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# Potential divider...

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#### Appleon

##### New Member
I know that u can use 2 resistors as potential dividers, but can anyone enlighten me on why a zener diode-resistor arrangement is a better solution, since I was told that commercially, a zener-diode potential divider is more popular. Any help on this?

Thanks!!
Appleon

Because the solution with zener not only voltage divider. The voltage on zener is stabilised.

Zener diode...

Hi, so am i right to say that with a zener diode, not only will there be less power dissipation, but also that the output will be more stable and constant? What about the choice of diodes? I read somewhere that the load resistance cannot be too big, but how do u find the impedance of a diode?

Plenty of thanks!!

Cheers!!
Appleon

Hi Appleon,

A simple resistor divider is ok for dropping voltage to a circuit that draws a constant current and does not require a precise voltage level.

A resistor-zener diode circuit will supply a circuit with a fairly constant current and voltage. You must know how much current and voltage your circuit requires before selecting the resistor and zener combination.

A simple and inexpensive solution is to use common 3 terminal voltage regulators such as LM78xx shown here:
**broken link removed**

Hope this helps a little, Tony

I see...

Hi Seeker,

thanks for pointing that out. If u were to use the regulator as mentioned, i think there will be loss of power as heat correct? Is there actually a more power efficient way? Thanks for helping anyway.

Cheers!!
Appleon

Hi Appleon,

As far as I know, there is always loss when converting voltages....and that loss is normally shown as heat.

I don't know of any *more* efficient voltage regulators.

If you give us a schematic of your circuit maybe we could help more.

Thanks, Tony

regulators and references

Whether a zener/resistor combination is "better" than a two resistor divider depends on what you're trying to use it for. Are you using the output as a voltage reference, or as a power supply regulator?

Output from a resistive divider is only as accurate as the supply or reference it's driven from (plus the tolerance of the resistors).

The zener diode/resistor will have a much lower output impedance than a resistive divider, on the order of a few ohms. A zener is a shunt regulator, and dissipates some power regardless of whether a load is connected. The 3-terminal regulator mentioned above is a series-type regulator, and also consumes some quiescent (standby) current. The low-dropout types can be very power efficient when the input voltage is close to the output voltage. For large differences between input and output voltages a switching regulator will provide the best conversion efficiencies.

So the important question is, what are you planning to use the circuit for?
Is it a reference or a power supply regulator. Does the output voltage need to be absolute or proportional to the input supply?

- CAL

Heat...

Sorry, I wasn't clear. I meant that the heat losses due to the regulator chip is usually much higher correct? I made the statement based on the diagram in the link refered to above as I have used a regulator chip fed with a 9V directly to convert to 5V and it became hot fairly quickly.

Actually, when we use a zener diode, we can get a more accurate output voltage, so the regulator is just to ensure a constant output with minimal quirks correct? Am I right to say that I am actually stepping down an external input voltage twice, once from the divider and once from the regulator, which usually requires 2V difference between the input and output? If I were to do this, can I say that the output is close to an ideal constant DC Voltage?

As for the zener diode, my aim is actually to use it as a reference voltage, so I gather that a resistive divider is actually more suitable as I don't need such an accurate value. Am I rght to say that for either arrangements, power will be dissipated, but if my chouce of resistor is reasonable, it will incur less losses than a zener diode arrangement? Let me read up on switching regulators for a better idea on how it works. Any reccomended readings?

Actually, I am trying to charge a battery using some form of AC, so after rectifying the AC to a DC, I need to put in the DC-DC converter to ensure a constant voltage and current. Since the power generated from my AC is fairly low, I am trying to minimise power losses as much as I can for efficient conversion.

Thanks!!

Cheers!!
Appleon

regulators and efficiency

I'm sorry, you meant the heat losses due to the regulator are much higher than what?

9V to 5V with a linear regulator isn't a very efficient setup, but it's often ok for low current uses. At 1 Amp, a regulator would have to dissipate (9 - 5)V x 1A = 4 Watts, you didn't mention what the current draw of your circuit was. A two stage approach using linear or shunt regulators doesn't improve efficiencies, but spreads out the power dissipation. A better approach for dealing with large input/output differences is to use a switching regulator. A quick web search will provide you with plenty of reading material.

Heat...

Hi,

I was thinking that there might be more efficient ways, since like I mentioned, the regulator got heated up fairly quickly, an indication that much power is lost as heat. I saw a simple circuit using a switching regulator, and since it is connected with an inductor-resistor, it should be more power efficient, so I think this is more efficient than a linear regulator, even if the current drawn is 1A. So if I were to use a switching regulator, I gather that a zener diode arrangement is not necessary to connect AC-DC conversion part to DC-DC conversion part, esp since this diode actually dissipates power. something not very efficient?

Cheers!!
Appleon

Input Divider

Hi i was just wanted to make sure that an Input Divider only works with the values of Current and Voltage.
Can someone tell me, please, what they are used for?

Much Appreciated

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