# Harbor Freight battery float charger questions

#### bigone5500

Apr 9, 2014
712
I have this: http://www.harborfreight.com/automatic-battery-float-charger-42292.html

I decided to take the case off and see what type of circuitry it was comprised of. The wall wart has an output of 15vdc @ 500mA. I have some questions now.

1) The input of the board has an ABS10 bridge rectifier...why do we need a bridge rectifier on DC voltage?

2) It also has a CW7805 regulator. I am assuming that it is for current regulation? - I figured it out. It's for the onboard LED indicator. DOH!!!

3) The board also has a MJD41C complimentary power transistor DPAK case. Does this transistor control the 'on/off' of the charging current?

I will get my phone and take some photos later.

Thanks.

#### bigone5500

Apr 9, 2014
712
My goal is to reproduce this circuit on paper. I suppose I'm bored...

#### bigone5500

Apr 9, 2014
712
Here are a couple photos.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,516
1) The input of the board has an ABS10 bridge rectifier...why do we need a bridge rectifier on DC voltage?
Because the input is AC 120 V / 60 Hz?
The board also has a MJD41C complimentary power transistor DPAK case. Does this transistor control the 'on/off' of the charging current?
You'll have to trace the full circuit to identify the function of each component. Possibly the two transistors operate in tandem to limit voltage and current.

#### bigone5500

Apr 9, 2014
712
Because the input is AC 120 V / 60 Hz?

You'll have to trace the full circuit to identify the function of each component. Possibly the two transistors operate in tandem to limit voltage and current.

Actually the input to the board is DC. The plug in transformer rectifiers it from 110vac to DC voltage. Which is why I am confused. If it is already rectified, then why does the board have a bridge?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,516
Actually the input to the board is DC.
O.k., I see. Then I can imagine two possibilities:
1. the board is used with another transformer, too, which outputs AC or
2. the bridge rectifier is a safety feature so the manufacturer doesn't have to care about the right polarity when soldering the wires from the transformer onto the board.
Edit: I see a third possibility: The rating on the transformer may be for the whole unit incl. the charger box. This is supported by the "CC = constant current" on the label and I'm quite certain that the CC feature is in the box, not the wall wart. It's easy to measure this with a multimeter.

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#### bigone5500

Apr 9, 2014
712
O.k., I see. Then I can imagine two possibilities:
1. the board is used with another transformer, too, which outputs AC or
2. the bridge rectifier is a safety feature so the manufacturer doesn't have to care about the right polarity when soldering the wires from the transformer onto the board.
Edit: I see a third possibility: The rating on the transformer may be for the whole unit incl. the charger box. This is supported by the "CC = constant current" on the label and I'm quite certain that the CC feature is in the box, not the wall wart. It's easy to measure this with a multimeter.

Harald, my apologies. I was naive and simply assumed that the wall wart had initial DC output. It does not. I had initially used my meter on the output of the board and found that it had a voltage of 14.8vdc. I assumed that the label of the wall wart was true of the output that I had measured. The input is 14.8vac. So now my confusion is alleviated. I can now continue with Project Boredom.

Lesson learned: use a meter...

Moderator
Nov 17, 2011
12,516
You're welcome.

#### bigone5500

Apr 9, 2014
712
I believe I have completed the schematic for the float charger. The parts list is as follows:

R1 - 1000
R2 - 3000
R3 - 5600
R4 - 5600
R6 - 4700
C1 - 1000uF, 25v
Q1 - MJD41C
Q2 - CW7805
Q3 - J3 (SOT23 package)
D1 - M7
LED - Generic Red LED
D1-D4 - Bridge Rectifier

MJD41C is an NPN Transistor
D1-D4 (ABS10) is a 5.4A Bridge Rectifier
J3 is an NPN Silicon Epitaxial Planar Transistor
M7 is a 50V, 1A Diode
CW7805 is a 1.5A, 5v Regulator

I drew this out to the best of my abilities and am pretty confident that it is correct. Please review the image and see if you can find any discrepancies. Please note that the transformer is just so I could have an input source.
*edit* I made a mistake and tried to change D5 to D1. I have deleted the file from my computer and re-uploaded it to the thread. However, it still says D5...??? I will make another reply.

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#### bigone5500

Apr 9, 2014
712
I am having to rework my schematic...please stand by...

Please see above post regarding the re-worked schematic.

Thank you.

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#### foobar1

Apr 6, 2021
1
This is the schematic. 7805 along with divider network R1+R2/R3 makes a 12V constant voltage source. Q2 is a switch which turns off if Q1 turns on under the over load conditions.

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• float.png
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