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ac to regulated dc help

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jamesburk

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Problem: I have a (5) 29Vac power signals from a air handler that I need to monitor with a device that can handle 3-12 Vdc and would like to keep the components down to a low cost low form factor.

First circuit idea that was given to me was a 1N4001 diode in series with a 100uF capacitor which netted me 39Vdc from cathode to common.I tried coming up with resistors to drop the voltage but ended up getting dc and ac as well. I was thinking about using zener diodes to regulate down to say
5Vdc but am unsure where to go with it.

Also thought about a 4 lead rectifier chip I saw at RS but unsure what to do with it.

I doubt the load is very much to trip the sensor module 500mA's I guess.

I am a little (or alot) rusty on as I got my BSEET 20 years ago and haven't put it to use.
 

dknguyen

Well-Known Member
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File:Diode bridge smoothing.svg - Wikipedia, the free encyclopedia
The 4-lead rectifier chip is the same as the 4 diode in the diagram in a single package.

A zener (with current limiting resistor would work). So would a linear regulator IC.
http://hyperphysics.phy-astr.gsu.edu/Hbase/electronic/zenereg.html

Resistors won't work alone because their voltage drop varies with current and input voltage. If current changes, output voltage changes. If input voltage changes, output voltage also changes- not what you want when regulating something.

THe simplest methods of converting 39VDC to 5V at 500mA will produce 17W of heat. For a typical chip, this would be a 1000C increase in temperature above ambient. With very very good heatsinking it's about 200C. Still way too hot for any components to survive. More efficient methods to convert DC-DC that produce less heat are much more expensive and complicated. For AC, it's easy so you should get a small transformer to step down voltage while it is AC as much as possible before running it through your diodes or rectifier so that the DC output is closer to 5V before running it through an inefficient zener or regulator.
 
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MikeMl

Well-Known Member
Most Helpful Member
Problem: I have a (5) 29Vac power signals from a air handler that I need to monitor with a device that can handle 3-12 Vdc and would like to keep the components down to a low cost low form factor. ...
I did something similar last summer. I incorporated a whole-house evap. cooling system into my existing 24V heat-cool thermostat. I needed to convert several of the 24V switched signals in my heating and airconditioning to CMOS logic levels.

Attached is a simulation of a single channel called "HEAT" in the schematic. The voltage controlled switch S5 is there just to make the 24V signal come and go for the simulation (see V(Heat)).

D6 rectifies the 24V signal, and stores the negative peaks in C8 (see V(X)). The voltage divider R6 R15 offsets V(X) to straddle the switching point of the CMOS Schmidt Trigger inverter (see V(y)), which normalizes the signal to a clean CMOS logic level (see V(Out)).

The 7805 voltage regulator is shown because it powers the CMOS and the voltage divider mentioned above.
 

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tcmtech

Banned
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Could you clarify a few things for us?

1: are you trying the power the sensor device off of the 29 AC source or just needing to get a signal from the 29 vac that this being converted to a voltage signal between 3 -12 vdc?

2: What does the sensor do? Is it a relay or an output device of some sort?
Does it have a manufacturer name and model number? For a specs sheet referance point.

3:What are you needing to monitor on this air handler unit?

I am just trying ot get a better idea of what you are attemping to do.
The more info you supply the better we can help! :)
 

jamesburk

New Member
tcmtech

I am just trying to get a signal so I know what the air handler is doing(ie. is it in heat or cool or 1st stage compressor ect.)
the 29Vac is going to relay's for each instance (W, Y1, Y2, R, O) so I am pulling a signal from across the 24Vac relay
I have 17 units to monitor with 5 signals on each one

the sensor is network based so I can monitor what a building's heating system is doing from offsight

I could just do it with relays but the cost was a little high
 

MikeMl

Well-Known Member
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pest thing Diode bridge you can do it easy.
James cant use diode bridges because all of his 24V circuits share a common connection to the 24V transformer, while simultaneously, his detectors need to share a common ground at the monitor panel.
 

MikeMl

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...
I have 17 units to monitor with 5 signals on each one the sensor is network based so I can monitor what a building's heating system is doing from offsight

I could just do it with relays but the cost was a little high
The circuit I posted will work; there are six of the Schmitt inverters in one package, so you could make it modular. Those are the same signals I needed to convert to logic levels...
 

Wond3rboy

Member
I did something similar last summer. I incorporated a whole-house evap. cooling system into my existing 24V heat-cool thermostat. I needed to convert several of the 24V switched signals in my heating and airconditioning to CMOS logic levels.

Attached is a simulation of a single channel called "HEAT" in the schematic. The voltage controlled switch S5 is there just to make the 24V signal come and go for the simulation (see V(Heat)).

D6 rectifies the 24V signal, and stores the negative peaks in C8 (see V(X)). The voltage divider R6 R15 offsets V(X) to straddle the switching point of the CMOS Schmidt Trigger inverter (see V(y)), which normalizes the signal to a clean CMOS logic level (see V(Out)).

The 7805 voltage regulator is shown because it powers the CMOS and the voltage divider mentioned above.

Hi Mike saw your circuits and liked them.I have one question though how did you set the resistor values in the above circuit..I have a problem very similar to this.
 

MikeMl

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Most Helpful Member
... how did you set the resistor values in the above circuit...
I simulated the circuit in LTSpice, which produced the schematic and the plots of the waveforms in the circuit. It becomes trivial to optimize the circuit by changing a value, and then observing the resulting effect the circuit behavior.
Here, the task was to offset the rectified voltage at the input of the Inverter so that it was below the trip point, and with no AC input, the pullup causes it to sit above the trip point.

Note that LTSpice is free for download at linear.com. There are examples and libraries of component models available at the LTSpice group on Yahoo.

Question for James: Would it be preferable to set up your monitor such that the inputs being monitored are optically isolated (do not share a common ground) with the refigeration/heating equipment. I suspect that that is why you may have considered relays. However, opto-isolators would work, too.
 

jamesburk

New Member
MikeMI

This is the device I am using to aquire the signal. As you will see it does have optically isolated inputs.

ControlByWeb: Five-Input Module Overview

As you stated I would have (5) 24-29Vac sources with 1 common ground.
Would it be to much to ask if you could draw me up a circuit as I described with component values.

Thanks James
 

Wond3rboy

Member
I simulated the circuit in LTSpice, which produced the schematic and the plots of the waveforms in the circuit. It becomes trivial to optimize the circuit by changing a value, and then observing the resulting effect the circuit behavior.
Here, the task was to offset the rectified voltage at the input of the Inverter so that it was below the trip point, and with no AC input, the pullup causes it to sit above the trip point.

Note that LTSpice is free for download at linear.com. There are examples and libraries of component models available at the LTSpice group on Yahoo.

Question for James: Would it be preferable to set up your monitor such that the inputs being monitored are optically isolated (do not share a common ground) with the refigeration/heating equipment. I suspect that that is why you may have considered relays. However, opto-isolators would work, too.

Thanks. I already use LTSpice.Just asking about circuits to improve my skills.
 
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MikeMl

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Most Helpful Member
This is the device I am using to aquire the signal. As you will see it does have optically isolated inputs.

ControlByWeb: Five-Input Module Overview

As you stated I would have (5) 24-29Vac sources with 1 common ground.
Would it be to much to ask if you could draw me up a circuit as I described with component values.

Thanks James
James, let's see, my consulting rate is $150 per hour. However just on this forum I have a deal for you...

I followed the link, and it appears that the device does have 5 optically isolated inputs. Based on the 12V @ 25mA input requirement, I guessing that each input of the WebRelay is a 470 Ω resistor in series with the LED part of a standard optoisolator.

Our task is to convert each of the 24Vac signals to a dc current capable of being detected inside the WebRelay as a clean logic one, while the absence of the 24Vac signal is a logic zero.

A simplified version of the circuit I already posted here will do the trick. It does not need a power supply. It is powered from the 24Vac signal itself. It uses only three components per channel. Set the WebRelay for the 3 to 12V input mode. The circuit delivers 5mA when the 24V signal is present, zero when not present. One common line serves five inputs...

Note that C8 needs to have a voltage rating of 45V min.
 

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tariq7868

New Member
File:Diode bridge smoothing.svg - Wikipedia, the free encyclopedia
The 4-lead rectifier chip is the same as the 4 diode in the diagram in a single package.

A zener (with current limiting resistor would work). So would a linear regulator IC.
Zener Regulators

Resistors won't work alone because their voltage drop varies with current and input voltage. If current changes, output voltage changes. If input voltage changes, output voltage also changes- not what you want when regulating something.

THe simplest methods of converting 39VDC to 5V at 500mA will produce 17W of heat. For a typical chip, this would be a 1000C increase in temperature above ambient. With very very good heatsinking it's about 200C. Still way too hot for any components to survive. More efficient methods to convert DC-DC that produce less heat are much more expensive and complicated. For AC, it's easy so you should get a small transformer to step down voltage while it is AC as much as possible before running it through your diodes or rectifier so that the DC output is closer to 5V before running it through an inefficient zener or regulator.
Geee...thanks alot..just what i needed...
 

jamesburk

New Member
MikeMI

I built the circuit as specified (except C8 is 35V for now) and it gives me a 3.6Vdc out which I ran for about 6hrs with no problems. I am guessing to increase Vdc out I need to lower the resistance.
Thanks again it seems to be exactly what I needed.
 
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