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Well, if you have 17 resistors 11 ohms each that is 187 ohms. The power is P=342^2/187=625.5 watts.
Now you short out one resistor, we are left with 16 resistors of 11 ohms each, so that's 176 ohms. The power now is 664.6 watts.
short out another, and we are left with 15 resistors so that's 165 ohms. The power is now 708.9 watts.
So that's not going to work. So let's assume that you only switch off one of the two 22 ohm resistors in each parallel set.
Starting with 17 resistors of 22 ohms each at 342 volts, that's 312.7 watts (all relays are open).
Turning on one relay at a time the power increases from that 312.7 watts up to a max of 625.5 watts.
So the question is, is the range 313 watts to 626 watts adequate for testing your power supplies?
I had assumed that you wanted to test from 0 watts up to 650 watts, but if a range of nearly 300 to nearly 625 is good enough then you should be ok.
Ok, so it should be putting out 1.9 amps continuously then? That means the voltage will only be 342v when at full load.
Then i guess you plan to short out each set of resistors to increase or decrease the load resistance. That sounds like it should work, so what's the problem then?
i mean, these relays are 30VDC max, and thats why i used 17 series resistors, ie, so that the max voltage would never be more than 30V........you see, this realy can switch 250VAC, but that refers to 50Hz, and we have 50KHz, and so we must use the dc voltage rating , because even though our AC goes through zero repeatedly, it does not extiguish the arc, because the voltage climbs back up again very quickly due to the high frequency.
Ok very good point there. But there's no way to know this without either some manufacturer's data or some experimental test data. So you either have to contact the manufacturer or else buy a couple and perform some careful tests. Alternately look around for a relay with a rating for higher frequency already.
There are relays made for signals rather than power line frequencies, but they are usually fairly low current.
OK thanks, that looks like a great relay, but is almost ten times more expensive than farnell 4444942.
The relay you kindly showed appears to have some transmission line type sturctures in it, that wont be a problem, but it wont be necessary for us at 50khz
Ok sure, and sorry if it turns out to not be what you are looking for, but i did say that you should test at least ONE before you buy a bunch of them. That's very important just in case we overlooked something. This way you will not be stuck with a bunch of relays you cant use just in case they dont work. A decent test would be in order too, like say 1000 or more opens/closures and maybe view the voltage across the contacts on the scope or something.
Also, another trick that is used in high power systems is that the system is shut down before the load is changed. That means you can use a cheaper relay too in most cases because it never actually has to dynamically SWITCH the load current, it only has to PASS the load current, and that makes a really BIG difference. Once the load is switched of course the system is turned back on. The drawback is that this does not allow testing ALL transient load conditions, but only the turn on and turn off transient of the system with various loads. That's not always a big deal though.
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