Hello
This evening I started my build.
I started with a TX board.
There was a problem with my component order from RS - they accidentally missed out the 470uF electrolytic capacitors I had ordered. This got me investigating whether I could make do with an alternative component.
From what I can see, this capacitor (C3 in the TXv3.12) is used on the ouput side of the 7809 linear 9V regulator. Wherever I looked online at example circuits for 78xx linear regulators, I see much smaller values for the output capacitor(s) - just 100nF or maybe 1uF - nothing like as big as the 470uF. In fact, I read that it's generally better not to have big capacitors on the output of the regulator, as they don't like there being voltage across the output when the input power is switched off. On the input side of the regulator, then this sort of size capacitor is more common, used to smooth out voltage ripple on the input supply.
And so I'm curious to know why that 470uF capacitor on the output side is in the design. The only explanation I can come up with is that it's for giving a bit of ooomph juice for starting up a fan motor (for a temperature sensor) from stationary. I'm reckoning that its got nothing to do with the relatively low power radio transmitter. So if I'm not having a fan, can I use just a ceramic 100nF? I'm going to look back at the archive of pre-fan circuits to see if I can get more evidence...
Could someone explain this? Maybe the chief designer himself?
Thank you
This evening I started my build.
I started with a TX board.There was a problem with my component order from RS - they accidentally missed out the 470uF electrolytic capacitors I had ordered. This got me investigating whether I could make do with an alternative component.
From what I can see, this capacitor (C3 in the TXv3.12) is used on the ouput side of the 7809 linear 9V regulator. Wherever I looked online at example circuits for 78xx linear regulators, I see much smaller values for the output capacitor(s) - just 100nF or maybe 1uF - nothing like as big as the 470uF. In fact, I read that it's generally better not to have big capacitors on the output of the regulator, as they don't like there being voltage across the output when the input power is switched off. On the input side of the regulator, then this sort of size capacitor is more common, used to smooth out voltage ripple on the input supply.
And so I'm curious to know why that 470uF capacitor on the output side is in the design. The only explanation I can come up with is that it's for giving a bit of ooomph juice for starting up a fan motor (for a temperature sensor) from stationary. I'm reckoning that its got nothing to do with the relatively low power radio transmitter. So if I'm not having a fan, can I use just a ceramic 100nF? I'm going to look back at the archive of pre-fan circuits to see if I can get more evidence...
Could someone explain this? Maybe the chief designer himself?
Thank you
