OT - Overvoltage protection on Serial Tx Circuit

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#1
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Hey there, currently I am working on a simple digital lighting controller to control some fadable fluros.

The circuit works as is (switches 12v @ 250mA regulated), but i would like to add overvoltage / overcurrent protection on the output transmit circuit. (the transmit circuits needs to handle 240v input across its outputs for long periods of time)

Currently I am looking at using a combination of a polyswitch (for current) and a TVS for overvoltage protection. I somehow need to incorporate this into my existing circuit design.

I was wondering if perhaps anyone on the forum with some experience with this sort of application could offer any assistance.

Attached is the digital lighting control interface circuit that needs the protection to be added.

Look foward to some replies.

odd

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I'm stumpped! I have no idea what those two transistors on the "output" side even do. The upper transistor looks like an emitter follower. Is the BC337 a positive-feedback element?

And, its even less clear (to me) what you mean by "handle 240V across its output for extended periods of time."

Is it supposed to do this when the switch is off?

Is it voltage between the output terminals (differential) or common mode (with respect to ground).

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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It appears the top transistor is an emitter follower that supplies voltage to the
load unless shutdown by the avr with the transistor on the left. The transistor
on the right works as a current limiter, throttling the emitter follower when
sufficient current is drawn through the 2.7 ohm shunt resistor. It looks like all
the avr does is turn the load off and on. Not sur at all what transients the OP
is worried about or where they would come from. ???

Tom Pappano
Tulsa, Oklahoma

Tom Pappano
Tulsa, Oklahoma

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the transistor on the left side simply acts as a buffer inverter.

the top transistor ascts as an emiter follower to reduce the output impedance of the inverter presumably to drive some kind of load.

the third resistor monitors the voltage developed by the load current across the 2.7 ohm sensing resistor. Once the current reaches aproximately 0.65/2.7 amps the transistor turns on to rob the base drive of the emiter follower thus acting as a "foldover" current limiter without affecting ( well significantly affecting ) the output impedance of the emiter follower under normal operating conditions.

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the third resistor == the third transistor

note to self..dont type after 2a.m.

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That's what the "Edit" function is for on the new Forum software. :)

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Ok, time to clear some stuff up.

Yes the circuit works using the shunt to turn itself off if excess current is being drawn from the output stage.

Ok, this issue with 240v.

The controller is used to interface to fluros, these fluros can be controlled using dali (my interface) or another method called 'Switch dim - both use the same terminals, however only one system is supposed to be running at the one time. Ie switchdim and dali can't be running in parallel.

Switch dim involves routing the active (240V) into one of the dali/switchdim terminals, while there is a contact (active input to switch dim) the fluro will change its fade level.

Although my controller does not use switchdim, i still need to design my output stage so that even if someone hooks it up incorrectly (ie switchdim on a dali network) things don't get broken / burnt. I want the system to go into a self resettable fail condition whilst incorrect operating conditions are present. You can probably imagine what would happen if you put 240v across my current tx output stage!

Here is some reference info
http://osram.com/pdf/service_cor...
Also some pics to help understanding.

Look forward to some help :)

odd

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