Topic: 20 Amp power supply

For all you folk out there that use a 20 amp dual output power supply sold by R*pm*x for powering your chargers, I have a word ( or two ) of warning....

I recently received one of these units to repair for one of the members..

Symptom was it wasnt working....Completely dead..

On receipt I opened it up and plugged into the mains. Switch on nothing....

The design of the power supply is based on the Half Bridge Switcher technology (same as a computer power supply ) with a TL494 pwm chip. On the primary side there are a couple of hefty switching transistors, 2SD209L. These have a Vceo rating of 700v and an Ic of 12 amps, more than adequate for a 270 watt power supply. The secondary side of the transformer is feeding 2  dual Schottky diodes rated at 30 amps each! This unit is certainly bullet proof you would think....

So onto the demise...What is dead?

Initally, apart from the toasted choke, which was rewound, 2 diodes blown and the 2 2N2222a driver transistors were shorted...After replacing them and a handful of capacitors to ensure reliable operation in the future, time to switch on....

Nothing, zilch, nada...not even a puff of smoke...

Further checks revealed the power switchers were shorted Base/Emitter, the base driver resistors ( 2ohm 1 watt) were open circuit too.

Basically the power supply has toasted itself, which is a surprise because looking further, the design incorporated current limiting circuitry..Obviously this didnt do anything to prevent the thing destroying itself.....Needless to say the unit is beyond economical repair, sourcing replacement power transistors being the main problem.

So my advice if you own one of these is not to overload it...Although it is supposed to deliver 20 amps into a load, I'd be inclined to derate it by 50% and limit loading to around 10-12 amps max, unless you are looking to buy a new one...

A quick update on this......

I've successfully repaired this power supply and will be returning it to its owner...

There's no apparent reason for the original fault...electronics do fail, especially power electronics where heavy currents are involved.

If I were to be in the market for a power supply for charging batteries, this one wouldnt make the list...

Actually I've just read over on RC Groups of a member who  has experienced the exact same failure...except the magic smoke managed to sneak out..

Have a read here if your so inclined....My power supply blew up! - RC Groups

Ok,will keep an eye on mine 
I have been drawing 22 amps out of mime from day one,even seem 24 at one time 
this has been run like this from 11 - 4 most times i go flying
been well pleased so far
will let you know when it goes bang 
good job i have two others as well running at the same time
all been fine so far 

Well mine died with a 0A draw. I turned it off at the wall, unplugged it and plugged it back and it was totally dead. No noise, no smoke, nothing And I didn't even have a load attached.

Richard

Richard,

pm sent.......

PM returned

Thanks, Chippie!

I have a ripmax 8.5 amp power supply the plastic case one with a cooling fan, bought it second hand and didn't give a lot for it, been using it for a few months no problem pulling 5 amps max, switched it on last night with a schulze 330 connected and bang, no smoke but a horrid smell.
i thought it might have been the power cable as i have had simaler on domestic appliances so opened it up but the power cable was ok and no obvious signs of any charred components, so it looks like these ripmax jobs are none to reliable, good job i have another couple of power supplys.

if i remember a post about a year ago the same thing happened to a rip-max power supply
Ron

An update on RGN's unit.....
Again both switchers were shorted B-E, this time the 2R resistors were intact, but instead, one of the 10R was open circuit. I'm thinking that if this resistor went open then this is possibly the reason for the transistors dying..

Anyhow, after wholesale replacement of many components it will live to power another day...Fortunately the choke in this unit hasnt been cooked...

Thanks, Chippie, I really appreciate this! 

Richard

Well, I received the unit back tonight and Chippie has breathed new life into it It's working again and is currently on a soak test with a 5A load and it is holding a constant 13.76V.

Judging by the bag of blown bits Chippie sent with it I'm amazed he got it going again.

Chippie, I really appreciate you help, thanks

Richard

I've got a couple of them. First one I bought, no trouble at all - had it about 3 years. The second - about 18 months ago - dead on arrival. Exchanged and occasionally seems as though it is cutting out if the ambient temperature is too high = twice in the last 6 weeks or so it simply refused to come on other than a brief flicker on switching on. My workshop gets pretty hot in summer and would have been well over 80F at the time. In the cool of  the evening it worked fine again.
Mike

Guys,

I have two Turnigy 8150 chargers that I would like to power. I would hope to be able to charge a 6S 5000 pack on each.

I had a fusion 200W twin power supply that went pop on me but even when it was working would power both charger doing 6S

what are my best options, without breaking the bank obviously.

I have a pro peak single output power supply powering my third charger which is what Iwould take to the field so size and weight aren't too much of an issue.

 

...I had a fusion 200W twin power supply that went pop on me...

So did mine - it replaced my Ripmax single which also went pop, and none of my chargers can pull more than 5A.  

Is it impossible to make a fairly reliable, reasonably priced power supply?

Mike

do a search for computer power supplies conversion  many have done it over the years
Ron
found one for you
http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

Hi,
I bought some of these off E-Bay and they are around most times for about £10 - £15. The mods. required are really simple and do not involve digging into the internal circuitry; just external connectors. See:-

http://computers.shop.ebay.com/Computers-Networking-/58058/i.html?_nkw=DPS-600PB&_npmv=3

If you look at the thread on RC Groups (My post 310) there is photo and details.
http://www.rcgroups.com/forums/showthread.php?t=1005309&page=21#post16193582

These units have the 12V 47A output as the main controlling output unlike the PC psus where the 12V outputs are auxilliary outputs and not intended as the main load. PM me if you need more details.

Alternatively if you just want to buy a psu, I would reccomend the bench units from Rapid which are variable, metered units 2.5V - 15V  @25A or 40A at £65 or £95 ( + VAT).  I have a 40A unit which is also useful, being variable,  for testing quite large motors (600W). I spent my working life in psu design and am impressed with these.

Wayne

Thanks Ron/Wayne.

I was aware of previous posts about PSU conversions when I posted, but they've always been a bit sparse on the actual "how to".  Your link Ron, seems to give all the "fix A to B" type instructions that an electrical simpleton like me needs!

rgds,

Mike

The failure mode of this type of power supply is quite common. You will find that the supply is dead after switching off after a successful use.  What seems to be the problem?  This type of switching supply regulates the output by limiting the drive to the mains transistors (pulse width).  If the mains is switched off with a load applied to the output, the regulation will try to maintain the output voltage by driving the input (mains) transistors harder. As the voltage on the input capacitor drops when the mains is switched off the transistors are driven harder and harder to try to keep the output regulated. The drive to the maintain the output as supply voltage drops can exceed the ratings of the transistors.  To prevent failures, make sure the output is disconnected before switching the mains off.  A permanent cure is a low value resistor (5 to 10 ohms) between the input filter capacitors and the main transistors. 

Arceenut,

Wondering if you are referring to ATX PSU conversion.
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I converted my 3rd ATX PSU the other week.....a new 500W one, rated 22A on the 12V rail.
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It ran fine for a couple of weeks, with 2 x iChargers loading it up to approx 22A, but today it suddenly failed on me, during use.
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There is not even a hint of the fan bumping slightly when switch on.
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The mains and PCB fuse is still good, nil sign of any electrical charring on the board.
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I realise I was running close to its limit, but I thought that these supplies were so bullet-proof, that you could even short the 12v rail and it would safely instantly shutdown.(indeed, this sometimes used to happen when initially connected the chargers, probably due momentary initial surge, but would always recover on cycling the power[/b].)
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PSU blown irreversibly now..... maybe I should try to run it 20% below its maximum rating?
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Please comment if you can.
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Many Tks, Steve

I know your question is directed at Arcenut but wonder if I may make a comment? General high volume PC PSUs can't sustain anywhere near their claimed wattage ratings. They're mainly aimed at spinning up hard drives which may take a few seconds of high current and then fall back to a lower level. The 800W-1200W high power server supplies are made to an entirely different standard however and can often withstand some considerable abuse. The build and design standards are usually of a much higher level and their purchase price when new reflects that. I would suggest that mass market PC PSUs are de-rated by at least 20-30% if you expect them to last. Charging batteries for an hour or more at full load is not representative of their expected usage. The fact that server PSUs seem able to withstand that duty is testament to their generous design margins and plentiful cooling.

Steve

I know your question is directed at Arcenut but wonder if I may make a comment? General high volume PC PSUs can't sustain anywhere near their claimed wattage ratings. They're mainly aimed at spinning up hard drives which may take a few seconds of high current and then fall back to a lower level. The 800W-1200W high power server supplies are made to an entirely different standard however and can often withstand some considerable abuse. The build and design standards are usually of a much higher level and their purchase price when new reflects that. I would suggest that mass market PC PSUs are de-rated by at least 20-30% if you expect them to last. Charging batteries for an hour or more at full load is not representative of their expected usage. The fact that server PSUs seem able to withstand that duty is testament to their generous design margins and plentiful cooling.

Steve

Steve

Glowfly sums it up pretty well.   Are you certain the 12 volt line is rated at 22 amps CONTINUOUS?  Often the rating (especially those made off shore is rated at something the supply can handle on a short duty cycle only  (they don't tell you that though).

Continuous.....to be fair, these are stated max outputs.
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I guess they are good at safely shutting down for instantaneous excessive current draw, but if we are torturing them over a long duration right on their limits, they keel over and die!!
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I think I am going to make a total of three, and use one on each charger for heavier charging tasks.
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Tks for your time to explain things.
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Rgds, Steve

Also....I suppose we have to remind ourselves how dirt cheap these ATX PSUs really are.
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Even if you were to try to source a small fraction of all the discrete components that sit on the PCB, this would easily outweigh the £10-15 I am paying for these supplies.
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Steve

Even if you were to try to source a small fraction of all the discrete components that sit on the PCB, this would easily outweigh the £10-15 I am paying for these supplies.

'Tis true. I've been given a 'free' Mastervolt 24v 75A battery charger/PSU that let out the magic smoke for use as a high power PSU. When new it cost around £1500 but just the replacement switcher semiconductors / drivers and rail caps will cost around £100 at small quantity prices.

Still much cheaper than I could buy an equivalent unit though.

Steve