# Powering a single load using multiple voltage sources (as fail-safe)

#### Zurn

Joined Mar 4, 2019
117
Hi everyone,

First off I apologize if the subject heading of this post is misleading, for some reason I'm having difficulty articulating my question...

I'd like to use three different microcontrollers, each with their own timers, to activate a single load. Each one is basically a fail-safe should the previous one not work as planned. They are all powered from different voltage sources. I'm just wondering how to connect them all together in such a way that they won't damage each other. Here's a my best block diagram in text form:

A - - |
C - - |

If any of the three goes high, the load is activated. Again, they're all powered from different voltage sources. Oddly enough it doesn't really matter what the voltage of the load is (sounds weird I know).

This seems like some very basic stuff but I'm difficulty wrapping my head around it for some reason. Even if someone can let me know how to research this kind of circuit, it would be most appreciated.

Last edited:

#### Ya’akov

Joined Jan 27, 2019
9,264
It would be very helpful if you could explain the goal rather than abstracting part of it. The problem is there are too many variables that can’t be eliminated with the theoretical version which the actual problem would eliminate.

In a thread like this, you will eventually say something like “well actually, I am trying to do X so I can’t use this solutions you are proposing”. You can cut to the chase if you explain what you are actually trying to accomplish, not how you are imaging you should do it.

Good luck.

#### Zurn

Joined Mar 4, 2019
117
My apologies, I'll make a more detailed post immediately. It was the concept I was after more-so than the precise solution. I realize this was a mistake.

#### Ya’akov

Joined Jan 27, 2019
9,264
My apologies, I'll make a more detailed post immediately. It was the concept I was after more-so than the precise solution. I realize this was a mistake.
It’s just a problem that when abstracting the concept from a position of ignorance (literally, not knowing the answer) you can’t know the aspects of the problem that affect what you think you’ve neatly extracted as a standalone, simpler version.

This is very common, there’s nothing terrible about it, not anything to apologize for. It’s the people that are intentionally hiding information they imagine is “valuable” that really torque folks off, not this sort of thing.

#### AnalogKid

Joined Aug 1, 2013
11,200
The concept is relatively simple. Depending on the load voltage, load current, and uC output pin current capability, it might be as simple as three diodes.

Critical question: Do the three uC circuits share a common ground? If not, the nature of the load becomes much more important.

Also, where are you located?

ak

#### Zurn

Joined Mar 4, 2019
117
So the more specific question would be:

I have three microcontrollers all being used to activate a burn wire for an ocean anchor. Each one is connected to a different voltage source and is a fail-safe of the previous one. When one microcontroller pin goes high (based on a timer), the burn wire circuit is complete (the salt water acts as the resistor completing the circuit) and the wire burns. The specific voltage level itself shouldn't matter, although all three microcontrollers output 3.3-5V.

How do I connect them all without wrecking anything?

#### Zurn

Joined Mar 4, 2019
117
The concept is relatively simple. Depending on the load voltage, load current, and uC output pin current capability, it might be as simple as three diodes.

Critical question: Do the three uC circuits share a common ground? If not, the nature of the load becomes much more important.
Yes, I figured it was just a matter of protecting the devices with diodes and connecting their grounds together. In fact, I imagine there would have to be a shared common ground in order for each circuit to be complete.

If a "fill in the blanks" style schematic would help, I could make one up.

#### AnalogKid

Joined Aug 1, 2013
11,200
What is the burn wire voltage?

What is the peak burn wire current?

Do all three control circuits share a common ground?

ak

Last edited:

#### Zurn

Joined Mar 4, 2019
117
Does anyone see any immediate risks with this?

What is the burn wire voltage?

What is but peak burn wire current?

Do all three control circuits share a common ground?

ak
The voltage through the burn wire would be between 3.3 and 5V. The current, I imagine, is fairly high, high enough to make a wire burn, though I believe the sea water is largely the cause of that.

This circuit hasn't been wired yet, so if it's there's no issue in it I could certainly connect all the grounds.

#### AnalogKid

Joined Aug 1, 2013
11,200
No immediate problems, although a more accurate representation is that each uC is controlling a switch connected directly to the DC source and the load. No uC can source the kind of current you are dealing with through an I/O pin.

The most simple method is to replace the switches and diodes with three logic-level p-channel power MOSFETs. A possible problem is that when one of them is on, current can go both ways through it, cross-connecting the batteries. If the code in the three uCs can prevent more than one being on at a time, then it should work.

ak

#### Zurn

Joined Mar 4, 2019
117
Thanks for the input, I was just thinking of MOSFETs.

Just to be clear, the switch in the above image is just to symbolize the uC pin going high, sending off a 5 V signal for a few minutes. I think I get what you're saying though: the logic level high on the uC I/O pin is just it's DC source, correct?

#### Ya’akov

Joined Jan 27, 2019
9,264
Is the μC pin actually supplying the burn current?

#### Zurn

Joined Mar 4, 2019
117
I had imaged it would, as it's important for the three different burn wire currents to come from separate voltage sources, in the event that the first two fail.

Does this a bad idea? I should probably test the burn wire circuit with one the microcontrollers (the least expensive!) first to see what happens when I only use the I/O pin. That'll have to wait until tomorrow though.

#### djsfantasi

Joined Apr 11, 2010
9,188
My comment is not really directly related to your question. But I wonder...

You have three power sources.
You have three control circuits.
You have three switch mechanisms.

You have one burn wire!

So with all the trouble you’ve gone through to have multiple redundancy, you have a major single point of failure. If either connection to the burn wire fails, you are out of luck.

Just sayin’

#### LesJones

Joined Jan 8, 2017
4,227
In post #6 you say "(the salt water acts as the resistor completing the circuit)". Can you show us in a schematic exactly how the burn wire is connected to the rest of the circuit ? Would it not also be worth having three burn wires so that you do not have a single point of failure on the burn wire ?
EDIT. I see that djsfabtasi beat me to the single point of failure comment.
Les.

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,088
it's like the stupid way:

Each mico has it's own relay.
The contacts of the relay are all paralled.
Those paralled contacts (any one of) provide the switch for the burn current.

Then there is each micro has say an OPTOMOS relay which is a high current optoisolator.
The paralled contacts activates a single contactor for the burn current or even a DC solid state relay. (3-30 V input)

Since you said sea water, then Maybe an SCR would be appropriate, but they have all sorts of requirements.

The problem is without knowing the limits.

Since you said reliable and water and particularly sea water it changes

#### Ya’akov

Joined Jan 27, 2019
9,264
I had imaged it would, as it's important for the three different burn wire currents to come from separate voltage sources, in the event that the first two fail.

Does this a bad idea? I should probably test the burn wire circuit with one the microcontrollers (the least expensive!) first to see what happens when I only use the I/O pin. That'll have to wait until tomorrow though.
Generally, you won’t source much current from a μC pin before frying it. I think you will probably want an optoisolator on the pin, and a separate source (could be from the same supply running the micro) to feed the burn circuit.

#### AnalogKid

Joined Aug 1, 2013
11,200
I had imaged it would, as it's important for the three different burn wire currents to come from separate voltage sources, in the event that the first two fail.

Third attempt:
What is the peak burn wire current?
ak

Last edited:

#### Zurn

Joined Mar 4, 2019
117
Okay, didn't get a chance to deal with this issue directly yesterday as I was still understanding the circuit with only one uC (clearly something I should have done BEFORE I asked my question here).

Here's a schematic of exactly what I'm doing right now:

As mentioned before, the sea water acts as a resistor, completing the connection from the battery to the collector. When the TTL pin on the uC goes high, the transistor switch closes, heating up the collector line which causes the burn wire to corrode, thus releasing the anchor.

When running the burn wire as a closed circuit directly from the battery (that is, without the uC or transistor), the current seems sit around 200 mA, peaking around 500 mA. This takes about 11 mins to corrode. However, the circuit depicted above should be running around 630 mA (if my calculations are corrrect), and only takes a few minutes to burn. The transistor is rated for 800 mA. I'd prefer to do this with an N-Channel Mosfet, but I don't have one immediately available to me.

So to restate my question: How could I redesign this circuit so that I could trigger this burn wire using three different microcontrollers powered by three different batteries?

Also...if users think I should abandon this thread and restate my question in the projects zone, please let me know.

#### LesJones

Joined Jan 8, 2017
4,227
So are you now saying that the "burn wire" (does not burn or melt) but is eroded away by electrolysis ? My original image was of some resistance wire wrapped around the anchor rope that burned through the rope. My new image is now a metal wire that forms part of the anchor cable. This metal is not normally eroded by sea water but it eroded when the wire is the anode of an electrolytic cell. Is this assumption correct ?

Les.