too much voltage drop- how to correct it?

[robismyname]

what does too much voltage drop do

I am providing 3.3 volts to the emitters of the 3906 transistors and 3.3 volts to the bases of the 3904 transmitters. As you can see I made a 3.3 volt bus in my circuit (see attached document) and the purpose of the document is to show you how i did it. The problem is that I measure 3.3 volts across the bus but when the transistors are connected to the MCU via connectors I notice my voltage drops down to .6 volts. Any idea why this is happening and how to correct it?

 

[FvM]

3.3 volt drop

There's no 3.3V bus at all, just a rather high impedance node. Also the operation mode of display drivers seem to be far from a meaningful design, but that's another topic.

Generally, the existing 5V regulated supply is most likely better suited to power the display driver.

 

[robismyname]

Re: too much voltage drop

There's no 3.3V bus at all, just a rather high impedance node. Also the operation mode of display drivers seem to be far from a meaningful design, but that's another topic.

Generally, the existing 5V regulated supply is most likely better suited to power the display driver.

I need 3.3 volts output. I was told that I can use a lm317 variable voltage regulator. If my input voltage is 10 voltage what value resistors do I use?

 

[haff99]

Re: too much voltage drop

You use the resistor divider to set the output voltage but not the way you are doing it. Basically take what you have and connect the node where the 2 resistors meet to the middle pin. The reference voltage is supposed is supposed to be 1.25V. Based on that you can can calculate what resistors you want to use to get 3.3V out. If you want to keep the 3.3K resisor the top resistor needs to be 5414. If you read the data sheet there is an error term that you may want to account for

To make a bus you need to drive off the OUT pin. As FVM said you have a high impedance node and not a bus. If you try to pull current out of that node, you are going to drop all the voltage across the 2.2K resistor. This is a simple calculation that should take a few minutes. The idea behind a regulator is the output impedance is low. You have an high output impedance.

I would suggest you read the datasheet

**broken link removed**

 

[weeb0]

Re: too much voltage drop

If you want to use voltage divider ( which is not the best way to do it in your case ) you should use an opamp + transistor using a circuit topology called super diode ( the feedback of the opamp is connected to the emitter) to make sure the impedance of the other part of the circuit does not influence the current through your voltage divider.

You can also use a 3.3 voltage regulator. I usually try to use them since the temperature coefficient can influence the resistor value.

 

[rajudp]

too much voltage drop

no wonder why the voltage is droping. you are using a 5v supply to make 3.3v the current is very less 0.0009A ie 0.9ma not even 1ma to drive led segment and base