airbrake?

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rockgod212
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Re: airbrake?

Post by rockgod212 »

ok, scratch the big 25ohm/ 200 watt power resistor- its too big for my box- doh.

question- is it the shunt resistor that takes the dummy load, if so would:
3x 8ohm 50watt power resistors in series, then paralleled with 2x 8ohm 50watt power resistors in series, then series with 2x 8ohm 50watt power resistors work better for power handling.

so it would be 24 ohm/ 16ohm + 16ohm= 25.6 ohm, what would be the then wattage of the circuit?
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pompeiisneaks
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Re: airbrake?

Post by pompeiisneaks »

It gets tricky with different resistance ratings in the resistors, in theory, they all take an equal portion of the load, but it's usually best to calculate for the weakest link, and then sum it all up based on the lowest rating.

i.e. a single 50 watt 8 ohm resistor is your weakest link, so use that and sum them all up. Basically, if things go wrong the lowest wattage and lowest ohm resistor will take a lot more heat/current than the others, and it could be over it's limit.

Example: 16 ohm 100W, 8 ohm 50W. It's probably best to call that 100 W due to the fact that more of the current would go through the 8 ohm than the 16 if you were pushing say 100W into it. It may, or may not get up over 50W but it's better to be safe than sorry. (for that math, treat the 100W like a 50W as well, because you don't want to overshoot the 8 ohms 50 w rating if there's too much of an imbalance).

Say you pushed 1A into it, at 100V. That's 100W right? in a perfect world, it should share pretty much equal current, across both resistors, but in reality, they're each going to slow the current in different ways, and therefore have differing current going through them. Due to lower resistance of the 8ohm it's going to allow a lot more current through it than the 16 ohm one. Look at ohms law, I= V/R . We have 100/8 = 12.5 Amps it 'could' drop whereas the 100/16 is only going to drop 6.25 amps. That means they would 'share' the load unequally, and that's the ratio in a perfect world. Sadly, though, components aren't perfect, and with current and heat, things can change.

That means if these resistors were rated for those amps at that voltage, they would take a total of 18.7A at 100V which is 187.5W (if you were to input 100V with no limit on current).

Also realize that if the 8 ohm was taking the full 12.5A, that's 125 watts, not 50! (of course we're only sending in 1A in the test case, but it shows the problem that 'can' happen in the perfect storm).

Another thing to think about, though, is if you've got series resistors, put the higher rated and valued ones first in the circuit, they'll effectively drop the voltage for the next resistor so it's only getting a lower current and voltage and can take a smaller amount at that point.

I think that makes sense the way I explained it, if not, please ignore ;)
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rockgod212
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Re: airbrake?

Post by rockgod212 »

ok yeah that makes sense, guess that way is out. I had these 8 ohm 50 watters laying around- I was trying to avoid having to buy another big power resistor, but guess I have to now. so does that shunt resistor take the dummy load, in the air brake its a 25ohm 100 watter. just curious how to bump up the wattage some, if its even possible with this circuit as it is.
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pompeiisneaks
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Re: airbrake?

Post by pompeiisneaks »

If you've got four, then do two in parallel, and set them in series. You end up with 8 ohms, and at least 100 watts

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romberg
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Re: airbrake?

Post by romberg »

Yep, wired like that you would have a pure dummy load of 8ohms at 200w. Then the trick would be to tap off some signal to drive a real speaker at low volume. My first guess would be to take your highest resistance rheostat (say 100ohms) and wire it in parallel with one of the 8 ohm power resistors. Use the center wiper connection to drive one 8 ohm real speaker. When the rheostat is all the way up you will more or less have two 8 ohms in parallel taking 1/4 of the power. So, 1/8 of the power goes to your real speaker. When the rheostat is dialed down then the real speaker takes less power.

I'm not totally confident with the rheostat idea. Just throwing it out there. It may be that because of how they are rated you may need one rated for more than 50w. I think that the 50w is for the whole rheostat. But if a portion of it is used then it is de-rated. But worse case I think you just burn up a rheostat. The amp should still be fine as it has a pure resistive load. And the load should be good for 200w.
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Re: airbrake?

Post by pompeiisneaks »

Yeah the problem with purely resistive loads is that they add tone suck really heavily. The dynamic ones I've heard of use a speaker coil in conjunction with some speakers to try and simulate a real impedance of a speaker to reduce tone suck.

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rockgod212
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Re: airbrake?

Post by rockgod212 »

I have a 25 ohm/ 100 watt rheostat in place of the 25ohm/100 watt variable bar resistor + the 150ohm 50watt rheostat for the bedroom setting, I plan on getting a 25ohm/150watt power resistor for the shunt. now how to wire this thing as I don't want to use the 6 way switch?
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RJ Guitars
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Re: airbrake?

Post by RJ Guitars »

I have a few of these being field tested and so far nothing but good news. Mine are all in smaller amps than you are talking about here... 35 to 45 watts. In my most recent implementation I have added a bypass switch so that you can send 100% of the power to a speaker if that is your preference.
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rockgod212
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Re: airbrake?

Post by rockgod212 »

I found the schematic I need to build it, im looking to use a 2pole 3position switch. whats the minimum current and voltage for the switch or does it matter, I found a 2p3p rotary switch grayhill but the current and voltage ratings are pretty low?
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Re: airbrake?

Post by RJ Guitars »

You could get a heavy duty 3 way from Mouser or Mojo or whoever - the same as many of us use for impedance switches. They are made to handle the current you are talking about.... however, you would have a momentary open circuit as you switch because this is a "break before make" switch. You might want some sort of safety load resistor on there to keep the voltage/current from trying to go to infinity and beyond as the resistance goes to zero.

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romberg
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Re: airbrake?

Post by romberg »

rockgod212 wrote: Wed Aug 16, 2017 11:22 pm how to wire this thing as I don't want to use the 6 way switch?
The attached is (I think) a schematic of the layout you posted on the first page of this discussion with the 6 position switch removed. The switch was serving two purposes. First it selected differing values for the series 100w resistor. It also switched in and out the 50w rheostat. Since you don't wish to use a switch, I've drawn the schematic three ways.

The first way (labeled "bedroom only") just connects everything as if the 6 way switch was in the bedroom position. The 25ohm rheostat is just used as a fixed resistor and any adjustments of it will have no effect. The load the amp sees will be 13 - 22 ohms. So, you probably wanna set your amp for 16 ohms when in the bedroom setting. Most of the current can go through the 25ohm 100w resistor. I don't think this will survive with a 100w cranked amp.

The second way (labeled "Fearless") does use the 25ohm rheostat as a variable component. However it is up to the user to correctly set each rheostat. Some settings could surely cause smoke to come out of various parts of the attenuator or the amp. So, only wire it this way if you have no fear.

The third way (labeled "switched bedroom") adds a single spdt switch to do one function the six way switch did. It connects up the bedroom rheostat to the full 25ohm series resistance.

A word of caution. I am guessing that their is a good reason why whoever designed the layout did not use a 25ohm rheostat for the series resistance. Maybe the contact of the center wiper can't deal with the currents. I am not sure. It could just be that a suitable rheostat was not available. It may also be unsafe (for the 25ohm rheostat) to be at really low settings (< 4ohm). The six way switch prevented this but there is no safety with a rheostat. The load the amp sees looks like it can vary between 21 and less than 4 ohms. Your amp may need to be adjusted to be happy about this. Lastly on extreme attenuation settings most of the current will go through the 100w 25ohm power resistor. I don't think this resistor will survive continuous use with a fully dimed 100 watt tube amp. I would never personally build this circuit or use it with any amps I own.
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rockgod212
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Re: airbrake?

Post by rockgod212 »

thanx for the info, this build is more for a 50watt amp though, even with a 100 watt amp im not trying to get to bed room levels- I like them loud, just wanna take a little hair off the old #36 marshall if I wanted to. here is the schematic I needed for the wiring, I can follow this. thanx for the help though.
Airbrake-Dr-Z-Schematic.jpg
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rockgod212
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Re: airbrake?

Post by rockgod212 »

update, I just bought a grayhill switch- 2 decks, 1 pole/ 3 positions per deck. according to mouser its rated at 5A, exactly what I needed.
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Guy77
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Re: airbrake?

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Re: airbrake?

Post by Guy77 »

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