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+5V from +35V

  • Thread starter Marco Trapanese
  • Start date
M

Marco Trapanese

Jan 1, 1970
0
Hello,

I want to obtain supply logic (+5V) from a main power supply (+24V -
+35V DC). The line is quite dirty because it's used by several stepper
motors.

I'm looking for a cheap and very low-count components way to regulate
the logic supply (Iout < 50 mA).

I tried with a resistor divider but the power rating needed is very high
(> 2W) for the series resistor. A DCDC converter would be nice but they
are expensive if no external components are needed, or cheap but they
will require several components.

There are a smarter way to do this? Also a filter against spikes will be
very appreciated... :)

Thanks in advance!
Marco / iw2nzm
 
J

Jacques

Jan 1, 1970
0
7805
TO-220 package with a small heatsink
 
M

Marco Trapanese

Jan 1, 1970
0
Jacques ha scritto:
7805
TO-220 package with a small heatsink


Are you sure?
If I have 35V on the input and 5V on the output the 7805 will have 30V
across both terminals. With an output current of 50 mA it should
dissipate 15W. I'm afraid the small heatsink can't help enough...

About the motor spikes? Would you connect directly the regulator to the
main power supply?

Thanks
Marco / iw2nzm
 
7805
TO-220 package with a small heatsink

Don't top post. And the 7805 isn't a good choice to hook up to a dirty
supply voltage that can run up to 35V and spike quite a lot higher.

The adjustable LM317HV can take a 56V volt differential, as can a
number of three-lead 5V linear regulators designed for the automotive
market. Dropping 50mA into a 5V rail from a 35V rail implies
dissipating 1.5W in the regulator. A 750mW 300R series resistor could
halve that, while still leaving 4V of headroom when the supply was
down to 24V.
 
M

Marco Trapanese

Jan 1, 1970
0
Marco Trapanese ha scritto:
If I have 35V on the input and 5V on the output the 7805 will have 30V
across both terminals. With an output current of 50 mA it should
dissipate 15W.

Pardon, it's 1.5W indeed.

Marco / iw2nzm
 
J

Jan Panteltje

Jan 1, 1970
0
Jacques ha scritto:



Are you sure?
If I have 35V on the input and 5V on the output the 7805 will have 30V
across both terminals. With an output current of 50 mA it should
dissipate 15W. I'm afraid the small heatsink can't help enough...

30 x .05 = 1.5
7805 is specified at 35 V in max, for 5 V out.
Else add a LM317 as pre-regulator to say 15 V, it will take the spikes.

35 V - LM317 - 15 V - 7805 - 5 V
Fewer watts and fewer volts per IC.
Or 2 x LM317.
 
P

Phil Allison

Jan 1, 1970
0
"Marco Trapanese"
Pardon, it's 1.5W indeed.


** Use a damn 16volt, 3W zener to fed the 7805.

Cheaper than chips.....



....... Phil
 
M

Marte Schwarz

Jan 1, 1970
0
Hi Marco,
I want to obtain supply logic (+5V) from a main power supply (+24V - +35V
DC). The line is quite dirty because it's used by several stepper motors.

ask google about roman black converter. I guess this will meet your needs.

Marte
 
M

Marco Trapanese

Jan 1, 1970
0
Marte Schwarz ha scritto:
ask google about roman black converter. I guess this will meet your needs.


Thanks to you too!
Marco / iw2nzm
 
M

Marco Trapanese

Jan 1, 1970
0
Spehro Pefhany ha scritto:
LM2574HV with a 45-50V TVS on the input. One or two more parts than a
linear regulator, BFD.


Thanks!
Marco / iw2nzm
 
M

mpm

Jan 1, 1970
0
Spehro Pefhany ha scritto:


Thanks!
Marco / iw2nzm

Spehro beat me to it. The National part would have been my
recommendation as well.
You might even get the 2575 for the same price? Little higher current
output.
I have seen them very competitively priced in the past.

For the TO-220 5-pin package, I've found the Avid 7136 heat sink is a
really nice compact fit.
-mpm
 
M

Marco Trapanese

Jan 1, 1970
0
mpm ha scritto:
Spehro beat me to it. The National part would have been my
recommendation as well.
You might even get the 2575 for the same price? Little higher current
output.
I have seen them very competitively priced in the past.

For the TO-220 5-pin package, I've found the Avid 7136 heat sink is a
really nice compact fit.


Thanks for the tips!
Marco / iw2nzm
 
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