Hello,

I am attempting to repair a Olympus BX40 microscope in Honduras with a BMET working here at the hospital. It has broken several times in the past, and the technician has repaired it by replacing resistors and MOSFETs. Right now, the burnt component is a resistor labeled "R10" on the board. It is in series with the gate of a MOSFET, and has a value of 0.33 ohms. It has burned three times now, but with several weeks or months between each burning.

Any idea what is going on here? It seems like too much power is being pushed through this resistor and into the gate of the MOSFET, since the resistor and connected MOSFETs are the components that burn. I'm not sure if this is a design flaw or a result of the length of service time and other conditions the machine has been subjected to. Would it make any sense to increase the power rating and/or resistance of the resistor? I've built MOSFET circuits before where there is no resistor in series with the gate, but in this case it seems like too much power is being pushed through the gate, so some sort of resistor seems necessary. Any ideas would help.

Justin

Dear Justin,
It seems that there is some overload on your resistor, you have mentioned the replacement of mosfet before, did you/they use the right part number or only the replacement? if it is not the original part number, try to increase the resistance in series with the gate it might not adequate...have you observed any changes in the brightness of the bulb before the resistor burned?...

Charles

Charles,

Thanks for the response. I believe the MOSFET was a local replacement found in Honduras of a similar component, not necessarily the exact transistor used in the original design. I'm not sure about any light brightness difference, but I'll ask about that tomorrow. I haven't actually seen it working myself, but I know it's gone through several repairs before. I will let you know what I find out.

Justin

It's difficult to be precise without knowing details of the circuit (layout, and what it is doing) ... but:- MOSFETs tend to fail short circuit. They are also quite sensitive to any over-voltage condition.

So my guess is that the MOSFET substituted is not at the same rating (voltage, especially) as the original ... it fails, after a while as you have mentioned, then zaps the associated resistor when short circuit current flows.

It would be nice if you could establish what the original component values were, and substitute accordingly.

Good Luck!

Thanks, Geoff. I'll follow and see if we can determine the exact component from the original design. It would be really helpful to have a service manual in this instance, but I'll see if we can get to the bottom of this locally.

If original component could not be available in case you have determined it already, you could also try to see the datasheet of the installed mosfet and see the ideal rating for its base voltage and current, with that, you could determine the approximate value of resistor to use in series with the gate to establish the right voltage and current requirement.

Thanks, Charles. I'll get back in a couple weeks to let you know what we find out.

Justin

Here's a nice source of info about microscopes.

There's some good stuff about Olympus kit on there, but not the BX-40 unfortunately. But I'm wondering if the details you seek are the same as (or similar to) other models (eg, the BH Series), Justin?