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Author Topic: Window comparator to connect/disconnect 500ma load  (Read 2884 times)

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sgking2

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Window comparator to connect/disconnect 500ma load
« on: January 25, 2012, 23:30:54 PM »
Hello,
I would like to operate a 12VDC, single supply circuit, as a rail to rail window comparator (or within 200mv). The inputs would be from a single variable voltage source in the same range, 0-12VDC, one comparator would trigger when the input reached a high of >= 11VDC and the low would trigger when the same input signal reached <= 6VDC. Basically, only 1 output at most could be 'on' (either high or low), or neither in the case the input was between 6-11VDC. The output of each, independently would need to switch, with decent isolation, a 500ma circuit ( a 12VDC 400ma fan). The high would connect the fan direct to 12V rail and the low would disconnect it and break the circuit from ground. When neither are triggered, the fan would be controlled variably by a TIP142 emitter-follower in the range 6V-11V.

1. What would that window comparator look like in terms of supporting components to have those trigger points with adequate hysteresis for slow moving inputs?, and

2. what would the outputs be to switch a 1/2A load (fan), could it be a FET of sometype or relay, or be driven directly by the comparator?

3. Since the TIP142 that would normally regulate the fan voltage (6-11V) might have some Base-emitter current, is there any reason to break that circuit and bypass the TIP completely at the emitter to GND leg when the TIP is bypassed and either FULL on with 12V rail or full OFF by breaking the circuit - the goal being near-ZERO power dissipation in the TIP in those 2 bypass cases.

** Stupid Question? I am new to this forum, how do I post a drawing or pic? If I click on insert image all I get is the IMG/IMG tags. No call out for a file location to paste in.

Many thanks! You guys are the best!

kam

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Re: Window comparator to connect/disconnect 500ma load
« Reply #1 on: January 26, 2012, 07:52:58 AM »
Quote
What would that window comparator look like in terms of supporting components to have those trigger points with adequate hysteresis for slow moving inputs?
If i got it right, this sounds a job for a schmitt trigger. The st has 2 thresholds, the high and the low. The low threshold is lower than the high (hysteresis). the output goes 1 when the signal is above or equal to the high threshold, and goes back to 0 when the signal is lower than the low threshold. Here it the theory:

The Schmitt Trigger



Quote
what would the outputs be to switch a 1/2A load (fan), could it be a FET of sometype or relay, or be driven directly by the comparator?
the ft is ok, a relay is also ok.



Quote
Since the TIP142 that would normally regulate the fan voltage (6-11V) might have some Base-emitter current, is there any reason to break that circuit and bypass the TIP completely
i do not quite get how you are going to use the tip in conjunction with the schmitt trigger. the base-emitter current is very small (compared to the collector emitter) and usually we do not get it into accounts during design time. 




Quote
I am new to this forum, how do I post a drawing or pic? If I click on insert image all I get is the IMG/IMG tags. No call out for a file location to paste in.
click "Attachments and other options" underneath the text area that you post your answer and it will appear

sgking2

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Re: Window comparator to connect/disconnect 500ma load
« Reply #2 on: January 26, 2012, 21:31:19 PM »
- I thought a Schmitt trigger required +/- rails ? ... or can it operate between 12VD and GND ? Also, does a schmitt trigger only have 1 threshold, if so, I need 2. I need a high at around 10.4-11.5 and a low around 5.2 - 7.8 (both can be fine tuned with a POT. Is there a way to have both thresholds in a single schmitt? If not, is there any benefit to 2 schmitt triggers over a dual (window) comparator?

- With just a single digital output (hi/low) how would I control 2 separate relays independently?

   The high level trigger would actuate a SPDT relay to connect the 12V rail to the Fan instead of the Emitter of the TIP142, bypassing its voltage drop and supplying the fan directly from the 12V rail. The single pole is the fan and the other 2 poles are the emitter of the emitter-follower, or the 12V rail directly.

    The low level trigger would actuate a SPST relay that would connect the above relay to the Fan or break the circuit open completely and prevent any current flow through the TIP142 emiiter-follower or the fan (to prevent stall and over-current).

- With a fan rated for 400ma, is it possible to use a solid state device inplace of the relays? Which type of device would be ideal? Would it be able to be driven directly by the output of the Scmitt trigger OR a window comparator? I think there are some comparators that can sink a load of up to 50ma but I do not know oabout the Schmitt triggers.

- With a scmitt trigger, it seems I would still need to invert the low level output to a high level so it could actuate the other relay or solid state device, unless you have an idea of one part that will "switch on" with a low level drive from the Schmitt trigger?

** FYI, the idea is that the TIP142 would act as a emitter follower for a voltage-controlled fan circuit (or fan-amp), and the small current from the variable voltage input would control the TIP to provide enough power/heat dissipation for the fan from 6V-11.3V. However, that same fan control can be fed into the window comparator or schmitt trigger and detect a level of <6V to break the circuit open completely and shut off the fan and TIP (no heat) OR >11.3V would bypass the TIP and provide the full 12V from the rail negating any single or double diode drops of a darlington transistor. Are there any problems with this logic or concept that you can think of ?

Thanks!

kam

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Re: Window comparator to connect/disconnect 500ma load
« Reply #3 on: January 27, 2012, 09:26:26 AM »
Regarding the st, there are +/- and single power supply methods. You should read the article to find what you need. And an st has 2 thresholds.

You can use a NPN and a PNP transistor to make 2 switches, one will operate with + and the other with -.

You will need to make a driver for the solid state relay of yours (and invert the signal as you wish). An st does not have absolute 0 in LOW, so the relay may not be off. A solid state relay is the best choice though.
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Regarding the "FYI" at the end:

Then forget that "schmidt trigger" part. At maximum output (11.3) you do not need to bypass the output transistor, since the only power loss will be the tiny I2R from the 400mA and you will not feel any heat.
For the 6volts, then you should better use a relay driven from a single comparator. This is the best. I do not recommend you use solid state relays unless you know what is hidden inside. Use a nice miniature relay to power off your circuit. There are also low current relays if you consider power loss.

Now, if you have a problem with the 11.3 instead of 12, you can add another comparator for this. But first measure the output of the gadget, it might be already above 12V. PC PSUs usually provide little above nominal voltage, so you may not need adding an extra comparator.








sgking2

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Re: Window comparator to connect/disconnect 500ma load
« Reply #4 on: January 27, 2012, 09:42:44 AM »
Very good points. Your feedback is very valuable. I have read much more on the schmitt triggers and have grown to like their benefits, especially with reponse and noise immunity to prevent bounce or rapid changes due to slow inputs. The hi trigger was just to provide maximum voltage possible, not a heat concern, but you are right my PSU is almost 12.6V - so I would be real close to 12V anyway. I guess I was afraid if I used a darlington pair that there would be (2) diode drops in that one package, not just one.
Thanks!

kam

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Re: Window comparator to connect/disconnect 500ma load
« Reply #5 on: January 27, 2012, 10:12:28 AM »
I guess I was afraid if I used a darlington pair that there would be (2) diode drops in that one package, not just one.

Didn't i mention that? Damn i forgot. Yes a darlington pair has more Vbe drop. Not double, but more, some 1.1 instead 0.65.