HGM5 Wiring

[GooeyGus]

After seeing all the questions/comments in Steve's thread about HGM equipment concerning the CW operation of the HGM5's, I figured I would start a separate thread.

I recently acquired an HGM5 (thanks again skyko!) that was already modified to run CW, but I have a few more questions.

When the unit is being used for it's intended use, it has a 'pointer beam' that runs constantly which is basically the tube idling. Then when the operator hits the footswitch, it gives the patient the business.

Currently I'm using the pot closest to the tube to control the current on my setup. I'm guessing that this is actually the idle control current for the tube. To turn the tube off, there has been a jumper installed in part of the thermal interlock circuit, so when the jumper is pulled the tube shuts off and the fan runs until it is happy with the temperature and then the unit shuts off.

You can see the loop here, it is the green wire looped back on itself.
http://www.photonlexicon.com/forums/...&pictureid=646

I'm curious as to if I could install some sort of switch so that the laser will be idleing most of the time, but when I hit the switch it will go up to the desired current level and when the switch is released it will go back down to idle and cool without shutting the entire unit off, much like how the foot pedal works on the unmodified unit.

Any ideas?

[Xytrell]

Doesn't your pot do that?

[GooeyGus]

My pot adjusts power directly in real time, I want to be able to adjust it to a high point, then be able to make it go back and fourth between the high setting and low setting (or idle) with a switch, without having to mess with the pot

[Xytrell]

Is it a miniature pot that requires a screwdriver to adjust or a pot with a knob on the front of the unit? You could install another pot and a selector switch:

[buffo]

I wonder what that current control circuit will do when the resistance goes to infinity for the short period of time it takes you to throw the switch? The tube might wink out...

I think it would be better to set R2 to the high current position (presumably this would be a low resistance on the pot), then add a second resistor in series with R2 that is large enough to bring the current down to idle.

Then use the switch to short around that other resistor. When the switch is open, the current control circuit sees the full resistance of both R2 and the fixed resistor you added, and thus the tube is idling. When the switch is closed, it shorts around the second resistor and thus the current controller only sees the resistance of R2, so the current ramps up.

Adam

[Skyko]

What buffo (Adam) says is the way I would do it. You will need to figure out how R2 is working in the circuit though. I am not sure if it is wired such that an increase in resistance lowers power or if an increase in resistance increases power.

[GooeyGus]

I wonder what that current control circuit will do when the resistance goes to infinity for the short period of time it takes you to throw the switch? The tube might wink out...

I think it would be better to set R2 to the high current position (presumably this would be a low resistance on the pot), then add a second resistor in series with R2 that is large enough to bring the current down to idle.

Then use the switch to short around that other resistor. When the switch is open, the current control circuit sees the full resistance of both R2 and the fixed resistor you added, and thus the tube is idling. When the switch is closed, it shorts around the second resistor and thus the current controller only sees the resistance of R2, so the current ramps up.

Adam

Great Idea. I love it!! I'll try this this weekend. I'm going to make a small 'remote" with tube current dispay, a pot to control tube current directly, another pot to set my 'high' and the switch to swap between high and idle.

[Xytrell]

If it were anything else, I'd say that instant during switching is negligible. But we ARE talking about quite an investment and I'd have to agree in this case. The other option wouldn't be entirely linear with respect to pot position and resistance, but the issue would be reduced to no more static that the supply would encounter under normal operation. That is to have two pots double the value of the original in parallel, and only switch the wiper.

[mixedgas]

If it were anything else, I'd say that instant during switching is negligible. But we ARE talking about quite an investment and I'd have to agree in this case. The other option wouldn't be entirely linear with respect to pot position and resistance, but the issue would be reduced to no more static that the supply would encounter under normal operation. That is to have two pots double the value of the original in parallel, and only switch the wiper.

WARNING WARNING WARNING, DANGER WILL ROBINSON

Do you still have the light sensor connected???? IF NOT:

You do know that the supply is running off the upper limit pot as there is NO current feedback in a model 5, right???? Your depending on the supply to run open loop PWM unless you add a hall sensor!!! So instead of being a closed loop regulated supply, the supply simply acts as a virtual resistor in series with the tube. THIS IS BAD! THIS IS RISKY! The tube current will change with the line voltage!!!!! ACK Pfut!!!!

ALL YOU ARE DOING IS CHNAGING THE UPPER LIMIT, THIS IS VERY BAD!

The is in the details..................

Hall sensors are 20$ or so at digikey or ebay, and drop right in, in place of the light feedback sensor board!!! All you need is a Hall and a little RC filter to prevent oscillation and become closed loop again!


Steve