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Old Dec 13, 2004, 05:28 AM
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Suntzu's Avatar
Canberra, Australia
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Got a new 2.5amp wall Power Supply, Need Help!

I got sick of my little 12v lead battery tiring out when charging my Swallow and lipos.

I found a new 2.5amp 12v psu with a normal 240v (wall connection in australia). I thought "Great", just what I need for my swallow charger as I only charge at about 1.5amps.

But the unit actually puts out 17V and I checked a few other "12v" units and they put out 17v as well. pfft! Even though they are rated for 12v.

My swallow chargers wants a max of 15v.

Whats the easiest and safest method of dropping a 17v 2.5amp supply to 12-15v??

I was thinking a 5w ceramic resistor but this is getting beyond my EXTREMELY limited electronics knowledge.

Can some one please advise me on this.?

Thanks
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Old Dec 13, 2004, 06:17 AM
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Sayre,PA
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It probably is an unregulated power supply. Under load it is likely that it will drop below 15V so it should be OK to use with your charger.
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Old Dec 13, 2004, 06:22 AM
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Canberra, Australia
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I thought the same also, until I tried it and it didnt drop one iota. The charger wont let me engage the charge before the error or "input voltage" message is generated.

Its a decent high quality new unit that I found in my shed, looks brand new, not a cheap one. I thought it would be ideal ..:O
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Old Dec 13, 2004, 06:42 AM
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I still think itís an unregulated supply and until the charger starts the charge there is not much of a load on power supply to draw it down. Try putting different resistors across the output and see how much they drop the voltage. I would start with something like 25 ohms of 10W or more. You should be able to find one that will make the charger happy without drawing too much of the current. An inexpensive 12V voltage regulator might also might work if the voltage under load doesnít drop too much.
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Old Dec 13, 2004, 06:43 AM
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Perhaps you could connect it to the lead battery to lower the voltage.
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Old Dec 13, 2004, 06:52 AM
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Another thought. 2.5 amps may not be enough output. My lipo charger needs 5 amps in to put out just 500ma.
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Old Dec 13, 2004, 07:09 AM
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Is your wall charger designed for charging lead batteries? I use a standard cheap car-battery charger permanently connected to my lead battery, so that the lead battery is being topped up at the same time as I'm drawing current from it.

If your wall charger isn't specifically for lead batteries, you might be able to make it work with your lead battery by putting a resistance in one of the leads. That way, your Swallow should 'see' something close to the voltage of the lead battery, not the voltage of the wall unit that's charging it. Just as a guide, my car-battery charger indicates that it's putting 1A into the battery when it's fully charged, and it goes up to about 3A when I'm drawing current with my flight battery charger.
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Old Dec 13, 2004, 10:12 AM
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Haralson County GA. USA
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Quote:
Originally Posted by Ron H
Another thought. 2.5 amps may not be enough output. My lipo charger needs 5 amps in to put out just 500ma.
That would be only 10% efficency ,meaning that to charge a 1200ma pack would take 12 amps. and to charge a 2200ma pack would require 22 Amp.
I think perhaps you are confusing the mininum required to charge anything to the mininum required to charge at the maxinum capabilities (output)of the charger.

Charles
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Old Dec 13, 2004, 12:21 PM
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Florida
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You should be able to put any good 12 volt three terminal device (on a heat sink) between the power supply and your charger to get a constant 12 volt to the charger. Check with any local electrical component supplier or check the web for 12 volt regulators.
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Old Dec 13, 2004, 12:54 PM
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United States, CA, Norwalk
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Quote:
Originally Posted by Rodney
You should be able to put any good 12 volt three terminal device (on a heat sink) between the power supply and your charger to get a constant 12 volt to the charger. Check with any local electrical component supplier or check the web for 12 volt regulators.
The problem with using a 3 terminal regulator is that most of the common ones (7812 LM317, etc) have a high minimum voltage drop. The voltage of your power supply will drop under load, and if it gets down under 15 volts, a 3 terminal regulator might allow the output voltage to get well below 12 volts. I think that Jimsp's suggestion is the best. Put enough of a load on your power supply to bring it under 15 volts. It probably won't take a whole lot of load. His suggestion of a 25 ohm, 10 watt resistor would be a great starting point.

Dan
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Old Dec 13, 2004, 03:13 PM
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Bishopville S.C.
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Quote:
Originally Posted by everydayflyer
That would be only 10% efficency ,meaning that to charge a 1200ma pack would take 12 amps. and to charge a 2200ma pack would require 22 Amp.
I think perhaps you are confusing the mininum required to charge anything to the mininum required to charge at the maxinum capabilities (output)of the charger.

Charles
I suppose I should have said it needs 5a in to operate. It doesn't vary with the ouput, which is limited to 1a.
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Old Dec 13, 2004, 05:05 PM
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Haralson County GA. USA
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Ron H,
What charger do you have which requires 5Amps input to operate at all(500ma) output?.
I think you missed my point the Triton requires 13amps at 12 VDC. to achive maximun potential however it will charge a 2 or 3 cell Lipos of up to 1200ma just fine off of a 2 12 v 2 amp power supply.

Charles
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Old Dec 13, 2004, 08:42 PM
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Celectra. It charges 1s or 2s only, 500 or 1000 mah. El cheapo, what should I have expected?
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Old Dec 14, 2004, 01:51 AM
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Quote:
Originally Posted by Suntzu
I thought the same also, until I tried it and it didnt drop one iota. The charger wont let me engage the charge before the error or "input voltage" message is generated.

Its a decent high quality new unit that I found in my shed, looks brand new, not a cheap one. I thought it would be ideal ..:O
I ran into same problem using a 12 volt powerr supply on the triton charger.
It was delivering over 15 volts. I can fool it and put a slight load on it , but once the charge process is started. in 55 seconds the trion does a volt check, and the voltage spikes back up to over 15 throwing oput the input volt error again.


Oh well not much of an answer for you, but I just use a different ps now for the trion.
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Old Dec 14, 2004, 02:35 AM
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Suntzu's Avatar
Canberra, Australia
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I tried a 10w 22ohm ceramic resitor just then. Problem is that it wont drop the volts until the load hits, but the charger detects and fails at 17v before the load can be applied. ARRGGH.

Im gunna use a bigger resitor next with higher ohms.

Thanks for the input, keep it coming. I learn something everyday in this hobby, thats whats great about it. Some of it s even useful!
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