Voltage quadrupler double check, please.....

[G._Hoffman]

So, I need to power the LED's for the front panel on my AC30.  I'd like to use some of the "extra" current from my heater tap.  The LED's are 3.3V_F, 30mA max LED's, in two strings of 5 with (I believe - I can't see any more) 1K current limit resistors, so I need to get the voltage up a bit to run them off of the heater tap.

(I did think about running them off of the mains, but that would take a 5 Watt voltage divider, and I'm not really liking that idea!)

So, the LED's look good at anywhere from about 20-40 mA total (measured), which takes anything from about 20-30 volts (maybe a bit more - honestly, it doesn't change the look much once you get up past about 22mA).  So, my figures are that a voltage quadrupler will give me (after diode drops) approximately 23V, (the voltage tripler was only about 18 volts, which isn't quite enough to make the logo look good), and should output about 30mA. 


Now, my big question - am I correct in assuming Ohms law holds true and this will then draw 120mA? 


None of the sources I can find online seem to talk about the current issues with voltage multipliers, but I need to be a bit careful with my heater tap, since the extra tube (EF86) has already got me up to 5.04A on a 6A tap.  120mA seems fine, but much more and I start eating into any remaining safety margin.

The other option is to put a mains plug inside the amp and find a wall wart that would work, but for a lot of reasons I don't like that idea!


Gabriel

[kagliostro]

To me, using a quadrupler, at the transformer the consumption is four times the consumption at the utilizer

so, assuming your LED has a consumption of 25mA, the transformer sees a consumption of 100mA

Franco

[G._Hoffman]

OK, the Sim is saying that for about 20mA on the LED's, it's going to draw about 220mA on the transformer.  What the hell?  Is this a sim error, or are the caps in there drawing a crap load of current?


Gabriel

[G._Hoffman]

And the current spike when you turn it on is like 1.6A!!!!!!!!!!!!!!!! 

Again, just according to the Mouser sim, but I don't want to try that if it's real.


Gabriel

[PRR]

The staring surge will not burn up a transformer.

Say two 22mA strings. 44mA at the quadrupled voltage. 176mA at the base voltage. This is DC. A peak-catching rectifier will pull an AC current 1.6X-1.8X times the DC current. ~~300mA.

I doubt 0.3A added load on a 6A winding already carrying 5A is a big deal. At worst it raises the long-term running temperature of the transformer a few degrees, takes a few percent off its statistical life-span.

[G._Hoffman]

Thanks, both!


Gabriel

[Paul1453]

I'm not sure what these LED strings are for,

but if just for lighting wouldn't it be easier to run them in parallel with just a dropping resistor on the heater tap?   

Added benefit of parallel wiring is if 1 LED goes bad it doesn't take down the whole string of LEDs.   

[G._Hoffman]

I'm not sure what these LED strings are for,

but if just for lighting wouldn't it be easier to run them in parallel with just a dropping resistor on the heater tap?   

Added benefit of parallel wiring is if 1 LED goes bad it doesn't take down the whole string of LEDs.   

It's for this:



And it's all wired up in parallel - and hard to get to - already. 


For what it's worth, I made the voltage quadrupler over the weekend, and it seems to be running fine.  I should probably add some bleeder resistors, and it turns out that even with some rather oversized vents I still am going to need a fan.  I let my nephew play through it for a few minutes earlier, and the air coming out of the middle vent was noticeably hot. 


Gabriel