those from non-english speaking backgrounds (I lost on both counts!).
People like that can't be helped, no matter how clear your instructions are.

With that guy, I'd almost forgotten about him until last weekend - I stoped at out local Coles (supermarket chain here in Oz) while doing a road test of a new Mercedes we had been putting in a fancy mobile data terminal into (the car is owned by the MD of large courier / taxi / truck company near here). I saw him - working as a trolley collector! :lol: :lol: :lol:

I couldn't resist tooting him and winding down the window to say hi... the look on his face made my day. :D

Anyway, back on subject - glad you got it all up and running OK and that my help was useful. Post some diagrams and pics when you get it finished.

Cheers

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Monday 13th Dec 2010, 13:21

I am happy to announce that the power supply is finally done! :dance: The Crowbar set at 16.4v works perfectly. The Startup Delay circuit as per diagram gives just over 2 seconds delay, which is fine.

Here is the diagram (corrected) (& simplified):
PSU_v4_diag.gif
Psu v4 Diag - corrected and now modified
(50.22 KiB) Downloaded 192 times

And here is a photo of the finnished supply:
psu_v4.JPG
psu_v4.JPG (75.54 KiB) Viewed 11156 times
Its quite untidy I know. :( I would have liked to make it neater, but with adding stuff, moving stuff and changing stuff to get it working good its ended up a bit untidy. That doesn't really matter though because it works very well now, only dropping by 0.02 volts with a 4 amp load... Thats better than my Zetagi power supply :shock:

I thought to keep the wires to and from the relay contacts as short as possible, so I soldered the NO contact of the relay directly to the output terminal. Thats how the relay is at a funny angle... also note my love of hot glue gun glue to secure things in place (its very handy for that :roll: )

If I was to build this circuit again, I would probably incorperate all three circuits (delay, crowbar and reg) into one board and that would make it neater, although keeping the three circuits seperate will make it easier to track a fault if it ever goes wrong.

The only addition I'm gonna add shortly is a common cathode bi-colour LED, connected to NO and NC of the relay and ground via 1k resistor... It will come on red when power supply is turned on then go green when the startup delay kicks in.

I plan to put a full story of this supply, what it started out like, and all the different stages it took to get to this state up on my own website soon. (I'll edit this post with link when i finally finnish and upload that)

I hope others can benefit from this topic, as I have, and everyone can build great power supplies in the future. Thanks to Sue for all your help!
Last edited by Warf135 on Thursday 16th Dec 2010, 7:57, edited 2 times in total.
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Re: Power Supply - Voltage drop under load

Post by lbcomms » Tuesday 14th Dec 2010, 0:50

only dropping by 0.02 volts with a 4 amp load
Good to see it up and running... only 2 small issues:

- The delay switching transistor is a BD681, not an 861
- The delay cap should be 35V, not 25.

When it's lightly loaded, the 25V will go close to 30 to 32V - although the 10K/1K will form a divider, limiting the voltage, it's generally considered bad design practice.
If the 1K, base, or emitter or their wiring ever goes open, result = messy explosion inside power supply :o
Image
note my love of hot glue gun glue to secure things in place (its very handy for that
That stuff causes no end of grief here. Had one at the end of last summer - in a cheapie Chinese UHF set, they has used that stuff to hold some wiring in place.
Nice hot day, inside nice hot car, glue melted and dribbled into the internal speaker between the cone and magnet. Audio dropped from several watts to several milliwatts...

I've lost count of the number of pots and switches I've had to replace because of that stuff!

Cheers

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Tuesday 14th Dec 2010, 4:14

The delay switching transistor is a BD681, not an 861
- The delay cap should be 35V, not 25.
Well spotted! There goes my dyslexia again... The cap I used was a 35v one, so I dont know why I put 25v on the diagram :doh:
with all things working correctly there is 10v at the cap. (I have corrected and uploaded the diagram again)

The hot glue gun glue I use melts at around 315c from what I've been reading. Thankfully the temprature over here doesn't get that hot, so it should be ok in my supply...
I was gonna use it to fix the thermal fuse to the transformer, but just in time remembered and stopped.. thats when I looked up the temprature of the glue gun. I ended up using an epoxy resin type glue to hold the thermal fuse in place. It maybe that the chinese factorys use "cool melt" glue guns which melt at a lot lower temprature
Norm

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Re: Power Supply - Voltage drop under load

Post by zodiac » Wednesday 15th Dec 2010, 12:26

Norm,
On your diagram you show the output of the transformer as being 25VAC, is that correct ?.
Dave.
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Re: Power Supply - Voltage drop under load

Post by Warf135 » Wednesday 15th Dec 2010, 13:33

Yes that is correct, the transformer does 27v with no load and 21v under load, so I put the average voltage of 25v on the diagram... I am thinking of updating the diagram and putting 15v - 25v on the transformer output, instead of 25v, so that anyone wanting to build it wont do their heads in searching for a 25v one if they have one in the 15-25v range.

As Sue states earlier in the topic, the lower the voltage the better as this will mean less heat generated at pass transistors. My 25v Transformer was already in the power supply case and it is (was) a very basic G-comm 12a power supply, which from what i can tell, were a pile of rubbish... now its a great little supply, i just need to put some stickers over the G-comm logo :lol:
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Re: Power Supply - Voltage drop under load

Post by zodiac » Thursday 16th Dec 2010, 4:56

Norm,
I would put a note in to explain that the secondary winding voltage is 15V AC before connection to the rectifier and capacitors with 25V DC after connection to both.
That should make it a lot easier to follow for those that have never done it before, it confused me the first time I built a PSU.
Dave.
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Twice it's length from halfway.

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Thursday 16th Dec 2010, 8:07

Hi Dave, Thanks for your input. I have now modified the diagram and re-uploaded it. While altering the diag, I found an error myself. The main filter cap was listed as 4700uf 25v when that should have read 4700uf 50v. That has now been corrected too.

Please re-view it and come back to me with any more suggestions you may have.
PSU_v4_diag.gif
(50.22 KiB) Downloaded 197 times
Edit:
Here is a diagram of the startup delay on its own if anyone wants to add it to a supply they already have:
delay-n.gif
Startup delay circuit
delay-n.gif (5.45 KiB) Viewed 11110 times
Norm

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Re: Power Supply - Voltage drop under load

Post by jimicorbet » Tuesday 12th Feb 2013, 0:19

Determine a good small transformer white and blue pot modulation. Super Swing radio, you will pull inductance, contributed to the RF driver. Looks like a resistor and inductor is green. You have to leave an inductor and a low voltage side at the top, pull-up. Installation of a hat, you want to play the resistor value.

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Tuesday 12th Feb 2013, 21:49

please note that it is not a good idea to re-open posts that are long-dead (more than 2 years in this case) as the original poster is usually long gone.

Jimicorbet, your post makes no sense at all, please clarify.
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Re: Power Supply - Voltage drop under load

Post by lbcomms » Sunday 17th Feb 2013, 0:35

Warf135 wrote:please note that it is not a good idea to re-open posts that are long-dead (more than 2 years in this case) as the original poster is usually long gone.
Hi Warf135, we are still around, though i don't get as much time for forums as I used to. How are things up there in the UK?
Determine a good small transformer white and blue pot modulation. Super Swing radio, you will pull inductance, contributed to the RF driver. Looks like a resistor and inductor is green. You have to leave an inductor and a low voltage side at the top, pull-up. Installation of a hat, you want to play the resistor value.
????? I'm not sure where he is from, but it's probably a good assumption that he is ingesting is not legal :lol:

As soon as I saw "super swing" I knew I was looking at a post by as clueless noob. What has the color of an inductor in the RF stages got to do with the power supply?

Cheers from Down under...

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Sunday 17th Feb 2013, 8:59

Hey there great to hear fro ya! Yea, I dont get on here as much any more.. Ithought the post was written by a 'bot' as it kinda talks about CB, but it makes no sense, and there is a link in the signature... "Determine a good small transformer white and blue pot modulation" determine a good small transformer is ok, but white and blue pot modulation?!? no human could of written that surely? :roll: Maybe i should just click the link and see where it goes :lol: (or perhaps not!)

My current project is setting up a PMR446 'gateway' on FRN (Free Radio Network)... its pretty cool to be able to use a license free 446mhz handset (doing 500mW!) and chat through the gateway to people all over the world... I just need to build a better antenna now to increase the range. Current thoughts are towards building an 8 section coax co-linear with a supposed 9DBi gain, taken from a tried and tested 70cm ham antenna design. (obviously re-calculated to 446mhz)

Hope things are going well for you ATM, Cheers from a cold, wintry Cornwall!
Norm

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