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Linear voltage Regulator L78xx

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darkfeffy

New Member
Hi all,
I am trying to design and test this circuit to power several ICs in my complete design model (please see .gif attachment). There are several 15V regulators, one 6V and a couple of 5V ones.

The problem is the power supply (as you can see in the .gif) is a 48Vdc battery. How do I step this down to say 24 - 30V for the regulators?

Well, I tried using two 100kOhm resistors as a voltage divider. Works pretty well when regulators are not loaded. When they are loaded, the output voltage drops to something unusable. I want to use 4 of the 15V regulators to power Mosfet gate drivers, so I expect the output currents of these regulators to go as high as 1A.

I suppose the problem is with the large resistors in the voltage divider which limit the current. I tried reducing their values, but this leads to other problems of large losses in the resistors (even when no load is connected to the regulators).

I used Proteus Isis

What can I do to solve this problem?
Thanks for listening...
Edwin
 

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darkfeffy

New Member
Hello Blueroomelectronics,
The problem with the LM2625 series is that the input limit voltage is 40Vdc. Given that I am using a 48V battery, I am faced with the same problem of voltage division (which is the original problem in the first place). Do you know of any other circuit with bigger input voltages?
Thanks
Edwin
 

dougy83

Well-Known Member
Then use the -HV variant. It works to 60V input.
 

smanches

New Member
Many linear regulators will work with higher voltages, but they can only DROP about 30-40v. This would allow you to use two in series; the first one to drop from 49v to 24v, and the second to drop from 24v to your desired voltage. Verify this is the case with the regulator you select though.

Although I agree with blueroom, using a SMPS as the first stage would be MUCH more efficient.
 

Sceadwian

Banned
Can't be kooky, I've done it twice to LED light bars so I don't throw money away on batteries.
 

Thunderchild

New Member
using linear regulators will be very wasteful, you could use a number of smps circuits that can take up to 60 v input (try the LT1074) or use one to drop the voltage to 15 V and then take the other voltages from it, much more efficient
 

Arumugam

New Member
Deva

For each voltage how much load current is required.
If it is high don't use linear regulator it will always give very worst efficiency.
You just give the load current values for each voltages then I will give the perfect solution to solve your problem.

Regards,
Deva
 

Hero999

Banned
How much current do you need?

The LM317 is fine providing the voltage differential between the input and output is <40 so for 15V you're fine as long as the input voltage is under 55V. This might be a problem if you're expecting to connect this whilst the battery is being charged so use the HV version if it's a problem.

Switching regulators don't have to be complicated or use proprietary IC, you can build a fairly efficient regulator using a couple of transistors.
Official Home Page of Roman Black
 

Thunderchild

New Member
comercial switchers are quite good nowadays if you want a hassle free solution, the amount of componenets required are minimal, if you consider the heasink space saved they are no more bigger than a linear reg and are easy to build even on matrix board
 
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