Switch ratings Q

[Chuck H]

I need to pick a rotary switch for an attenuator project I'm working on. The silly big switches that are clearly up to the task are very expensive. I notice that many units use a switch smaller than these two and three deck Frankenstien's lab type switches. The same applies to the impedance switches of amps. So my question is:

I see switches rated at, say, .5 amps @ 125VAC. Can I exceed that current rating at a lower voltage? Is this switch capable of 62.5 watts no matter how the voltage/current ratio comes together as long as it doesn't exceed 125VAC? Or is a switch with this rating limited to .5 amps no matter what the voltage is?

I did try to look this up on google but I didn't find any articles that answered this.

Thanks

Chuck

[loudthud]

Look on the Hoffman site in the parts listings for his impedance switch. Mouser sells the same part for about half what Doug sells them for. I think it's made by Panasonic. There is a thread somewhere with the part numbers.

Switches are usually speced for "switching current". They can handle more as long as you don't change the switch while current is flowing. If you can find a good manufacturers spec sheet, it will usually go into more detail than what you find in catalogs.

[loudthud]

Go to Mouser and look for this part number:

633-HS13-Y-RO

[black_labb]

i have seen switches with ratings that say "2A @ 120V, 1A @ 240v" so that may be the case.

im looking for a impedance switch (for an oddly wound hammond transformer which cant just have simple impedance taps). i dont know if i'd trust those little rotary switches for an amp that could well put out 400w. im trying to find a switch that will work suitably for my amp too.

just saw loudthuds response. will have a look

[tubeswell]

i have seen switches with ratings that say "2A @ 120V, 1A @ 240v" so that may be the case.

im looking for a impedance switch (for an oddly wound hammond transformer which cant just have simple impedance taps). i dont know if i'd trust those little rotary switches for an amp that could well put out 400w. im trying to find a switch that will work suitably for my amp too.

just saw loudthuds response. will have a look

You need those old NOS rotary switches with the aluminium or brass shafts. I got me a stash of 3-pole 4-way switches. But you could try looking in an electrical wholesaler (That's where I found mine). Sometimes they have stock that they've had sitting around for years, never been used, the sort of stuff that was made for high voltages.

[Chuck H]

Thanks guys.

Loudthud, I will check some more data sheets but the switching current isnt always spec'd from what I've seen so far. Thing is I'm looking for a shorting switch so perhaps the contact current rating IS the switching current rating. I spent about 40 minutes on google looking into it but I guess I have more looking to do.

Tubeswell, this switch will be a spec'd part for manufacturing so surplus, over run or obsolete parts won't do for this one. But I have found some really great switches for projects like this (think high current 2 pole 11 position...Very cool) in the past through wholesalers and salvage distributors. Sometimes these are old military or medical equipement parts and they can't be beat.

Black_labb, I hope your right about the ratings. I do know that Hammond has recently introduced straight wound secondaries. So no more goofy secondary switch needs. A tech at Hammond told me that the way they are designed, they should sound just like the old ones. The new ones have the same part numbers but with an "A" at the end.

So, I don't need a high voltage switch. Does anyone know if I can exceed the current rating with the lower voltage?

TIA

Chuck

[Robert M. Martinelli]

So, I don't need a high voltage switch. Does anyone know if I can exceed the current rating with the lower voltage?

TIA

Chuck

Hi Chuck, hi all,

The Pd function is not linear, we know that the switch's dissipated power is R*(I^2), ( and this formula is voltage-independent, the voltage rating has to do mainly with the switch arcing when operated ) so, if R is the switch's contact resistance, doubling the current actually increases the power dissipated across the switch by a factor of four.....

You can probably exceed the switch rating a little, but even 20 % more current makes for a 44 % dissipated power increase.

To ensure a longer switch lifespan, I would stay well within the nominal current, if possible half of that, as, per the above formula, halving the current REDUCES the dissipated power by a factor of four.

Just a thought.....What about a switch operating several relays? That way the switch's lifespan would be maximized, as it would have to deal with the coils' current only, and you could throw in relays with an appropriate current rating to maximize their lifespan as well.

HTH

Best regards

Bob

[tubeswell]

These are the ones I was meaning

Sorry they aren't solid shaft , they are splined split shaft 3-pole 4-way - just the thing for a 2R, 4R, 8R, 16R impedance selector. Remember that you are only using about 20-30 volts on your typical bassman in that location (and with 50W that is only a couple of amps at most) - Sorry if I'm wandering off the topic a bit

[Steve Conner]

Hi all

There are two factors in a switch rating: I2R heating of the contacts, and arcing. The I2R heating depends on current, and the arcing depends on both voltage and current, and also whether it's AC or DC.

It's because of arcing that the current rating goes up as the voltage goes down. But eventually it hits the I2R limit and stops going down, so it's not a constant number of watts as the OP stated.

The arcing thing is no joke. I once tried to use a plastic rocker switch meant for 220V AC in a circuit carrying 4 amps at 600V DC. When I flipped the switch off, it started to melt and spew smoke, while the circuit stayed very much on.

To confuse you even further, some switches have a horsepower rating, too.

To get back on topic, maybe you could get a switch with more poles than you need, and parallel them up. That doesn't help the arcing rating (you'd need to series them for that) but it will help the I2R losses. And it will help reliability, since now two contacts need to fail before the switch goes open and takes out your customer's original Partridge OPT and the screens on his GEC KT66s.

Of course, better to arrange the circuit so the amp is still somewhat loaded even if the switch does fail, and then you don't need to worry about the customer working the switch while the amp is on.

[Bruce / Mission Amps]

Here's what I use on my 20-45 watt amps... and well before Hoffman came into the picture.
I used to sell them to MOJO before they were so readily available... read that as "they found where I was getting them from". ha ha

http://www.missionamps.com/images/ga...d_switch_l.jpg

[black_labb]

Black_labb, I hope your right about the ratings. I do know that Hammond has recently introduced straight wound secondaries. So no more goofy secondary switch needs. A tech at Hammond told me that the way they are designed, they should sound just like the old ones. The new ones have the same part numbers but with an "A" at the end.

i saw those. would have bought one of those, but they are alot more expensive (around 100$ more for the 1650wa vs the 1650w) so i decided to use the cheaper one with the confusing secondaries. i'm thinking i may use 1 or 2 spdt/dpdt/3pdt switches meant for mains power. using one switch i can go from 4 to 8, or 8 to 16. using both i can get 4,8 and 16, but the last combination causes a short across one of the OT windings. i'm thinking that i can add an extra pole to each switch, so that when that setting is on it cuts off the high voltage, or just the power to the grids and the preamp. does anyone have a different idea to make the 4th setting (lets say with both up) from happening?

personally i dont think i would need the 4 ohm tap, as i have 2 16ohm 1x15 cabs, but alot of bass gear is 4 ohms, due to the large amount of solidstate gear, so making that available would be good if borrowing a cab or something similar.

[Steve Conner]

The series/parallel secondaries give better bandwidth than the tapped ones, AFAIK, so it's worth the hassle of the weird switching arrangement, for a hi-fi amp at least.

I think I once figured out how to do it with two DPDTs. Both switches down was 4 ohms, both up was 16, and one up, one down gave 8. It didn't matter which was up and which was down, you got 8 ohms either way. Or maybe I was high on solder fumes that day and imagined it.

[black_labb]

i'll have another look, may be a way i can do it that works like that. aside from the voltage switch i only need a spdt, so there may be a way to do it. if you have any tips/brain farts please share. even if they are a bit smelly

[black_labb]

i should be able to put a switch to swap from 4ohm to 16 ohm as per the drawings ( http://www.hammondmfg.com/1608.htm ) and then put a switch to swap from the green/yellow tap to th yellow tap. this would turn the 4 to 8 ohms, and the 16 to 24 or 32? (impedance is the turns squared?). whatever it is, i dont think i'd use that impedance. when i drew it up before i thought to swap from 4-8 or 8-16 without thinking about how to make it all work without the tranny killing setting.

edit: the next setting is 24ohms. may be useful to get less of a p-p load with the 16ohm cab. i may use 2 jacks and 1 switch instead of switches and one jack.