MINIMUM UNSTICK CURRENT FOR Cambridge 6800 ?
Could some one with a known good and scanning Cambridge 6800 run a test for me? I need to know the threshold current they start moving at. I have three on my bench, that I question the origin thereof. I'm concerned that they are demagnetized by a oscillating galvo amp, before I got them. In the group buy of used 6210 we had a few duds with demagnetized rotors, and they would barely move at 250 mA. So I just want to know the minimum unstick current. Unstick means it defeats the friction in the bearings and resistance to motion caused by the magnetic field.
For Example, DTs start moving full field at ~60 mA. They will certainly move rapdily full field at 100 mA. Same for 6210, I can see good movement at 50 ma, and rapid full field at 120 mA. In a Cambridge, the stop is a rubber pad behind the position sensor, so it is not wise to inject more then say 200 mA, as this will PEG the rotor.
The way you do this is configure a constant current bench power supply for 50 mA, and then hook meter probes to it. Disconnect the galvo from the amp. Move its mirror to one side, till you can feel it just stop on the rubber pad. You inject the current to the galvo coil by touching the probes to the coil. You do not have to leave the current on for long, just touch the one probe till you see movement. If it does not move at first, reverse the probes as needed. Reverse the probes a few times to confirm you have good movement. Again, you do not have to do more then just touch the probe for less then one second. Increase current as needed, but do not exceed 200 mA.
Obviously this is for a PLer with a good test bench. For the group buy, I used a LM317 set up as a constant current souce with 6.2 and 12.5, and 25 ohm resistors. This ran off a 12 volt stack of AA cells. I also have a fairly state of the art adjustable current bench supply. Nine to twelve volts seems to be a good number for the initial voltage on the constant current PSU.
Doing this on the energized scanner amp defeats the purpose, the amp will source up to ~20 volts at nearly unlimited current to start the movement.
After testing 24 galvos in the group buy, with 1/3rd of them damaged, I have found this to be a decent, sane, and safe test before connecting to the amp. The next test after this, if you have a suspect galvo, is to disconect the drive to the coil, power up the amp, and watch the position sensor output with a scope as you move the rotor by hand. OUT of the 24 galvos in the group buy, ZERO had damaged leds or photodiodes.
If anyone could do this for me, it would really be appreciated.
Steve
NOTE for Electronics Newbies. DO NOT TEST THE GALVO WITH A RAW BATTERY WITH NO CURRENT LIMITING, YOU WILL POSSIBLY DAMAGE or DEMAGNETIZE THE GALVO.
Last edited by mixedgas; 03-06-2013 at 16:00.
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