# Isolated variable resistor function?

J

#### Joerg

Jan 1, 1970
0
Hi Folks,

Does anyone know a part that is small, can be used to command a certain
resistance and can be talked to across the usual mains-isolated barrier
of office equipment and the like? I need to set resistor values on the
mains side from a uC on the low voltage side. Has to be:

Linearity error from 0V to about 1V excitation should be <5%.
Absolute resistance tolerance <20% if possible.
Bandwidth for resistor value changes up to a few kHz.
No limit on the number of changes (meaning no EEPROM).

So far I've considered:

a. FET optocouplers such as the H11F1. A bit pricey at over $1 but most of all has the problem that the linearity goes to pots as soon as there is more than 20-30mV across the FET, plus it needs a ton of drive power for low resistance: http://www.fairchildsemi.com/ds/H1/H11F1M.pdf b. Digital potmeters such as the MCP4011 series. These are nice, tons of bandwidth, cheap and small. But they do need an additional two-channel optocoupler to get the command data across and the absolute resistance value tolerance is borderline at 20%: http://ww1.microchip.com/downloads/en/DeviceDoc/21978c.pdf c. Servoed FET plus opto. The servo gets rid of the non-linearity. Done it before but this gets old because it needs a lot of parts and real estate. d. LDR plus LED: Very nice, but fairly large and most of all LDRs are banned in some countries on account of their Cadmium content. Any other miracle parts out there? K #### [email protected] Jan 1, 1970 0 Hi Folks, Does anyone know a part that is small, can be used to command a certain resistance and can be talked to across the usual mains-isolated barrier of office equipment and the like? I need to set resistor values on the mains side from a uC on the low voltage side. Has to be: Linearity error from 0V to about 1V excitation should be <5%. Absolute resistance tolerance <20% if possible. Bandwidth for resistor value changes up to a few kHz. No limit on the number of changes (meaning no EEPROM). Addressable across isolation barrier. So far I've considered: a. FET optocouplers such as the H11F1. A bit pricey at over$1 but most
of all has the problem that the linearity goes to pots as soon as there
is more than 20-30mV across the FET, plus it needs a ton of drive power
for low resistance:

http://www.fairchildsemi.com/ds/H1/H11F1M.pdf

b. Digital potmeters such as the MCP4011 series. These are nice, tons of
bandwidth, cheap and small. But they do need an additional two-channel
optocoupler to get the command data across and the absolute resistance
value tolerance is borderline at 20%:

c. Servoed FET plus opto. The servo gets rid of the non-linearity. Done
it before but this gets old because it needs a lot of parts and real estate.

d. LDR plus LED: Very nice, but fairly large and most of all LDRs are
banned in some countries on account of their Cadmium content.

Any other miracle parts out there?

Does it have to be four-quadrant? Floating? There are some cute fake resistor
circuits around but the ones I've tried using need one end of the pot at
"ground". That and an isolated DAC (DAC on the far side of an isolator, even)
will solve your tolerance problem. You probably won't like the cost, though.
;-)

J

#### Joerg

Jan 1, 1970
0
John said:
Any chance you can PWM a resistor?

Would be nice but the part where the resulting signal goes into is
blazingly fast (has to be). So the PWM would cause an undesired modulation.

But it is an idea, maybe I can slow something down in there. And go in
with several meggeehoitzes. The uC we have would moan and groan though.

J

#### Joerg

Jan 1, 1970
0

That's similar to what John suggested. Thing is, this variable resistor
is going to be the bottom resistor of a voltage divider and has to be
varied between zero and 10k or so.

J

#### Joerg

Jan 1, 1970
0
Does it have to be four-quadrant? Floating? There are some cute fake resistor
circuits around but the ones I've tried using need one end of the pot at
"ground". That and an isolated DAC (DAC on the far side of an isolator, even)
will solve your tolerance problem. You probably won't like the cost, though.
;-)

Actually single quadrant is ok and one side of it can be grounded. Cost
is a factor but not like in most of my other design. Meaning a buck is
acceptable but two bucks would be a stretch.

J

#### Joerg

Jan 1, 1970
0
Jan said:
PIC with ADC on one end with RS232 out one opto
PIC on other end with PWM DAC out (assuming you need some control voltage).

I don't need a control voltage but a control resistance. That's the
challenge. Getting a voltage across is way easier: You'd just send PWM
out of the uC that we already have, onto an optocoupler, and then
RC-filter this into a DC voltage one the other side. Could be done for
<30c. Of course they'll throw coffee mugs and paper wads at me because I
would have to file a request for the 2nd timer in there. Analog guys
always want the timers. Well, those plus beer and marzipan.

K

#### [email protected]

Jan 1, 1970
0
Actually single quadrant is ok and one side of it can be grounded. Cost
is a factor but not like in most of my other design. Meaning a buck is
acceptable but two bucks would be a stretch.

How good of a resistor do you need? Is an opamp fast enough?

J

#### Joerg

Jan 1, 1970
0
How good of a resistor do you need? Is an opamp fast enough?

Yes, it is fast enough. But it has to behave like a real resistor, up to
several kHz.

J

#### Joerg

Jan 1, 1970
0
Jan said:
OK, very long plastic rod with potmeter.

That would require shipping a trained mouse with every unit. Too
expensive because of the food, veterinary bills and so on.

J

#### Jon Kirwan

Jan 1, 1970
0
Would be nice but the part where the resulting signal goes into is
blazingly fast (has to be). So the PWM would cause an undesired modulation.

But it is an idea, maybe I can slow something down in there. And go in
with several meggeehoitzes. The uC we have would moan and groan though.

Some of the MSP430's have Timer D, which will run PWM at
256MHz -- if I read correctly.

Jon

J

#### Jon Kirwan

Jan 1, 1970
0
<snip>
Some of the MSP430's have Timer D, which will run PWM at
256MHz -- if I read correctly.

I should have said "clocked at 256MHz," but you already know
that the count per period would divide this down for the PWM
frequency. Sorry about not being precise in writing that.

Jon

J

#### Joerg

Jan 1, 1970
0
Jan said:
I have made very nice 'resistors' with MOSFET with feedback resistor network to gate.

So have I, but tough to achieve a sustained 5% linearity over production
runs. I've done it in servo fashion but with all that you quickly reach
a point where a 30-40c digital potmeter is simply the better solution.

J

#### Joerg

Jan 1, 1970
0
Jan said:
No no, it saves a micro.
This assumes that the equipment is used by a human.
Why the electronics?
he / she can twiddle the pot directly.

The uC has to do the twiddling.

J

#### Joerg

Jan 1, 1970
0
Jon said:
I should have said "clocked at 256MHz," but you already know
that the count per period would divide this down for the PWM
frequency. Sorry about not being precise in writing that.

A MSP430 clocked at 256MHz? Shazam! That sounds like a Suzuki Alto with
a Hayabusa engine in there.

K

#### [email protected]

Jan 1, 1970
0
So have I, but tough to achieve a sustained 5% linearity over production
runs. I've done it in servo fashion but with all that you quickly reach
a point where a 30-40c digital potmeter is simply the better solution.

Why is linearity an issue (feedback, ya' know)? Drive it with a good (5%
isn't "good") multiplying DAC.

J

#### Joerg

Jan 1, 1970
0
John said:
"Blazingly fast" and "10K" may not get along.

Well, of course not while at 10k

J

#### Joerg

Jan 1, 1970
0
Why is linearity an issue (feedback, ya' know)? ...

The signal going into this divider can't be distorted more than 5% and
with a FET-optocoupler it would be.

... Drive it with a good (5% isn't "good") multiplying DAC.

Sure, but then I might as well just use my solution "b", a digital
potmeter. Reduces that function to one part costing 30-40c, outside the
optocoupler to xfer the data lines which I'd need either way.

J

#### Jon Kirwan

Jan 1, 1970
0
A MSP430 clocked at 256MHz? Shazam! That sounds like a Suzuki Alto with
a Hayabusa engine in there.

Hehe. I was just needing a faster PWM (wanted to use a very
cheap RC, 20db/decade rolloff, low-pass, but needed an audio
spectrum. Couldn't get there with 1/62500 PWM period of
standard MSP430. So I looked and... cripes!! They had
something else!

of capture operation by half. In addition, the combining
compare blocks help to control both rising and falling edges
of the PWMoutput signal. The Timer-Event-Control-block offers
external triggering options as well as internal
synchronization of timer instances."

See the MSP430F51x2 parts.

http://www.ti.com/product/msp430f5172&DCMP=msp430&HQS=430timer

Jon

J

#### Joerg

Jan 1, 1970
0
Jim said:
John Larkin wrote: [snip]
Any chance you can PWM a resistor?
Would be nice but the part where the resulting signal goes into is
blazingly fast (has to be). So the PWM would cause an undesired modulation.

But it is an idea, maybe I can slow something down in there. And go in
with several meggeehoitzes. The uC we have would moan and groan though.

In this case several MHz. In the world of switch mode power supplies
that counts as "blazingly fast"

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