Help with diy bench power supply - not getting enough current.

Thread Starter

Tyler Simmons

Joined Mar 30, 2019
22
I made a bench power supply using a transformer from an uninterruptible power supply rectified to about 10.4 volts DC, then wired to a LTC3780 buck/boost converter board. The problem is I am getting nowhere near the current that it is capable of. LTC3780 is rated at max 10A and I have tested this transformer with a 6A load and it does not even get warm. I have tried increasing the capacitor from 3300uF to 13200uF with no change. I have tried some different loads with no change. I have two of this transformer, both react the same when wired to the ltc3780.

Here is a picture of the exact transformer. The primary coil is small black/white, secondary coil is white/red thicker wires on the bottom, red and black wires on top measure 120v.
upload_2019-4-19_18-41-23.jpeg

This is similar to the one I use.
https://www.aliexpress.com/item/Free-Shipping-LTC3780-DC-5-32V-to-1V-30V-10A-Automatic-Step-Up-Down-Regulator-Charging/32706496336.html?spm=a2g0s.9042311.0.0.27424c4d3W3hwq

kbj5010 is the rectifier
http://www.asemi.com.cn/upLoad/down/month_1407/201407261642596508.pdf
 

crutschow

Joined Mar 14, 2008
23,480
Hard to tell what's wrong from your description.
What was the output voltage with a 6A load on the transformer?
What was the voltage input value to the LTC3780 when it was connected?
How much current out do you get before the output voltage starts to drop?
What do you mean "no change"? Change from what?
 

Thread Starter

Tyler Simmons

Joined Mar 30, 2019
22
What was the output voltage with a 6A load on the transformer?

What was the voltage input value to the LTC3780 when it was connected?

How much current out do you get before the output voltage starts to drop?

What do you mean "no change"? Change from what?
10.4v is the voltage from the transformer after it is rectified and smoothed.

Voltage input to the LTC3780 is 9.8-10v.

The output voltage drops as soon as I add my load which is a 12v vehicle light bulb. The voltage drops to 0, but the fault light does not yet come on the LTC3780. I am no longer able to get more than .7A. Earlier I could get 2A and at least partially light the bulb before the fault light turns on. I have changed nothing in my circuit.

By "no change" I mean the output current is still lower than i need it to be.
 

Thread Starter

Tyler Simmons

Joined Mar 30, 2019
22
Very true.
I didn't notice the finger. :rolleyes:
That size transformer would likely be good for less than an amp.

Tyler, how did you measure the 6A load?
I measure current with a DMM. The negative probe connects to negative output, and the positive probe connects to negative of the load.

Edit: It is fro ma 550VA supply if that means anything.
 

crutschow

Joined Mar 14, 2008
23,480
What happens when you connect the bulb directly to the rectified output?

You didn't answer as to how you measured the 6A transformer current.
 

Thread Starter

Tyler Simmons

Joined Mar 30, 2019
22
What happens when you connect the bulb directly to the rectified output?

You didn't answer as to how you measured the 6A transformer current.
I measure current with a DMM. The negative probe connects to negative output, and the positive probe connects to negative of the load.

Edit: It is fro ma 550VA supply if that means anything.


The bulb directly to the rectified output powers just fine and a second bulb to get more amps works just fine.


I have burned up a bulb, but with resistors I can still get over 6A. I now got 7A from the transformer.

Is the only answer that I have a defective LTC3780?
 
Last edited:

Thread Starter

Tyler Simmons

Joined Mar 30, 2019
22
6600uF capacitor for smoothing. I now see that my voltage drops below 5v with the load which is the minimum input for ltc3780. I have tried this with 13000uF. So would more capacitance keep this voltage up enough to power to LTC3780?
 

crutschow

Joined Mar 14, 2008
23,480
6600uF capacitor for smoothing. I now see that my voltage drops below 5v with the load which is the minimum input for ltc3780. I have tried this with 13000uF. So would more capacitance keep this voltage up enough to power to LTC3780?
If it's dropping down to 5V then it's likely the transformer and more capacitance will not affect that significantly.
As previously stated, that transformer does not look large enough to supply over an amp of current.

Measure the transformer output winding resistance.

Measure the transformer output voltage with a known load.
How big are these bulbs?
 

Thread Starter

Tyler Simmons

Joined Mar 30, 2019
22
Ahh, well thank you. They are 65 and 75wbulbs. No biggie, just another failed project! I'll need to wait till I find a atx psu it something.
 
Top