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Old 13th September 2006, 12:42 PM   #21
Roushon is offline Roushon  India
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Default Re: ... and another thing ...

Quote:
Originally posted by FastEddy

Here is a neat trick however ... you can use a daisy chain of diodes to "knock down" the incoming voltage = each diode = 0.6 VDC throw away = 10 diodes = 6 VDC down. [/B]
to be on the safe side i will use your idea to drop the voltage, as i have some MUR diodes with two diodes in one package. but i have one query. can this daisy chain of diodes be put just after the rectifier and before the filter caps or i have to put them after the filter caps? which one is better?

thanks
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Old 13th September 2006, 12:57 PM   #22
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Default Re: Re: ... and another thing ...

Quote:
Originally posted by Roushon


to be on the safe side i will use your idea to drop the voltage, as i have some MUR diodes with two diodes in one package. but i have one query. can this daisy chain of diodes be put just after the rectifier and before the filter caps or i have to put them after the filter caps? which one is better?

thanks

Before the cap they have to conduct the very high charging pulses into that cap. After the cap they only have to conduct the load current. The latter makes life easier for those caps and gives less possible interference of pulsed current that *may* radiate depending on the layout.

Jan Didden
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Old 13th September 2006, 04:04 PM   #23
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Default extra parts ...??

Re: snubbed regulator in your circuit diagram ...

1) I never understood why the value of R1 & R2 was so low in this circuit (2.2K ohms) .... it bleeds off the power supply rails continuously = wasting power and generating heat. (This circuit would fail current production power supply restrictions in Europe and soon, here in the states. = an environmentally incorrect design, etc .... A value about 5 times greater, 10K to 22K ohms would suffice ... or none at all).

2) Likewise, I believe that R3 & R4 should be 910R ohms (instead of 91R ohms) and R5 & R6 should be 22K ohms (instead of 2.2K ohms) ... saving a little more in wasted heat, etc. I would also suggest that for audio work that R3, R4, R5 & R6 be metal film type and held to precision of +/- 2% or +/- 1%. (1/8 watt rating is OK here if R5 & R6 >= 10K.)

3) For capacitors C3, C7, C4 & C8, I would recommend the use of plastic (polystyrene) or comparable "audiophile" quality capacitors ... most especially at C4 & C8. (SoniCaps ?). These capacitors are used here as "snubbing" caps, reacting to and filtering off the higher frequencies and "popcorn" noise associated with fixed voltage regulators and RF from external sources.

4) I always add a Ferrite Bead to the output legs and ground legs of these kinds of linear supplies ... especially if intended for a sperate enclosure. ( http://en.wikipedia.org/wiki/Ferrite_bead )

5) Transformer: if independant 30 VAC secondary type is unavailable, then a "center tap", +/- 28 to 30 VAC secondary may be used (although some here might quibble about retaining the "split supply" nature of the original). The center tap can be connected directly to "G" or anywhere upstream of "G" and downstream of the (-) negative LM338 at D2.

6) If output power is expected to exceed ~ 40 watts (output >= 40 VA) ... then a heat sink should be added to both LM338 regulators. If close to the maximum rating of the transformer (200+ VA) then the heat sink(s) on the LM338 should be quite large indeed.

(Alternate part number for LM338 = pin for pin substitution = uA7805, although some here feel that this substitution will be noisier and is of older design.)

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Old 13th September 2006, 08:22 PM   #24
Roushon is offline Roushon  India
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Default confused!!

uA7805 is a 5V, 1.5Amp regulator. am confused!
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Old 13th September 2006, 09:05 PM   #25
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Default confused?

" ... uA7805 is a 5V, 1.5Amp regulator. am confused! ..."

As previously mentioned the uA78xx series can be "stacked", all pins connected in parallel, for just about any current rating you want.

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Old 15th September 2006, 06:12 AM   #26
Roushon is offline Roushon  India
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Quote:
Originally posted by wine&dine
>
Diode drops are fine when you need a small voltage drop. In Roushon's case, an extra transistor is much safer. See http://www.national.com/an/LB/LB-47.pdf for a circuit.
following the circuit in the above page a schematic is designed (attached). my aim is to safeguard LM338 from possible high in-out voltage difference (in case of output short) . the plan is to keep this difference fixed around 6v. please give me your suggestion for the points below.

1. value of the darlington pair transistors.
2. is only one transistor enough instead of the darlington pair?
3. value of the resistors.
4. also if the other resistor values are OK.

or any other concern....
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File Type: png safe-35v-lm338-ps.png (7.3 KB, 290 views)
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Old 15th September 2006, 06:54 AM   #27
clem_o is offline clem_o  Philippines
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Quote:
Originally posted by FastEddy


As previously mentioned the uA78xx series can be "stacked", all pins connected in parallel, for just about any current rating you want.


This is new - is it in the data sheet? Seem to have missed that!!

Fasteddy, where did you get this info? Thanks!
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Old 15th September 2006, 12:11 PM   #28
Roushon is offline Roushon  India
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Default ????

any suggestion...?

thanks
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Old 15th September 2006, 04:57 PM   #29
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Default Stacked 78xx ??

fasteddy: " ... the uA78xx series can be "stacked", all pins connected in parallel, for just about any current rating you want. ..."
clem-o: " ... This is new - is it in the data sheet? Seem to have missed that!! Fasteddy, where did you get this info? Thanks! ..."

I have been using this scenario since about 1980 = parallel 78xx and 79xx. When I built my first computer, I used ten (10) Fairchild uA7805 in the TO-3 can in parallel on two large heat sinks to power the bus, I/O & processor boards in a rack of S-100 bus boards. In the supply there were also two (2) uA7812 in parallel, one (1) uA7912, 0ne (1) uA7905 on the sinks. Result = fairly clean power + 5 VDC @ ~50 Amps "rating", - 5 VDC @ ~ 5 A., +/- 12 VDC @ 10 A. and 5 A. respectively. No problems, total load was actually ~ 25 Amps, mostly on the +5 VDC side, heat sinks ran at about +5 to +15 deg. F. above ambiant. I still have this supply and it still works as does the whole computer. (The -78xx in the TO3 can had a rating of 5 Amps back then, each, which was a little generous as it implied an "infinite" heat sink, so 1/2 power loading for thermal reasons was/is a good idea.)

I have built a number of other digital and analog projects using "stacked", parallel 78xx including a bench supply with four (4) each uA7815 and four (4) uA7915 = +/- 15 VDC @ max of ~20 Amps "rated", each rail, although I have seldom used it to do more than 2 A. loads.

Please note that others posting about the original, primary question ... that the LM338 is a better regulator than the old uA78xx series = better noise filtering, etc. ... and I am not sure that it is a good idea to "stack" or parallel the LM338 ...

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Old 16th September 2006, 01:36 AM   #30
clem_o is offline clem_o  Philippines
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Fasteddy - thanks, very enlightening!

Cheers

Clem
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