Making a Voltmeter into a Current meter - Page 5

anatech · 14th July 2006, 06:55 PM

Hi imix500,
What value load resistors are you using? Contact resistance will be variable, so it looks like you have to use hardware and switch them yourself.

You could use relays and measure each leg individually to calculate the total load, that's the only other way I can think of. Use Mercury wetted if you can find them.

-Chris

imix500 · 14th July 2006, 07:54 PM

Hi anatech, the resistors are 6.6 ohms 600W. Kind of a strange value but I couldn't pass them up for the price. The relays I have are 40A automotive type.
Considering the max current across any one of them with a max input voltage of 100V would be 15A they should do fine. The only thing I'm wondering about with using them at that voltage is arcing across the contacts when I take them out of the circuit.
I might just pick up some ice cube types off ebay.
Why mercury wetted?

anatech · 14th July 2006, 08:10 PM

Hi imix500,

Quote:

Why mercury wetted?

1. They are environmentally unfriendly if you open them.

2. The mercury prevents the contacts from arcing. The arc takes place on the mercury. Much longer contact life.

I would not count on the contact resistance with any switch or relay at those currents unless you are using contactors. Even then .... Breaking 15 ADC at 100 VDC is not trivial! Switching AC is much easier as the arc self extinguishes 120 times a second. Some of the other members are well versed in this application. I know enough to say I don't know enough to advise you fully.

My advise: Turn the supply off to change the load, do so by connecting with a screw into a heavy terminal block. Not even the familiar Jones barrier strips are rated for your full current. Use a lockout switch so a co-worker does not energize you!

Is there an industrial electrician in the house? An electroplating guy would be ideal I think.

-Chris

imix500 · 14th July 2006, 08:20 PM

Yeah, I may have to look around for relays. If I can find them surplus, dc solid state relays would be cool.
Powering down before changing the load is probably what will happen most of the time, but I'd hate to burn a relay if the unexpected happens.
The barrier blocks I have are the ones found on the back of a Crown MA5000. I don't know the exact rating, but they are beefy, waiting to hear back from crown on that. It will also have the new NLT4 connector, for load testing some amps at work.

poobah · 14th July 2006, 09:44 PM

Look here

Not that spendy either... I have used thousands of these guys.

anatech · 14th July 2006, 09:53 PM

Perfect for current events poobah!

(I guess I'll be flayed for that one

)

-Chris

imix500 · 14th July 2006, 10:07 PM

Geeze, puns-a-plenty here at diyaudio. I just bought some 35A mercury contactors, they should do the trick.

anatech · 14th July 2006, 10:12 PM

Hi imix500,
Occasionally check the voltage drop across the contacts as you raise the current. You have added another variable to the load.

Otherwise, I don't think you could reasonably do any better.

-Chris

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14th July 2006, 06:55 PM	#41
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi imix500, What value load resistors are you using? Contact resistance will be variable, so it looks like you have to use hardware and switch them yourself. You could use relays and measure each leg individually to calculate the total load, that's the only other way I can think of. Use Mercury wetted if you can find them. -Chris

14th July 2006, 07:54 PM	#42
imix500 Proud Union Member diyAudio Member Join Date: Mar 2004 Location: Nashville, TN	Hi anatech, the resistors are 6.6 ohms 600W. Kind of a strange value but I couldn't pass them up for the price. The relays I have are 40A automotive type. Considering the max current across any one of them with a max input voltage of 100V would be 15A they should do fine. The only thing I'm wondering about with using them at that voltage is arcing across the contacts when I take them out of the circuit. I might just pick up some ice cube types off ebay. Why mercury wetted?

14th July 2006, 08:20 PM	#44
imix500 Proud Union Member diyAudio Member Join Date: Mar 2004 Location: Nashville, TN	Yeah, I may have to look around for relays. If I can find them surplus, dc solid state relays would be cool. Powering down before changing the load is probably what will happen most of the time, but I'd hate to burn a relay if the unexpected happens. The barrier blocks I have are the ones found on the back of a Crown MA5000. I don't know the exact rating, but they are beefy, waiting to hear back from crown on that. It will also have the new NLT4 connector, for load testing some amps at work.

14th July 2006, 09:44 PM	#45
poobah diyAudio Member Join Date: Nov 2005	Look here Not that spendy either... I have used thousands of these guys.

14th July 2006, 09:53 PM	#46
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Perfect for current events poobah! (I guess I'll be flayed for that one ) -Chris

14th July 2006, 10:07 PM	#47
imix500 Proud Union Member diyAudio Member Join Date: Mar 2004 Location: Nashville, TN	Geeze, puns-a-plenty here at diyaudio. I just bought some 35A mercury contactors, they should do the trick.

14th July 2006, 10:12 PM	#48
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi imix500, Occasionally check the voltage drop across the contacts as you raise the current. You have added another variable to the load. Otherwise, I don't think you could reasonably do any better. -Chris

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