[ARC5] running ARC-5 receiever fillaments on AC.

[Peter Gottlieb]

That is why you need a capacitor after the rectifier.  It holds up the DC going 
into the regulator between AC peaks.  The 24 V secondary, full wave rectified 
with a capacitor, is fine for a 7824.  By your calculations below, at low line 
you have 30.5 volts, let's say you use a capacitor which yields 1 V P-P ripple 
under load, the lowest voltage the 7824 sees is 29.5 volts, which is plenty of 
margin over the ~26 volts minimum it needs on the input. You're going to need a 
reasonable capacitor, just guessing in the range of 10,000 uF.

Peter
kb2vtl


On 8/26/2017 10:04 PM, Brian wrote:
> Using a 24 Vac transformer is a very poor idea. You need to ensure that the 
> regulator under full load does not see dips in the rectified DC from the raw 
> rectified AC. You also need to allow for AC mains droops; the electricity 
> supply authorities are permitted 10% droop in mains supply. Suggest at least 
> 34 Vac nominal.
> Here’s the maths on your suggestion:
> 24 Vac rectified –> very rough 33.94 Vdc peak, less 10% supply droop –> 30.54 
> V peak, less two diode drops (FWB) at full load –> 29.14 V peak value. The 
> average will be less. Doesn’t leave any headroom for the 7824. You can do the 
> maths on 34 Vac.
> You’ll find the same design ‘philosophy’ used in very good quality telephone 
> exchange battery supply and back-up systems. Where this design philosophy is 
> ignored, eg, the exchange is run by an accountant, you will hear hum in 
> telephone handset earpieces throughout the whole exchange area.
> And just a little point – no ARC-5, ATA, ARA, SCR-274N equipment uses tubes 
> with filaments – all their tubes have HEATERS.
> 73 de Brian, VK2GCE.
> *On* Sunday, August 27, 2017 11:29 AM, you said:
> <snip>
> I'm going to rectify the 24VAC to 33.6 VDC  and pass that through a LM7824 to 
> get back to 24V.
> That should keep the oscillators happy!
>
>
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