# Alt tester newby needs help plz

Discussion in 'Power Electronics' started by JodyBC, Mar 29, 2021.

1. ### JodyBC

3
0
Mar 29, 2021
Hello All, I am looking for some help on a homemade test device that is probably 30= years old and the transformer appears to have burned up and needs to be replaced.
It has as an input: 110V AC and 12V DC. The inputs are run through a switch and then the 110V passes through a transformer, then through diodes - as in the drawing. Then it passes on to the resistors and caps.
Could someone please explain how this suppose to work? I assume the power input from the 110V line is varied to induce a load on the alt. What I need to do is determine the size and voltage output of the transformer as it is kiplut.... Any help would be appreciated. I can tell you the large resistor is 1 ohm, the small resistor is 2 ohm the large cap is 71000mf small is 32000mf.

Thanks again for any help!

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2. ### Martaine2005

3,308
904
May 12, 2015
I doubt that your drawing is correct.
It won’t have AC 110V connected to DC 12V via the same switch as you show.
I suspect it’s a 12V (110V-12V) transformer given the fact it has a 12VDC input.
But..... use a multimeter to measure voltages when it’s powered by the 12V jack.
You might be better off posting actual photos rather than a wrong drawing.

Martin

3. ### bertusModerator

1,611
638
Nov 8, 2019
Hello,

It looks like a CT (Center Tap) connected transformer as there seem to be two diodes for the rectification.
The transformer could have a 2 X 12 Volts output.

Are the resistors parallel to the caps?
That would make a heavy load.

Bertus

4. ### Martaine2005

3,308
904
May 12, 2015
Actually, that switch is probably a double pole.
Anyway, the transformer is 110VAC input. It reduces the voltage to 12VAC. The diodes rectify the AC to DC.
The 12VDC input is direct.
I don’t understand what is happening with the resistors or capacitors. Obviously a load, but how it’s wired is not clear. A photo could clarify.

Martin

5. ### JodyBC

3
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Mar 29, 2021
Thanks for the help the drawing is correct, it is a double pole switch, it is the load bank part of an automotive 12V alternator testing machine. I could take a picture but it's messy, in a homemade box, and wouldn't reveal anything, the resistors are in series with each other as the picture shows, looks like the caps are parallel to each other and parallel to the series resistors. assuming it's a 120V-12V transformer, what size should it be? Someone also told me it could be a 110VAC to12-0-12DC transformer?

6. ### Martaine2005

3,308
904
May 12, 2015
Yes, that’s what @bertus said. A centre tapped transformer. Meaning 12-0-12. So 0-12 for 12V or 12-12 for 24V (if it is).
The max output current would be difficult to say. Measure the dimensions and look up similar sizes. But going larger won’t hurt as the load is already there.

Martin

7. ### JodyBC

3
0
Mar 29, 2021
the owner sent me some pics so I have attached them, I suspect the 110Vs power is regulated.. 'll see tomorrow & order XF. After talking more with the owner, owner says it's to test voltage regulators....guess that's what goes in the center box. I suspect this may be for older vehicles
(box obviously needs rewired heavy)

Last edited: Mar 30, 2021
8. ### Bluejets

4,604
977
Oct 5, 2014
I very much doubt that.

Your diagram needs to be redrawn correctly.

Last edited: Mar 30, 2021