[A. Shuford]

Several days ago, I sent a post requesting info on determining the leads on some Minebea 8 lead motors. Tachus42 replied with a most logical answer and I tried it. I tied the motors series bipolar and did a test with my new Xylotex driver. Everything seemed to work perfect. I set the reference voltage to 2.88 for the test on the 2.5 amp motor. I so elated.

I then turned off the power supply and connected my Tektronic 465 scope to the A phase of the motor (on the leads, not the board) leads. When I turned the system back on, I heard that snap that people do not like to hear. No smoke just a slight pop.

Now, everything is isolated from earth on a fiber electronics table. I realize that the first thought is that there was a short somewhere between the system and the scope, but I do not think so. I could be.

I had only turned this system on one time and did my motor test. I am wondering that when I turned the system off, after the original test that due to no bleed resistor on the 24.8 volt motor power, that the 24.8 volts was (standing) on the board when I reapplied to 5.5 volt logic power. I have two switches so I can apply logic voltage first as I did on the original test.

If anyone has had this experience, I'd certaily like their input.

Regards,

Dick Shuford
Pleasant Hill, La
[email protected]

[sbrpollock]

The first possibility that comes to mind:

If you check out your scope, you should find that the ground lead on each probe (the small short alligator clip) is connected to the groung/neutral that you plug into the wall. So, you must realize that whenever you hook this alligator clip to something, you are hooking it to the ground/neutral of your shop/house/lab/etc. If there is not complete isolation in the circuit that you are testing, this can lead to a short-to-ground.

Second possibility that comes to mind:

If you use both chanels of the scope, remember that BOTH of these ground leads are connected to the ground/neutral, hence they are connected together. If you hook them up to two diferent parts of the circuit that are at different potentials, you are creating a dead short. As a rule, only one of the probes that I have connected to my scope has the ground lead attatched. I take the ground lead off of the second channel probe and leave it off to make sure I don't inadvertantly hook them both up and short something out.

I hope this info helps you track down the short that caused the problem.

Keep us posted.

[A. Shuford]

Thanks Patrick,

The earth ground on the scope is the only earth potential as both supplies are floating for the moment. Both of these are fed through isolation transformers. Anyway the driver has to go back to the manufacture.