[nightanole]

Is there a simple way to limit inrush current without resorting to 10-20 components on a protoboard?

Lets say i wanted to limit inrush power to 300 watts. at 120vac i would need a 48ohm thermistor thats rated at 2.5 amps correct? I could then bypass the mains with a relay. I could put the relay coil with the correct resistor on the secondaries of the transfomer and it would kick when the caps are charged to the correct voltage. Or does all this seem wrong?

 

[bidoux]

The resistor used as current limiter are more after the diode bridge, aren't they ?

 

[amb]

bidoux, no, for startup current surge limiting you'll want it on the primary side of your power transformer, because the transformer itself is also a source of that surge.

nightanole, if you're going to use a delay-relay, you could simply use a resistor instead of a thermistor. The thermistor (without relay) is the simplest solution, but it has its drawbacks (i.e., its resistance will not go to zero, and may be slowly modulated by the load current. It will also have to run hot to do its job).

 

[cfcubed]

amb has discussed what I was going to mention - thermistors. Think most (all?) of Nelson Pass' FirstWatt amp PSs use them as inrush limiters. I used these in my F1 & F2 builds.

 

[nightanole]

Ok heres another idea i came up with

a 40ohm thermistor on the mains (300watt load on the wall)

put the relay coil across the secondaries with a series resistor.

Now when i turn on the mains:
the current goes through the thermistors
the caps start charging
the coil triggers when the secondaries are at voltage
the relay then shorts the thermistors

The use of thermistors is for protection incase the relay doesnt trip. they can handle the load forever. 5-10 watt resistors cant.

Would that work?

 

[amb]

First, just putting a relay at the transformer secondary will not work well enough. The relay will close in a split second after power is applied, before the surge current has a chance to settle down. You need a delay of at least a few seconds, which requires that you add some delay circuitry to close the relay, or use a special time-delay relay.

Second, thermistors work by varying its resistance with respect to temperature. The idea is to use a NTC (negative temperature coefficient) thermistor so that immediately after power is applied, it inserts enough resistance to absorb the current surge, but as the thermistor warms up due to the load current, its resistance will drop to reduce the current limit.

Assuming that the application is for a high-power audio amplifier, the problem is that you need to pick a thermistor that would get hot enough under the desired load to drop in resistance, and hopefuly it would go as close to zero as possible so it doesn't become a current bottleneck. The fact is you'll have a hard time finding one that has enough resistance change that would give you a large enough current limit at turn-on and still get close to zero ohms (after all, the way thermistors work is by having some resistance. and the voltage drop on that resistance multiplied by the current draw is what causes it to heat up).

To complicate matters, unless you're talking about a pure class A amp with steady-state current draw, class AB amps draw very little current while idle, but could change to many amperes based on the music signal (depending, of course, on the power output of the amp and the connected speaker load). So the thermistor will cause the AC voltage (as seen by the power transformer primary) to fluctuate according to changes in current draw. Not a good thing! Plus it would further complicate the selection of the thermistor -- you need to select one based on the maximum current draw, but then at idle it probably won't get hot enough.

If you're going to implement a relay to short out the thermistor after some delay, then why bother with a thermistor at all? Just use a standard power resistor, let it absorb the surge, then have the relay disable that resistor and be done.

 

[digger945]

Omit the VN01 and use an IRF part for the pass device. Page 167, AoE.

Maybe use some RC timing like in the Sigma if needed(or maybe as an alternative)
I've got this up in LTspice with what parts come with the software(generic pnp and pmos), and it simulates a straight line for current using a 200ohm load and 2000uF with 12v supply.

 

[nightanole]

This isnt making sense to me. Now go with me on this

Im not using a thermistor for a thermistor. Im using one incase the circuit fails. Imagine if i was just using a 5-10watt resistor instead of a thermistor. If the timer circuit failed then its bye bye resistor and odds are worse. a 2 buck thermistor can handle 10amps or what ever and not grenade in the case. Odds are the amp will act alittle funky too with a 10-20 ohm load in series with it.

Now heres were i dont understand why a relay coil on the secondaries wont work. you have a 120v mains, across and amp and resistor/thermistor. On power on the amp is like a short. So shouldnt that mean almost zero volts across the amp? As the caps charge the amp will "open", so most of the mains voltage will be across the amp, so that should mean the secondaries are up to voltage, so a relay (with a proper diode/resistor combo that has the lower trigger voltage the same as the secondaries voltage) would close.


How can there still be surge current going on, and the secondaries voltage up to operational level, at the same time?

 

[digger945]

Would you like to share with us what kinda amp your building, and how much capacitance you're planning to use?
Perhaps all you need is a slow blow fuse.

 

[nightanole]

Holly crap, ive never looked at a beta24 now!

Anyway way planning a simple gain clone from like chipamp.com.

So im looking around 160-200va transformer and 10,000 uF -20,000uf in capacitance. I kinda wanted to get the tech down on something small before i move to something that has more power. If i have to make something with 10-20 components on a bread board for a 500ms delay i guess i gotta do it. I allready blewup a smps and the amp/controller in a 41hz kit by not having a current limit circuit.

 

[amb]

Quote:

Originally Posted by nightanole /img/forum/go_quote.gif Im not using a thermistor for a thermistor. Im using one incase the circuit fails. Imagine if i was just using a 5-10watt resistor instead of a thermistor. If the timer circuit failed then its bye bye resistor and odds are worse. a 2 buck thermistor can handle 10amps or what ever and not grenade in the case. Odds are the amp will act alittle funky too with a 10-20 ohm load in series with it.

You just need to select a resistor rated high enough to do the job. If you're not using the NTC aspect of the thermistor, then it is just a resistor (albeit its resistance changes with temperature). Btw, there are plenty of commercial designs using a power resistor (and a delayed relay) to implement power-up surge suppression. You wouldn't be inventing anything new there, it's a proven method.

Quote:

Now heres were i dont understand why a relay coil on the secondaries wont work. you have a 120v mains, across and amp and resistor/thermistor. On power on the amp is like a short. So shouldnt that mean almost zero volts across the amp?

Unless the power supply has some slow ramp-up feature (most don't), the supply voltage will rise very quickly (indeed that's why there is such a large surge current -- the charge-up of the bulk cap(s) is very fast).

Quote:

As the caps charge the amp will "open", so most of the mains voltage will be across the amp, so that should mean the secondaries are up to voltage, so a relay (with a proper diode/resistor combo that has the lower trigger voltage the same as the secondaries voltage) would close.

As I said, unless your relay is driven by some additional time-delay mechanism, it won't do what you think. If you don't believe me, just try it.

Quote:

How can there still be surge current going on, and the secondaries voltage up to operational level, at the same time?

Sure it could be. That's the very reason why there is such a large surge current. If the capacitor is forced to charge slowly, then there won't be a large surge.

 

[sandbasser]

A lot of the PSU designs on diyAudio forum use SL60-type Thermistors for in-rush current limiting. Browse around over there and I'm sure you'll find lots of info. Look at the PassLabs area.

MODS, if it's verboten to post advice to look at other 'brand' forums please excuse the infraction and delete this post.

 

[nightanole]

you can send people to other web sites. You can not say buy my stuff though. So if you had a time delay circuit to sell, you couldnt say that. You would have to get a upgraded manufactures account.


PS for just plain and dirty

http://www.factorymation.com/s.nl/it.A/id.761/.f


it has 120vac trigger so shouldnt this be all i need period?

 

[digger945]

Rod Elliott/Elliott Sound Products(ESP) of Australia, used to sell pcbs for a soft start circuit, if you really wanted to go this route.
Some interesting reading if nothing else.

 

[nightanole]

At power-on a 10Ω resistor is in circuit and limits the current into the smoothing capacitors. When each supply voltage exceeds the zener voltage the FETs will trigger turning on the relay which shorts the current limiting resistor. The p-channel FET is harder to turn on so a lower voltage zener diode is used. In normal operation the soft-start will cut out in about 0.5 sec


A Gainclone for the PC Age


This looks simple enough. Its only about 4 passives and 1 fet.

All the pcbs ive found are $20 + buying the components. For that price i might as well just buy the industrial controller timer relay thats good for 15 amps.

Im trying to keep this project as cheap as possible since its my first. I have the case and the gain clone kits are about $70-90. I have the rest of the hardware from my blown up/ scavenger amps. All thats left is getting a transfomer of proper voltage. I have 2 unknowns that are huge and center tapped from scrap recievers. However alot of those turn out to be 50+ vac...

Maybe i could make a 3 way switch for 5 bucks that goes from off to resistors to ready.