I need to make a heater circuit more intelligent.  The existing design uses
a low-side switch with with a simple logic feedback to check for proper
operation.  An objective is to replace that with a device that will provide
a voltage output proportional to the heater current.

I have found a few high-side switches that will do the job and I am starting
to lean that way.

On Mon, 14 Mar 2005 08:21:39 -0800, Luhan Monat wrote:
> Richard Henry wrote:
>
>> I need to make a heater circuit more intelligent. 
>
> Ok, but to do exactly what?

To monitor the current going to the heater, as he says below.

>   The existing design uses
>> a low-side switch with with a simple logic feedback to check for proper
>> operation.
>
> Some safety feature, perchance?

Does it matter in the context of the question?

>    An objective is to replace that with a device that will provide
>> a voltage output proportional to the heater current.
>> 
>
> Not proportional to temperature?  Just why do you need to know the current?

Can you truly not think of any good reasons for wanting to monitor
the heater current?  Ah well chalk it up to either a lack of imagination
or a lack of experience.  How is the reason he wants to monitor the
current relevant to the question?

>> I have found a few high-side switches that will do the job and I am starting
>> to lean that way.
>> 
>
> Don't mean to be rude here, but this is a great example of 'how not to 
> ask a question'.  Asking the right questions here will generally get you 
> very good answers.

I can't personally think of a clearer way to ask "does anyone know of
a low-side switch with current monitoring?"  Perhaps you could reprase
it in a way that makes it clearer, or did you reply the way you did
just to make yourself feel smart?  'Cause all you've done is make yourself
look like a bit of a dope.


-- 
Trevor Barton

Richard Henry wrote:

> I need to make a heater circuit more intelligent. 

Ok, but to do exactly what?

  The existing design uses
> a low-side switch with with a simple logic feedback to check for proper
> operation.

Some safety feature, perchance?

   An objective is to replace that with a device that will provide
> a voltage output proportional to the heater current.
> 

Not proportional to temperature?  Just why do you need to know the current?

> I have found a few high-side switches that will do the job and I am starting
> to lean that way.
> 

Don't mean to be rude here, but this is a great example of 'how not to 
ask a question'.  Asking the right questions here will generally get you 
very good answers.

-- 
Luhan Monat (luhanis 'at' yahoo 'dot' com)
"The future is not what it used to be..."
http://members.cox.net/berniekm

Sounds like a "sensefet".  Does IR still make those?

In article <39lnlhF60b04mU1@individual.net>, rphenry@home.com says...
> I need to make a heater circuit more intelligent.  The existing design uses
> a low-side switch with with a simple logic feedback to check for proper
> operation.  An objective is to replace that with a device that will provide
> a voltage output proportional to the heater current.
> 
> I have found a few high-side switches that will do the job and I am starting
> to lean that way.

IR website search engine doesn't return anything for "sensefet".

One of the parts I am looking at is the IR3312S.  From the data sheet and
appnote, it requires an external FET to translate the input signal from
logic level, a slight disadvantage.  A compensating advantage ( something I
was not originally looking for) is that the feedback current resistor can be
used to set the overcurrent limit.


"Ol' Duffer" <DontSend@MeSpam.net> wrote in message
news:MPG.1c9f881284e477ea9896ac@news.bright.net...
> Sounds like a "sensefet".  Does IR still make those?
>
> In article <39lnlhF60b04mU1@individual.net>, rphenry@home.com says...
> > I need to make a heater circuit more intelligent.  The existing design
uses
> > a low-side switch with with a simple logic feedback to check for proper
> > operation.  An objective is to replace that with a device that will
provide
> > a voltage output proportional to the heater current.
> >
> > I have found a few high-side switches that will do the job and I am
starting
> > to lean that way.

Richard Henry wrote:
> I need to make a heater circuit more intelligent.  The existing design uses
> a low-side switch with with a simple logic feedback to check for proper
> operation.  An objective is to replace that with a device that will provide
> a voltage output proportional to the heater current.
> 
> I have found a few high-side switches that will do the job and I am starting
> to lean that way.
> 
> 
Why not just a big MOSFET with a small resistance in the source lead, 
plus a low-offset opamp?

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

On Mon, 14 Mar 2005 07:07:28 -0800, "Richard Henry" <rphenry@home.com>
wrote:

>I need to make a heater circuit more intelligent.  The existing design uses
>a low-side switch with with a simple logic feedback to check for proper
>operation.  An objective is to replace that with a device that will provide
>a voltage output proportional to the heater current.
>
>I have found a few high-side switches that will do the job and I am starting
>to lean that way.
>

What sort of current are you talking about ? Maximum voltage ? 
Philips, ST Electronics and International Rectifier has these sort of
devices.

Regards
  Anton Erasmus

On Mon, 14 Mar 2005 07:07:28 -0800, Richard Henry wrote:

> I need to make a heater circuit more intelligent.  The existing design uses
> a low-side switch with with a simple logic feedback to check for proper
> operation.  An objective is to replace that with a device that will provide
> a voltage output proportional to the heater current.
> 
> I have found a few high-side switches that will do the job and I am starting
> to lean that way.

http://www.google.com/search?q=current+sensing+power+mosfet&btnG=Google+Search

Good Luck!
Rich

In article <39ltgaF6404f3U1@individual.net>, rphenry@home.com says...
> IR website search engine doesn't return anything for "sensefet".

I guess I'm dating myself...
It appears they have changed the name to "hexsense", and
their search engine isn't very useful.
They make 5-leg TO-220 and TO-247 package devices,
IRC730 for example.
Download AN-959 from their archives for an overview.

Anyone figure out if this is an AC or a DC current sensing requirement ?
What kind of Amps too ?

Makes a big difference how to constuct what is required !

Jay

Trevor Barton wrote:
> On Mon, 14 Mar 2005 08:21:39 -0800, Luhan Monat wrote:
> 
>>Richard Henry wrote:
>>
>>
>>>I need to make a heater circuit more intelligent. 
>>
>>Ok, but to do exactly what?
> 
> 
> To monitor the current going to the heater, as he says below.
> 
> 
>>  The existing design uses
>>
>>>a low-side switch with with a simple logic feedback to check for proper
>>>operation.
>>
>>Some safety feature, perchance?
> 
> 
> Does it matter in the context of the question?
> 
> 
>>   An objective is to replace that with a device that will provide
>>
>>>a voltage output proportional to the heater current.
>>>
>>
>>Not proportional to temperature?  Just why do you need to know the current?
> 
> 
> Can you truly not think of any good reasons for wanting to monitor
> the heater current?  Ah well chalk it up to either a lack of imagination
> or a lack of experience.  How is the reason he wants to monitor the
> current relevant to the question?
> 
> 
>>>I have found a few high-side switches that will do the job and I am starting
>>>to lean that way.
>>>
>>
>>Don't mean to be rude here, but this is a great example of 'how not to 
>>ask a question'.  Asking the right questions here will generally get you 
>>very good answers.
> 
> 
> I can't personally think of a clearer way to ask "does anyone know of
> a low-side switch with current monitoring?"  Perhaps you could reprase
> it in a way that makes it clearer, or did you reply the way you did
> just to make yourself feel smart?  'Cause all you've done is make yourself
> look like a bit of a dope.
> 
> 

I stand by my remarks.  Like many things I've seen on this NG, it lacks 
some overall context for the questions being asked. At least, it 
honestly seemed that way to me. If I'm wrong here, well, thats the way 
things work here - after some back-and-forth bantering, some useful 
information usually emerges overall.

-- 
Luhan Monat (luhanis 'at' yahoo 'dot' com)
"The future is not what it used to be..."
http://members.cox.net/berniekm

Trevor Barton wrote:

> Can you truly not think of any good reasons for wanting to monitor
> the heater current?  Ah well chalk it up to either a lack of imagination
> or a lack of experience.  How is the reason he wants to monitor the
> current relevant to the question?
> 

Would it not be better to know whether the heater here is used to heat a 
fish tank or regulate blood temperature in a heart-lung machine?

I can *imagine* altogether tooooo many posibilities here.  If this were 
a paying client asking the original question, I would certainly want to 
know exactly what the heater was part of.  For liablility reasons for 
starters.

-- 
Luhan Monat (luhanis 'at' yahoo 'dot' com)
"The future is not what it used to be..."
http://members.cox.net/berniekm

In article <39lnlhF60b04mU1@individual.net>, rphenry@home.com says...
> I need to make a heater circuit more intelligent.  The existing design uses
> a low-side switch with with a simple logic feedback to check for proper
> operation.  An objective is to replace that with a device that will provide
> a voltage output proportional to the heater current.
> 
> I have found a few high-side switches that will do the job and I am starting
> to lean that way.

Given the description I suspect Allegros devices are too large and 
probably too expensive but you may want to have a look.

http://www.allegromicro.com/hall/currentsensor.asp

Then again maybe not, I see they go as low as 5A now.

Robert