OT - Circuit Mods for output TX

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#1
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Hello.

Currently I have a lighting controller that is used to control DSI controllable fluro fittings. The unit is controlled using an ATMEGA8. Problems however exist with the output circuitry - its output voltage becomes too low once the load increases- this is due to the circuit design used on the output stage (refer to attached pic).

What I was wondering is if anyone has a simple solution to this problem that could perhaps be used on the existing PCB (with minimum modifications)?

DSI signalling is simple +12v(1) and 0v(0) and can operate down to around 10v before the system stops working.

The current output configuration circuit pic is attached.

Ideally I want be able to switch up to about 80mA@12V for the output.

Look foward to some responses.

Thanks in advance,

odd.

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look at the pic attached, you will need to invert the logic levels how you control the output in reference to your circuit.
In my opinion, AREF should be connected to AVCC.
HTH,
/Martin.

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Some ideas:

a) Keep your existing circuit, but add another PC board after the output of your original, probably with a PNP transistor or P-Channel MOSFET to switch its output directly up to the 12 volt supply;

or b) Try to adjust the existing component values. Here's some calculations off the top of my head (so possibly wrong - don't trust them blindly), just using typical values and not adjusting for real world factors (e.g. temperature etc.). To deliver 80mA at a minimum of 10V, you'll want a maximum voltage drop of 2V accross R12, which corresponds to a maximum value of 25 ohms for R12. To get your (0) level signal close to 0V, you're going to need the transistor switched on hard. Your output won't go below Vce(sat) which for this transistor at the expected Ic with R12=25 should be around 1.2V, i.e. Ic will be about 430mA. It's within the safe operating area chart on the datasheet, but it's a lot of power for nothing when the output is in its 0 state - ~0.5W for Q1 to dissipate, and ~5W for R12. It means you're going to have to use a fairly chunky power resistor for R12, which might not fit on your board. Under ideal conditions, you're already delivering enough current into the base to push Q1 into saturation (BD681 has a high Hfe), but forward gains can vary and it wouldn't hurt to also drop the value of R11 to increase Ib.

or c) As above with respect to R12, but replace Q1 with an N-channel mosfet that happens to have the right pinout to fit on your board. This doesn't solve your problems with the amount of current through R12 in the 0 output state, but if the fet has a low enough static Rds at least it could get your 0 state output voltage closer to 0V, and dissipate less power than the existing transistor.

or d) Re-do your board with something that switches up to 12V.

Personally, I'd probably go with option a) or option d), after having got another opinion. Options b) and c) are an ugly way to get the output signal you desire. Are you sure you need 80mA? If it was less, this would be easier.

Michael

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Thanks for the responses Martin and Michael.

I will do some experimenting today and tell you how it goes.

Thanks,

odd