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Old 12th January 2003, 08:29 PM   #1
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Question Adjustable power supply questions

Hello, I have a few newbie questions about adjustable power supply. Shematic is taken from National's LM117 datasheet for 5A constant voltage/constant current regulator.

- They are using LM317K - in TO-3 package. Can I use normal LM317T instead, because K is some 8x more expensive and they are both rated at 1.5A?

- What should be power rating of resistor R1?

- If I want to make it for 3A, could I use BD250C instead?

- Is LM301's V+ connected to +6V since V- is to -6V?

- What's a good replacement for 1N457, 1N4148 maybe?

- If I want to make negative regulator, except using LM337, and MJ802 (or BD249C), what else should I change? Should I reverse some diodes, or switch + and - inputs of op-amp maybe, or something else?

Thank You.
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Old 12th January 2003, 10:18 PM   #2
halojoy is offline halojoy  Sweden
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Default Re: Adjustable power supply questions

Originally posted by rendisha
- They are using LM317K - in TO-3 package. Can I use normal LM317T instead, because K is some 8x more expensive and they are both rated at 1.5A?

- What should be power rating of resistor R1?

- If I want to make it for 3A, could I use BD250C instead?

- Is LM301's V+ connected to +6V since V- is to -6V?

- What's a good replacement for 1N457, 1N4148 maybe?

- If I want to make negative regulator, except using LM337, and MJ802 (or BD249C), what else should I change? Should I reverse some diodes, or switch + and - inputs of op-amp maybe, or something else?
------------------------------------------
The Current LIMIT resistor is 0.33Ohm - makes 2A go into LM317K
0.7v /0.33ohm= 2.12Ampere.

For LM317T (TO220), I would make R= 1.0 ohm
Gives max 0.7V/1.0= 0.7 Ampere in LM317

Any calculations for IC Transistors is depending on
WORST CASE you should use this supply.
The original components were chosen to SURVIVE this.
- If input voltage is 35V, output is 2.0V and LOad is taking 5 Ampere.
That means, 35-2= 33Volt 5A, will be on REgulator
33x5= 165 WAtts!!!!!!!!!!!!!!!
THat will mean very Hard job. For Regulator Transistor and HEATSINKS.

So to give you advice is impossible without knowing
Your WORST CASE
If your worst case is not so much Current
At low voltage output,
than a lot of those things you suggest will do.
-------------------------------------------

It is like this in all circuit design.
Doing the math for worst case.
The worst case you can think of.

An amplifier should be built for LOWEST impedance the load has.
At the highest Watt output.
And the power Supply must be able to Deliver worst CASE currents.
at Worst Case Voltage.
That makes worst Case Watt =Power!
as Voltage x Current is POWER (Wattage)

-----------------------------------------

And the Worse your worst case is,
the more your components, IC-transistor Resistors ...
will have to be able to Survive or work good at.
----------------------------------------------------------------

/halojoy
- End of lesson one the law of worst case

- "if there is a worst case possible - it will happen -
most probably sooner than later - just for to upset you"

(halojoy's law of worst case)

or

"Give me your worst case
and
I will give you your circuit"
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Old 13th January 2003, 12:05 AM   #3
UrSv is offline UrSv  Sweden
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Default Re: Adjustable power supply questions

Quote:
Originally posted by rendisha
Hello, I have a few newbie questions about adjustable power supply. Shematic is taken from National's LM117 datasheet for 5A constant voltage/constant current regulator.

- They are using LM317K - in TO-3 package. Can I use normal LM317T instead, because K is some 8x more expensive and they are both rated at 1.5A?

- What should be power rating of resistor R1?

- If I want to make it for 3A, could I use BD250C instead?

- Is LM301's V+ connected to +6V since V- is to -6V?

- What's a good replacement for 1N457, 1N4148 maybe?

- If I want to make negative regulator, except using LM337, and MJ802 (or BD249C), what else should I change? Should I reverse some diodes, or switch + and - inputs of op-amp maybe, or something else?

Thank You.
#1
LM317T is fine.

#2
Halojoy mistakingly it seems thought it was 0.33 Ohms and that all the current would go through the reg. It seems is is 33 Ohms which seems more reasonable. The power rating is (based on the fact that the voltage over it is the forward drop of the transistor) P= U*I = U * U / R = 0.7*0.7/33 = 0,015 W which means take whatever you have but a standard metal film is perfect.

#3
Yes, BD250C would probably be OK.

#4
Don't know...

#5
I think that would be OK.

#6
A bit more complicated. Why not make 2 identical and connect in series?
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Old 13th January 2003, 01:17 AM   #4
halojoy is offline halojoy  Sweden
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#2
Halojoy mistakingly it seems thought it was 0.33 Ohms and that all the current would go through the reg. It seems is is 33 Ohms which seems more reasonable. The power rating is (based on the fact that the voltage over it is the forward drop of the transistor) P= U*I = U * U / R = 0.7*0.7/33 = 0,015 W which means take whatever you have but a standard metal film is perfect.

They are using LM317K - in TO-3 package

UrSv is mistaken.
Why On Earth would you Use a TO3 REgulator
for to pass only 0.700/33= 21mA.
If the regulator had been LM317L in TO92, i would have guess on 33 ohms and 20 mA

I beg my pardon or yours....
Hope you can give me an answer....
I do not think you have done your math alright
dear friend UrSv

/halojoy is right! 0.33 not 33 ohm, is what it is in that schematic
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Old 13th January 2003, 06:20 AM   #5
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Halojoy, UrSv might be right. As you said, with .33 it will pass 2A through LM317K, but it is also rated for "only" 1.5A!

UrSv, about #6, how much complicated, and what exactly, if I don't ask too much

Thank you both.
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Old 13th January 2003, 08:51 AM   #6
UrSv is offline UrSv  Sweden
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Quote:
Originally posted by halojoy
#2
Why On Earth would you Use a TO3 REgulator
for to pass only 0.700/33= 21mA.
If the regulator had been LM317L in TO92, i would have guess on 33 ohms and 20 mA

I beg my pardon or yours....
Hope you can give me an answer....
I do not think you have done your math alright
dear friend UrSv

/halojoy is right! 0.33 not 33 ohm, is what it is in that schematic
I think they the TO-3 because it is an application note and that is the regulator in question. The regulator is NOT used as the main current provider but as a regulator (suprise?). I have downloaded the datasheet again and looked closely and as far as I can see the resistor is 33 Ohms just as clearly as in the picture posted earlier.

As for #6 I would not be the right person to tell you how to do it and if I did then it would take a long time and most likely be incorrect. I do have a basic knowledge of electronics but I have clear limitations...
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Old 13th January 2003, 11:32 AM   #7
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Default adjustable power

Rendisha,

You must also take care of the opamp power supply, indicated as "-6V" and "V+". Since the output and inputs of the LM301 float up and down with the output voltage, the supply should be referenced to the output voltage I think. Did the app note say anything about that?

Jan Didden
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Old 13th January 2003, 11:42 AM   #8
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No, just schematic
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Old 13th January 2003, 12:52 PM   #9
UrSv is offline UrSv  Sweden
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Default Re: adjustable power

Quote:
Originally posted by janneman
Rendisha,

You must also take care of the opamp power supply, indicated as "-6V" and "V+". Since the output and inputs of the LM301 float up and down with the output voltage, the supply should be referenced to the output voltage I think. Did the app note say anything about that?

Jan Didden
Exactly what I thought but it does not show in the app note as far as I could see. This is however the way one of the Elektor designs did it so I would try that to.
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Old 15th January 2003, 04:57 PM   #10
Jeff R is offline Jeff R  United States
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I belive you can convert it to a negative supply quite simply if you use a dual secondary transformer.

For the positive supply, feed it normally, with the ground being tied to the center tap and the +35 volt input tied to the positive rail from the rectifiers.

For the negative supply, connect this ground to the "+35 Volt" input, and connect the "gound" to the negative rail. You still use an LM317.

Rember, voltages are all relative, so both supplies still see 35 volts across them.

The main downside is that the voltage ajustments for the two will be different. I mean, if turning one fully CCW gives minimal voltage out (i.e. zero volts), then that same position will give maximum absolute voltage on the other (i.e. -30 volts).

If you an LM337, you need to change polarities on everything. I think there should be an application note on this.
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