Re: Liquid level indicator



MarkMc wrote:
> Hi Chris
>
> Thanks once again for a very detailed response. I really do appreciate
> you giving me your time on this.
>
> In response to your post.
>
> Thanks for the insights regarding EMI. I was aware of the EMI produced
> by the pump, and I managed to buy a joined 100R resistor and (can't
> remember) I think 0.1uF capacitor rated at 400v in one package,
> specifically for the purpose of protecting relay contacts. Hopefully
> these will be ok.
>
> I'm not sure what current rating the pump is, but it's afairly hefty
> s/s cellar pump. I bought it reconditioned, but the home brew shop I
> bought it from said that I should fit a 5A fuse. The pump is indeed
> A/C, 240v. I'm going to buy a controller from RS to control the flow
> rate, so I think this fits in series with the pump load IIRC.
>
> Of course, the heater, pump 'logic' is a perfect solution, as is your
> suggestion of a 3-way switch for the on/off/override requirement - such
> simplicity! I have about 14 years experience in my job (computer
> programmer), and one of the first things I try to teach inexperienced
> guys is "simple is best".
>
> You use the short-hand "FF", but I can't see for the life of me what
> this is refering to! Can you explain?
>
> Also, I have no idea what a MOV is, can explain what this does?
>
> Following your advice, I'll place the 240v stuff and relays in one
> metal enclosure, and mount this as far away as possible from the 12v
> logic stuff, which I'll obviously place in a separate enclosure - I
> assume metal will be best for this too?
>
> I amazingly knew to put a bypass cap from VCC to gnd to help with
> voltage fluctuations, but thanks again for the professional advice.
>
> Regarding the FE33L, I already have one of these, but the documentation
> is pretty poor. The 'probe' that can be bought to fit the device is
> soldered (by the user-i.e. me) on the the board, so I was wondering if
> I could make a food safe probe of my own by fitting the wires to a
> piece of stainless steel rod?
>
> I've been planning on testing this thing in the lab before using it in
> anger with water and beer from a can. I assume I'm best to include a
> preset resistor in the sensor ccts, and make any adjustments by
> adjusting this?
>
> Thanks once again for the help.
>
> Cheers!
> Mark

Welcome back, Mark. This type of conversation should be of some use to
a lot of beginners in electronics.

Your pump is a lot beefier than I had guessed. Your snubber of 100
ohms + 0.1uF may help some, but if you remove the cover from the relay
and watch it switch off, you'll still get an arc. It might be best to
move the relay physically to another enclosure if you have problems
with electrical noise.

We mentioned in an earlier post that "FF" is shorthand in electronics
for Flip-Flop, the basic memory storage unit in electronics. It holds
1 bit of information, and can be a logic 0 or 1. The RS (Set-Reset,
and don't ask why it's always called RS instead of SR, I don't know) FF
is the most basic one that can be cobbled together, with only two logic
gates. You can use NOR or NAND gates to make one. They operate a
little differently from each other, but that's another story. You've
got a NOR RS FF in your control circuit.

An MOV is a Metal Oxide Varistor, a device developed by the wizards at
General Electric Semiconductor during the early '60s, when men were men
and women were glad of it. An MOV is a device that doesn't conduct at
all until its in its breakover region. At that point, it conducts and
acts something like a bidirectional zener. It's commonly used as
device protection across the line. The problem is that the crystalline
structure of the zinc oxide breaks down when it absorbs energy, so they
can be destroyed instantly if there's no series impedance to limit the
current in the event of a line voltage surge. An MOV is a wear part,
and should be replaced occasionally.

Since you've got a 400V cap, you want an MOV that will conduct before
the voltage across the cap gets to that voltage. The bigger the MOV
the better, but you might want to look at Maplin HW13P Suppressor
250VAC 308 in Stock £0.69. Here's a reference which starts in on an
explanation of the MOV:

http://www.currenttechnology.com/paper_5.htm
http://www.currenttechnology.com/paper_6.htm

GE wrote the bible, an applications manual on MOVs, but I don't think
it's available online.

For your application, just put your snubber across the relay contacts,
and put the MOV in parallel with the snubber, and you should be OK.

Using a metal enclosure for both boxes should be OK. Be sure that, if
you've got line voltage in a metal box, you make sure to earth the
chassis as well as fusing as necessary. Use a GFCI is mandatory where
water and line voltage are in proximity. Make sure that, even if the
box drops on the floor, there won't be loose wires with line voltage in
the box. You also might want to get someone knowledgeable to take a
look at your work before you plug it in.

Your probe might be a bit of a problem, as we mentioned before.
Anything used in a food service application should be able to be washed
down and sterilized. The probe shown in the .pdf article doesn't look
like it will fill the bill. If you have a description of the sensor
and/or part number (is it an LM35?), I might be able to give you more
help.

The .pdf article on the FE33L is somewhat descriptive, but it isn't a
data *** and it doesn't have a schematic of the controller itself.
Possibly if you have a better reference, I might be able to give you a
hand.

* Heater Wattage?
* FE33L additional documentation?
* Sensor type? (If it looks like a TO-92 transistor, you could just
copy down the numbers)

I'm looking forward to seeing your work in progress on your website.

Cheers!
Chris

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