They are the cheapest components in any product. (Look in any parts catalog, where they are practically free in bulk)
So why forgo them?
My new scanner has no on/off switch. I have to reach to the wall socket to plug and unplug it every time I use it. (The old ones, of the same make, all had switches)
I just bought an automobile battery charger that had no on/off switch. This means you have to set everything up and then use the cord plug to extension cord connectors to turn it on or off. (The old one, same make, had a switch)
Perhaps they were to easy to trigger? Maybe people could too easily start up or shut down the product. Although I don’t see why the company could not simply change the format of the button/switch.
I’ll make the leap of faith and assume you mean a computer scanner, not a police band scanner. I think the reason for no off switch is that it uses so little electricity, it can be left on. Also, ANY way to cut the cost is factored in. A power switch may only cost a few cents, but when you’re shipping millions of units, t adds up.
Ya, the switch only costs $0.75.
The wires going the switch only cost $0.10.
The connectors on the wires only cost $0.10.
The extra space on the circuit board for the switch wires only cost $0.10.
Assembly of the switch and wires cost only $1.00.
The extra line in the manual to explain the switch cost only $0.005.
The extra weight of the switch, wies, circuit board increases shipping by $0.005 per unit.
The extra cost in product returns caused by failure of the switch average out to only $0.10 per unit.
Because if you can save 2 bucks on a product when making 50,000 of them, you just increased your profits by $100,000.
If you only charge $50 for the scanner, you just increased profits by 4%.
It’s not just a question of switches being cheap, but of UL (and related international safety organizations) regulations.
To truly turn a device off means that its incoming mains power is disconnected - in other words, the switch needs to be on the 120 or 240 volts wiring. All of that high-voltage stuff needs to be done safely and certified. Switches and insulation aren’t expensive, but certification is.
Why can’t you have a switch on the DC power input? Does the UL have to certify that as well, or are you saying there is little benefit in cutting off the DC power because the transformer would still be on?
Anyway, another reason for omitting power switches is that most computer peripherals can shut off automatically when not in use. Printers and scanners go into standby mode after a few minutes. The manual doesn’t tell you to unplug it every time, does it?
Of course that only explains why manufacturers are justified in omitting the switch. billy has already explained why they want to omit it. (Though I think his estimates are a bit high.)
“All of that high-voltage stuff” needs to be done safely and certified whether there is a switch or not.
On a high volume product like a scanner, the difference in certification fees for one with a switch vs one without is exactly $0 provided the switch is used correctly and already has the needed certification. Even if you decided to use a switch that was not previously UL listed, you could get it done for use in your product for a one time fee of probably less that $10,000
The decision to use a switch would have nothing to do with certification unless it was specifically called out in the standards. Obviously these switches are not called out because they are no longer used.
What drives the design of products is customer design specs, reliability, safety and cost. If the customers don’t need a switch, the switches reduce reliability, and increase cost with no effect on safety, why put them in?
Actually, I can’t comment really on something like a high volume scanner very accurately. I design machines where, on a good year, we sell 100 of them.
In low volume applications like ours, where the wiring is assembled in the US, we would pay about $20 for the switch, connectors, and two wires hanging off of it plus the cost of parts. This is because we buy 10 at a time. For a high volume cable, made in China, I took a guess and divided by 10.
The company that I work for makes printers. For a while (don’t know if we still do, don’t work in tech support any more) they started making printers without power buttons. One of the explanations that I was given was that the printers uses virtually the same amount of electricity when it’s off as it does when it’s on but not printing.
There are other reasons:
It’s a ~$2 part that adds no benefit (not to mention the power LED)
The power button generates tech support calls from people who can’t print because they didn’t turn the printer on.
It’s a safe bet that these are competitive goods, where revenues roughly equal expenses. By losing the switch (no matter how much it costs) your profits may double or you may now make a profit where you wouldn’t have before.
Also realize that when you get cheap power switches, they are often the first thing to fail on a device. I imagine that many people get upset when their $150 scanner is unusable because somebody used a 50 cent power switch.
A lot of people have mentioned “cost” as a reason for not including a power switch. This may be true, but it doesn’t tell the complete story.
It really comes down to design requirements. Before the design is frozen, a list of customer requirements is drafting using a standard quality procedure (QFD, etc.). It is at this stage where the decision is made to include or not include a power switch. There may be a safety requirement to have a power switch, in which case it’s obviously included. If this is not the case, then the sales team has to decide if it’s a customer requirement. One of the three is concluded based on marketing research:
The customer wants a power switch
The customer doesn’t want a power switch
It makes no difference to the customer
A power switch is included if #1 is true. A power switch is not included if #2 is true. If #3 is true a power switch is probably not included based on cost.
I admit this is an oversimplification, as I’ve left out some of the details. But the above is essentially how it’s done.