I plan to use an smps to power 2 similar amplifiers (connected in parallel). As I always do when I power up a build for the 1st time, I've got a lightbulb current limiter in series on the mains.
When I connect to either amplifier on it's own it powers up without issue and the bulb doesn't glow. When I connect both amps, the bulb lights up (fully luminous) as though the circuit was shorted... I should mention that I've already tested this circuit with a linear supply without issue. I've simply swapped out the linear supply with a smaller, more powerful switcher...
I'm thinking that there's really no problem with the circuit and that the SMPS just doesn't like the voltage drop that's being caused by the bulb limiter while it tries to regulate the output voltage... Can anyone out there confirm that this is likely the case? Is there a better way for me to safely test this?
When I connect to either amplifier on it's own it powers up without issue and the bulb doesn't glow. When I connect both amps, the bulb lights up (fully luminous) as though the circuit was shorted... I should mention that I've already tested this circuit with a linear supply without issue. I've simply swapped out the linear supply with a smaller, more powerful switcher...
I'm thinking that there's really no problem with the circuit and that the SMPS just doesn't like the voltage drop that's being caused by the bulb limiter while it tries to regulate the output voltage... Can anyone out there confirm that this is likely the case? Is there a better way for me to safely test this?
Probably so. The bulb tester is just a simple way to avoid damage upon initial power-up.
Once it passes that test, don't use it any more.
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You may need to use a larger bulb then, say twice the wattage. The bulb tester assumes that
the DUT's normal current draw is significantly less than the bulb's current at its rated voltage.
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not mentioning what wattage the supply is as well as omitting the wattage of the limiting lamp and the fact that the DUT worked with a linear supply(precluding the need to "re-test) is where i got the impression that it was a potential "troll"
testing smps with a DBT can be of dubious value as the in-rush at start up can make them perpetually re-start regardless of selected limit value.(with the exception of extreme values as in 1000 watt lamps at which point the DBT is no longer able to protect the supply or the load connected to it!)
testing smps with a DBT can be of dubious value as the in-rush at start up can make them perpetually re-start regardless of selected limit value.(with the exception of extreme values as in 1000 watt lamps at which point the DBT is no longer able to protect the supply or the load connected to it!)
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150W bulbs may still be available, although harder to find.
You need to be aware that a lamp limiter works by using the PTC nature of hot metals. Trying to draw twice the current will make the bulb dissipate a lot more than four times the power so its resistance will shoot up and disrupt the SMPS.
You need to be aware that a lamp limiter works by using the PTC nature of hot metals. Trying to draw twice the current will make the bulb dissipate a lot more than four times the power so its resistance will shoot up and disrupt the SMPS.
tungsten filament bulbs have a 1:12 to 1:10 cold:hot resistance ratio.
Halogen versions, because they run slightly hotter, can appraoch a 1:15 resistance ratio.
Take a 60W bulb and assume a 1:12 ratio.
The hot resistance is given by Vac²/Power rating = 240V²/60W = 960ohms
The cold resistance can be measured and should come out at ~80ohms (that 1:12 ratio)
The hot operating current is ~250mAac
any load that draws anywhere close to half that will significantly heat the filament and the glowing brightly temperature will have increased from the cold value of ~80ohms to maybe somewhere around 300ohms to 500ohms.
That 500ohms will drop most of the mains voltage leaving only a small portion to start up the equipment.
Ordinary transformers will operate at that low input voltage.
SMPS and other circuits that NEED a defined voltage will generaly operate improperly.
You can use two bulb holders wired in parallel. For general start up duties use one bulb only.
For heavier duty you can use two bulbs in parallel.
That would give me a range of bulb wattages from 28W to 300W, but I have never (yet) needed more than 100W
Halogen versions, because they run slightly hotter, can appraoch a 1:15 resistance ratio.
Take a 60W bulb and assume a 1:12 ratio.
The hot resistance is given by Vac²/Power rating = 240V²/60W = 960ohms
The cold resistance can be measured and should come out at ~80ohms (that 1:12 ratio)
The hot operating current is ~250mAac
any load that draws anywhere close to half that will significantly heat the filament and the glowing brightly temperature will have increased from the cold value of ~80ohms to maybe somewhere around 300ohms to 500ohms.
That 500ohms will drop most of the mains voltage leaving only a small portion to start up the equipment.
Ordinary transformers will operate at that low input voltage.
SMPS and other circuits that NEED a defined voltage will generaly operate improperly.
You can use two bulb holders wired in parallel. For general start up duties use one bulb only.
For heavier duty you can use two bulbs in parallel.
That would give me a range of bulb wattages from 28W to 300W, but I have never (yet) needed more than 100W
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Not really. There are too many unknown variables. (Almost everything.) There can be many cause of this behaviour. First I'd study the "paralleling". (I don't know what do you mean by this. Common power supply, or you tied the outputs together also? How similar? Exactly what amplifiers?) Only if it seems to be error-free, then I would test it. The type of SMPS can also makes difference. There are many different details and error-mechanisms can explain the experienced result.
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Agreed. The following power supply is the one being used
350W Digital Power Amplifier Switching Power Supply 36V9A 12V2A Dual Output | eBay
The two amp boards are as follows:
Amp 1: TDA7498E based amp (2x150W dual BTL)
Amp 2: TDA7498 based amp board (2 x 80W BTL)
Both amp's power terminals are tied to the 36v out on the smps.
My guess is that the current requirement for the initial surge isn't being met because of the to the bulb limiter. The SMPS couldn't start up at all with the 60W bulb and it kept re-starting with the 100W.
All is well though, it starts up fine when connected directly to the mains and has been running stable under load
D
I also tested a very similar SMPS (the same, but without 12V), and it didn't show any abnormal behaviour. With slowly increased input voltage at a point it simply turned on, without any artifact. Also it worked flawlessly at overload. However I didn't use the famous Bulb Limiter.
I have to ask again: what about the outputs? Are they also tied together? What about load and input?
Whatever...
This SMPS is a fly-back converter. It can work with fairly low input voltage, but as all SMPS at low input voltage it requires higher input current to produce the power consumed by load (I=P/V). In your case the load is 2 amplifier with probably quite high idle current. It seems to require higher current on mains side at this low input voltage than the operating current of the bulb. Quite funny way this is exactly the kind of problem one of my colleque ran into last week with a POE module.
I bet the modules are working while the bulb is lit.
Being unable to start is acceptable for some cases, but loading input with high current during restart cycles is not. I'm almost sure the SMPS is working in the state you saw.
The extent and cause of high idle current worth further investigations though.
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