Still not got it right anyone any ideas

[Scarr]

Hi all,

Well I have be trying to sort a problem with a comms probs / injector I have made, I have attached the schematics from my original (working) design and the new design that attempts to solve a problem I have whilst sometimes hitting a high voltage line (24v) or GND whilst transmitting and blowing parts up.

This circuit is used to monitor comms on a test board and occasionally inject a command overriding anything else on the RX/TX line by forcing the line high and low in time with my command, the line is held high (5v 50ma) and all other devices are connected with bipolar transistors. When another device is transmitting and I want to override it I have to be able to supply 1.5A to ensure it's held high I do this with the FMMT717.

The new design attempts to limit the voltage with a 5.1v zenner and a current limiting resistor 100R, this is derived from 24v / 250ma = 96R as the zenner can only handle 250ma max. You will also notice a change in base resistor on the FMMT717 but I think this still is incorrect and should be 10R (5v) to ensure MAX output of 2.5A.

This has been driving me mad! I have had some great advice so far but tried to do it myself so I learn, I think I'm almost there and maybe it's only the use of a FMMT717 and not a FMT720 (capable of a base voltage difference of -40v and not -12 as with the FMMT717)

So, to clarify, what I want to do is, monitor comms, "force" a command onto the line, not blow parts when either hitting a 24v line or GND whilst transmiting a command.

Hope this all makes sense.

Thanks

p.s. The Pullup control and RX/TX are connected to a MCU

 

[kchriste]

Your circuit doesn't really make sense to me. If you are trying to drive a "24V COMMS IO" buss, why are you driving it with a PNP transistor (Q3) sourced from +5V? Is the COMMS IO some sort of standard communication buss? If yes, what one?
Yes, using a FMMT717 with a 24V signal would be a problem because it's VCEO rating is only 12V. Are you sure that the other line drivers on the buss can handle being forced? I've clarified your diagram a bit to make it easier for myself and others to understand what you've drawn:

 

[Roff]

Which device(s) are damaged when you accidentally put the probe on 24V?

 

[Scarr]

Replies

kchriste:
It's not a 24V I/O bus it's a 5v comms line and I accidently hit a 24v / GND line sometimes, the COMMS are RS232.

Roff:
I think the only compoment now damaged is FMMt171 I am replacing this with a FMMT720 today to see if this solves my problem, before (in original circuit) it was the FMMT717 /BCR22PN's and sometimes the MCU damaged different each time and sometines multiple ones damaged.

 

[Roff]

kchriste:
It's not a 24V I/O bus it's a 5v comms line and I accidently hit a 24v / GND line sometimes, the COMMS are RS232.

Roff:
I think the only compoment now damaged is FMMt171 I am replacing this with a FMMT720 today to see if this solves my problem, before (in original circuit) it was the FMMT717 /BCR22PN's and sometimes the MCU damaged different each time and sometines multiple ones damaged.

Regardless of which transistor you use, your PNP will suffer base-emitter breakdown if you connect the collector to +24V, because the collector-base junction becomes a forward-biased diode. You should be able to avoid this by adding a diode in series with the collector.

 

[Scarr]

To Roff

Hi Roff,

Can you beleive it I almost got this right, well OK I have it working using a diode, what I did was put the diode between the supply (5v) to the emitter not sure if this has any consequences but it appears to work? Your idea is much better and I will amend it

Thanks everyone, at the moment I have a working project again.

P.S. One last question, what size type doing would I use in this situation, hold off 24v whilst allowing 5v, 1.5A out. I am going to look at this and see if I can work it out but if someone again answers this and I get it close I will be sort of learning slowly

EDIT: OK here is my stab at the correct diode MULTICOMP|S2A|DIODE, STANDARD, 1.5A, 50V, SMD | Farnell UK

Steve

 

[Roff]

Hi Roff,

Can you beleive it I almost got this right, well OK I have it working using a diode, what I did was put the diode between the supply (5v) to the emitter not sure if this has any consequences but it appears to work? Your idea is much better and I will amend it

Thanks everyone, at the moment I have a working project again.

P.S. One last question, what size type doing would I use in this situation, hold off 24v whilst allowing 5v, 1.5A out. I am going to look at this and see if I can work it out but if someone again answers this and I get it close I will be sort of learning slowly

EDIT: OK here is my stab at the correct diode MULTICOMP|S2A|DIODE, STANDARD, 1.5A, 50V, SMD | Farnell UK

Steve

The diode will have about 1V forward voltage drop, and dissipate around 1.5W, while current is flowing.