# How to combine 36-60VDC source and a regulator IC with 5-25VDC input range ?

Discussion in 'General Electronics Chat' started by nqchanh194, Aug 21, 2014.

1. ### nqchanh194 Thread Starter New Member

Jul 5, 2013
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1
At first, I would like to describe the situation I'm stuck in. Here I've got a regulator circuit with the input range 5-25VDC and now I would like to put it on a device which has the voltage supply range 36-60 VDC.
My initial idea is a voltage dividing circuit using resistors, but are there any potential problems for this way ?
Another alternative is to use a step down voltage regulator. However, it's really expensive.
Hope you might give me some suggestions.

Last edited: Aug 21, 2014
2. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,924
602
The voltage divider by itself would work very badly. Why? Because the load will be seen as another resistor in parallel with one of the resistors that form the voltage divider. Lets say you have a voltage divider formed by two resistors, 1 kOhm and 2.2 kOhm. Now you put a load whose resistance is 150 Ohm in parallel with 2.2 kOhm. What is the equivalent resistance of these two parallel resistors? 140.4 Ohm. So your 2.2 kOhm resistor has turned into 140.4 Ohm resistor. That is not good and your voltage divider is no good.

What you could do is put a buffer (op amp configured as buffer or transistor circuit configured as buffer) between the resistor of voltage divider and the load.

Next question is how much current do you need. An op amp that is used for a buffer can source some current, but not a lot.

Last edited: Aug 21, 2014
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3. ### nqchanh194 Thread Starter New Member

Jul 5, 2013
27
1
In this case, my circuit's current consumption is about 0.2 for maximum !
I think a step down regulator is also appropriate !

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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How about a simple zener regulator - given such a low current is required.

Is the 30-60V supply adjustable to a reasonably stable value? Such as 30V without loss of regulation under light loading? Or is the output unpredictable within the 30-60V range?

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5. ### nqchanh194 Thread Starter New Member

Jul 5, 2013
27
1
the voltage output is fixed to a reasonably stable value without loss of regulation under light loading !

6. ### ScottWang Moderator

Aug 23, 2012
5,450
858
Using LM317HV in series with your another regulator, the Vout of LM317HV set to 25V, and then connecting to your regulator.

30~60Vin → LM317HV Vout=25V → your regulator (Vin=5~25V)

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