[sdiy] Quick and Easy Power Question

[Ben Stuyts]

Hi Shawn,

To answer your original question: having two of these on the same transformer is OK.

To echo what others have already said: C1/C2 seem low to me. Put at least some 4700 uF or 10000 uF / 35 V types in there, even for 1 Amp output. If you have a scope, have a look at the voltage across C1 and measure the ripple voltage. The lowest value over C1 should be at least 14.5 V, at full load. Any lower and the 7812 will not regulate.

But I do not understand how you manage to pull 2 Amps from this supply. I cannot find a datasheet for the 7812TV, so unless it is a DC/DC converter in disguise I assume it is a regular 7812-style analog regulator. 2 Amps is really A LOT for any 7812. My concern is about power dissipation: The input voltage will be around:

( 24 * sqrt(2) - 1.2 ) / 2 = 16.4 V.

Therefor the voltage drop over the regulator will be 16.4 - 12 = 4.4 Volt. At 2 amps, this gives a power dissipation of 4.4 * 2 = 8.8 Watt. Thermal resistance from junction to case is 5 K/Watt. Add to that the thermal resistance of the SK104 which is 14 K/W, so total thermal resistance is 19 K/Watt. As said, you dissipate 8.8 Watts, so temperature increase (above ambient) is 19 * 8.8 = 167 Kelvin. Assume an ambient of 21 degrees C, the chip will have a temperature of 188 degrees C. The internal regulator protection will shut down long before that.

So, just not possible to draw 2 Amps from that. (Perhaps those low 1000 uF caps are actually helping a bit because the average input voltage might be lower than 16.4 V, but at the cost of a huge ripple on your 12 V output. Or your transformer voltage might drop at 2 Amps.) How did you verify that it is actually capable of delivering 2 Amps?

Ben


> On 2 Nov 2020, at 02:03, Shawn Rakestraw <shawnrakestraw at gmail.com> wrote:
> 
> Here is the circuit for the PSU. You guys / gals helped me iron this one out almost a year ago. This schematic is for 1 amp on the +12 and -12, but I have built a couple now with 2 amps on the +12 and it does not get too hot for the regulator. The transformer would be your standard cube shaped iron core center tap transformer (120V to 24V). Circuit is based on Electronotes.
> 
> My main concern was that I want to have 2 of these psu boards off a single transformer. I suppose I could also have a transformer for each psu board and just split the mains before the transformers too.
> 
> As long as I am posting the circuit, I will ask one more question. This one, much more generalized. My filter caps are only 1000 uF and the circuit seems to work perfectly fine. I see others using an outrageous number of caps in parallel with much higher values. Is this advantageous or is it pure overkill and bordering on being a problem due to not fully charging the caps?
> 
> <psu.png>
> 
> On Sun, Nov 1, 2020 at 7:16 PM Ben Stuyts <ben at stuyts.nl <mailto:ben at stuyts.nl>> wrote:
> 
> > On 2 Nov 2020, at 00:49, Shawn Rakestraw <shawnrakestraw at gmail.com <mailto:shawnrakestraw at gmail.com>> wrote:
> > 
> > If I use a transformer that is say 24V and 6 amps, can I connect 2 of my power supply boards to that transformer? There's nothing wrong with that, right?
> 
> Hard to say without any additional info. It really depends on the topology of the power supply. For example, if you have a single wave rectifier on the input of the psu, and you have connected both psu’s in reverse, you will probably short-circuit your transformer.
> 
> Ben
> 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20201102/18fab0a7/attachment.htm>