120 to 24volt step down transformer

wsmm

Member
Bought a 120 to 24 volt step down transformer to power my 18 volt battery charger. I'm thinking the transformer is bad as when I measure the output with out a load it's measuring 35 volts!.. Going to try measuring it today with a load on it and see what I get. I do know that it charged the battery a NiCad to an excess of 18 volts. Any thoughts or comments?
Thanks,
Bill
 
Could be a number of factors that would cause that. #1 you have a cheap uncalibrated meter. #2 input voltage maybe higher then 120 volts. #3 being unloaded you could also be getting an incorrect reading
 
You need the rated load on it . Remember , the Delco Alt. we all have on our tractors has an open circuit voltage of over 90 volts....
 
Are feeding a full-wave rectifier with it to charge batteries?

Do you have an understanding of peak-to-peak AC voltage, RMS AC voltage, and DC voltage?

In other words, the transformer is rated in RMS Volts, which, when rectified, under light load, would "look" more like the peak-to-peak value of the AC that was rectified, less voltage drop across the diodes... likely HIGHER than the nominal AC voltage output rating of the transformer.... which is EXACTLY what you are seeing.

U need a voltage regulator and/or a slightly lower output voltage transformer.

Clear as mud?
 
If your meter is meassuring RMS AC sine wave voltage,,,,,,,the Transformer has a 5 to 1 turns ratio (its a 120 to 24 transformer) ,,,,,,,,,,the meter reads 120 VAC in, Id expect the same meter to read 24 VAC out unloaded,,,,,,,,,,Even if loaded and within the power and amperage rating of the transformer, Id still expect to see that same 5 to 1 ratio of AC voltage difference......As Bob correctly notes, once the output is full or half wave rectified youre NOT gonna get the same voltage reading, the above figures ar for the AC to AC transformation and RMS values..... You cant compare apples to oranges,,,,,,theres AC RMS and theres straight pure DC and theres half wave or full wave rippled DC and theres filtered DC.....

John T Too long retired EE and rusty on this so no warranty, Im better at AC Distribution or tractor DC stuff
 
Thoughts or comments.

First, read below about chargers and Fires.

Are you trying to design an 18v Ni-cd charger? If so, you will have to find a way not to over charge and cause a fire. You will need to limit the charging current. There are two types of Ni-cd chargers. Many years ago I bought a cheap 18 v ni-cd black & decker drill. It had a 500 ma transformer and took almost 24 hours to charge at a very low ma. I have 18v dewalt chargers that are fast chargers. I've never measured the amp draw of a fast charger, but I can say I fried the wiring in my truck's cigarette lighter by using a 12v in dewalt charger which is designed to charge all sizes of dewalt batteries.

I'm not an expert, but the new chargers, that can charge batteries ranging from 7.2v-18v, simply monitor the charging current and shut off when it drops to a certain lever. My 12v truck charger has to have an inverter circuit to change 12 volts in to around 20 volts to fast charge 18v batteries.

So, thoughts and comments, go buy the charger you need unless you are desinging something special. In that case, go to your local college and sign up for an electronics class. One of the first things should be making power supplies, which is basically the same as making a charger.
George
 
Most chargers put out a higher voltage when unloaded, even my solar panel puts out close to 20 volts without load. A laptop charger might work for you.
 
Most digital multimeters do not measure true RMS voltage. Also, all transformers introduce a certain amount of harmonic distortion. This distortion is called hysteresis or "crossover distortion" and will result in a different peak voltage for a given RMS voltage than the nominal 1.4 ratio. So it's not uncommon to get a measurement error when reading a transformer's output using a typical peak-averaging multimeter.

However, you haven't been particularly forthcoming about your actual setup. As Bob and John T pointed out, if you are reading the DC output of a rectified AC signal, the voltage is not going to be the same as the RMS voltage but will rather be the PEAK voltage of the signal. For a 24 volt rms sine wave, the peak voltage is around 34 volts. It's details like this you need to understand before trying to roll your own battery charger.
 
All transformers will read high on power tool chargers.The dc output has to go thru a regulator.You do not understand what you are doing so beware.A defective transformer will not read high.It will read low and overheat.Since these power tool chargers often catch fire I WONT do any repairs on them.An 18 volt battery needs 20 volts to charge it.A 6 volt battery needs 7.3 volts, a 12v battery needs 13.7 volts to charge.
 

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Back
Top