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Bench Power Supply

davenn

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well being old school taught we learnt to think of the diode symbol as like a speaker
and when you do that electron flow coming out the front of a speaker makes sense :)
 

chopnhack

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well being old school taught we learnt to think of the diode symbol as like a speaker
and when you do that electron flow coming out the front of a speaker makes sense :)
That is a great way to remember electron flow, thanks Dave!! I was always thinking of the arrows and would confuse myself sometimes when thinking of electron flow. Either way is fine, just need to work your way through a circuit one way or another - by checking with the opposite method you can catch mistakes.
 

chopnhack

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upload_2015-4-15_20-0-30.png

Tonight's question!
Here is the schematic for producing a negative supply voltage from the datasheet. I assume that the ground symbol is true earth ground and not common.
1. Can someone confirm this for me. I am relatively sure of it.
2. Does this arrangement make more sense? Should the negative regulation work as shown?

I think that this or the other schematic will require some breadboarding for confirmation of function.

upload_2015-4-15_21-11-5.png
 

davenn

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Tonight's question!
Here is the schematic for producing a negative supply voltage from the datasheet. I assume that the ground symbol is true earth ground and not common.
1. Can someone confirm this for me. I am relatively sure of it.
2. Does this arrangement make more sense? Should the negative regulation work as shown?

in the case of a single supply rail, the Ti datasheet part page you posted is ok and the GND symbol is just your zero volt rail

the problem is when you try and incorporate that into a dual rail supply as you have, you no longer have a dual rail supply with the common 0V rail
you have 2 separate supplies and it cannot be used for a dual rail supply like what you are probably aiming for so that you can supply IC's that need a +V, 0V and -V rails

The final point being, They produce -V voltage regulators for a reason .... if you could easily produce dual rail supplies without them, what would be the point of manufacturing -V regulators ??

Dave
 

chopnhack

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in the case of a single supply rail, the Ti datasheet part page you posted is ok and the GND symbol is just your zero volt rail

the problem is when you try and incorporate that into a dual rail supply as you have, you no longer have a dual rail supply with the common 0V rail
you have 2 separate supplies and it cannot be used for a dual rail supply like what you are probably aiming for so that you can supply IC's that need a +V, 0V and -V rails

That is probably why I was having such difficulty in producing a schematic, there was something that just didn't seem right, thanks for the explanation!

The final point being, They produce -V voltage regulators for a reason .... if you could easily produce dual rail supplies without them, what would be the point of manufacturing -V regulators ??

Dave
Valid points, I saw some information in the datasheet which led me to believe it was possible to do. Lack of experience brought me down this path, but at least I read the datasheet, LOL ;-)

Also, I hate to just copy and paste others work - I don't learn anything by doing that.


I didn't find a complementary chip, I think I will research negative v- regulators and use that as a starting point.

Thanks Dave!! :)
 

davenn

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I didn't find a complementary chip, I think I will research negative v- regulators and use that as a starting point.

Well I did mention the LM333 in a post on page 2 .... its capable of 3A, to increase the current capability, you have 2 choices
1) a current pass transistor or
2) parallel 2 x LM333 's

either way will work

Dave
 

chopnhack

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Well I did mention the LM333 in a post on page 2 .... its capable of 3A, to increase the current capability, you have 2 choices
1) a current pass transistor or
2) parallel 2 x LM333 's

either way will work

Dave
Thanks Dave, I will look at in depth.
Is the large amount of power wasted not really much concern with linear regulators?
Konstantine mentioned that linear is the way to go with bench supplies because there is less noise and that becomes very important with amplifiers, but the inefficiency leaves me questioning this approach.
 

davenn

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inefficiencies in this case isn't really a significant problem ... the system is mains powered
its when using linear reg's with battery supplies that one really starts worrying about the wasted power from the battery

if not well filtered, switching supplies can be noisy, ya just have to do your design well to take that into account :)


D
 

chopnhack

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inefficiencies in this case isn't really a significant problem ... the system is mains powered
its when using linear reg's with battery supplies that one really starts worrying about the wasted power from the battery

if not well filtered, switching supplies can be noisy, ya just have to do your design well to take that into account :)


D
Thanks Dave, off I go! :)
 

HellasTechn

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Thanks Dave, I will look at in depth.
Is the large amount of power wasted not really much concern with linear regulators?
Konstantine mentioned that linear is the way to go with bench supplies because there is less noise and that becomes very important with amplifiers, but the inefficiency leaves me questioning this approach.

Well i used audio amplifiers as an example. in general Yes linear is the way to go if you are most interested in the quality of your psu's output. not to mention that the circuits are much more simple.

Why are you so concerned about losses within your psu ? It is really not a big deal since it is mains powerewd and think about it you will only be useing it for short time periods. (power up the psu, connect the device under test, run for a while then turn all off) after a year how many hours in total you think your psu will have run ?
 

chopnhack

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Well i used audio amplifiers as an example. in general Yes linear is the way to go if you are most interested in the quality of your psu's output. not to mention that the circuits are much more simple.

Why are you so concerned about losses within your psu ? It is really not a big deal since it is mains powerewd and think about it you will only be useing it for short time periods. (power up the psu, connect the device under test, run for a while then turn all off) after a year how many hours in total you think your psu will have run ?
I was thinking more along the lines of having to rid the IC of heat - specifically at the extremes of low voltage and high current.
 

davenn

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use of heatsinks isn't really a problem :)
 

HellasTechn

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Yes normally that is indeed not a problem. Unless your power supply delivers high current like mine that uses 4x 2n3055 transistors and delivers up to 10 Amps. In that case you will need a really large heatsink !
 

chopnhack

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Yes normally that is indeed not a problem. Unless your power supply delivers high current like mine that uses 4x 2n3055 transistors and delivers up to 10 Amps. In that case you will need a really large heatsink !
Aside from just a heatsink, have you thought of using computer fans to assist the cooling?
 

chopnhack

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Well I did mention the LM333 in a post on page 2 .... its capable of 3A, to increase the current capability, you have 2 choices
1) a current pass transistor or
2) parallel 2 x LM333 's

either way will work

Dave
Found out that the LM333 are obsolete. Digikey had a listing with "call" and TI's datasheet are marked obsolete.
 

davenn

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o
Found out that the LM333 are obsolete. Digikey had a listing with "call" and TI's datasheet are marked obsolete.
ooohhh bummer

try looking at the availability of a LT1033 3A adj neg reg
 

chopnhack

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That one is also obsolete. I am looking at the LT1083 - 7.5A max, three terminal device. I haven't thoroughly searched yet, but the datasheet didn't seem to have anything about using the regulator as a negative supply. If I can not find a negative complement, there is a 5 pin LT1085/LT1185 +/- regulators that I can look at next.

I like TI's datasheets better. They seem to have more options and examples of how to use their product.
 

chopnhack

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The LT1185 would not be a good choice, the unit operates at +2.5-25v above the reference voltage which is 7v. That means the psu would start at ~10V! That just wont work...

Back to the LM350/LM338!

Would it be wise to get one of each regulator from the same manufacturer? My thought was that if there were any small deviations, if the chips were from the same company they would be closer together in their variances.
I see that the datasheet states built in short circuit protection - would it be prudent to add foldback current circuitry to prevent the chance of the IC being overheated?
I would like to have current control and voltage control per channel, is that something I could achieve using this IC?

The other option I am weighing is a pcb and parts for about $10, it wouldn't be a negative rail, but for $20 I could have two channels of pos. voltage. The transformer I was looking at is below as well.
PCB
Transformer

Your thoughts as always!
Thanks in advance :)
 

chopnhack

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Post #41, Figure 26 - look at the bridge.

ak
Hi AK, I think I see what you mean in post #43 - the input of the negative regulator needs to be fed from the positive side of the bridge rectifier, while the output of the reg. is held to ground, this would leave the bottom rail to 'go more negative' as the regulator adjust to keep its Vref stable.

If I got it correct, I will draw up a new schematic and post it.
 
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