Incandescent lamp inrush current vs. regulated power supplies

If the switcher has fold back protection, it's likely to latch itself into protected mode. The few laptop power supplies I've tested shut down or fold back if overloaded.

Some suggestions:

1. Turn the lamps on, one at a time, to cut peak inrush current in half.
2. Add a series "inrush current limiter" to reduce peak inrush current. I've never tried using these at 12V and have no clue if they'll work at 12VDC. Watch the energy (watt-seconds) dissipated by the device.
3. Add a BFC (big fat capacitor) across the output of the Tripp-Lite power supply. When faced with a high inrush current, most of the initial energy will come from the capacitor, not the power supply. You might be able to squeeze by with that.

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"John Nagle"

** You need a regulated supply that goes into current limit, when overloaded, to overcome the problem.

Fed with 3 amps or so the two 12W lamps will then light up slowly - in about

2 seconds.

Fraid there is no way to find out if a given SMPS does this ( or shuts down) other than trying it.

.... Phil

Is this the PR-3/UL? It has some margin (like 5A momentary, with undefined time). If inrush is really 10x, you are doing good, since that is a 10 amp surge and the device is specd for 5 amp surge.

"John Nagle"

** Errr - why are you NOT using a for of SMPS **DESIGNED** for lamp loads ???

Halogen lamp " electronic transformers " are sold in the millions.

Dirt cheap too

Wot is your problem ???????????????

... Phil

One approach (which also leads to long life for your bulbs) is to PWM them gradually on (the digital equivalent of the inrush current limiting devices - I'm not over-fond of those myself without adding something to bypass them after startup.)

We have a cheesy (110V) alarm clock that gradually ramps up an incandescent this way - thing must be 15 years old, and the bulb has never needed to be changed yet. The ultimate in soft-starts.

In the specific case of two or more bulbs, you could also try a setup that ran them in series for a few seconds at turn-on to warm the filaments, and then switched to parallel operation.

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e

You can buy LED lamps made for replacing the 12 volt auto lamps. They are not too expensive and as a friend of mine says " works good, lasts a long time. "

Dan

Some switchers current limit and some "burp", namely shut down quickly on overload and try again later, a few times per second maybe. The latter don't pull up loads very well.

I've had that problem with USB 5-volt supplies, especially when driving electronic loads that have switchers - negative impedance - loads inside.

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tough one. I've run into this very problem with linear power supplies.

What's happening is the cold filament looks like something close to a short, and then the power suppply drops the output voltage to back off a bit. It doesn't matter what voltage it's trying to output, it will never heat the filament up enough to increase it's resistance to match the load rating of the power supply.

You might be able to defeat the overcurrent protection which is causing this- find the current sense resistor and slap another across it.

--
Dunno. 

Are you tied to a bare single supply or can you stand some external 
soft-start circuitry?

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

I find they work better. A [continuous] foldback limit is guaranteed to get stuck forever, each and every time the load is the same. That's boring, and dangerous besides.

Example: suppose you have a converter which draws 12V, 1A. Maybe it's a

5V buck converter or something, doesn't matter. Set your benchtop supply to 1.5A current limit, bring it down below 8V, and just try to get it back up without fiddling with that current limit knob.

The peak current delivered in "burp" mode is typically 1-2 times the full load current, enough to push most loads through.

Additionally, if the heavy load is due, in part, to charging capacitance, a few "burps" will deliver enough charge to start it up. The current limited supply can do this, too, but if it doesn't deliver enough to cover steady state at reduced voltage, it's still screwed.

Maybe all the "burp" supplies you've bought just suck.

Tim

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You need an adjustable current limit type supply. Cost more than most. A typical supply will need, say, 10 times the current capability with a built in shut down mode. A non regulated supply can work, as well as a variac controlled supply.

Greg

Yeah, a known voltage, load, R,C and transistor, I think I can make a slow start.

George H.

"gregz"

** Why ??

** Why ????

Long as the 12V supply does not shut itself down at switch on it will work fine - providing of course it has at least 2 amps current output available.

..... Phil

You can keep the surge down to barely more than the normal operating current. Just pre-warm them with about 10% of normal. (I did such a thing during my investigations of ways to do the sequential turn signals for the Ford T-bird... but I ended up using SCR's and kissing off worrying about the surge :-) ...Jim Thompson

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ide quoted text -

I did a slow start thing for laser diodes... where you care about the diode. The circuit was copied from someone else.. (Little of what I do 'original'.)

George H.

Switchers don't need to fold back, and I haven't seen one that does.

We evaluared a bunch of USB switchers. The 700 mA Samsungs pulled up our loads, and some "1 amp" burpers wouldn't.

We've been careful to have our electronics soft start, to make it easier on the wall wart switchers at startup.

Could be. There's a huge number of bad power supplies out there.

Phihong and Samsung seem to make good warts. MeanWell makes good open-frame switching supplies.

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John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

Phil has it right.

For a less elegant approach that may work with your existing power supply, try this:

5ohm 10W +12 ---+---/\/\/---+---+---[Lamp]---+ | | | | +---o o----+ +---[Lamp]---+ \ | | Gnd --------------------------------+

Turn the switch on to short out the resistor after a couple of seconds warm up. If it works to your satisfaction you could replace the switch with a delay circuit later on.

Ed

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

We've had good results with MeanWell. Also one of the few manufacturers with a 480V input unit (size 100W and up).

Tim

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Deep Friar: a very philosophical monk. 
Website: http://seventransistorlabs.com

Some switchers do need to fold back.

Here's why: (Assuming a steady output current) at low output voltages the duty cycle will be low. This means the synchronous rectifier fet is carrying a much larger RMS current than it normally would at full load.

Adding foldback to reduce the output current when the output voltage is low can save money on thermal management for the lower fet in certain designs.

Of course, foldback can lead to all sorts of system problems and should be used with caution.

Regards, Allan