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Old 27th May 2010, 01:43 AM   #1
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Default DC-DC Converter

I have a Rotel amp with trashed transformers (one completely missing, one shorted). I have shoehorned in a toroid transformer, and I have it back up and running. But, the old transformer had a 5.5V winding to power a couple of lamps in the front panel power meters. The new transformer has nothing of the kind.

The rectified voltages are at +/-60V, and to use a simple resistive current limiter would mean a rather large resistor and a major waste of power (8+ watts). I was looking at a small 3W DC-DC converter, but I've never used one. Cincon makes a model EC3SA-48S05 that can take from 36 to 75V input, and output 5V with a max current of 600mA, and sounds like it'll do the job nicely.

But...the data sheet is pretty sparse with info, and the website isn't any better. With the pinout, I assume the '-V' input simply will be connected to ground? I also assume that the control pin is left unconnected? What about noise? I don't want to see a bunch of RF leaking into the preamp or amp circuitry.... Is there also supposed to be any additional filtering components connected to the output?

If someone has a bit of real experience using these small DC-DC converters, I'd much appreciate a bit of guidance.
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Old 27th May 2010, 02:24 AM   #2
infinia is offline infinia  United States
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control has internal pulldown so open is ON.
RFI is what I'd worry about on the secondary and the primary is conducted noise or hash. Some DC/DCs are worse than others. Strange that these are fixed ripple at 75mV pp regardless of output voltage.
What I'd do is try hand winding seperate secondary on the toroid for a dedicated AC circuit. Say 12-24 turns and check the voltage with some light load.
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Old 28th May 2010, 12:28 AM   #3
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Well, I was wanting to learn something about a class of devices that I've not used before. Thought there might have been a good number of members here with experience using these devices.

I finally measured the current required (120mA @ 5V), and settled on a 1.5W TDK CC1R5-4803SR-E. At least their data sheets are much more informative.

Last edited by EchoWars; 28th May 2010 at 12:31 AM.
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Old 28th May 2010, 12:31 AM   #4
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Would it be possible to wind a few turns onto the toroid to make a 5.5V winding?
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Old 28th May 2010, 12:34 AM   #5
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Actually, I did that. It seemed to work (as far as I went), but the usually very-quiet mechanical humming/vibration of the transformer got considerably louder. I didn't like that, and abandoned the idea.

I don't think I'll ever use these cheap-*** Avel-Lindberg transformers again.
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Old 28th May 2010, 12:55 AM   #6
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For those lamps you can get 5.5V directly from mains, using a series capacitor, one or two resistors, a rectifier and a zener diode, if you intend to feed them with DC (although could be used AC as well), providing the necessary isolation of wires.
I will try to draw the circuit and post here.
Best wishes,
Max.
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Old 28th May 2010, 01:07 AM   #7
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Default LED under 220V

I often use this configuration to light an LED directly from mains.
You may change the values of capacitor for 120V and the zener diode for 5.5V. The LED will drive the forward current, while the zener diode the reverse one.
You may insert the lamps in place of the LED, providing necessary zener and resistor values, but take care of the maximum current that capacitors can draw. If necessary, use more than one in parallel.
I think some changes and it will work.
Or you may also try to replace lamps with bright white LEDs (e.g. 3.6V).
Best regards,
Max
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File Type: jpg 220v-led.jpg (11.0 KB, 98 views)

Last edited by smartx21; 28th May 2010 at 01:10 AM. Reason: ...
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Old 28th May 2010, 02:14 AM   #8
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The lamps are inside the meters. Axial-lead lamps with a long skinny body.

If I drove it off the mains as you suggest, that is even more components than using a DC-DC converter, and I'd need to fuse it.
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Old 28th May 2010, 02:57 AM   #9
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Quote:
Originally Posted by EchoWars View Post
Actually, I did that. It seemed to work (as far as I went), but the usually very-quiet mechanical humming/vibration of the transformer got considerably louder. I didn't like that, and abandoned the idea.

I don't think I'll ever use these cheap-*** Avel-Lindberg transformers again.
Hmmm somethings not right then. Did you use thin stranded hook-up wire?
Simple solution using this instead of SMPS. Measured volts/turn?
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Old 28th May 2010, 02:59 AM   #10
jaycee is offline jaycee  United Kingdom
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it's also highly dangerous!!
Seems a bit odd that winding only a few turns onto the core made it growl.
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