non E-bike schematic mod help required

I wonder if any one could take a look at the attached schematic for me.
It is a PSU board on a David Clarke aviation noise cancelling headset. Original schematic DC themselves sent to me many months back.
It is designed to run from either a 6volt battery supply, or external power 10-32 volts.

The original plastic battery case has now become brittle and is scrap. so i could just re build in another case and continue with the 4 xAA pack..but I already have other kit that runs from 9V PP3 so to run this from 9v would be more convenient

The board is designed to take either 6 volt from the internal pack or 10-32v from external aircraft source, and feed 12 volt out to the headset NAR units.

U2 the MAX761CPA produces the 12 volt out, and can take up to a 16.5 volt input. So as far as that goes 9 volt is no issue


The alternative supply from the aircraft panel supply is via U1 LT1176CN8 This accepts from 10-32 volts.
U1 outputs 6 volt to feed U2, which then converts back up to 12volt which is fed to the headset ANR unit.


I have tried 9 volt in at the 10-32 volt input and this is a little too low to be stable, and drops out.
Since U2 can take up to 16.5 volt in, then I suspect that sticking in 9v where it expects 6v would be OK.


Except for the one issue that I can see, that this question is about.
I think I know the answer, but no being an EE I want someone that actually knows about these things to look first and see if I have missed anything that would cause me to damage the unit. Replacement is $220 USD!!)

The resistive divider network R4 and R5, drops the 6 volt input down to 2.3 volts, to feed the battery status indicator LED, via the Op-amp U3.
Obviously if I stick in 9volts, that is going to put this R4 - R5 junction to 3.5 volts or so..higher than the spec printed on the schematic..of 1.5 to 3 volts.

So my guess is that if I change R4 to 150kΩ and R5 to 51kΩ ( or swap out R4/R5 with a 200k pot), that is the only change I need to make the battery feed circuit work at 9 volt and keep the battery indication working correctly.

Have I missed anything?

you should put a 300k 1/8W resistor in parallel with R5. that get the equivalent resistance to 60k so a 9V source would give you 3V on that signal line to the indicator led. the range is printed there to indicate the values you would find with a normal 6V battery, and if the battery discharges too low the indicator would flash to indicate low battery.

if you have some lifepo4 pouches or small lifepo4 cylindricals then you could use 2S and use those without modification of the divider bridge.

dnmun said:

if you have some lifepo4 pouches or small lifepo4 cylindricals then you could use 2S and use those without modification of the divider bridge.

Click to expand...

I wanted to make it 9v so it is the same battery type as the other headsets, and a head torch I have, in order to keep the spare batteries I I have to carry in the cockpit flight bag to a minimum.

Yes, I saw the 1.5-3 volt indicator, hence the suggestion of the values I calculated..keeps the over all divider network the same value, so current flow stays the same.adding a parallel resistor is going to ( ok not by much) reduce the value so increase current flow a bit.

But apart from that section..any other area that will be affected by the 9v instead of 6?

the two resistors only determine when the low battery indicator trips... so you don't need to change those.
for the rest it looks like it should work. my main concern would be blowing out the LT1186 via
its pin #4 but the resistive divider r2/r3 should prevent that. I don't get the circuitry around
Q2 though, what's that about ?

Lebowski said:

the two resistors only determine when the low battery indicator trips... so you don't need to change those.

Click to expand...

you do if you want the low battery indicator to trip.. I guess a 200k pot to replaceR4 and 5 would make the indicator level adjustable.

Lebowski said:

for the rest it looks like it should work. my main concern would be blowing out the LT1186 via
its pin #4 but the resistive divider r2/r3 should prevent that.

Click to expand...

Can guess that you mean LT1176, but that sjhould be OK, as it is good for 32 volt when powered from the aircraft power source.

Lebowski said:

I don't get the circuitry around
Q2 though, what's that about ?

Click to expand...

I wondered too..but now you mention it and I look again, I am thinking maybe a shut down feature should the 12 volt output fail. ?Since it is the 12volt output that drives the gate. Would ahve thouht having it cut if it went over voltage would be more appropriate

Yeah but how does it start when there's no 12V yet ?

Ah yes..good question, it does actually work.???

The business around the fet is probably some sort of soft startup circuit, like the precharge switch some of us have.

Have some balls, put in earplugs and try it

I would do if I do not need it for work and they were not charging so much for a new one.

Am sure it will be OK and no one has come back with a big "don't do it' so there is probably nothing else obvious that can cause an issue

don't remove the resistors there, just add the 300k in parallel by soldering the leads to the leads of the 75k resistor, then it can go back to the 4AA later if you have to and less chance of damaging the pcb.

the way i see it the LT1176 is used as a buck converter to guarantee 6V going into the MAX761. the Max761 is a boost type converter to step up the 6V input to the 12V required by the rest of the circuit.

just plug your 9V battery into the input jack used for vehicle power. you don't need to change a thing then. the LT1176 will drop the 9V down to 6V and the Max761 will boost it up to 12V to power the audio part of the circuit.

rick

i skimmed right over that part about 9V dropping out. so you are probably correct that using 9V at the battery input should work with the resistor changes you calculated. with those values the cutoff should be at a battery voltage of around 5.85V.

but to be on the safe side you could temporarily disconnect the Orange wire from the ANR modules and measure the output voltage with a 9V battery. may need to load the circuit with a 1K to 5K resistor from the Orange wire to ground for the regulator to work though.

Looking at Q2. that works as an "Ideal Diode" reverse polarity protection. connect the external supply in reverse and the FET body diode blocks power. connect the correct polarity and initially the body diode conducts. there will be a 1V drop across the diode though. once the buck/boost regulator chain starts up Q2 will be biased hard "on" and the drop across the diode will be close to nil.

that Q2 circuit may be what is keeping the buck/boost circuit from starting reliably with only 9V attached to the input.

rick

Cheers rick..will ahve a look in the future..that is on hold for now..burnt LiFePO4 pack build on the go at the moment for a mate that should not be let near wiring at all

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