new product design, some questions for the designers here..

[Svart]

Since I'm out of work for a little while I have decided to work on a project that will hopefully be a basis for an audio design company, a proposed collaboration between myself and another forum member. Since this *might* be commercial( but low volume) I'll understand if you don't want to give free information. I'm just interested in what other designers would want since I know you guys would just dig right into the box to see how it ticks, i'd rather give you something that you'd rather NOT mod but would enjoy as is.

First question: If you had to choose between a power transformer and an offline switching power supply which would you choose? Why?

2: I need a standby power source for a circuit that needs to be powered regardless of the main power being on/off. I have the choice of another offline switcher which needs an inductor and schottky diode, or I can use a diode/R/C/zener regulator which would need at least a 1w resistor.

3: Audio switching. Relays or FETs for switching? Done properly FETs are still a bit cheaper than relays since Relays need the FETs/freewheeldiodes also, but FETs give an ever-so-slight distortion. What about Audio POWER switching?

 

[gyraf]

Depends on who your target customer is. If it's one like us - in the pro audio area - my guesses are:

1) Transformer "classic", because switched-mode supplies are notoriously unpredictable when it comes to noise - electronic and acoustic. Also the added weight of the power transformer will probably make your product feel nicer

2) How much standby power/voltage/current? For what? Can't that be avoided? Like recall of monitor- or speaker-select switch position with the help of a flash ram, or powered by a goldcap or lithium battery?

3) I'd take relays for audio switching any day. Fet's are nice when they behave right, but will mess up in unpredictable ways. Think serviceing ssl4000 consoles... Relays are predictable and will help legitimize your product as serious high-end audio electronics.

4) Power audio switching - like speaker protect/mute relays - should always be relays imo.

..but we'll probably need more info in order to answer well..

Jakob E.

 

[Svart]

well the standby voltage need might be able to be overcome with a supercap due to the low current need but it's part of the mains switching and I would need a small bootstrap for it since initial charging of the cap would be a problem if I did that.

I only mentioned a switching psu because of the very low current needs of this application. I could do some other nifty things and lower the current needs even further though.

I agree, small sealed signal relays were my first choice in an application like this but i still have questions about the power switching and the requirements for the relays.


more info later..

 

[Boswell]

Is the current needed for your standby circuit more than a few microamps?

 

[pstamler]

[quote author="Svart"]First question: If you had to choose between a power transformer and an offline switching power supply which would you choose? Why?[/quote]

I'd go for an offline transformer -- in other words, a wall-wart or line-wart. Why? Because switching supplies, as others have mentioned, can insert all kinds of crud into the circuit.

As for why the transformer should be outside the box: Aside from noise pickup issues, having it outside the box means you can outsource the wallwart and won't have to apply for UL certification; you can buy a wall-wart that's already certified. This saves thousands of dollars.

2: I need a standby power source for a circuit that needs to be powered regardless of the main power being on/off. I have the choice of another offline switcher which needs an inductor and schottky diode, or I can use a diode/R/C/zener regulator which would need at least a 1w resistor.

Do the analog regulator thing. Noise.

3: Audio switching. Relays or FETs for switching? Done properly FETs are still a bit cheaper than relays since Relays need the FETs/freewheeldiodes also, but FETs give an ever-so-slight distortion. What about Audio POWER switching?

Relays. Be sure you choose a relay that's absolutely bog-standard in specs and board layout (or socketing), to provide at least a little safeguard against the part disappearing.

For audio power switching, use a higher-powered relay.

Peace,
Paul

 

[Svart]

You mentioned analog regulator, would you rather see a zener shunt powersupply or a zener follower supply for the standby rail?

The current needed is around 26uA for standby, around 50uA for startup. not much at all but I need to guarantee that it will turn on every time, something I cannot do when I use a supercap or battery.

I didn't like the idea of a wallwart but I'm actually warming up to that idea.. Problem is that most wallwarts are now switchers of poor quality...

the relays would standard Omron G6S2 SMD parts, just about any SMD part will fit it's footprint.

 

[pstamler]

If it won't be powering anything except the turn-on circuit, I think a shunt zener would be adequate.

DC, analog wallwarts are still pretty common. Or you could just make it a transformer-wart (transwart?) and do your recitfying and filtering in the box. It's still okay with UL as long as there's no line voltage in the box.

Peace,
Paul

 

[Svart]

ok, it should work out ok, except the regulations that passed that require all wall-warts to become switchers in the next few years.. :shock:

it might have to be a multi-output wallwart if I can't use AC into the box itself and derive my own diff rails from it..