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Steve Conner

Mon Sept 03 2012, 11:15AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The LM317's "adjust" pin sources a few microamps, but you also have a current flowing through the resistor string, as determined by the supply voltage and Ohm's law. If you use the recommended 200 ohm resistor between Out and Adj, this works out about 5mA, so ordinary 1/4 watt resistors will be fine.

Ash Small

Mon Sept 03 2012, 11:44AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve Conner wrote ...

The LM317's "adjust" pin sources a few microamps, but you also have a current flowing through the resistor string, as determined by the supply voltage and Ohm's law. If you use the recommended 200 ohm resistor between Out and Adj, this works out about 5mA, so ordinary 1/4 watt resistors will be fine.

Thanks Steve. I wasn't sure if the resistor string, R1, R2, 'sees' the input or output voltage, or what the 'trickery' inside the LM317 actually does.

I've a couple of small 500 Ohm presets, but I'm not sure of the wattage rating.

Steve Conner

Mon Sept 03 2012, 12:02PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The resistor string sees the output voltage, and the "trickery" drives the "Out" pin in such a way as to maintain 1.25V between "Out" and "Adj".

I think you can safely assume that the small presets are rated at least one-eighth watt.

Ash Small

Fri Sept 14 2012, 07:35PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Well, Klugesmith was correct, as usual (Thanks Rich

).

I'll use 5k pots (presets) accross the meters, and ONE LM317T to drop the ~12V supply for the fans down to 2V, which is then divided over a string of 4 x 1 Ohm, 10W resistors to give 0.5V for each thermistor/meter.

I'll use 2 x 470 Ohm resistors in parallel to give 235Ohms for R1, and 3 x 470 Ohm resistors in parallel to give ~157 Ohms for R2 for the bias on the LM317T, with an additional 500 Ohm preset or two in parallel for 'fine tweaking'. (for starters, anyway).

I couldn't find a suitable 'wall wart' to power the cooling fans, but I did find a 15V 6A toroidal transformer, a 35A 1000V rectifier and a couple of 4700uF 50V 'lytics in the shed, so I'll try that as a supply.

Do I need the 0.1uF and 1uF (if I remember correctly) accross the input and output rails on the LM317T or not? (I have some 400V MKP's I'll probably use, but some sources say they aren't really needed).

I should have all the resistors by mid-week, in the meantime, here are a couple of photo's of my 'bread board'.

Conundrum

Sat Sept 15 2012, 07:16AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

LM317 uses a 1.25V internal reference.
It can be configured to output a constant voltage or constant current depending on circuit, and yes you can replace the bottom leg of the resistor divider with a thermistor.
It burns a lot of power being a linear device but does have failsafes.

BTW this setup looks a bit like the portable vortex generator off "Sliders"

What the heck are you trying to build anyway?

Ash Small

Sat Sept 15 2012, 11:09AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Conundrum wrote ...

LM317 uses a 1.25V internal reference.
It can be configured to output a constant voltage or constant current depending on circuit, and yes you can replace the bottom leg of the resistor divider with a thermistor.
It burns a lot of power being a linear device but does have failsafes.

BTW this setup looks a bit like the portable vortex generator off "Sliders"

What the heck are you trying to build anyway?

I'm not replacing the bias resistor with a thermistor, I'm setting the output to 2V, then using a string of four 1 Ohm resistors to drop the voltage to 0.5V, then feeding 0.5V into each thermistor/moving coil meter.

At 100 degrees C, the combined resistance of each thermistor and meter is ~5k Ohms, which will have virtually no effect on the voltage accross the 1 Ohm resistors (~0.02%). so accuracy will not be affected by having the four thermistors and meters in series with each other.

It's part of this project:

Ash Small

Mon Dec 10 2012, 01:27AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Well, I've finally got back to work on this project after a couple of months of 'not much spare time'.

I filled an enamelled cast iron saucepan with cheap cooking oil and put it on the aga with a thermistor connected to my DMM set to the Ohms setting, added a thermometer, stirred it continuously, and started plotting a graph of temperature V thermistor resistance.....

EDIT: I've now done some maths and I think the rest of this post is incorrect. Please see post ten posts down
....and have realised that the graph curves the 'wrong way' (I wanted a non-linear response, so that the meters don't move much until the thermistors start to get 'hot').

I guess the way around it is to put the meters in parallel with the thermistors instead of in series, and turn the meters upside down (and re-do the 'scale') so that the meters are at FSD @ room temp, but as the temp of the heatsinks (and thermistors) rises, the current through the meters falls. That way I should get the non-linearity I want. (In the right direction.)

I'll probably also need to add a resistance in series with each meter, to control the rate of 'rise' so that I get an acceptable 'curve'.

I can probably still use most of the rest of the circuit as described above, but will probably have to set the voltage supplied by the LM317T to a different voltage to the 2V described above (I've not done the maths yet).

I've only used one thermometer so far (up to 50C), I have a 'jam thermometer' which goes from 50C to over 200C to finish the 'plot')

Hopefully I'll get a chance to make a bit more progress tomorrow.

EDIT: Ignore the cross ar 170 C, 75 kOhms, it's not supposed to be there.

Proud Mary

Mon Dec 10 2012, 07:02PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I use these inexpensive 1000V 25A bridge rectifiers for plasma electrolysis.

I use an industrial isolation transformer in ALL my experiments, but this is not essential.

I then have a big variac, the bridge rectifier above, and some big fat 10,000 uF 450V electrolytics for whatever amount of smoothing seems sensible.

I have the bridge rectifier bolted to a big black heat sink with a smear of heat sink compound, and that's all there is to it.

As my stuff is running off a 13A mains ring, with 13A fused plugs, there wouldn't be any point using any more powerful stuff than this. With all the arcing, bumping, boiling, and evolution of gas with 250 VDC going into a 500 ml beaker full of electrolyte, it's frightening enough as it is, without a lot of palaver about rectifier cooling. The fuses will blow long before the 25A maximum rectifier rating is exceeded. Just use a heat sink of the rating specifed in the rectifier datasheet and let the cooling take care of itself, or you'll never get anything done!

Ash Small

Mon Dec 10 2012, 08:27PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Proud Mary wrote ...

I use these inexpensive 1000V 25A bridge rectifiers for plasma electrolysis.

I will be using this for mains rectification.

I also used my last one for various electro-chemical processes, including electro-polishing, electro-etching, etc, where I often run it from a welding transformer, sometimes more than one in parallel.

I blew up my last one using it for DC TIG welding, where hundreds of amps can be flowing.

If I was just using it for mains rectification, I'd do the same as you. (I'm actually using one of those 35A, 1000V rectifiers (GBPC3510, as opposed to the KGBC2510 you linked to) for rectifying the 12V supply from the toroidal Xformer, for powering the cooling fans for the heatsink and 'instrumentation system' as it's all I had lying around).

The 300A, 1200V stud diodes cost me hundreds, so I'm not taking any chances this time.

I also plan on implementing temperature monitoring on the heatsinks of some of my other projects as well, so this research should come in useful for other projects as well.

EDIT: My stud diodes are Schottky's, so should also be ok for rectifying the output from high frequency ferrite cored Xformers as well, if I ever need to.

klugesmith

Mon Dec 10 2012, 09:37PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Ash Small wrote ...
....and have realised that the graph curves the 'wrong way' (I wanted a non-linear response, so that the meters don't move much until the thermistors start to get 'hot').

I guess the way around it is to put the meters in parallel with the thermistors instead of in series, and turn the meters upside down (and re-do the 'scale') so that the meters are at FSD @ room temp, but as the temp of the heatsinks (and thermistors) rises, the current through the meters falls. That way I should get the non-linearity I want. (In the right direction.)

Good work, measuring the component.

1. Does the response agree with what you expect from datasheet?

2. Regarding the "wrong-way" nonlinearity:
Can you show us a chart of predicted meter current with respect to temperature?
It certainly won't be proportional to the thermistor resistance.
Dr. Slack's original recommendation mentioned a reasonably linear voltage variation around the temperature where NTC and fixed resistor divide the reference voltage in half.

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