electronics smarties- power supply question

[pne]

I have a pricey nimh/nicd charger/conditioner that I just purchased, and it requires an external 10-15v power supply. The switching 10amp 12v power supply the store had was a hefty $140, so I said no thanks. I came home and tried to run it off a 12v 500ma adaptor, no dice. The amperage draw drops the voltage too low and the charger starts beeping like mad and locks up (a safety function). I try an old SLA 12v battery which is 5 amps, and still the beeping. I'm quite fed up but try one last trick. I pick up an old AT computer power supply, rated for 9 amps into 12v. Plug her in, and just when I thought it was about to work, beep beep beep. With all these sources the amp draw temporarily dropped the voltage below the minimum operating voltage, setting off the built in low voltage detector.

Question: is there any way to raise the output voltage of this computer power supply, by transformers or other methods, to around 15v, so that hopefully the voltage drop will not fall below 10v? Is there another cheap viable option for a clean power supply that will handle 12v and around 10 amps?

 

[mono]

I'm not sure what you mean by "SLA 12V battery which is 5 amps". The typical SLA battery can produce far more than 5A, even if only a 5AH battery... and any of decent size should be able to stay above 10V for awhile at least.

Was that switching supply at the store meant for this charger?
What exactly is it charging, what cells? 10A at 12V is a he!! of a lot of current to charge NiMH/NiCd cells unless you're trying to charge a dozen in 15 minutes. What is the base charger, do you have specs or perhaps an online product link you could provide?

No you can't raise the power supply output on the 12V rail as when the current draw causes voltage drop, that's a matter of wattage... in other words if you up the voltage you just drop the current it can supply. While it's not quite straight VA, if it can supply 9A @ 12V it's likely to manage 10A @ > 10.0V.


However, it's more likely you could get away with using the computer power supply if you put a load on the 5V rail, as that is usually how they regulate. By drawing more 5V current you increase the 12V current available. That should allow it to work, BUT it's not ideal- if the PSU is rated for peak rather than continuous output then it should not run for lengthly periods outputting at it's max.

 

[Garbz]

You need 10A for a battery charger

Are you sure about this? The specs should be written on the charger. A short in the device can also cause this power sag.

 

[pne]

thanks for the replies guys.
http://www.duratrax.com/caraccys/dtxp4170.html
here is the link to the charger. Instruction manual says it requires 19 amps between 10-15 volts to charge at 8 amps.

I am trying to charge at 7.2 volts, 3.6amps.

The SLA is 9.5AH (my mistake) with an internal fuse which for some reason I kept thinking was 5amps. The batteries are sub-c cells meant for rc cars/boats, and can handle 3-4 amps of charge current. I have tested the charger on a car battery and confirmed that it does work. I will try putting a load on the 5v rail, thanks for the suggestion.


edit: mono-thanks! your suggestion was the ticket, I would never have thought to load the 5v rail. Seems that drawing an amp on it increases the 12v current by far. Thank you very much!

 

[mono]

Well even with a load on the 5V rail you're not going to reasonably squeeze 19A out of it. IMO, you just need a larger SLA battery to power it as that keeps it portable and then you'd recharge the SLA cell off a lesser supply.

If you really want an AC -DC adapter to power it directly the most cost effective (for 19A) would probably be one of of the newer generations of ATX computer power supplies in a 400W+ range... about $50, a lower-cost generic may fall short on sustainable capacity. You might still need add a load on the 5V rail though, even when the newer ones monitor the 12V rail there's still the potential for (5V) overvoltage shutdown circuits to kick in if left floating or only internally (lightly) loaded.

 

[floodo1]

ok, first up i race r/c cars for like ummm 8+ years now. i own www.kgbracing.com so this isnt some bunk advice.

no r/c charger is going to require 19 amps to charger at 8 amps. for an 8 amp charge a 12 amp constant power supply is more than enuf. not to mention a 12 amp supply will also let you use the 10a motor run feature with no problems.

it shouldnt be hard to find a 12v 12 amp power supply on ebay or someplace else. if you want an r/c marketed one you can find an integy or competition electronics (20amp).

also most power supplies even under load are going to be putting out >13v (or can be adjusted to)

trust me 12a power supply is fine for that charger, as long as you're using 8 cells or less. if you're trying to charge 9 or 10 cells 12ish volts wont be enuf. i know my 8 cell packs peak at > 11volts.

what ARE you using this to charge? ICE is a great charger, so versatile!

 

[ericj]

There's no way you need even the 12 amp supply.

How big are the cells we're charging?

Edit: Oh, i see.

Well, here's the thing. Batteries don't get rated in amps. You are not holding a 3.6 amp battery in your hands. You have a 3.6 amp HOUR battery. Meaning that if you theoretically drew 3.6 amps from it, it would theoretically last an hour. Theoretically. Whether or not it's safe to use it like that is another question entirely.

How fast is this battery supposed to get charged? I think if you work out the math, it'll become clear that a 16v 1.5a power supply for an old laptop will probably be fine.

 

[pne]

thanks erick and floodo1.

ericj-I know about the amp hour vs amp difference, I was referring to an internal fuse built into the SLA to prevent discharge at too high a current. In practice the charger isnt 100% efficient so the formula's don't work out. Also the peak circuitry in the charger requires as clean a power supply as possible so the best option is an sla or switching supply, perferably not transformer type.



floodo1- this computer supply is rated at 9 amps so I think it will be all I need. The reason I did not buy an RC oriented power supply is they are so bloody expensive. The computer power supply cost me all of 7 dollars at a computer parts shop, and it does the same job, perhaps with not as clean DC. Since you're quite experienced, mind answering a question? I just peaked a 4 cell pack of aa's rated 2500mah. I set charge controls for 2500mah capacity, 1c charge current, 15mv sensitivity, and 4 cells. It peaked at 7.3 volts, way too high. Is this normal? My 5 cell receiver packs are peaking way above 6.0 volts as well.

 

[ericj]

Quote:

Originally Posted by pne thanks erick and floodo1. Also the peak circuitry in the charger requires as clean a power supply as possible so the best option is an sla or switching supply, perferably not transformer type.

This both (a) can't possibly be true, and (b) is upside-down.

There's no way the pulses used to rapid-charge the pack need to be clean. The battery can't possibly care.

Switchers give you a hideous high frequency sawtooth with very high frequency noise under load. Worst case out of a linear supply is a 120hz wave. Anybody with an oscilliscope can show you this.

It sounds to me like you need either more volts, or a bigger storage capacitor in the power supply, or both.

As for efficiency, that charger doesn't seem to be large enough to hold the heatsink you'd need if it were horribly inefficient.

Perhaps the charger is intentionally designed to sell overkill switchmode power supplies?

 

[pne]

Quote:

Originally Posted by ericj This both (a) can't possibly be true, and (b) is upside-down. There's no way the pulses used to rapid-charge the pack need to be clean. The battery can't possibly care. Switchers give you a hideous high frequency sawtooth with very high frequency noise under load. Worst case out of a linear supply is a 120hz wave. Anybody with an oscilliscope can show you this. It sounds to me like you need either more volts, or a bigger storage capacitor in the power supply, or both. As for efficiency, that charger doesn't seem to be large enough to hold the heatsink you'd need if it were horribly inefficient. Perhaps the charger is intentionally designed to sell overkill switchmode power supplies?

I believe how the manual refered to it was AC interference from a transformer type power supply, and perfered is a switching because it puts out only DC.

 

[ericj]

Quote:

Originally Posted by pne I believe how the manual refered to it was AC interference from a transformer type power supply, and perfered is a switching because it puts out only DC.

Yeah, see, they lied. People do that sometimes.

There is no "ac noise" on linear DC power supplies. There's some ripple - how much depends on how much current you're trying to draw vs. how much capacitance is available to fill in between peaks.

There's some noise picked up from the grid, but your battery does not, and cannot, care at all.

Some sorts of noise can cause problems with the charging circuitry, and a responsible manufacturer would install some strategically chosen capacitors to filter it out.

Switching supplies will give you as much if not more of it. Especially where a lot of switchers actually connect the 'ground' rail directly to the neutral lead coming in from the wall.

When they're under a lot of load, switching power supplies often exhibit much more ripple than linear supplies, and the ripple they output is a sharp sawtooth, not a nice smooth sine.

Here's my guess. This charger doesn't have a storage capacitor of it's own, so it cannot directly service the pulses it uses to rapidly charge the packs. Furthermore, these pulses go on long enough that consumer supplies that are technically able to supply the current unfortunately don't have enough capacitance to keep from rippling.

fwiw, switchmode power supplies don't like to drive capacitative loads, because an empty capacitor is almost a dead short, as far as the equivalent series resistance is concerned. They like resistive loads.

But they still ripple under load - unless your supply is rated for some crazy number of amps.

Building a rapid charger with insufficient capacitance would be a perfect way to sucker people into paying $120 for an unnecessary power supply . . . .

I bet if you hooked it up to a fully charged car battery - which can supply enough amps to do some serious damage - it would still complain.

I would also suggest that you take the cheapest DMM you own (just in case something goes wrong), switch it into the highest amperage ammeter setting it has (and plug the probes into the appropriate sockets), and hook up the charger to said battery through the dmm.

I think you will find that it is not drawing a whole amp when it starts complaining.

 

[mono]

I don't know that we've established that any attempt had provided sufficient current until the PC PSU had the 5V load added. "Old SLA" may be showing it's age, and the formerly unloaded PC PSU might have had very low 12V amperage. PC PSU manufacturers normally spec around 2A 5V current just for basic guaranteed regulation, let alone running one at full load.

I doubt the charger needs a capacitor added to it. They might've simply overstressed the supply quality as a way to shift RMA costs from themselves towards parts costs for the consumer just to rule out that minor % of people using something really poor as a supply.

 

[ericj]

Quote:

Originally Posted by mono I don't know that we've established that any attempt had provided sufficient current

I think that's an excellent reason to use an ammeter to see how much current is being drawn.

If pne has more than one DMM, put the cheaper one in as an ammeter in series with the positive rail, and use the better one as a volt meter between the rails. Alligator clip jumpers will come in handy.

Most halfway decent DMMs, when switched to AC volts mode, will show you the peak to peak ripple on a DC line. And of course in DC mode will show you how many volts are making it into the charger.

I would be shocked - totally floored - if this thing can draw more than about 5 amps at the most.

I'm aware that RC car people have liked using PC power supplies to drive their chargers for at least 15 years. Keep in mind that 15 years ago harddrives were much bigger and sucked a lot of 12vdc just to spin the platters, and the 5v rail didn't see much action.

It's the other way around now. the 12v rail is hardly used and the 5v rail gets all the load.

So an aincent AT power supply will give you more juice on the 12v rail than a brand new supply of any kind.

Hey, here's a thought, I have this 25 year old unix server with an enormous power supply. The main power output hookups are literally gold plated lugs with 8awg wires bolted on. I will sell it to you for only $60 + shipping . . . .

 

[mono]

Quote:

So an aincent AT power supply will give you more juice on the 12v rail than a brand new supply of any kind.

Not at all, check the last few revisions of the ATX specs. Today a supply with 18A 12V current is deemed by some to be "weak" on the 12V rail while 18A was extremely uncommon on a "PC" AT supply unless it was really a server part.

There's still quite a bit of current required to spin up modern drives, generally up to 2.25A per, though closer to 1.25A avg, plus now any gamer's video card is drawing at least 1-3A 12V, and CPU another half dozen amps or more through board supply subcircuits.

 

[floodo1]

what are you guys talking about pulse charging batteries?

steady CONSTANT current with SUPER fine peak detection and temperature controlled charging........thats what is fun

anyway, computer power supply is definately cool with duratrax ice charger. just google "homemade 12v computer power supply" or something like that. there will be tons of tutorials on how to easily make it nice.

then just hook up the ice to that. main reason i suggest a 12 amps is because its should be EASILY availabe in a computer power suply. and its enough to suply the 10A motor run feature. however if the motor's being run wont draw that much current then you can decrease the power suply down. the current loss through the charger is super safe to be 2A with quality r/c charger like duratrax ice.


thats just what i know about the pros and daily life of r/c electric car racing