managing inrush current advice

[black_labb]

thinking about it a bit more i'm leaning towards putting the plate supply onto the standby switch and putting a resistor across it as well. it will still be running 450VAC but with a half wave (does that make it better?) but i think having the resistor it will tend to draw the current through that instead of sparking. the resistor will charge the caps and there wont be much of a voltage across it after a couple seconds anyway. if that doesnt work then i'll go with the thermistors.

[mooreamps]

thanks steve and merlin, seem like a good idea. i think i will explore it a bit more

Unfortunately he fails to give you suggested values as a starting point. Maybe you can figure these yourself, but perhaps there are others on this forum who would have need for a little more "explanation" than just some vague circuit posted on a forum with nothing more than, "OK, here is what it looks like. Good Luck !! "


Therefore, for the benefit of the others, I'll type in a brief circuit analysis of how this thing works. For example, if the filter cap is the typical value of 22 uF, and if the cap will charge to full value in 5 time constants, it will take approx 1.1 seconds for the cap to charge to the full value of B+ ; or in this case we could say "+300 volts". But the zener will fire the relay at 12 volts.

Well, if in the first time constant the cap charges to 96 volts ; if the given B+ is 300 volts ; then the relay will fire even before the first time constant ; R*C ; which is 10K times 22uF ; or 10 e+3 times 22 e-6 ; which equals approx 0.22 seconds. Not much of a delay, is it ???


-g

[Merlinb]

Well, if in the first time constant the cap charges to 96 volts ; if the given B+ is 300 volts ; then the relay will fire even before the first time constant ; R*C ; which is 10K times 22uF ; or 10 e+3 times 22 e-6 ; which equals approx 0.22 seconds. Not much of a delay, is it ???


-g

You misunderstand the circuit operation. The relay will only fire if the current through it is sufficient. The incoming voltage is not constant- it is pulsing.

When you switch on, the cap charges slowly, but the inrush current to the cap can be made small, so that it doesn't fire the relay. It is only once the valves have warmed up and start drawing load current that the relay will fire. Obviously the limiting resistor may need a little adjustment depending on circumstances. For example, if R is too small then the relay might pull-in during the initial cap inrush, then drop out again almost immediately, also known as chatter.

[mooreamps]

You misunderstand the circuit operation. The relay will only fire if the current through it is sufficient. The incoming voltage is not constant- it is pulsing.

When you switch on, the cap charges slowly, but the inrush current to the cap can be made small, so that it doesn't fire the relay. It is only once the valves have warmed up and start drawing load current that the relay will fire. Obviously the limiting resistor may need a little adjustment depending on circumstances. For example, if R is too small then the relay might pull-in during the initial cap inrush, then drop out again almost immediately, also known as chatter.

I'm still not so sure I would have drawn it this way. You would need to find a relay that would have it's coil fully energized with the value of plate current, and I'm not so sure I would put a 12 volt coil on a 300 volt, or in some cases a 600 volt DC power line. Instead, I would use a 12 volt dc power supply ; and my 12 volt DC regulator 9 second voltage ramp circuit to do the time delay for the relay coil, and then use a pair of relay contacts themselves to do the switching.


-g

[Merlinb]

I'm not so sure I would put a 12 volt coil on a 300 volt, or in some cases a 600 volt DC power line.

The relay never sees more than 12V across it thanks to the zener, no matter what the load current. In this sense its task is actually less stressful than if the coil were fed from a low voltage PSU while the contacts worked in high voltage areas, because in this case the coil and the contacts are all much closer in potential.

[mooreamps]

I don't know....... I'm just not seeing the time delay in this...


-g

[Enzo]

Ask yourself this then: When the amp is first flipped on and that B+ first filter charges in an RC instant, but the tube heaters have not warmed up yet and are not conducting, how much voltage is dropped across the relay coil?

yes, there is B+ ON the relay coil, but not across it. If the tubes are cold, they are not conducting, so there is no current drawn through the relay coil. Once they start conducting, current starts to flow through the relay coil, and when sufficient current flows through it, it pulls in and shunts the limiting resistor.

At least that is what I see there.


Contrast a separate delay circuit that fires up after X seconds no matter what, consider that if the heater fuse is open, your delay still applies B+ to cold tubes, while the self actuating circuit leaves in the resistor.

or perhaps I am missing something.

[mooreamps]

Ask yourself this then: When the amp is first flipped on and that B+ first filter charges in an RC instant, but the tube heaters have not warmed up yet and are not conducting, how much voltage is dropped across the relay coil?

yes, there is B+ ON the relay coil, but not across it. If the tubes are cold, they are not conducting, so there is no current drawn through the relay coil. Once they start conducting, current starts to flow through the relay coil, and when sufficient current flows through it, it pulls in and shunts the limiting resistor.

At least that is what I see there.


Contrast a separate delay circuit that fires up after X seconds no matter what, consider that if the heater fuse is open, your delay still applies B+ to cold tubes, while the self actuating circuit leaves in the resistor.

or perhaps I am missing something.

You, sir ; are not missing anything . . . . .

In fact, I pulled a few data sheets of typical 12 relays. I found the nominal coil resistance at 1000 ohms. So, you would need a plate current of 12 mills to pull in the relay contacts. But, as Enzo so assutly points out, a cold power tube does not start conducting until what ; 14 or 16 seconds after initial power on, in which case "no" current is flowing and the full potential of B+ is still felt upon the plate of the power tube. In essesence, the circuit achives "nothing"...

Secondly, a separate PS for the relay coil can be tied to filament power, especially since if you run your filaments at "12 volts" . . . . .

-g

[Merlinb]

in which case "no" current is flowing and the full potential of B+ is still felt upon the plate of the power tube. In essesence, the circuit achives "nothing"...

The purpose of the standby is not to stop the voltage being on the plates (the plates don't care a jot, they're just bits of metal). It's purpose is to limit the inrush current to the PSU caps, which is does very well indeed.

[mooreamps]

I would not really care about in-rush on such a "small" value of capacitance. Further, I would still not be wanting to put a 12 volt relay coil on a high voltage power line, along with the noise maker zener. I'd do it the old fashion way by using tube rectifier, and allowing the 8 to 10 sec warm up time to ramp up the high voltage "gracefully".

-g

[defaced]

So... a hand full of diodes later and I most definitely had inrush current problems with the attached schematic. Right now I'm using a series resistor which is bypassed by a relay. I'm ok with that solution because I plan to use a relay for standby anyway, but I would also like to learn a bit, which brings me to my question: how does one choose a thermistor for an application? Meaning, should a 5 amp thermistor be used in a 3 amp application? Should it be rated for maximum current or typical idle current? You know, all that fun stuff. If there's an idiots guide website or something I'd be interested, I was unsucessful in finding one.

Note: on the schematic I'm missing a 1N4007 diode between the relay contact and the first filter cap, 4th figure down The Valve Wizard.

[black_labb]

i ended up using a 47ohm resistor across the standby switch for the plate voltage (should run around 630v at idle, was at around 660v with 2 tubes instead of 6). the 47ohm (5w wirewound) resistor seems to have failed and gone open, but it could have been a cold solder joint (it worked at first though)

i had assumed that a 5w resistor would have enough thermal intertia to withstand the charging of 340uf with 670v. i may have been wrong. i'll have a closer look and change to a smaller resistance or a 10w resistor.

[defaced]

With the 100k resistor I'm using the B+ comes up to about 150v during standby, still low enough to mute the amp. By my math, that has to dissipate about 3 watts during initial start-up which falls to 1.6 watts once the B+ settles to 150v.

Why a smaller resistor? Seems to me you need to go the other way (larger resistor) to further limit current. Duncan's PSU simulator can accommodate for transformer winding resistance, which is what your series resistor effectively is.

Another question, black labb and I are both using toroidal power transformers. Because of their increased efficiency over EI transformers, is that alloying them to generate higher current spikes at turn on? As mentioned way earlier in the thread, 130 volts lower and 50uf higher with an EI transformer and I did not have this problem.

[black_labb]

i actually havent had any problems. the 47ohm resistor went open and the amp has run fine without it. i'm thinking of the longevity of the caps. and possibly blowing fuses on startup. hasnt happened yet though.

and i thought about doing it higher resistance. i either have to make it higher or lower. i might go 100k+. the grids and preamp supply is running the high resistance method. i thought i would try the low one for the plates as i didnt want much voltage on the switch for long as it is rated for 250vac and i am running 450vac rectified into it. the idea is that there is a low resistance across the switch so the switch has no need to spark over. then again, 100k resistor is better protection than a 47ohm resistor that has gone open.

on another note, what do people/manufacturers do for standby switches for amps running high voltages? surely they cant underrate a switch on something that has to be safety approved?

[Steve Conner]

Hi guys

Yes, toroidal transformers have worse inrush than EI types. This is for two reasons:

They're more efficient than EIs (lower leakage inductance and winding resistance) so they can dump more current from the mains into your caps at startup.

They have more inrush current in their own right. The core often saturates on startup if you turn on the mains switch at just the wrong instant. All transformers do this, but toroids do it worst: the tapewound core saturates hard, and the low winding resistance allows lots of current to flow. 50-100 amps wouldn't be surprising for a big toroid running off 120V (25-50 amps for 240v)

Combine these two effects and you can get quite a bang on startup.

Standby switches for high-powered amps are quite tricky. The safety approval ratings are a real issue, and even if they weren't, most switches are rated for AC and struggle to break high-voltage, high-current DC. I like to use a double-pole switch and put the two poles in series for better arc breaking, like Fender did in the 300PS. Putting the switch in the screen circuit would help, because the voltages and currents are smaller.