Heat control gurus, Q for you

[BigSwede]

Rather than pollute the 50+ page thread any further, I thought I'd drop this Q here.

I'm building my controller, it's looking good. I've got a Crydom 40A SSR in there, using resistance as the control - it's designed for a 1 megohm pot, which I have.

I have a combo meter I'd like to add, a digital volt/amp/watt meter. Neat little gadget, but it needs a minimum voltage to turn on, and then it goes through a self-test. I don't want to be causing it to turn on and off. I can't see myself doing any distilling below the turn-off point, so I'd like to somehow keep the SSR energized to a minimum.

If the response of the SSR is linear, it should look something like this:

1 Megohm = 0 volts, essentially off
500K = 120V, half way
0 Ohm = 240V, fully on.

If I simply put a 600K resistor across the leads, it'd give me about 90V. Now I add the potentiometer, and I've got two resistances in parallel, and the math gets weird.

The question is basically, how do I keep this thing to no less than , say, 60 volts output, yet still have control above that? Thanks!

[bellybuster]

1 meg plus 600 k is 375k
http://www.sengpielaudio.com/calculator-paralresist.htm

[kaziel]

Add resistor in series with trim-pot. If it's added in parallel when trip-pot is @ ~0ohm it will bypass other resistor (as if there was no other resistor), on the other site if tri-pot is @ 1MOhm and R2 = 600k you've got total resistance of something like 370k. Connect them as I've describe and measure the resistance (with no power applied) you will see what I've mean.

[superdaveva]

I hooked my meter up before my on off switch, reads 220 when i plug it in, amps go up and down.

[BigSwede]

Thanks guys, I guess I really need to get off my rear and simply bench test. I appreciate the replies.

[kaziel]

What was the price of that SSR? If you don't mind telling.

[BigSwede]

What was the price of that SSR? If you don't mind telling.

Found it on eBay (new old stock) for $32. Happy as a clam! I know Crydom makes good stuff. I think this model is well north of $100 new.

Plenty of guys are having success with Chinese versions, others are getting ripped off with false labeling, like a 10A unit packaged as a 40A device.

[kaziel]

What was the price of that SSR? If you don't mind telling.

Found it on eBay (new old stock) for $32. Happy as a clam!

Sweet. Was the heat sink included? I'm using 30 triac based digital controller but my electric installation fuse is only 10A .

[BigSwede]

This thread is a bit old, but I wanted to post the solution... it's easy and may help someone with a potentiometer-SSR rig.

These SSR's are full ON when the potentiometer is zero ohms, or you simply short the input with a switch. They shut off at some resistance built into the SSR at the factory.

Problem - 1 Megohm pot, dialed back, reduces voltage and turns off my fancy digital power meter. I didn't want that because it boots up slowly and I don't want to be rolling the voltage back and forth through its shut-off point. Needed to find a maximum resistance to keep the meter on.

Easy solution - Ran the unit, slowly rolled the power back until the digital power meter shut off. The value was 50 Volts. I kept the potentiometer frozen in that spot, then took it out and measured the resistance. It was 600 kilohms.

If I had a nice 500K pot, I'd use it. Not wanting to wait, I decided to add resistance in parallel to create a 500K pot out of a 1 Meg pot. Just a little research gave me the answer... I feel stupid for asking the original Q. in this thread. Goes to show a little leg work is better than spewing questions here.

I had a drawer full of 1/2 W 1 megohm resistors. Wanted 2 watts, so I used four, two each in parallel, then both "2-packs" in series. This gives me 1 megohm, 2 watts, which I put in parallel with the 1 meg potentiometer. Result - a 500K pot, unit powers up at 60 volts, and the digital power meter is happy.

There are resistance calculators online that make the job easy. If someone has a quirky rig, and needs to fine tune a potentiometer, adding resistances works nicely.



Kaziel, sorry I missed your post. The heat sink wasn't included, but it was a $5 Chinese job. I know they can make a fine heat sink, but I'm a little suspicious of some of their super-cheap proportional-control SSR's out there. A new 40A Crydom costs something like $125, yet a Chinese equivalent is $12. Hmmmm.

I'm using 30 triac based digital controller but my electric installation fuse is only 10A

Well, 10 amps at 240VAC is 2,400 watts, should be more than enough to power a boiler, especially if it's insulated. Is there any way to upgrade your service panel?

[Jimbo]

Are you sure your fancy meter is powered off the voltage its reading?? Very odd. I suspect you have seperate supply lines, and should power them off a fixed voltage.

[BigSwede]

Are you sure your fancy meter is powered off the voltage its reading?? Very odd. I suspect you have seperate supply lines, and should power them off a fixed voltage.

There's no doubt. It is potted in epoxy, and there are two screw terminals poking out for the AC volts, and a toroidal transformer for amperage hanging on 2 wires, nothing else.

I would have preferred a separate power input like 5VDC, but nope, not there.

The data sheet says "85 volts min" but in real life, it reads good down to 50VAC or so. Very nice meter, as it should be for the price.

[sambedded]

"e="Jimbo"]Are you sure your fancy meter is powered off the voltage its reading?? Very odd. I suspect you have seperate supply lines, and should power them off a fixed voltage.[/quote]
"

There are lots of self powered voltmeters available. They are using same lines to power itself and to measure.

[bellybuster]

Are you sure your fancy meter is powered off the voltage its reading?? Very odd. I suspect you have seperate supply lines, and should power them off a fixed voltage.

There's no doubt. It is potted in epoxy, and there are two screw terminals poking out for the AC volts, and a toroidal transformer for amperage hanging on 2 wires, nothing else.

I would have preferred a separate power input like 5VDC, but nope, not there.

The data sheet says "85 volts min" but in real life, it reads good down to 50VAC or so. Very nice meter, as it should be for the price.

I think what Jimbo is getting at is the meter is powered by a separate non regulated source, not the regulated power for your element. A solid 110 volts branched from your 220 would solve any issues from shutting off.

[Jimbo]

"e="Jimbo" Are you sure your fancy meter is powered off the voltage its reading?? Very odd. I suspect you have seperate supply lines, and should power them off a fixed voltage.
"

There are lots of self powered voltmeters available. They are using same lines to power itself and to measure.

Hmmm, not in small signal world. Kinda hard to measure millivolts that way. Small signal DC and radio frequency electronics I do is a different world from power electrics. I admit I closed my eyes and said a small prayer before flipping the switch for the first time on my 240 electric conversion. A 'smoke test' takes on a different meaning when the sparks and pops are 10000 times bigger

[sambedded]

Millivolts are not applicable for still heating. As you may noticed that self powered voltmeters require minimum 60-80 volts minimum.

[bellybuster]

I might be missing something her but...
Your meter has a current transformer right (toroidal)?? Plus a connection for meter power? The meter power should be solid power, constant. The power for the meter should not sway with your power adjustment or you will have the issues you are having.

[bellybuster]

sorry BigSwede, I looked up your meter and it needs the regulated power in order to give you voltage and power readings. I was wr...wro...wro....mistaken.
Nice meter by the way

[BigSwede]

No prob BellyBuster. On the back of the meter, there are literally only two places to connect wires. Everything else is in the epoxy except for the dangling toroid. Yeah it would have been better IMO to have 4 terminals, two for device power, two for process measurement, but nope, not in this case.

The darned toroid is a bit small. 10 gauge wire fits through with ease, but not the size of crimp connector I would have preferred to use. I had to take a round connector and file the sides a bit to get it through. I didn't want bare stranded copper clamped in the female twist-lock receptacle I used for the output.

Despite the price, I am tickled, it works very well, showing volts, amps, watts perfectly. Also added a $5 Chinese analog volt meter as a backup.

[Jimbo]

If youre like me, at the end of the day you wont even look at the meters. I have 3 pieces of tape cut to points for 2000, 3000 and 4000 watts. 5500W is of course full tilt. These 4 settings cover all my needs and take 1 second to dial up with the marks.

[bellybuster]

when I'm running UJ I know exactly where to set my controller by amps, 9.7amps = 3litre/hour. A piece of tape would have been just as good but I like shiny things. Thinkin on adding some redundant flashing lights and maybe a siren.....or two and a mirror ball??


would love to see pics of it up and running BigSwede.

[Jimbo]

LOL belly!

[bellybuster]

wait till you see my new brew rig controller Jimbo, gonna need sunglasses to run the damned thing.

[Jimbo]

haha, cant wait, posty picy's

[BigSwede]

wait till you see my new brew rig controller Jimbo, gonna need sunglasses to run the damned thing.

Bling... is GOOD!

Yeah I figured what I have on my box is overkill as well, but I like it. I also have an eBay PT100 temperature display, (4 channels!) that was ridiculously cheap yet works great.

This thing: http://www.ebay.com/itm/-/251518378916