Beginner needing help

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Tclayfox

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I am a beginner in the whole circuit board world and need some help. Well alot really. I am trying to make some to help me in work as Ac tech. I have a drawing of what I am trying to make but not sure the best way. I have checked Amazon for different parts I need but think it will be more compact it I make t from scratch. I have a super rough drawing and was hoping someone could guide me through how to make this thanks in advance

The center rectangle is the board with everything I need to build into it
 

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I assume,
1) assume the 28v just needs a switch, to connect/disconnect.
2) The 9v battery lowers the switch and 5-minute timer
3) the 28v is DC
4) the 5-minute on-State starts when you turn the switch on. Then turns itself off after 5 minutes and stays off until you move the switch to off then on again.
5) you want the battery to last for a year or so.
 
So ultimately the 28 volts coming out will be for testing wires to see if they are shorted/grounded. Therefore tripping the resettable 3 amp fuse. Normally the wires have A.C. voltage but I assume the concept still works with DC as far as seeing if they are shorted. And yes the 5 minute timer starts when power is turned on and turns off and stays off unil the device is turned back on
 
Fuses are only needed when there's nothing else to limit the current. The circuitry that creates the 28 V can easily be made to limit the current without problems in case of a short, so there is no need for the fuse.

It would be very difficult to make a circuit that created enough current from a 9 V battery to blow a 3 A fuse anyhow.
 
So I work in hvac. Normally the low voltage wires have 24-28v. There are contactors, circuit boards and wires inside and out. So when we normally have to test for shorted we have to keep running inside and out cause the 24v come from the inside and shorts happen outside. So I wanted something I could carry and send the voltage threw each part to test what part is tripping the breaker/fuse.
 
The board on the furnace/ac unit typically sends 28v to the thermostat and the thermostat opens/closes the switch. So, to be clear, you would disconnect the thermostat wires before injecting this voltage to the thermostat, correct? Injecting 28v into the furnace board could cause a catastrophic failure. Note that the thermostat w/9v battery does not create the 28v on a standard furnace.

Also, measuring resistance is going to be a lot safer and more of an industry standard than pushing 3+ amps to pop a 3amp fuse on purpose.
 
I think that a multimeter to measure Ohms or continuity would do what you want. A dead short won't need 28 V to detect it.
 
Ohm won't work on like a contactor as far as I know due to there is a coil of wires that make a magnetic field so it would obviously ohm out
 


Yes I'm fully aware how the system receives 24v. The 3 amp resettable fuse I wanted in my contraption was chosen cause most furnaces use 3amp or 5. I choose smaller one. Ac low voltage doesn't pull 3amps unless shorted. So again that's why I chose 3 amp.
Also been in ac for 8 yrs and have a pretty good understanding of residential low voltage. Just have no knowledge in building a board hahaha
 
Tclayfox said:
Ohm won't work on like a contactor as far as I know due to there is a coil of wires that make a magnetic field so it would obviously ohm out
Click to expand...
A coil that runs on ac (alternating current) will have dc resistance. A multimeter will measure the dc resistance. When the dc resistance is measured, that won't tell you what the current and power will be when the ac supply is connected. However, the dc resistance of the contactor coil will tell you if it is a short circuit or an open circuit or a normal value. You can certainly compare it against another contactor of the same type.

I've not got a contactor here to measure, but I found an Omron relay, it's a 230 V ac MKS2PI. The coil resistance measured 4160 Ohms. That won't tell me the current that it takes, because I don't know the inductance of the coil, and the coil current is much less than a 4160 Ohm resistor would take from 240 V.

However, a look at the data sheet (https://assets.omron.com/m/083eb9ae43a4c783/original/MK-S-Datasheet.pdf) shows me that it's close enough to the expected value, and it's not shorted or open circuit, therefore the coil circuit is almost certainly good.

If you want a 28 V supply that is capable of operating a contactor, you will need a bigger power supply than can be run from a 9 V battery. Obviously a multimeter won't operate a contactor or relay. You won't need 3 A. The contactors won't take that much current, but without knowing exactly what they are, it's not possible to know the operating current.
 
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I have been int he ac field for 8 yrs. I don't know any tech that memorizes the ohms for every contactor, condenser delay boards, furnace boards and relays from every brand. Seems very reasonable and practical to have a small battery operated device to replicate the 24v sent from the furnace. I there is someone here who could just help me with the idea that would be great if not I appreciate all the input. Pretty much I can figure it out by trail and error and buying parts off Amazon but figured I could just ask for guidance from someone more knowledgeable than me in the board field
 
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I'm not sure if you want to detect that the contactor coil is there, or to activate the contactor.

If you want to detect the coil, a multimeter will be fine. It doesn't matter that the multimeter measures the dc resistance, or the correct value isn't known, because a short or an open circuit can be detected very easily.

If you want to activate the contactor, you could just put three small 9V batteries in series.
 
A contractor for an Air conditioner is typically 5 to 25 ohms. At 24 to 28v, that is a pretty good current flow of 3 amps. Way more than a 9v battery can muster, even 3 in series won't do that without blowing the bottom off of the battery in a few seconds. If you only need it for 0.1seconds to hear/measure that the contractor triggered, then you might be ok but, you'll be way better off with a stack of three 9v Lithium batteries, (e.g. Energizer Ultimate Lithium). Those can have 1 amp draw with spurts much higher than 1 amp. A standard alkaline or HD 9v battery can only do a few 100 mA and peaks to 1 amp.
 
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