# How to modify a switch mode power supply.

#### lawrence101

Joined Oct 23, 2017
29
This thread is a carry-over from one that i started earlier on a SMPS that wasn't working. The problem with that supply has been solved and is now working fine. My next project is to build a battery charger using that supply, only problem is the output is +31.5 volts DC which is far too high for a regular 12 volt battery. The next step is to modify the circuit to lower the voltage to about 14 volts ( or ~13.8 ) .

I had a plan of how to do that and have since drawn a schematic of my proposed idea. Since a picture ( or schematic ) is worth a thousand words, and i'm a slow typer , i've attached a screen capture of the layout as i was working on it. Has they say down south , What y'all think ?

P.S. Thought I should mention ,the components to be added are not surface mount.

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#### crutschow

Joined Mar 14, 2008
34,392
What y'all think
That should work, but your R10 and U1 value allow adjustment up to 35V output.
Using 20kΩ for U1 would allow finer adjustment to a more reasonable 20V.

Better still might be to have R10 = 10kΩ with a 10k pot for U1 to give an adjustment range of about 10V to 17.5V.
The circuit regulates when the voltage at R7 is 2.5V.

#### Dodgydave

Joined Jun 22, 2012
11,300
To get 14 V out the ratio of the resistance ( R10 +Vr1) to R7 is 4.6, so with R7 at 3.3K , then the total resistance of Vr1+R10 is 15K .

So as said earlier make VR1 10K and R10 a 10K or 7.5K.

#### lawrence101

Joined Oct 23, 2017
29
That should work, but your R10 and U1 value allow adjustment up to 35V output.
Using 20kΩ for U1 would allow finer adjustment to a more reasonable 20V.

Better still might be to have R10 = 10kΩ with a 10k pot for U1 to give an adjustment range of about 10V to 17.5V.
The circuit regulates when the voltage at R7 is 2.5V.
Thank you crutchow for responding,
I believe you meant 20K for VR1 , surprised by that because that was what i had for VR1 at first and decided instead to try an match the existing circuit values as close as possible for now as it's only experimental . The pot will only be used temporarily , its purpose is to find the exact value or nearest standard value of fixed resistor to get an output of ~13.8 volts or there-a-bouts. It's good to know i'm at least in the ballpark.

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#### lawrence101

Joined Oct 23, 2017
29
To get 14 V out the ratio of the resistance ( R10 +Vr1) to R7 is 4.6, so with R7 at 3.3K , then the total resistance of Vr1+R10 is 15K .

So as said earlier make VR1 10K and R10 a 10K or 7.5K.
Thanks Dave for responding,
I will certainly give that a try but have to see what pot values i have left in my parts bins, i think i have a 25K not sure yet. We are digging out from a winter storm here today so don't have much time for anything else right now.

#### crutschow

Joined Mar 14, 2008
34,392
I believe you meant 20K for VR1
No.
A 10k resistor in series with a 10k pot connected as a variable resistor, will give a maximum nominal output voltage of about 17.6V.

#### lawrence101

Joined Oct 23, 2017
29
No.
A 10k resistor in series with a 10k pot connected as a variable resistor, will give a maximum nominal output voltage of about 17.6V.
Ok i see. Any way i was also thinking to maybe add a selector switch in the circuit to switch between charging either a 12 volt or 6 volt battery, some commercial chargers have that feature .
I've ordered a couple of cheap ( $6.99 e ) analog type meters on amazon should arrive by early next week , plan is to use these to monitor the charging cycle when in use. Haven't got around to designing that part of the project yet but open to ideas. Last edited: #### Dodgydave Joined Jun 22, 2012 11,300 Ok i see. Any way i was also thinking to maybe add a selector switch in the circuit to switch between charging either a 12 volt or 6 volt battery, some commercial chargers have that feature . I've ordered a couple of cheap ($6.99 e ) analog type meters on amazon should arrive by early next week , plan is to use these to monitor the charging cycle when in use. Haven't got around to designing that part of the project yet but open to ideas.
What type of switch-mode chip does it use, as some chips won't work below a certain voltage, can you show the rest of the primary side?

#### crutschow

Joined Mar 14, 2008
34,392
i was also thinking to maybe add a selector switch in the circuit to switch between charging either a 12 volt or 6 volt battery, some commercial chargers have that feature .
A simple way to calculate the resistor values is, since the output is in regulation when the TL431 control voltage across R7 is 2.5V, you can calculate the current through R7 for 2.5V across it.
Since this current also must go through R10 and VR1, you can calculate their values for the desired output voltage minus 2.5V across them for that current.

Thus for R7 = 3.3kΩ, its current is 2.5V / 3.3kΩ = 757.6µA.
For 13.8V output, the value of R10 and VR1 in series would then be (13.8V-2.5V) / 757.6µA = 14.9kΩ.

#### lawrence101

Joined Oct 23, 2017
29
What type of switch-mode chip does it use, as some chips won't work below a certain voltage, can you show the rest of the primary side?
The Regulator IC on the primary side is a SRT-F6138-LF1352 , cannot find much info on it but i believe its made by Sangen ( or Sanken ? ) electronics ? I'd really like to know what its max current handling capability is tho. After looking at some other similar regulators I'm thinking it's around 6 amps ? The line fuse is a 8amp/125v so there's that.

I've attached the schematic of the primary side but more is available on another related thread , link here

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#### lawrence101

Joined Oct 23, 2017
29
A simple way to calculate the resistor values is, since the output is in regulation when the TL431 control voltage across R7 is 2.5V, you can calculate the current through R7 for 2.5V across it.
Since this current also must go through R10 and VR1, you can calculate their values for the desired output voltage minus 2.5V across them for that current.

Thus for R7 = 3.3kΩ, its current is 2.5V / 3.3kΩ = 757.6µA.
For 13.8V output, the value of R10 and VR1 in series would then be (13.8V-2.5V) / 757.6µA = 14.9kΩ.
Thanks , ive been playing around with the on-line calculators for the TL431 but none of them tell what value(s) of fixed resistor(s) are needed. I noticed on the board itself the designers used a combination of three resistors connected in series to get the desired output voltage( for a total of 4) . I will prolly have to do the same.

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#### Dodgydave

Joined Jun 22, 2012
11,300
Thanks , ive been playing around with the on-line calculators for the TL431 but none of them tell what value(s) of fixed resistor(s) are needed. I noticed on the board itself the designers used a combination of three resistors connected in series to get the desired output voltage( for a total of 4) . I will prolly have to do the same.
The output voltage is set by the two resistors on the gate of the TL431, the formula is Vout=1+( R1/R2) x 2.5V,,, where the upper resistor is R1, and lower resistor is R2.

So if you use 15K for R1 and 3.3K for R2, then the output voltage will be 13.8V.

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#### lawrence101

Joined Oct 23, 2017
29
Thanks Dave for taking the time to calculate that much appreciated , noted and written down
.
Currently modifying ( and upgrading ) a battery monitor/cut-out circuit i found from an on-line search will prolly lose a volt or so passing thru that part of the circuit. So may have to settle for a 14 volt output or so from the SMPS when everything is finished.
For clarity attached is a screen shot of the monitor circuit as i was working on it.
The circuit has not been built or tested yet it's strictly in the drawing stage for now but i have the parts to build it as it is right now.

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#### Dodgydave

Joined Jun 22, 2012
11,300
Thanks Dave for taking the time to calculate that much appreciated , noted and written down
.
Currently modifying ( and upgrading ) a battery monitor/cut-out circuit i found from an on-line search will prolly lose a volt or so passing thru that part of the circuit. So may have to settle for a 14 volt output or so from the SMPS when everything is finished.
For clarity i've attached is a screen shot of the monitor circuit as i was working on it.
The circuit has not been built or tested yet it's strictly in the drawing stage for now but i have the parts to build it as it is right now.
Why don't you use another TL431 to trip the relay, same as the output of the power supply,, use the TL431 to pull in a pnp transistor , and use the Collector to drive the relay coil, that way will be more accurate.

#### lawrence101

Joined Oct 23, 2017
29
Yes thanks Dave, that's another good idea . Looked at several circuits using the TL431 but i don't have that particular IC in my parts bin . I was interested in this circuit 'cause i already had the parts to build it . Rather not remove one of the 431's from the SMPS board as i might find a use for the other outputs ( there are five in all ).
Just wondering but does accuracy matter much for a battery charger ? I keep hearing 13.8 volts for a 12 volt battery but my store-bought charger that ive had for years puts out ~14 volts DC .

#### Dodgydave

Joined Jun 22, 2012
11,300
13.8V is for float charge, so you can leave it on this voltage for weeks, whereas 14V to 15V are for 1 or 2 hours of charging.

#### lawrence101

Joined Oct 23, 2017
29
I see. The charger has a max output of 12 amps @ 14 volts but there's a slider type selector switch for a 2 amp trickle charge. So the trickle charge is the 13.8 output ? I haven't checked that with my DMM yet busy with other things .

I said earlier that i didn't have a TL431, well i was digging thru my parts bins today and came across two M5236L voltage regulator IC's . They are practically identical to the 431 , they are both in a TO-92L package and looking at the data sheet it shows an example circuit in which it drives a PNP transistor. I believe you mentioned something about using it to drive a relay if i'm not mistaken ?

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#### crutschow

Joined Mar 14, 2008
34,392
They are practically identical to the 431
No.
The 431 is a shunt programmable voltage reference, and the 5236 is a series voltage regulator.

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#### lawrence101

Joined Oct 23, 2017
29
No.
The 431 is a shunt programmable voltage reference, and the 5236 is a series voltage regulator.
Ack !, yes it is , thanks crutschow for pointing that out , its a linear regulator so definitely not a drop-in replacement for the TL431.

#### Dodgydave

Joined Jun 22, 2012
11,300
I see. The charger has a max output of 12 amps @ 14 volts but there's a slider type selector switch for a 2 amp trickle charge. So the trickle charge is the 13.8 output ? I haven't checked that with my DMM yet busy with other things .

I said earlier that i didn't have a TL431, well i was digging thru my parts bins today and came across two M5236L voltage regulator IC's . They are practically identical to the 431 , they are both in a TO-92L package and looking at the data sheet it shows an example circuit in which it drives a PNP transistor. I believe you mentioned something about using it to drive a relay if i'm not mistaken ?

Using this TL431 relay circuit ( ignore the buzzer), use a 10K preset and 1.5K resistors as shown, the voltage will give you 15V in the middle of the preset.