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Old 14-04-2013, 14:11   #1
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120v To 24v transformer

If have an appliance that goes from the plug to a 120v to 24vac transformer inside. I don't want to use an inverter, I want to take out the transformer and put 24v from my battery bank directly to it. I have a feeling the secondary side of the transformer is alternating current and it won't work with my 24v direct current. Am I right? If so do they have transformers that goes from 24vdc to 24vac?
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Old 14-04-2013, 14:17   #2
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Re: 120v To 24v transformer

Are you sure that the transformer inside outputs 24V AC and not 24V DC? Do you have a multimeter with which you can check this if you are uncertain?

I think that there might be such a 24V DC to 24V AC converter out there, but you will end up paying quite a bit more for a such an item than an off-the-shelf inverter for 120v; which will also regulate the output voltage much better. This means that when the batteries are charging you'll have 27 or 28VDC on the bank, and when they are almost empty you'll be at 25 or perhaps even lower; an inverter will ensure output of 120V AC regardless of input (within limits, of course).
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Old 14-04-2013, 14:17   #3
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Re: 120v To 24v transformer

24v AC on the secondary of a power supply transformer usually becomes 27 to 30 v DC once it's rectified. So you need to find out what the actual DC voltage used by the circuit is, by measuring the device's DC supply rail, after the rectifiers.

A schematic will be essential, unless you have alot of experience with power supplies.
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Old 14-04-2013, 14:51   #4
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Re: 120v To 24v transformer

What I'm getting is a used machine with a compressor. The schematic shows 120v to 24vac transformer that controls all the circuits. the compressor is 120v but i want to put a 24vdc in its place,which is not a problem, the schematic shows a 24vac relay for the compressor.I'll run the DC power thru that relay or get a different one. I'm doing this just for fun so i don't want any alternating current going to it. The fuse on the secondary side of the transformer is 3 amps, so I'll probably need a 100watt 24vdc to 24vac transformer.
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Old 14-04-2013, 15:31   #5
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Re: 120v To 24v transformer

What is the machine and what exactly are "all the circuits"?

Can you just replace the AC relay with one spec'ed for a 24vdc coil?
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Old 14-04-2013, 16:24   #6
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Re: 120v To 24v transformer

Transformers do not work on DC. A transformer works by magnetic flux continuously expanding and collapsing, which can only be created by AC. The output of a transformer can have a rectifier on it so that the output is converted from AC to DC.

You can't just run 24 VDC through a 24 volt AC circuit if that circuit was engineered for 24 VAC, for a number of possible reasons including the fact that AC and DC motors are two completely different designs that can only accept the type of voltage they were designed for.
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Old 14-04-2013, 18:25   #7
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Since its a little unclear. Ill preface my suggestions with a concern.

If the 24vac circuit is control cir unit, you MAY be able to feed in DC , but in reality you need to understand the circuit. If the circuit just uses the 24vac and rectifies it to DC , then yes you can use DC. If it has specific functions that expect AC then no.

I don't quote understand why you are doing this since you as I understand it the motor is 120vac anyway.

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Old 15-04-2013, 03:30   #8
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Re: 120v To 24v transformer

It's a refrigerator with control circuits that come off the 24vac side of the transformer. By the print the only thing that runs 120v is the compressor,fan and lights. I have no 120v to plug it into so i wanted to change it over to 24v and use solar panels,which would be my next topic i'll post in a new title. Anyway it sounds to me i'm going to have to use an inverter, I just wanted to be all 24vdc.
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Old 15-04-2013, 03:37   #9
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Re: 120v To 24v transformer

Quote:
Anyway it sounds to me i'm going to have to use an inverter, I just wanted to be all 24vdc.
yes I know the feeling , I wanted to marry Cameron Diaz en all !.

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Old 15-04-2013, 05:04   #10
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Re: 120v To 24v transformer

Quote:
Originally Posted by tuberider View Post
... I don't want to use an inverter, I want to take out the transformer and put 24v from my battery bank directly to it...
... do they have transformers that goes from 24vdc to 24vac?
Devices that convert DC to AC are called inverters.
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Old 15-04-2013, 10:20   #11
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Re: 120v To 24v transformer

Quote:
Originally Posted by tuberider View Post
It's a refrigerator with control circuits that come off the 24vac side of the transformer. By the print the only thing that runs 120v is the compressor,fan and lights. I have no 120v to plug it into so i wanted to change it over to 24v and use solar panels,which would be my next topic i'll post in a new title. Anyway it sounds to me i'm going to have to use an inverter, I just wanted to be all 24vdc.
Can you post a pic of the print?
It still might be possible to run the thing on 24 vdc.

(I repaired electronics professionally for 30 years in a previous life)
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Old 15-04-2013, 10:35   #12
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Re: 120v To 24v transformer

Quote:
Originally Posted by tuberider View Post
It's a refrigerator with control circuits that come off the 24vac side of the transformer. By the print the only thing that runs 120v is the compressor,fan and lights. I have no 120v to plug it into so i wanted to change it over to 24v and use solar panels,which would be my next topic i'll post in a new title. Anyway it sounds to me i'm going to have to use an inverter, I just wanted to be all 24vdc.
If the compressor motor runs on 120V AC power then you still have to have 120V to run the refer even if you can find a way to power the other circuits on 24V AC or DC.
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Old 16-04-2013, 08:58   #13
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Re: 120v To 24v transformer

My point was, are you SURE the compressor runs on 120?
I used to have a boat fridge that converted 12 volts to 3 phase pulsating DC @ 22 volts to run the compressor.
It's not a good idea to just assume unless you can read some voltage documentation on the schematic.

Skipmac is right if the compressor indeed does work on 120 vac; why mess around with the control circuit at all?
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Old 17-04-2013, 11:25   #14
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Re: 120v To 24v transformer

Here is some inside information.
24 volts AC is a standard control voltage for heating, air conditioning and refrigeration controls.
It is usually a small transformer that doesn't run any big loads like motors, but just switches relays and solenoids.
You can't use 24 volt DC instead of the 24 volt AC because when run on DC the relays and solenoids will draw way too much current and overheat.

BUT there is a trick I've used many times in the past.

It so happens that a 12 volts DC supply instead of the 24 volts AC works just right and operates the relays and solenoids without overheating.

SO

If it is just a small transformer, thoroughly analyze all the items running off the 24 volts AC in the device. If there are no devices that REQUIRE AC such as cooling fans with motors, and it is just a control circuit with relays, switches, solenoids etc., then try it on 12 volts DC while monitoring the components to see that they work OK and are not over stressed.
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