Astrodyne RS75 75 Watt Switch Mode Power Supplies 13.5 volt down to 12 volts

[mantha3]

I have a question on running an Astrodyne RS75 75 Watt Switch Mode Power Supply at 13.5 volt down to 12 volts. Would I just run 1,2 or maybe 3 diodes to get down to 12 volt? Any specs on good diodes I should buy

some background:
I am building a boom box/book shelf stereo. I will have some 12 Volt components in this (12 Volt Fan, 12 Volt LED light strip) that I'd like to run so I thought it was a good idea to get the 13.5 volt down to 12 volt for these items… I'd like to run the AMP6 at the full 13.5 Volt
I am building a boom box/book shelf stereo. I plan to run this on a 12 volt battery and also have this 13.5 volt power supply for when in the home.

I am going to put in the main compartment a 12 volt set of LED lighting to light the inner electronics area so people can see what is inside. I also will run a 12 volt PC fan (not needed but I have it, why not)
Here is the specs on the power supply:


Model Number Output Voltage Output Amps Load Regulation Ripple and Noise Over Voltage Protection DC Voltage Adjust (typ.) Efficiency
RS75-15 15 Volts(DC) 5.0 Amps ±0.5% (Note 4) 120mV pk-pk (Note 1) 17.25 ~ 20.25Volt(DC) Hiccup mode, auto recover 13.5~16.5Volt(DC) 0.82

This is what I was thinking of building and the yellow box is where I'd put the diodes:



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[FFF]

Y. is ok. Will also prevent current from battery>smps. And a low drop diode to prevent the reverse might be useful too. (smps>battery), unless the smps is supposed to charge the battery of course.

[mantha3]

Would you recomend any particular Diode or Diode spec? I go online and look at places selling Diodes and I'll see 85,000 different types to choose from. I'm just learning. I was looking for input on the voltage reducer diode(s) mostly but I think your idea of doing a diode to protect the batter is also a good idea!

Thanks for taking the time to respond to this! I'm a newbie.

[FFF]

For leds+fan:
- 1N4001, glass-type 50V,1A 15 eurocent each. (I assume 1A is plenty)
- 1N4002, non-glass-type, rest=same
- 1N4007, 1000V, rest=same

Else just use 3x inline to Fan + same to LEDs (separate feed)
If using more serious Fan and LEDs for whole home, then: 1N5404, 3A, 400V, 45 eurocent here.

Try a bit, you can get 10x1N4001/2/7 for 1 euro.

All types should do.

[mantha3]

I'm reading some online content on "Zener diodes" and "General purpose signal diodes such as the 1N4148 are made from silicon and have a forward voltage drop of 0.7V. "

I think I'd go General Purpose and use 2 of these or so. I could dial up the Power supply to 4.1 volt output and use 3 of these General Purpose.

Time to buy a voltage meeter.

This is fun stuff.

[FFF]

Zeners are used parallel to a load, to keep it at the zener-rated voltage (for more power it's hooked to a big transistor).
You just need a voltage drop.
Get a cheap DMM (digital multi meter) from a not-electronics related shop, 5 euro or less. it will do.
Try with more diodes (4-5) first to test, then maybe take 1 or 2 off.
Do not count on 0.7V, can be more or LESS too. and over time, add 1 extra in-case-of.

[tuo]

Well, yes, they do. But 1N4148 is a general purpose signal diode, whose current rating is only 200 mA. This is not enough for a power amplifier, so if you use these, they will blow eventually. FFF has listed better options, and those diodes are still very cheap and do have similar ~0.7 V forward voltage drop.

[Scratchy]

It's great to see you are planning ahead for your build.
However, there is really no need for the fan. This is just adding noise (both electrical and mechanical).
If you feel you must use the fan, then there is really no need for those diodes. This is just adding complexity.
A 12V fan will run fine at 13.5V (just a little faster).

As for the LEDs, technically there is no such thing as a 12V LED.
LEDs have current limiting resistors designed to limit the current according to the input voltage.
There is a great LED resistor calculator linked in this post:
http://www.41hz.com/forums/showthrea...tor-for-an-LED

[mantha3]

Hey FFF! Thanks for the detail you just added. I looked these up and clear as day. Thank you for the response.

[FFF]

But: it is fun blowing up diodes! The bigger the better, just like electrolytic caps!
And have one of you guys tried to 'charge' ordinary zinc batteries? You can kill someone with the carbon rod shooting out (try with 12V).
Also with 24V and two carbon rods 2mm (refills from refillable pencil), you can make the tips glow so hot it will actually melt glass!
those days.....wait! I can do it now too!