Inrush Relay Q's..

[EchoWars]

Adding an inrush relay to an amp. Relay is an Omron 24VDC 30A, using a 7.5 ohm 10W resistor for the current limit. Nominal DC voltage from the filter caps is 64V.

So I get the power from the big filter caps through a 1.1K 3W resistor, which provides the relay with about 23.5V. However, I noticed that on power-up, the relay closes almost instantly. A little experimentation shows that the relay, although rated at 24V, will actually close with as little as 13.5V on it.

So my concern here is that with such a low voltage necessary to close the relay, that it ends up switching the power resistor out of the circuit much sooner than it should...perhaps even at a point where current is close to maximum.

So...three thoughts:

1) Increase the resistor size and reduce the voltage on the coil to something lower than the 24V spec.

2) Add an electrolytic cap across the relay coil to delay the contact closing (I'm afraid that the size may be a problem (470µf+), but have not experimented yet...)

3) Figure that, even though the relay closes fairly quickly, the inrush current has already dropped to a reasonably low value. In other words, don't worry about it...

What say ye?

[EchoWars]

Well, since everyone was jumping in at once, I decided to see what other amp designers had done.

What I discovered was that adding a cap across the coils to delay the closing of the contacts was a fairly common affair. Values ranged from 220µf to 1000µf.

On this amp, adding a 1000µf 35V cap results in a delay of about 500mS for the relay to close. Much better... Now I know the circuit is serving its purpose.

[johnnyx]

Many of the switch - mode power supplies that I see through my work have an NTC thermistor instead of a relay and fixed resistor.
NTC thermistors come in different current ratings, the cold resistance being different for different currents. The cold resistance of one rated at 1A is 10R, as it heats up with the nominal current, the resistance drops to less than 1R.
I thought that this would be a simpler and cheaper solution to the inrush problem for audio amps.They are available in higher resistances, 1A of mains is a lot of power for an audio amp.
What do you think?

[slowhands]

Originally posted by johnnyx
Many of the switch - mode power supplies that I see through my work have an NTC thermistor instead of a relay and fixed resistor.
NTC thermistors come in different current ratings, the cold resistance being different for different currents. The cold resistance of one rated at 1A is 10R, as it heats up with the nominal current, the resistance drops to less than 1R.
I thought that this would be a simpler and cheaper solution to the inrush problem for audio amps.They are available in higher resistances, 1A of mains is a lot of power for an audio amp.
What do you think?

I've been using them for years. They are a simple and reliable current inrush limiter. Thermistors are not totally foolproof, since if they are already hot (say you just turned the amp off), they won't limit current, and they do waste a little power.

This current inrush limiting function is necessary for reliability of many electronic devices, including higher power amps. It gives most protection to power switches and bridge rectifiers. You can bet that if it weren't useful, it would not be in the super cheap PC power supplies and TVs, where every penny is shaved on component cost.

There are plenty of more complex methods, the resistor switched out by a relay or thyristor being the major alternative. You see relays often in Japanese amps. Guess what, when an amp has problems with the relay, it may not even turn on properly. And thyristors can generate noise, just like rectifiers. A good designer can work around these issues, but must take them into account.

[sam9]

Regarding NTC thermistors, do they cause any problems with sudden peaks in the source signal? Like a few minutes of soft music followed by a full blast from an orchestra that brings the voltage near the rails. To make it worse, suppose this happens in in band where the speaker impedance dives to 4 ohms or less.

It just occurs to me the normal dynamics might interact with the NTC thermister to limit current at the wrong time.

[thylantyr]

Originally posted by sam9
Regarding NTC thermistors, do they cause any problems with sudden peaks in the source signal? Like a few minutes of soft music followed by a full blast from an orchestra that brings the voltage near the rails. To make it worse, suppose this happens in in band where the speaker impedance dives to 4 ohms or less.

It just occurs to me the normal dynamics might interact with the NTC thermister to limit current at the wrong time.

I was wondering the same thing, why would I want to limit
my power supply current during normal listening ? But thermistors
make sense for class A where current draw is all the time
, but what about other classes of amp where little current is
drawn during soft or no music (thermistor gets cold again). ?

[johnnyx]

The idea is that they only limit current when cold, at switch - on.
Once they are at their normal operating temperature, hot, then they have a very low resistance <1R. So they will be in their low - resistance state when any music is playing, not just peaks. In this state they have comparable resistance to a fuse, and the resistance reduces as current increases, so fears of current limiting during program peaks are unfounded.
I think I'll just have to try it in an amp with normal quiescent conditions.

[peranders]

Have you seen this thread?
http://www.diyaudio.com/forums/showt...809#post231809

First: Why do you want to limit the inrush current? Is it for the fuses in the wall or is it for saving the rectifier diodes or what? If you don't have the answer, I suggest that you try to get it first.

[sam9]

"Why do you want to limit the inrush current?"

Well I have several amps getting switched on at once, or at least I will after a couple of projects are completed. Main speakers Biamped 4x200W, Surrounds 2x100W-150W, Center channel 200W (2x200W is biamped) and a subwoofer amp many, many W. Ontop of this there are smaller demands on the same circuit.

Calculations say a 20A circuit can handle this, except if they all turn on at once. An alternative to consider will be relays with staggered time delays: click, click, clickety, click , click ,click.

Better still will be to have an electrician add a circuit but that can cost as much as a DIY amp!

[EchoWars]

Originally posted by peranders
First: Why do you want to limit the inrush current? Is it for the fuses in the wall or is it for saving the rectifier diodes or what? If you don't have the answer, I suggest that you try to get it first.

I work on a lot of old 70's gear. Two not-uncommon failures are a stuck power switch on a unit where the switch takes the full inrush current, and old leaky power supply filter caps.

With the caps, it is quite often mandatory (for space reasons) to find another cap of the exact physical size (sometimes length is not so much an issue, but diameter almost always is). With advances in electrolytic cap chemistry, a 50mm x 80mm cap of 1975 might be 15,000µf 63V, wheras a modern replacement of the same physical size is likely to be 22,000µf 63V. With the likelyhood of power switch failures on these old units, it only makes sense to protect the switch (and the diodes too, although they can probably handle the current) from a failure from the increased current that they will likely have to deal with if the caps are replaced with larger units.

Does that sound like it has been thought out enough?

BTW, thanks peranders for the email and link to your inrush circuit. I had concocted something similar, though not nearly as well designed, several years ago. The old receivers I'm struggling with now don't have the room for additional PC boards, but if I were to build from scratch, this is surely the way to go..