Reply by Benderthe.evilrobot November 4, 20162016-11-04
"Phil Allison" <pallison49@gmail.com> wrote in message 
news:d8e3f471-eded-4785-b99f-acaf76f55bbf@googlegroups.com...
> Benderthe.evilrobot wrote: > >> >> > >> >> >> >> I disagree that timeout relay design is difficult - a relay means most >> >> of >> >> the circuitry is on the secondary side. A basic monostable will do the >> >> job, >> >> you just use a steering diode to clamp the timing capacitor to a >> >> secondary >> >> side Vcc rail - when that goes down; it resets the timing circuit. >> >> >> >> >> > >> > ** There are too many different scenarios for one scheme to be the best >> > solution. Some PSUs must start under full load while others can have >> > that >> > load delayed until things settle. Audio amplifiers are an example of >> > the >> > latter and it is normal to mute the inputs until a few seconds after >> > switch on. >> >> The OP wants it for the SMPSU section of things like oscilloscopes - the >> scheme I suggested would fit that bill. >> > > ** But is more complicated to design and install than needed.
Probably nearly half of commercial electronics come out the factory with just the basic NTC, probably more in the case of professional gear. Usually delay relays turned up on CRT VGA monitors for the degauss posistor. Controlled by the front panel micro - sometimes also a manual degauss button. You only need a similar delay for the inrush NTC if petty little weenies spend all their time picking fault with every suggestion put forward.
Reply by Phil Allison November 3, 20162016-11-03
Benderthe.evilrobot wrote:

> > > > >> > >> I disagree that timeout relay design is difficult - a relay means most of > >> the circuitry is on the secondary side. A basic monostable will do the > >> job, > >> you just use a steering diode to clamp the timing capacitor to a > >> secondary > >> side Vcc rail - when that goes down; it resets the timing circuit. > >> > >> > > > > ** There are too many different scenarios for one scheme to be the best > > solution. Some PSUs must start under full load while others can have that > > load delayed until things settle. Audio amplifiers are an example of the > > latter and it is normal to mute the inputs until a few seconds after > > switch on. > > The OP wants it for the SMPSU section of things like oscilloscopes - the > scheme I suggested would fit that bill. >
** But is more complicated to design and install than needed. The OP to wants to upgrade existing designs that work well enough - cos he is paranoid about imagined large inrush surges. If he merely replaced the old supply side PSU diodes or bridges with larger ones and maybe the on-off switches too, the upgrade is done. ..... Phil
Reply by Benderthe.evilrobot November 3, 20162016-11-03
"Phil Allison" <pallison49@gmail.com> wrote in message 
news:0188aa0e-f3ba-4d02-b0dc-fedcde7110a6@googlegroups.com...
> Benderthe.evilrobot wrote: > >> > >> >> >> >> ** A resistor does not change its value, so will guarantee a known and >> >> constant surge current limit every time the unit is switched on. >> >> >> >> An NTC is a poor solution, as it only works as intended the first >> >> time >> >> unless given many minutes to cool down OR works in conjunction with a >> >> relay bypass system that must be carefully designed to drop out soon >> >> as >> >> the AC supply is removed. >> >> >> > >> > I would put in enough fixed resistance so the peak current is within >> > spec, >> > then also add an NTC to further reduce the peak during a normal start. >> >> Using both is pointless - the downside of a fixed resistor is increased >> volt >> drop if current increases for any reason. >> >> The NTC is better, but as Phil says; an outage re applies current with a >> hot >> NTC, so no inrush protection for that event. >> >> I disagree that timeout relay design is difficult - a relay means most of >> the circuitry is on the secondary side. A basic monostable will do the >> job, >> you just use a steering diode to clamp the timing capacitor to a >> secondary >> side Vcc rail - when that goes down; it resets the timing circuit. >> >> > > ** There are too many different scenarios for one scheme to be the best > solution. Some PSUs must start under full load while others can have that > load delayed until things settle. Audio amplifiers are an example of the > latter and it is normal to mute the inputs until a few seconds after > switch on.
The OP wants it for the SMPSU section of things like oscilloscopes - the scheme I suggested would fit that bill.
Reply by Phil Allison November 2, 20162016-11-02
Benderthe.evilrobot wrote:

>
> >> > >> ** A resistor does not change its value, so will guarantee a known and > >> constant surge current limit every time the unit is switched on. > >> > >> An NTC is a poor solution, as it only works as intended the first time > >> unless given many minutes to cool down OR works in conjunction with a > >> relay bypass system that must be carefully designed to drop out soon as > >> the AC supply is removed. > >> > > > > I would put in enough fixed resistance so the peak current is within spec, > > then also add an NTC to further reduce the peak during a normal start. > > Using both is pointless - the downside of a fixed resistor is increased volt > drop if current increases for any reason. > > The NTC is better, but as Phil says; an outage re applies current with a hot > NTC, so no inrush protection for that event. > > I disagree that timeout relay design is difficult - a relay means most of > the circuitry is on the secondary side. A basic monostable will do the job, > you just use a steering diode to clamp the timing capacitor to a secondary > side Vcc rail - when that goes down; it resets the timing circuit. > >
** There are too many different scenarios for one scheme to be the best solution. Some PSUs must start under full load while others can have that load delayed until things settle. Audio amplifiers are an example of the latter and it is normal to mute the inputs until a few seconds after switch on. If the inrush surge is so large it must never happen, as is the case with large toroidal transformers, then it is worth using a separate transformer just for the relay. It is then easy to have the relay de-energised immediately the AC supply is disconnected. ... Phil
Reply by Benderthe.evilrobot November 2, 20162016-11-02
<makolber@yahoo.com> wrote in message 
news:d6761122-a54f-4fe2-892f-397fc6433229@googlegroups.com...
> On Tuesday, November 1, 2016 at 9:58:35 PM UTC-4, Phil Allison wrote: >> Cursitor Doom wrote: >> >> > >> > >> > MOVs are subject to cumulative damage, though. They should really be >> > replaced periodically. I would say NTC thermistors, suitably rated, >> > would >> > be a better bet. >> > >> >> ** MOVs and NTCs do different jobs, one is not a better bet than the >> other. >> >> >> > Or Phil's suggestion of a current limiting resistor. >> >> ** A resistor does not change its value, so will guarantee a known and >> constant surge current limit every time the unit is switched on. >> >> An NTC is a poor solution, as it only works as intended the first time >> unless given many minutes to cool down OR works in conjunction with a >> relay bypass system that must be carefully designed to drop out soon as >> the AC supply is removed. >> >> .... Phil > > tastes great vs less filling > > I would put in enough fixed resistance so the peak current is within spec, > then also add an NTC to further reduce the peak during a normal start.
Using both is pointless - the downside of a fixed resistor is increased volt drop if current increases for any reason. The NTC is better, but as Phil says; an outage re applies current with a hot NTC, so no inrush protection for that event. I disagree that timeout relay design is difficult - a relay means most of the circuitry is on the secondary side. A basic monostable will do the job, you just use a steering diode to clamp the timing capacitor to a secondary side Vcc rail - when that goes down; it resets the timing circuit.
Reply by November 2, 20162016-11-02
On Tuesday, November 1, 2016 at 9:58:35 PM UTC-4, Phil Allison wrote:
> Cursitor Doom wrote: > > > > > > > MOVs are subject to cumulative damage, though. They should really be > > replaced periodically. I would say NTC thermistors, suitably rated, would > > be a better bet. > > > > ** MOVs and NTCs do different jobs, one is not a better bet than the other. > > > > Or Phil's suggestion of a current limiting resistor. > > ** A resistor does not change its value, so will guarantee a known and constant surge current limit every time the unit is switched on. > > An NTC is a poor solution, as it only works as intended the first time unless given many minutes to cool down OR works in conjunction with a relay bypass system that must be carefully designed to drop out soon as the AC supply is removed. > > .... Phil
tastes great vs less filling I would put in enough fixed resistance so the peak current is within spec, then also add an NTC to further reduce the peak during a normal start. This way, 99% of the time when there is a cold start, the surge will be very low... and those 1% re-rush corner cases, the fixed resistance will keep the peak safe. Many times, the hot resistance of the NTC is sufficient and a separate fixed resistor is not needed. mark
Reply by Phil Allison November 1, 20162016-11-01
Cursitor Doom wrote:

> > > MOVs are subject to cumulative damage, though. They should really be > replaced periodically. I would say NTC thermistors, suitably rated, would > be a better bet. >
** MOVs and NTCs do different jobs, one is not a better bet than the other.
> Or Phil's suggestion of a current limiting resistor.
** A resistor does not change its value, so will guarantee a known and constant surge current limit every time the unit is switched on. An NTC is a poor solution, as it only works as intended the first time unless given many minutes to cool down OR works in conjunction with a relay bypass system that must be carefully designed to drop out soon as the AC supply is removed. .... Phil
Reply by Cursitor Doom November 1, 20162016-11-01
On Mon, 31 Oct 2016 23:02:12 -0700, dakupoto wrote:

> Metal oxide varistors(MOV) are very good surge suppressors - we use it > for power line surge suppressors, satellite TV surge suppressors, wall > plug mount LED lamps etc., > Very easy to incorporate in any circuit.
MOVs are subject to cumulative damage, though. They should really be replaced periodically. I would say NTC thermistors, suitably rated, would be a better bet. Or Phil's suggestion of a current limiting resistor.
Reply by November 1, 20162016-11-01
On Tuesday, 1 November 2016 05:15:38 UTC, Phil Allison  wrote:
> Chris wrote: > > > Phil Allison wrote: > > > > > For a load of say 1A rms, a WW resistor of 4.7 ohms rated 10 watts > > > should be fine. Other resistances in the AC circuit and the filter > > > electro's ESR will bring the effective value up to 8 to 10 ohms total. > > > > Well that would be great, as I already have dozens of these type of > > resistors in my junk box. I'm just wondering what's providing the > > protection here, though: the resistance of 4.7 ohms or the reactance > > arising from the inductive nature of WW resistors? Or both?? > > > > ** Any reactance is negligible at the frequencies involved, so the resistance alone is doing the job. > > You say you have seen "surge damage" to SMPSs in scopes etc - but is that linked to *inrush surges* or something else ? > > The only components likely to suffer from inrush surges are the rectifiers acting on the incoming AC supply - something designers have know about since Noah was a boy. Very likely, the SMPSs you are dealing with already have inrush surge suppression in the form of WW resistors, NTC thermistors or simply the resistance of common mode chokes used in line filters. > > It is only necessary to limit the worst case surges to about 30amps peak to protect the sort of diodes and bridges that might be involved and this means the total resistance in series with the AC supply needs to be 10ohms. > > With no resistance included at all, peak surge currents would be well over 100amps and destroy diodes immediately and/or trip supply circuit breakers at switch on. > > .... Phil
The problem is gradual degradation of the NTC Rs. I suppose one could always replace one with 4. NT
Reply by November 1, 20162016-11-01
On Sunday, October 30, 2016 at 4:32:15 PM UTC-4, Chris wrote:
> Hi, > > What is the best choice currently for protecting SMPS circuitry in the > event of surge at mains switch-on? I'm talking about the PSU sections of > oscilloscopes, typically, so under normal working conditions less than > one Amp drawn at 240V (as it is here in europe). I read somewhere that > the front end of an SMPS momentarily looks like close to a short circuit > resulting in very large inrush currents at turn-on. How best to limit > these, please? > > Chris.
Metal oxide varistors(MOV) are very good surge suppressors - we use it for power line surge suppressors, satellite TV surge suppressors, wall plug mount LED lamps etc., Very easy to incorporate in any circuit.