how to drop voltage

[surfstu]

Pulling my hair out here, am on my second transformer. Bought a 9Vac transformer to feed tps7a4700 LDO regulator set at 5.4V for raspberry Pi. This ran the TPS too hot which caused the Pi to crash. Then bought an 8Vac toroid which gave me 10.3Vac on the input to the TPS board.

The TPS board has 4 diodes for recitification which should give me 1.4V drop and then into a 3300uf cap which should allow for approx 2Volts Uripple which I don't quite understand as a voltage drop....

I then bought a LM7806 simple board from the bay:
L7806 LM7806 three terminal voltage regulator module 6V voltage regulator - UK | eBay

I bypassed the diodes onthe TPS board and now the LM7806 runs hot and the TPS board runs cool.... but the Pi is stable!!

But, I have a feeling the sound is not as good.... and that I could have unecessary components.

I would like to know if there is a way that I could get rid of the LM7806 and just drop a couple of volts prior to the TPS board diodes?

Can i put more diodes in to get rid of a few volts?

 

[rayma]

Can i put more diodes in to get rid of a few volts?

Yes, as long as they are used at less than their rated current.

 

[surfstu]

So I could put a forward facing diode at each input (transformer secondary to AC input of TPS board) to give me a further 1.4V drop?

Could I put 2 on each input to give me 2.8V drop?

Would you choose this method over a pre reg circuit?

 

[surfstu]

Thought some more and realise there needs to be a return path for the current. Don't think I can add forward facing diodes before a bridge rectifier.

Would still like to know what the best thing to do would be sound quality wise, I'm sure I lost some quality in sound when I used the 6V regulator board from ebay as a pre regulator

 

[rayma]

So I could put a forward facing diode at each input (transformer secondary to AC input of TPS board)
to give me a further 1.4V drop? Could I put 2 on each input to give me 2.8V drop? Would you choose
this method over a pre reg circuit?

There's no real problem doing this on the DC side, but on the AC side you'd need an external FWB
to drop the voltage. A linear preregulator would be best, especially when the input AC is not well known
or can vary somewhat. In theory the preregulator should improve the LDO regulator's performance.
Maybe your preregulator is too low in voltage.

 

[surfstu]

Thanks Rayma, I will go with pre regulation, have attached a pic of the lm7806 reg circuit, can I simply swap in a 7808 regulator or would I need to change the resistor or any cap value?

 

[rayma]

Thanks Rayma, I will go with pre regulation, have attached a pic of the lm7806 reg circuit,
can I simply swap in a 7808 regulator or would I need to change the resistor or any cap value?

Hopefully the preregulator has an output capacitor on the board.
Then this should be ok with the 7808.

 

[surfstu]

Maybe L7808CV?

No, the diagram above is not correct, both capacitors on my board are pre lm7806

But, this will then feed the bridge rectifier of the TPS board which is followed by 3300uf

 

[rayma]

Maybe L7808CV?

Sure, most of these are equivalent.

 

[MIKEVO]

Stu, the Rasperry Pi should run at 5.1Vdc and optimally the power supply is best specced at 2.5amps.

The 7806 board is only rated at 1.5 amps as far as I am aware.

Also using diodes to drop the voltage may cause issues later as the diodes may vary their voltage slightly depending upon current draw.

Don't forget to put a smaller cap in Parallel with the main reservoir cap to give you a low impedance at higher frequencies and reduce noise.