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Question about using voltage divider

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sita

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My friends,

My topic is in very basic section- "Voltage Divider"

As part of academic project, I need 5V, 3.3V, 0.5V supply accurately without any change. I planned to implement a simple power supply generation circuit of +5V and thereafter use Voltage Divider to get 3.3V, 0.5V. Approximate current taken by regulator will be 20mA-50mA, and we are using 50mA rating Stepdown transformer. IC also needs current requirement of max. 50mA. Will there be any problem. Will the same 20-50mA current will be available practically at divided voltage section?

For better accuracy, will implementation using high precision resistors (in divider section) be sufficient or should I need to use separate reulator for other voltages?

Waiting for your reply :|,
sita
 

Re: Voltage divider

Hello

If you're using the voltage divider just for generating voltage references, you should be alright with precision resistors as long as the output of the divider is buffered, but if you are going to supply power to some IC from the output of the voltage divider, then they won't be enough because the output voltage of the divider will change according the current drawn from its output. The reason of this is, when you connect some load to the output of the divider, you actually add another impedance in parallel with the bottom resistor of the voltage divider, reducing the value of that resistor.

The rule of thumb is that if the load impedance is many times that of the output load divider then the load will have little effect on the voltage and thus the divider will do its job.

Regards.
 

Re: Voltage divider

Hi,
Voltage divider will be a problem due to its internal resistance. You can get 5V and 3.3V using zener regulators and for 0.5V , use a forward biased diode. The unregulated DC voltage from the transformer /rectifer combination should be about 3V more than the required output. The transformer current rating should be around 200 to 250 mA, to reduce the winding drops when the DC current is drawn and also to reduce the heating of the coils if marginally rated to carry the current.
Regards,
Laktronics
 

Re: Voltage divider

Voltage divider is prone to loading effect as soon as a load is connected to the output
 

Re: Voltage divider

hello,

Oh you mean to use a reverse biased 5.1V, 3.3V zener in series with forward biased diode? And can I use a 230 to 12V step down transformer, so that unrectified output voltage will be:

(12*√2*2/Π) - 1.2V
1.2V =>drop in 2 diodes
So voltage will be around 9.603V. Will that be sufficient?
Why did you mention specifically 3V greater than output?

How can I design 3.3V and 0.5V with regulator IC LM7805?

Thanks for your reply, Waiting again for next reply:|,
sita
 

Re: Voltage divider

Hi,
You are on the right track. Using the regulator IC is the best approach.
 

Re: Voltage divider

To obtain a 5V or a 3.3V nowadays it's more simple to use some dedicated IC regulators. Low voltage dropout or normal ones, fixed voltage or adjustable, there’s a large IC series to be used. LM2940T, LM3940 or the classical LM7805 are some examples.
And for the 0.5V here's what might be the problem if you need an accurate reference. A simple diode won’t help to much. Any normal silicon diode have at least 0.6V at 20-30mA. Probably some schottky diodes will help. Try instead the 1N4148 two 1N5822 in series to obtain at 30-40mA the 0.5V required.
 

Re: Voltage divider

Hi Sita,
What does "wirhout any change" mean? There is always going to be some change question is how much is acceptable for your design? Power supplies are one of the most versatile things you could ask question about.
To help you specify problem here are few questions. Answer for each voltage individually.

- initial voltage tolerance
- maximum change of voltage in long time, e.g. hours and days
- maximum change of voltage in short time e.g. seconds to micro seconds.
- does load current change, how much and how quickly?
- how much voltage noise can power supply generate?

Might be better to describe what is it you are trying to power, be specific, with as many details as possible. It is likley that you just need regulated power supply and for that you already got some examples of IC's that can do very good job for you. 0.5V is little odd, but it is not to big of the problem.

Fixed regulators are widely available in3.3V and 5V versions. 0.5V is available in some adjustable regulators.
 

Re: Voltage divider

Hi, I think the best way will be not to use the potential divider, rather use voltage regulators. There are regulators having wide dynamic input and output tranges and can drive upto a few amps.
Use the 5V supply as input and make the settings of the regulators for desired output. LM117 will be appropriate for your use I guess.
 

Re: Voltage divider

Friends,

Actually, there's a slight modification in my requirement. Actually, I need a -10V, +5V, 3.3V.

Will this circuit send as attachment be sufficient.

What must be input current to get atleast 50mA at each outputs?

Regards,
sitra
 

Re: Voltage divider

Hi,
Inorder to get -Ve voltage, you need a -ve Voltage input and a -ve voltage regulator like 79XX type.
Regards,
Laktronics
 

Re: Voltage divider

hi
i think the best way is too use a voltage divider by opamps
 

Re: Voltage divider

hi
i think the best way is too use a voltage divider by opamps
 

Re: Voltage divider

sita
For schottky barrier diodes as for any diode, the forward voltage dropout Vf it’s related to the direct current flow trough diode If. Here are some examples of Vf/If, like 1N5820, BAT54 and DSF07S30U. This means you can obtain as 0.15-0.2V low voltage but the current must be kept in some limits in order to have very low variations and also this voltage it’s temperature related.
 

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