optiphot transformer replacement

[iconoclastica]

incandescent bulbs have a very low cold resistance and only reach rated resistance when at full temperature. What you experienced with an initial over-current shutdown due to the startup cold bulb resistance is not uncommon. If you want to stay with a voltage mode for your bulb you could slowly ramp up the voltage, that should keep the initial current lower.

The smallest quantity of power that I can supply in this setup is 19.35V during 1/255 of 1/490 second. If smoke starts signalling within that period, I can't ramp up any easier than that.

I thought of relaying to a parallel warming up circuit. But for that I would need a high power low-ohm resistor that wasn't in my box today. The additional 5 ohms of the relay itself wasn't sufficient.

But hopefully it is solved already. The on/off switch is at the bottom of the potentiometer, so it always starts low (if the mosfets are not broken and wide open). For the last few hours it has steadily started flawlessy.

Wim

[Saul]

Why do not use this:
https://www.ebay.com/itm/Switch-Dimmer- ... 2772987180 ?

All my leds are controlled by these cheap dimmers ...

For an entirely external light such a dimmer is excellent. But in this case, the on/off switch and power control potmeter are integrated in the stand and I want to continue using them.

You can easy integrate them - your switch can be used to disconnect external power supply. Pot-meter - just find right one (value, dims etc) which will fit original .

[iconoclastica]

Yes, I considered similar solutions. As I have it now, I should work well. In fact, for my epi-illuminator just this afternoon (Eur) I ordered something like that. For the stand, I still have hope that I can do away with the external power supply and have everything neatly integrated within the stand's foot. The downside of that is that very little space remains and then such pre-manufactured boxes wont fit.

From the very beginning of this project I have been in two minds about external/internal PS. Given the disappointing results with the arduino I now am strongly inclined again towards an internal PS as it originally had. Only I hope I chose one strong enough to survive that first second.

Wim

[mawyatt]

Wim,

Just thought of another option, just insert a inrush limiting resistor with the bulb that has a highly negative TC. These start out as higher resistance and as they heat up the resistance drops.

Here's an example.


https://www.ebay.com/itm/NTC-Thermistor ... SwhAJci1MB

Best,

[iconoclastica]

Hm, interesting. Need to find a 5 amp version though. I think it may be not a good idea to include it in the present power line for I'd be afraid to inctoduce a time-lag effect. But a one second relay switched warm-up circuit isn't too difficult.

Another thought I had laying awake in bed last night: Now I have this temporary 19.35V external power supply. When the lamp's initial resistance is 2 ohm, the power consumption is roughly 200W (V*V/R). Once the intended power supply arrives I will have 12V, so about 70W (144V/2ohm). Even though it's a 100W PS instead of the 136W now in use, it should be up to the rush.

Wim

[iconoclastica]

Here's an update, after working about a year with the mosfet regulated original halogene lamp. For some time it worked quite well. It seems that turning up the power slowly was sufficient to control the inrush current. But then there was a bad connection in the 12V power line causing the lamp to off and on randomly. I guess this was what fried al remaining mosfet modules. If so, the same thing could have happened operating a central switch.

So now it's on a trafo predated from the floor lamp downstairs with a standard halogen dimmer before the transformator. It definitely doesn't have a professional look, but it works and it's simple.

[PeteM]

FWIW, I've entirely replaced several older scope power supplies with a cheap (Chinese) PWM supply inside. The older ones usually had bulky transformers, the newer PWM ones can be fairly compact. No failures to date - but I oversize them if space permits.

Even the under $20 ones can be had with built-in potentiometers. Just open up the new supply's case, disconnect the built-in pot, and solder leads to the original (or if need-be, replaced) scope potentiometer. Resistances were very similar in the cases I've done. Only had to replace the pot in one scope.

One of these cheap supplies, for example, fits inside a Leica DMLS. A 50 watt supply running a 30 watt bulb. The only exterior difference is a couple screws at the back holding the supply (inside the stand) in place.

Another larger supply fits inside a Leica DMLB. A 100 watt supply running a 100 watt bulb.

I'm pretty sure the Optiphot 1 has enough space in either the base or inside the upright to fit at least a 50 watt supply inside.

This doesn't solve the other scope control issues, but is another option to keep the original cord, switch, and dimmer -- by replacing the entire power supply.

[Smokedaddy]

Would love to see pictures of the actual thing once it's all figured out.

[iconoclastica]

Even the under $20 ones can be had with built-in potentiometers.

This is an interesting approach that I didn't consider last year, for I had big plans that would have been better served with a constant PS to drive the lamp, the epi illuminator's LED, the stepper motor and micro-controllers. However, since each step took much more time than foreseen, I am scaling down my ambitions for now
But I still want to replace the lamp with a bright LED, so for the moment I am happy with my solution. But just in case: could you post link to such a power supply?

Would love to see pictures of the actual thing once it's all figured out.

Mine or PeteM's?