I am working on a battery charger that uses 2 potentiometers to control battery undervoltage setpoint and charge current. When the pots are adjusted they affect the 5 volt reference, that is accurate when the power stage is not connected to the rest of the circuit, which in turn affects my 2.5V reference. The 5 volt reference moves with with different potentiometer settings from 5v to ~3.7v. I am at a loss for how to fix it without completely redoing the power stage by splitting all the outputs of the transformer and use diodes. I just realized that this was happening and, although, the circuit still functions it affects performance. I would appreciate any help!!!

 

Can you show on the diagram what voltages are being affected?

 

Can you show on the diagram what voltages are being affected?

I cannot illustrate on the diagram itself at this time, but on the Power Stage diagram the LT1021 5V reference (labeled +5) should be producing a constant 5V, but when the two variable resistors which connect to the connector (most center) on the Full Circuit schematic are adjusted the 5v reference at LT1021 drops to 3.3-4.5 volts depending on resistor settings. The LT1009 produces the 2.5V reference (labeled Vref) is downstream of the LT1021 and is affected similarly because the 5v reference is feeding it. The circuit uses a 28V/720mA transformer that feeds an LT4320 Ideal diode bridge controller. Without the LT4320 I could have split all outputs of the transformer and isolated the 5v leg more, but I liked the benefits of the controller. The 28VDC goes into a DPDT switch after C2 on the Power Stage schematic that sends positive power to the constant current circuit (middle circuit on Full Circuit) or constant potential circuit (top circuit on Full Circuit). It feeds the constant current circuit (middle circuit to right of power stage) before the power mosfet. I see that the png files are hard to see as they don't zoom much, sorry about that. Is there a way to draw on the diagram with this site? I hope my description helps!

 

How much current are you drawing from the 5V rail when you connect the two potentiometers?
When the LT1021 output drops, what is the voltage at the LT1021 input?

 

the lot1021 is only rated for a maximum of 10 ma. it is a reference source, not a high current regulator. the output will change with load above 10 ma.

 

How much current are you drawing from the 5V rail when you connect the two potentiometers?
When the LT1021 output drops, what is the voltage at the LT1021 input?

Ok, I did some measurements and got the following:

LT1021-5:
No Load - IN_V: 45.7V
IN_I: 25mA
OUT_V: 5V

Load - IN_V: 28 to 45.7V as Circuit Current adjustment Potentiometer increases battery charge current
IN_I: 126mA (startup)
110 to 112mA as load increases
25 to 126mA as Circuit Current adjustment Potentiometer increases battery charge current
OUT_V: 5V (startup)
4.1V (steady-state) load makes no significant difference like ~2/1000 and Circuit Current adjustment Potentiometer that increases battery charge current makes no significant difference. The undervoltage potentiometer that set adjust setpoint between green/red LED if batter (load) not at specific voltage (resistance) creates a +/- 0.2V difference from 3.9 to 4.1V at LT1021-5 output.

I changed from a 36V transformer to a 28V because I was worried about voltage in of the LT1021, but input voltage is a differential of 35V with input defined at 40V. Appears I am exceeding the current input rating. Do you think just adding resistance before the LT1021 input would be a solution?

Thanks!

 

If R4 is 8.6k then it is much too large. I would choose something like 2 mA for ref so R4 would be about 1k2.

 

If R4 is 8.6k then it is much too large. I would choose something like 2 mA for ref so R4 would be about 1k2.

I know its hard to read, but it is at a 3.6k. I will try adjusting it to 1k2 and see what happens.
Thanks!

 

If R4 is 8.6k then it is much too large. I would choose something like 2 mA for ref so R4 would be about 1k2.

Going from 3.6k to 1.2k at R4 made a nice difference in the 2.5V reference. It went from less than 2 v to 2.58v. Didn't make any difference with the +5V reference. Thanks for the suggension Bernard?

 

Can you re-export a bitmap of the circuit, setting the DPI to at least 600? That schematic is impossible to read.

 

is it hard to understand that the lt1021 is only good for 10 ma output? any more and the output voltage drops. like I said before.

 

I agree with Alfacliff. Not only is the current draw way above the rated 10mA, but the input voltage and the in/out differential are both above the Absolute Maximum ratings ! You need to rethink the circuit. A simple resistor dropper won't restrict the load current demand so is not the answer.

 

is it hard to understand that the lt1021 is only good for 10 ma output? any more and the output voltage drops. like I said before.[/QUOTE

The LT1021 is not providing the output for the circuit, just the 5v reference for the chips that evaluate the signals. The output of the circuit to the 24 volt battery is before the LT1021, so what happens going into the LT1021 does not affect the performance of the circuit that charges the battery...it just affects the performance of the comparators that need constant +2.5V and the chips that may not get a constant +5v at their voltage input I have been testing the circuit continuous for over 8 months and the LT1021 has not failed, even when I had a 36 volt transformer powering it. I switched to a 28 volt transformer recently to protect the +5V and +2.5V circuit bus more. The battery charge portion of the circuit is achieving good voltage and 0 to 800 mA.

 

I agree with Alfacliff. Not only is the current draw way above the rated 10mA, but the input voltage and the in/out differential are both above the Absolute Maximum ratings ! You need to rethink the circuit. A simple resistor dropper won't restrict the load current demand so is not the answer.

Thanks for your comment, please see my other post for the explanation concerning the 10 mA. My understanding of a voltage differential is that the difference in the output voltage of the LT1021 and the input voltage of the LT1021 cannot exceed the defined differential voltage, which on this chip is 35 volts. I am not exceeding this. However; the 40V max input voltage of the LT1021 has concerned me, which is why I dropped the transformer down from a 36 volt to a 28 volt. The problem is if the max current of the transformer is not being all used then the voltage is higher than its rating, because of Ohm's law. I added a 1k2 resistor after the output to the battery charging circuit and before the LT1021 and changed two resistor values; one on the 5V reference and one on the 2.5V reference and achieved better results for the these outputs. Now when I adjust the current adjustment potentiometer and the undervoltage potentiometer the LT1021 output is 4.7 to 5.1V as current is adjusted from 10 to 160mA and the LT1009 is 2.500 to 2.509V. Much better then before, but if I do redesign it to get better isolation on the +5V and +2.5V bus I think I will remove the LT4320 Ideal diode bridge and just separate all transformer outputs combining with regular diodes and semi-isolate the main 28V output from the +5/+2.5V bus. The 28V battery output is inserted into the battery main charging circuit between a power mosfet and a transistor and I believe this will provide good isolation from ground if I semi-isolate the two busses. It is the sinking of current in the main battery charging circuit that is affecting the +5/+2.5V references because I am using an Ideal Diode Bridge Controller.