Adjustable Linear Regulators (reguladores de tensão)

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Adjustable Linear Regulators

The Other Voltagespage showed how a fixed voltage regulator like the LM7805 could be rigged to

give a variable output, ad this is !ust what "al#a do i their little $a#ate %% cotroller&This

regulator sets the #ii#u# output at 5V, #axi#u# is about '&5V below the igoig suppl( level&

Maxi#u# load will deped o the si)e of heatsik fitted up to the chip*s '+ li#it& The $a#ate

ol( has a s#all sik, li#itig loads to 0&5+ -./ so (ou #a( wat to build a #ore powerful

versio ador #ake oe #ore suited to pael #outig&

The circuit is straight fro# the Texas %stru#ets datasheetad %*ve icluded the reco##eded

s#all capacitors 1 these should be fitted ear to the regulator iput ad output pis to prevet itfro# oscillatig uder so#e circu#staces& The "al#a circuit does*t bother with the#, but it ol(

saves a few pece&

Output voltage is give b( the for#ula

Vout= 5(1 + VR1/R1) + (Iq * VR1')

where %2 is t(picall( about 3#+ 0&003+/ ad V4'* is that fractio of V4' i use, so with 4' 'k

ad V4' 'k, the #axi#u# theoreticaloutput of over '6V allows for so#e adverse tolerace o

the resistors ad regulator& +s said, (ou*ll be luck( to get over '0&5V fro# a '6V suppl(&

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Construction

Construction Guide Component side view

1 2 ! 5 " # $ % 1& 11 12 1 1! 15

1

2 0V !' '

! V4'b/

! '6V 4egi/ '

5 4egc/ 4' ! V4'w/

" Var9 4ego/ 6 !6

# !3 V4'a/

$ 0V !' 6 !3

% !6 4'

%*ve used a piece of stripboard : strips wide, '5 holes log which icel( suited the screw;ter#ial

blocks ad heatsik used, a Maplis 4

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The ter#ial blocks % get fro# [email protected] Mapli sell si#ilar 6;wa( 5&08## pitch but theirs are #uch

#ore expesive& Or it #a( suit (ou to solder wires direct to the board ad take the# to a #olex

suppl( ad fa coector&

heck the fiished board for a( errors or solder bridges betwee tracks, fire up ad test&

arts List4g' LM7805 5V regulator

' 0$ 0&u$/ cera#ic or fil#&

6 '00$ 0&'u$/ cera#ic or fil#

4' 'k 0&65. 5E or better

V4''k li, '-## F?;#outig

potetio#eter

MiscTO660 heat;sik G paste, coectors,

cotrol kob to suit&

The #ore co##ol( used but slightl( #ore expesive/ ad!ustable regulators such as theLM'7or

L600are essetiall( si#ilar, the LM'7 beig at heart a '&65V regulator, the L600 a 6&77V device&

Their advatages over the 7805 iclude higher curret hadlig up to '&5 a#ps with a '7T, 6+

with the L600/, a #uch lower 2uiescet curret %2, ad a lower #ii#u# voltage, though that*s of

little iterest i a fa cotroller&

Most published regulator circuits give a output dow to the #ii#u# possible 1 with fa cotrol

there*s little poit i goig below about -V, as (a)the fa #a( ot start ruig at lower voltages,

ad (b)(ou*re wastig a lot of potetio#eter track that could be used for better sesitivit(& +ddig a

fixed resistor i series with the pot achieves this&

The LM'7 has a 4eferece Voltage of '&65V betwee the output ad ad!ust#et pis, ad with the

circuit show, i si#ilar vei to the previous exa#ple,

Vout= 125(1 + R2+VR1' / R1) + (Iq * R2+VR1')

+s said, the %2 value is ver( #uch lower t(picall( 50u+ &00005+/ ad ca usuall( be igored i

calculatig the output&

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.ith the values show i the circuit above, the calculated output will be fro# -&5V up to ''&7V,

ideal for fas& The resistor co#biatios for other output voltages ca be calculated with this '7

[email protected] for exa#ple, #akig 4' 6004 ad 46 -804 will give a 5&5V1''&8V rage with the

'k pot&

+s poited out earlier, so#e volts are lost across the regulator trasistor !uctios, so fro# a '6V

iput the output is li#ited to aroud '0v i a( case (10.25V on mine, with a 0.2A fan)& %*vedesiged for ''&7V because potetio#eters have a prett( broad tolerace rage H60E is co##o/,

so there*s a bit of leewa( if the actual pot resistace is o the low side of o#ial&

Construction

arts List

%' LM'7T

''00 0&'u$/ '-V cera#ic or #(lar

fil#&

6 'u$ '-V tatalu# bead

4' 6304 0&65. 5E or better

46 'k 0&65. 5E or better

V4''k li, '-## pcb #outig

potetio#eter

Misc TO660 heat;sik G paste, cotrol kob

Construction Guide Component side view

1 2 ! 5 " # $ % 1& 11 12 1 1! 15

1 0V ' !'2 br/ 46 !6 V4'

'6V ' 4egi/

! $a9 69/ 4' 4ego/ br/ !6 V4'

5 4' 4ega/ 46

" $a1 61/ !' V4'

#

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The circuit was built o a piece of veroboard, 7 strips wide, '5 holes log& Two track cuts as show

b( the red spots&

+ 0&65I -##/ drill bit ca be used to #ake these, a few turs b( had will re#ove eough copper

to break the track&

% used a sli# coductive pol(#er potetio#eter for V4' but a '-## F?;#outig carbo t(pe

will [email protected] the lik ad resistor, if ecessar(/ will go uder the pot barrel&

The '7T re2uires a heat;sik 1 a 65>. t(pe will ru fas rated at up to about '0. at '6V/ as at

the worst case it will be droppig -V at 0&36+, i&e& 6&5.&

% like the Mapli 4

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Fhoto shows a earlier desig to use up so#e rather log heat;siks 1 % had to bed the regulator

legs to fit a drive ba(, the Mark6 is si#pler to build ad eater&

Lo ,ro-.ut Regulators

+ #a!or drawback of the '7T ad si#ilar stadard regulators is that '&716 volts are lost fro# the

'6V suppl(, so the fa ca ol( ru up to about 85E of its full speed ad flow ratig& +lthough this

site is ai#ed at keepig oise dow, there #a( be occasios whe (ou wat to tur the wick right

up&

ou ca get #uch earer the '6V iput with the ewer low;dropout regulators, ad with the

cotroller show below #( voltage loss ruig a 600#+ 6&3./ fa was ol( '0#V '6V i,

''&87V out, ::E full speedN/&

The( cost a fair bit #ore tha the hu#ble '7T 1 4apid Alectroics price for a M%6:'56 is &'0

co#pared to 8p for a '7T 1 but whe (ou factor i the other parts, especiall( with a fac( cotrolkob, it*s ot such a big differece overall, ad well worth it %MCO&

%*ve used Micrel*sM%6:'56?T regulator, which is rated at '&5+ ad co#es i a 5;pi TO;660

case& %f (ou eed #ore curret, the M%6:06?T is idetical but rated to +, or the si#ilar

M%6:506?T will carr( 5+& ou #a( eed a bigger heat;sik with the bigger bo(s& Dee the heat;

sik pagefor the details&

Klike the '7, the 4eferece Voltage is across 46, ot 4', ad the calculated voltage output is

Vout= 12!(1 + R1 + VR1 / R2)

.ith the resistors show, the calculated output rage works out at -&:V to '6&5V, givig a bit of

headroo# if co#poet toleraces ru the wrog wa(& (The 20! tolerance ban" on #ots can #la$

ha%oc with calc&lations ' if $o&r 10k #ot is below abo&t k $o& won*t get the f&ll #otential. The

ratio of (+1V+1)+2 m&st be o%er ./1 ' increase +1 to 12k or re"&ce +2 to 2k)

The two capacitors esure stabilit(, particularl( useful with log suppl( leads& Do#e alterativeresistors are give below to suit differet #ii#u# voltages&

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Construction

arts List

4eg' M%6:'56?T

' '00 0&'u$/ '-V cera#ic

6 '0u$ '-V alu#iiu# electrol(tic4' '0k 0&65. 5E or better (see below)

46 6k6 0&65. 5E or better (see below)

V4''0k li pot, '-## pcb #outig (see

below)

MiscTO660 heat;sik 60165deg./ G

paste, cotrol kob

Construction Guide Component side view

1 2 ! 5 " # $ % 1& 11 12 1 1!

1

2 '6V 4eg'/ !' ' br/ ! V4'

4eg6/ !'

! 0V 4eg/ !6 ' br/ 4' ! V4'

5 $a9 4eg3/ 69/ 4'

" 4eg5/ 46 V4'

# $a1 !6 61/ 46

The stripboard is 7 rows x '3 colu#s, two track breaks as show i red& ?ed the regulator leads

carefull(& % foud the best wa( was to support the outer legs as ear the case as possible with a

thu#b;ail ad bed outwards slightl(, the support agai o the other side ( &se a "art ti#, also

%er$ &sef&l for c&rling resistor or "io"e lea"s for %ert