Quick DC-DC Question - 35V to 12V?

[Dave]

Can anybody let me know the easiest way to drop 35V DC to 12V DC? All I
need is 85mA. I could easily drop that load to 55mA by removing three
status indicator LEDs if that will make the circuit more efficient.
Continuous load is only 18mA which could be dropped to about 10 by removing
an indicator LED.

Would a resistor be more efficient or a 78L12 regulator? It's 5:50am and
I've been up all night so my brain isn't working as it should.

Thanks,
Dave

 

[Rich Grise]

Can anybody let me know the easiest way to drop 35V DC to 12V DC? All I
need is 85mA. I could easily drop that load to 55mA by removing three
status indicator LEDs if that will make the circuit more efficient.
Continuous load is only 18mA which could be dropped to about 10 by removing
an indicator LED.

Would a resistor be more efficient or a 78L12 regulator?

Either way, you'll have to dissipate (35 - 12) * .085 = almost 2 watts;
Drop a watt or so of that in a resistor of P = I^2 R,
1 = (.085 * .085) * R, therefore R = 1/.007225 = 138.4, so a 120 - 150
ohm, 2 watt resistor will keep your 7812 from getting way hot. I'd check
the max. input voltage on the 7812, 35 sounds kind of marginal. You could
also use a series zener, or even a shunt preregulator.

Good Luck!
Rich

 

[Jonathan Kirwan]

Can anybody let me know the easiest way to drop 35V DC to 12V DC? All I
need is 85mA. I could easily drop that load to 55mA by removing three
status indicator LEDs if that will make the circuit more efficient.
Continuous load is only 18mA which could be dropped to about 10 by removing
an indicator LED.

Would a resistor be more efficient or a 78L12 regulator? It's 5:50am and
I've been up all night so my brain isn't working as it should.

"Simple" means not-switcher, I think. And that means you have to
dissipate the 2W that Rich already mentioned -- that is, to supply the
85mA at 12V you have to drop away 23V also at 85mA. Whatever you can
cut current requirement that down to, goes in your favor in terms of
dissipation, of course. The continuous load of 18mA, if that's the
bulk of it, means that 18/85 of 2W or about 400mW is all you'd need to
burn off. More tolerable.

The 78L12 usually has an absolute maximum specification of 35V, so
your supply is worrisome. Normally, these are used with voltages
above 14.5V or so, but well below 35V. And if your 35V can be 36V,
then you'd be above the absolute max. Not so good.

Resistors aren't more efficient than a 78L12, or less efficient. It
doesn't matter how you throw away the voltage -- it's all equally bad,
unless you use switching. But resistors don't regulate well and with
the load you mentioned varying its current requirements all over the
map, I don't think a resistor would be the regulator you want it to
be.

Could consider a 12.6V zener, a current setting resistor for it, and a
BJT emitter follower. Three parts. I think you are in the right
ballpark, load-wise, for something like that. Still need to dump the
wattage, though, so a BJT in a TO-220 on a small heatsink.

Jon

 

[,]

Could consider a 12.6V zener, a current setting resistor for it, and a
BJT emitter follower. Three parts. I think you are in the right
ballpark, load-wise, for something like that. Still need to dump the
wattage, though, so a BJT in a TO-220 on a small heatsink.

Do people still say, "Duh"? ;-)

Thanks!
Rich
(that's me saying, "duh", by the way)

 

[Dave]

Thanks everybody for your help, glad I didn't try hooking something up at
6am when I was working on it. I kept thinking 'there must be a simpler way
of doing this!' but due to the voltage being 35V (higher than standard
regulator maximum input voltage) and the need to dissipate 2W of heat, plus
the varying current load it wasn't as simple as a dropper resistor.

I like the zener idea, very elegant. If I have a spare 12.6V zener sitting
around I'll hook something up. Otherwise, I just found a spare 100mA 12V
wall-wart, probably easier to just use that instead! It's only going to be
used for a couple of weeks anyway. It is being used to control a door
opener, and the door motor operates at 35VDC.

Thanks,
Dave

 

[Jasen Betts]

Can anybody let me know the easiest way to drop 35V DC to 12V DC? All I
need is 85mA. I could easily drop that load to 55mA by removing three
status indicator LEDs if that will make the circuit more efficient.
Continuous load is only 18mA which could be dropped to about 10 by removing
an indicator LED.

Would a resistor be more efficient or a 78L12 regulator?

no. it'd be about equal. if the load is constant, or precision of the 12V
not important use a resistor, else use

the 7812 with a resistor in the input picked to drop the imput voltage
into the allowable range.

Bye.
Jasen

 

[Byron A Jeff]

"Simple" means not-switcher, I think. And that means you have to
dissipate the 2W that Rich already mentioned -- that is, to supply the
85mA at 12V you have to drop away 23V also at 85mA.

The 78L12 usually has an absolute maximum specification of 35V, so
your supply is worrisome.

Resistors aren't more efficient than a 78L12, or less efficient. It
doesn't matter how you throw away the voltage -- it's all equally bad,
unless you use switching. But resistors don't regulate well...

Exactly. However resistors can easily be use as a preregulator for
a 7812. Create a resistor divider with 5W resistors that cuts the
35V down to 16-17V, then feed that into the 7812. Then of the 23V
that needs to be dropped, the resistors handle nearly 18V of it and
the regulator only needs to handle the last 5V or so, of which 3V
are required in headroom.

Could consider a 12.6V zener, a current setting resistor for it, and a
BJT emitter follower. Three parts. I think you are in the right
ballpark, load-wise, for something like that. Still need to dump the
wattage, though, so a BJT in a TO-220 on a small heatsink.

The 7812 has thermal protection and the like. I think it's better to
use a regulator as opposed to trying to emulate one.

BAJ

 

[Roger Dewhurst]

"Simple" means not-switcher, I think. And that means you have to
dissipate the 2W that Rich already mentioned -- that is, to supply the
85mA at 12V you have to drop away 23V also at 85mA. Whatever you can
cut current requirement that down to, goes in your favor in terms of
dissipation, of course. The continuous load of 18mA, if that's the
bulk of it, means that 18/85 of 2W or about 400mW is all you'd need to
burn off. More tolerable.

The 78L12 usually has an absolute maximum specification of 35V, so
your supply is worrisome. Normally, these are used with voltages
above 14.5V or so, but well below 35V. And if your 35V can be 36V,
then you'd be above the absolute max. Not so good.

Resistors aren't more efficient than a 78L12, or less efficient. It
doesn't matter how you throw away the voltage -- it's all equally bad,
unless you use switching. But resistors don't regulate well and with
the load you mentioned varying its current requirements all over the
map, I don't think a resistor would be the regulator you want it to
be.

Could consider a 12.6V zener, a current setting resistor for it, and a
BJT emitter follower. Three parts. I think you are in the right
ballpark, load-wise, for something like that. Still need to dump the
wattage, though, so a BJT in a TO-220 on a small heatsink.

What about a string of diodes in series each dropping a bit of voltage?

R

 

[Jonathan Kirwan]

<snip>
What about a string of diodes in series each dropping a bit of voltage?

Aside from the fact that it's not likely to have good resolution
(roughly .7V per step), the voltage of each varies on the order of
60mV per decade of current. To drop some 23V, he'd need some 33
diodes. With current varying from say 18mA to 85mA depending on LEDs
and perhaps other things, this would be a rough factor of 5 -- about
42mV per diode. Times 33. Almost 1.4V variation. Not so good.

The old:

: +35
: |
: |
: \ R1
: / 2.2k
: \ 1/2W +35
: / |
: | |
: | |/c Q1
: +-----| TO-220
: | |>e
: ---/ |
: / ^ '----->
: / \ 12.6V
: --- zener
: | 1/4W ,---->
: | |
: gnd gnd

Would be better. Base would vary less than 10mV. Vbe would also vary
no more than 40mV, I think. So about 50mV over the load variations.
Double all that and call it 100mV. Still a lot better than a diode
string. And you can dissipate 2W in a TO-220 easily.

Jon

 

[Dave]

Thanks Jon, that's definitely the best answer yet!

Unfortunately this is all now a moot point, but I'll keep that schematic of
yours for if I need it next time for something else. I hooked up the remote
control box (the thing that needed the 12V supply) up to the door control
box, but unfortunately the receiver causes a lot of interference and stops
the box from working! Very strange but I've seen it happen a couple of
times before. There isn't enough shielding on the motor control circuit or
the receiver box (or both) so I have to now mount the two in different
corners of the room with an interconnecting control wire. It'll be much
easier to just use a separate plug-pack now, as there is a powerpoint
mounted in a convenient position. Running the wire along the ceiling will
now be the tricky part.

Thanks again for your help, I'm sure I'll use this for something else soon,
it won't go to waste!

David

 

[Tim Auton]

Exactly. However resistors can easily be use as a preregulator for
a 7812. Create a resistor divider with 5W resistors that cuts the
35V down to 16-17V, then feed that into the 7812. Then of the 23V
that needs to be dropped, the resistors handle nearly 18V of it and
the regulator only needs to handle the last 5V or so, of which 3V
are required in headroom.

That could work, but you're dissipating more power (in your divider)
with that scheme than with the regulator alone. The extra couple of
watts may not matter much for a mains-powered device though.

A single resistor in series with the regulator can also work, with the
same power dissipation as the regulator alone, but only over a limited
range of currents. Specifically, you need to ensure that you have
sufficiently high minimum current for the voltage drop across the
resistor to keep the input to the regulator below its maximum Vin, and
sufficiently low maximum current that the drop across the resistor
keeps the input to the regulator above 15V or so.

Oh, and a 78L12 hasn't got a hope in hell of dissipating the required
power, even though the required 80-odd mA is below the 100mA current
limit of the 79Lxx series. I find it's quite rare in practice that I
get to design for the nominal 100mA current limit of 78Lxx regulators,
rather than the thermal spec. With a thermal resistance for the TO-92
junction-ambient of 230C/W and a max junction temperature of 125C, you
can only dissipate (125-25)/230 = 0.43W and that's with the
unrealistic assumption of a 25C ambient and no safety margin on the
junction temp. 0.43W at 0.1A means a maximum voltage drop of just
4.3V, even with that unrealistic ambient temp and zero safety margin.
Bump the ambient temp a bit and try to keep the junction below, say,
100C and you're struggling to dissipate enough power to pass the rated
100mA at the minimum voltage required for regulation.

A heatsink helps, of course, but you're not going to find anything
very substantial to fit a TO-92 or any of the other packages you can
find a 78Lxx in.


Tim

 

[Jasen Betts]

What about a string of diodes in series each dropping a bit of voltage?

how many diodes?

that's why not.

Bye.
Jasen

 

[Roger Dewhurst]

how many diodes?

that's why not.

Quite a lot, but they are cheap.

R

 

[Roger Dewhurst]

how many diodes?

that's why not.

I know of someone who uses a 7.5 volt drill pretty continuously in the
field. He has modified the battery pack to hold a bunch of diodes and to
connect to the 12 v car battery.
It works apparently.

R

 

[ehsjr]

Can anybody let me know the easiest way to drop 35V DC to 12V DC? All I
need is 85mA. I could easily drop that load to 55mA by removing three
status indicator LEDs if that will make the circuit more efficient.
Continuous load is only 18mA which could be dropped to about 10 by removing
an indicator LED.

Would a resistor be more efficient or a 78L12 regulator? It's 5:50am and
I've been up all night so my brain isn't working as it should.

Thanks,
Dave

I'm surprised no one drew a schematic of this idea
that someone mentioned.

------
35V + ---[165R]---+-------+---In|LM7812|Out--+---> +12
| | ------ |
[1K] [.22uF] | [.1uF]
| | | |
Gnd --------------+-------+---------+--------+--->Gnd

The 165 ohm is 5 watt, the 1K is 3 watt, both values based
on availablity at from Allelectronics at 3 for $1.00.
(The 165 will dissipate a bit over 2 watts, the 1K a bit
under 1 watt). The LM7812 in a T0220 package can handle
1 amp; in the circuit at .085 amps it will dissipate about
..34 watt. Allelectronics can also supply the LM7812 at
50 cents, .1uF cap at 17 cents each, and a .22 uF cap at
5 for $1.00.

Ed