Rob Gray :: ontheroad :: wothahellizat :: wot2 :: diaries :: issue-014


Tue 27 Nov 2007

While we're still a ways away from doing any wiring I do have a small job to do with the inverter.

In Wothahellizat 1 the inverter was mounted under the floor and we had a small trapdoor that allowed access to the control panel. This worked OK but I always wanted to add a remote panel so we could change settings without groveling on the floor.

However at over $500 for a remote panel it never happened.

Then a few months ago we had to get the thing fixed so while it was not being used I had a poke around. I found that the control panel is a separate circuit board (PCB) which is screwed to the inverter's enclosure and connected to the main PCB with a simple 20-way ribbon cable.

Hmmm, this looks promising. I should be able to make my own cable that's long enough to allow me to remotely mount the display.

The control panel, the inverter's case has been removed.

Here we see the control panel (yellow line) and the original connecting ribbon cable (blue line).

As the cable only interfaces to an liquid crystal display, a few LEDs and some pushbuttons there are almost certainly no high-frequency signals that will be upset by a longer cable, so I head down to the local electronics store to buy some 20-way cable and two connectors.

No luck, they have 40-way cable but no connectors. I buy a metre of cable ($0.81) and return home figuring to pirate the connectors from the cable I'm replacing.

When I return though I realise that there are two other cables that are used for functions we don't require, ie the official remote display connection and a connection for combining multiple inverters.

I remove the IDC (Insulation Displacement Connectors) connectors (yes I know that's tautologous, like PIN number) from these cables, split my newly purchased 40-way cable in half, and press the connectors onto it.

Pressing a connector onto the cable. I'm sure there are special tools for this, but even in my days working in electronics we simply used a vice.

Having made my new cable I plug it in and power up the inverter. Everything seems to work, so for the time being I'll consider this a good idea.

So there you have it, if you have one of the large Trace inverters you can buy the official remote display for $500+, or make your own for 81 cents.

NOTE: Obviously this will void any warrantee, and be very careful about the pin 1 orientation of the cable as getting that wrong could prove to be very expensive.

Sat 1 Dec

I'm installing the heavy wiring from the batteries. This means running some pretty fat wires through the lounge room to the shunts in the electronics area.

Shunts? What are shunts?

Shunts are very accurate resistors used to measure large currents.

For small currents you normally just connect the wires directly to the meter.

For small currents the meter can be connected in series with the wire carrying the current.

But with larger currents, usually over 20 amps or so, it's common to let the bulk of the current bypass the meter and just allow a small (and proportional) amount to be tapped off to activate the meter's mechanism.

This is partly because the meter would be too large if it were built to handle large currents, and also because the meter is often mounted remotely from the wire through which the main current is flowing. Usually such wires should be kept as short as possible and it doesn't make sense to detour to a meter several feet or even yards away.

Therefore we employ a shunt. A shunt is just an accurate resistor, when a current passes through it a proportional voltage is developed across it. This is why they are specified as for example a 100A 50mV shunt, which means that if 100 amps are flowing through the shunt you will measure 50 millivolts across it.

Therefore a meter with a 50-millivolt movement that is connected across the shunt will fully deflect the needle when there is 100A flowing, half deflect at 50A etc.

A schematic of a shunt with a remote meter connected.

And a couple of shots of the real thing. You can also see the inverter in the background and two 175A circuit breakers (one for each battery bank) on the left of the top photo.

The actual resistor part can take many forms and may show file marks, cuts, solder dabs etc. This is not rough manufacturing, it's how the shunt is tuned to exactly the right resistance.

Sun 2 Dec

Peter is cleaning his excavator today. It hasn't been cleaned in four years and the pile of muck left behind in the wash bay is enormous.

Not all the muck is on the ground, Peter looks a little grubby after cleaning the excavator.

Mon 3 Dec

Back onto wiring the batteries today. We want to get a rudimentary system running without doing too much temporary wiring that has to be removed.

The left hand battery bank. The wires emerging from the floor are from the bank on the right hand side of the lounge room.

The heavy wires running from the battery banks up to the circuit breaker and shunt area.

The shunt area doesn't look so neat now. Despite not wanting to do much temporary wiring most of this will have to go.

A wider view showing the solar regulator.

Tue 4 Dec

We finally have enough infrastructure in place to charge batteries with the two solar panels from the Landcruiser.

The Landcruiser's two Kyocera 120W panels can now be used to charge our batteries in the truck.

Wed 5 Dec

I've finished most of the plumbing up to and including the pump and external connections. I've never seen so many hose clamps in my life, I already had dozens from Wothahellizat 1 plus I purchased several a few days ago and today I took delivery of another 80!

Due to an oversight I have to run the tank outlets through the thick steel body bracing.

It's not hard to see where all the clamps have gone I guess, there's eight in this tiny area.

The accumulator, pump and control valves.

A pseudo schematic of the plumbing in the tank and control valve area. "In" comes from the outside world and "Out" goes to the pump, accumulator, various taps, HWS etc.

Thu 6 Dec

Peter is still servicing his excavator. I'm still working on the plumbing.

Fri 7 Dec

Well done all those who spotted an apparent fault with my high-current wiring from the batteries. Although it wasn't all that obvious the implication of some of the previous text was that I had batteries, several feet of wire, then the circuit breakers. Something like this.

Now this is not a very clever setup, you should protect wires as close as possible to the current source, (in this case the batteries) not eight feet away. What happens if a short circuit develops somewhere within that eight feet?

I do in fact have some Mega fuses bolted directly to the positive battery terminal of each bank, the only reason I didn't show them before is that I stuffed up the photo.

A 175A Mega fuse bolted directly to the battery terminal.

The circuit breakers are really just used as isolation switches. As you can see the fuses are 175A which is the same as the breakers, it would make better sense to use fuses rated a little higher so that in the event of a problem further downstream of the breakers they would trip before the fuses blew which of course would be cheaper and easier to rectify. But I'm using what I've got.

While on the subject of using fuses to protect wires let's look at a common scenario, that of running a thick wire DC bus around the vehicle and tapping off the main circuit with small wires for lights, 12v appliances etc.

When the wire shorts out it tries to conduct too much current, it will overheat and possibly catch on fire.

Most appliances are fused but this may only protect the appliance, what about the wire? It's typical for such wires to be run all around the place on their way to the appliance. If it should for example pass through a steel bulkhead at point "A", vibration can cause the insulation to become worn (because you didn't use a grommet :-) and the wire to short circuit to the steel.

It almost certainly won't be up to the job of conducting the available current and the result will be a meltdown and/or fire.

Therefore it makes sense to add a fuse at the point the small wire branches from the large one. This fuse should be rated well below the capacity of the wire.

Now when the wire shorts the fuse blows instead of the wire.

Sun 9 Dec

Still mucking around with plumbing, it's not that the system is overly complicated, but there just seems to be a lot of detail to attend to. For example we have a water meter to measure our usage, but for some reason no plumbing fittings like meters and valves have built in mounting points. Maybe they are designed to be used with old-fashioned steel pipes that would hold things in place, but in these days of nylon connections and hoses you need to provide some extra mechanical support.

Like the meter below, I want the face to appear through a panel, but with no mounting points on the device I have to bodge up something myself.

The rear of the meter showing some timber spacers that allow the dial to protrude just the right amount through the panel, and a steel strap to hold it in place.

Tue 11 Dec

I've got some good news and some bad hews. The good news is that you can buy generators really cheaply these days. The bad news is that that's just what we will probably have to do, buy a new generator.

Here in the workshop we often use a 6.5KVA diesel generator to top up the batteries on overcast days like today. It's a noisy thing and we stick it outside so we only hear an annoying hum.

But when that hum turns to a pop and fades to nothing I run to investigate.

The machine has stopped and there's a whiff of smoke in the air. That's OK, it has been smoking a little lately. But the smoke doesn't stop, in fact it gets worse.

The bloody thing is on fire.

I disconnect it and push it further from the shed, then run inside and grab the first fire extinguisher I can think of, a tiny powder unit.

This has little affect so I grab another, the new extinguisher is larger but the problem is access, because this is an enclosed "silent" generator there is no ready access to the internals, and it's the internals that are on fire.

I poke the nozzle in through air vents and an access hatch but it has little affect. Then I start removing the side panels while Chris tries to direct some powder at any flames that appear.

It soon becomes apparent though that the fire is dying out of its own accord, thank goodness this is a diesel generator, with a near full tank of petrol I may not have been so keen to get close.

Two dead fire extinguishers and a dead generator.

Some of the internal wiring.

All this makes me think about access, despite having an endless supply of fire extinguishers they were of little use because we couldn't get access to the source of the flames.

Because this is a "silent" generator it's fully enclosed, and although you can access everything it takes tools and time, neither of which are in abundant supply when a fire is on the loose.

Now I always make things with access and maintenance in mind, most panels are removable, brackets bolted rather than welded etc. But I think about spending a few minutes with a spanner while maintaining, not a few seconds while fighting a fire.

So from now on I think I'll pay more attention to this aspect of the design.