×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

# A Current Balance Circuit for LED Strings in Parallel with Polyswitch Protection

## A Current Balance Circuit for LED Strings in Parallel with Polyswitch Protection

(OP)
Power Supply:
Gophertc CPS-6011

LED’s:
108x Cree XP-G3

LED Configuration:
6x 18 LEDs in Series.

XP-G3 LED's = (2.74 Vf 150mA @25C) - (3.11 Vf 2000mA @25C)

Voltage Range = 49.32 to 55.98v
49.32v + .2 Transistor drop = 49.52v

BD139 Transistors have Maximum Collector-Emitter Voltage of 80.
Max Collector Current (DC) = 1.5A

The Minimum Voltage could be about 50v up to the Supplies Maximum of 60v!

Work out Voltage at V1
V1 = Resistor1 x 1.1A

Bipoloar NPN transistors 0.7v drop across.
0.7v. R1 = (0.7v / 1.05A) = 0.6666666666666667 ohms.

The voltage at the anode of D7 will be (2 * 0.7v) or 1.4v.

1mA down through diodes should bias them to the 0.7v on state.
0.5mA to bias each transistors well into saturation point, turned on.

6 Transistors = 3mA current
1mA through D7/D8 Diodes + 3mA for Transistors.

4mA of current required through Rr.

Rr can be calculated as (53.64v -1.4v Diode drop) / 0.0040A = 13.060 ohms.

-----------------------

Resettable Breakers

I have a 60V 11A adjustable Power Supply and wanted to safeguard against someone adjusting to 11A and blowing up Current Balancing Transistors.

Thermal Derating Curve
You will see below the Raychem RXE-065S1 trigger current changes 19% between 20 and 40C Ambient Temperatures.
The Littlefuse RKEF065 has very similar specs with tighter trigger Current variation of 13% over 20 to 40C Ambient Temperatures.

Time-to-trip
The other thing is time to trip in seconds, the RXE-065S1 is more like a Slow Burn Fuse at 5.3 Sec.

Which makes me think this might not be suitable for my purpose, but would protect from a slowly rising current?
The RKEF065 has a faster, 1 second trip time.
But would it trip before damage if someone dialed up the current on Power Supply.
Anybody know?
-

Here's some Specs for those who are interested, 40C Trip Currents are derived from graph - double check with data sheets before use.
-------

RXE-065S1 60V - Raychem

% of rated hold and trip current for Ambient Tempretures.
Thermal Derating Curve (81% of hold and trip @40C)

Hold Current (A) 0.65 @20C
Hold Current (A) 0.47 @40C

Trip Current (A) 1.3 @20C
Trip Current (A) 1.053 @40C

Time-to-trip 5.3 Sec
Time-to-trip with Fault Current of 10A = .1 Sec

Min Resistance 0.31 Ohm
Max Resistance 0.48 Ohm
---------------------------------

RKEF065 60V - TE Connectivity or Littlefuse

% of rated hold and trip current for Ambient Tempretures.
Thermal Derating Curve (87% of hold and trip @40C)

Hold Current (A) 0.65 @20C
Hold Current (A) 0.54 @40C

Trip Current (A) 1.30 @20C
Trip Current (A) 1.131 @40C

Time-to-trip 1.0 Sec
Time-to-trip with Fault Current of 10A = .09 Sec

Min Resistance 0.250 Ohm
Max Resistance 0.450 Ohm
---------------------------------

RKEF050 60V - TE Connectivity or Littlefuse

% of rated hold and trip current for Ambient Tempretures.
Thermal Derating Curve (87% of hold and trip @40C)

Hold Current (A) 0.50 @20C
Hold Current (A) 0.38 @40C

Trip Current (A) 1.00 @20C
Trip Current (A) 0.87 @40C

Time-to-trip 0.8 Sec
Time-to-trip with Fault Current of 10A = .06 Sec

Min Resistance 0.320 Ohm
Max Resistance 0.529 Ohm

----------------------------------------------------------------------------------

Brief Explanation:
Basically we have 6 strings of 18 LED's each.
Each set of 18 LED's uses 1050mA and has a Current limiting Transistor.
The power supply is 11A max, hence it is capable of providing 1.83A per string of 18 LED's.
As a precaution, I am looking at protect each string from exceeding about 1.3A Current.

Questions:

Current Balancing Transistors?
Can anybody help with BD139 Transistors alternative?
I need 6 Transistors in a single package for the thermal connection properties!

PolySwitches?
Anybody using these Polyswitches, do you have a known Source Current reference to give some real world experience.
A device like RXE-065S1 would be perfect If the trip Current stayed within the 1.3 to 1.053 Amp variation (20 to 40C).

But when the Data sheet mentions Min Hold Current is 0.65A and Max Trip Current is 1.3A there is a very large gap between.
So the question is, how tight is the Trip Current in real use.

The RKEF065 has a faster, 1 second trip time but would it trip before damage if someone dialed up the current on Power Supply. Anybody know?

### RE: A Current Balance Circuit for LED Strings in Parallel with Polyswitch Protection

Well then.. Seriously?

You are really going to cheap-out on everything after dropping more the four hundred dollars on those LEDs? Exactly why?

Don't bother with PTCs, use actual little fuses from Littlefuse that look like resistors with leads. Use them suspended between terminal blocks. Use one on each string. Recognize that the instant one blows they will all have to blow. Don't bother with breakers and you should not attempt this without using at least one actual LED driver. A very large Meanwell perhaps? Then you divide its CONTROLLED current between the strings. You don't use a standard power supply ever!

Or conversely do it right and use a LED driver on each sting so when one string has an issue only one string is affected rather than the entire fixture going completely dark.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: A Current Balance Circuit for LED Strings in Parallel with Polyswitch Protection

(OP)
Hi Keith

Quote:"You are really going to cheap-out on everything after dropping more the four hundred dollars on those LEDs? Exactly why?"

Perhaps!

My plan was/is to use 9 Mean Well LDD Step Down Converters and a 48v Power Supply.
1 LDD per string of 12 LED's.

It was suggested I should use an adjustable supply to experiment with different drive Current for LED's.
Although I do not really see a need to experiment and could purchase sets of LDD sizes, they are cheap and handy to have around!

Long story short, it was suggested to go down the adjustable path and I decided to ask around the forums for feedback.

In regards to polyswitches, I found a post were someone was using them and decided to look into the specs.
I am not aware of the particular Littlefuse you refer to, but was thinking on similar lines, just standard fuses.

The original plan is the best.

Thank you for the input.

Merry Christmas

### RE: A Current Balance Circuit for LED Strings in Parallel with Polyswitch Protection

Hi Tom,
I was one of the 'first responders' in the very DIY aquarium world working with the original Cree high power LEDs(HPLED). I worked up how to do it cheaply but dependably.

This was what I came up with and was followed by hundreds of DIYers.

That was quickly formalized by commercial sites into this drawing:

If you want to go inexpensive this works. I have this lighting my living-room and am typing by it now.

I'm about to build a replacement for my kitchen ceiling as yet another #\$%@@*^ fluorescent tube as failed and I never want to see or deal with another one. However, I'll just use a couple of individual drivers so the thing eventually fails gracefully.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: A Current Balance Circuit for LED Strings in Parallel with Polyswitch Protection

(OP)
Thank you Keith,

I see the bottom drawing also uses XPG3 LED's.

There are a lot of options, each with its pro's and con's.

I am looking to build PWM circuits that could be optionally connected to the Driver during experimentation.

The default operating current will be 1050mA from driver, a 555 PWM can be connected with values from 50 to 99% duty.
Or 500mA to around 1A adjustment.

The question still hanging is how best to balance 2x 150v strings of 50 LED's @1050mA.

Cheers.

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!