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8ch DMX dimmer pack

 

This single phase diy dimmer pack was designed for small size and maximum stability. Though it is quite cheap because of its few parts, it has a firing table for power linearization and a 'switch mode' option for each channel integrated. An internal heat control prevents failures in the power units.

A dimmer pack consists of:
1     DMX-Transceiver
1     zc detection
1-8 power units

DMX-Transceiver (Rev. 3.01)

picture of transceiver rev.3.01With this modul DMX data can be received or transmitted.

Because of the complete usage of the RS485 converter, a bidirectional transfer of data is possible (i.e: RDM as described in ANSI E1.20). Till now this feature is only supported by a few expensive devices.

Though the circuit is only made of an AVR and few other parts I recommend a diy etched board.

 

 

 


parts

  IC1
  IC2
  IC3
  B1 
  LED1
  LED2
  R1
  R2,3,4
  C1,2
  C7
  C5,6
  SW1
  Q1
  connectors
ATmega8515-16PU
75176B
7805
rectifier (round)
LED 5mm red
LED 5mm green
10k (PT10-S)
390 Ohm
27pF
100nF
100F
DIP-Switch (10)
8MHz (HC49)
pin header (single)

 

 

 

 

 

 

 

 

 

 

 

As you can see, the schematic is quite simple: The heart of the controller is the firmware of the mcu (IC1). The start address is set with SW1. The LEDs indicate the status of the controller. The parts around IC3 are responsable for an exact Vcc of 5.0Vdc. IC2 is the RS485-Transceiver. It allows our controller to communicate via DMX512 with other equipment.

For a dimmer- or switchpack you have to connect the pins of "output" with the "-"pins of the dimmer-/switchmodules because a mcu can drive bigger loads as a current sink. The "+"inputs of all modules must be connected to vcc.

The "spare" port is used for additional pins or to jumper different modes.

AC1&2 have to be connected with a power supply of 9-12V ac or dc. 3-5W should be sufficient.

The following schematic shows you how to connect the transceiver with the DMX bus:

this is the layout:

The board is 48 * 76 mm^2. The resolution of the picture above is 300dpi.

placement:

On the resources site you can find a manual for programming AVRs. The 8MHz crystal has to be selected as clock source by changing the fuse bits.

After setting up the clock source, the dimmer firmware should be written to the AVR's flash.

zc detection

This zero crossing detection is needed for syncing a microcontroller with the mains. The more precise the circuit works, the better is the resolution of the dimmer pack. Because of this, I tested lots of such devices...  

In my opinion, this simple circuit is one of the best:

parts:

OK1
D1
R1A
R1B
R3
4N33
1N4007
27k / 1W
27k / 1W
10k

The OK1-LED is powered via R1 in the positive part of the ac cycle. (D1 is for limiting the reverse voltage in the negative part.) If OK1 is conductive, it pulls ZC to GND otherwise ZC is pulled to VCC via R3. At the output, there should be a square wave with 50% duty-cycle. The edges indicate a zero crossing (but only the falling edges are used in the firmware).

The zc detection is part of the new power modules.

And here's the placement :


power units

With this dimmer modul you can dimm inductive loads up to 1kW or resistive loads up to 3kW per channel. (depends only on the used triacs)

The temperature must be between -20C and +80C.

The schematic is based on application notes of Teccor and was enhanced with information published by ON-Semiconductor. 

schematic:

parts for the dimmer modul:

OK1
T1
R1
R2
R3
R4
R5-S
C1-D
C2
OUT
MOC3022
TIC 2XX
390Ω
360Ω
470Ω
39Ω / 0,5W
not used
47nF / 630Vdc (MKS4 / RM7,5)
10nF / 630Vdc (MKS4 / RM7,5)
6.35mm Ribbon con. (Print, vert.)

If the LED of OK1 is turned on, the optotriac discharges instantly C1-D into the gate of the main triac. A snubber network to limit inductive surges is made of C2 and R4.  The optotriac is filtered by R3 and C1-D. The modul is powered via the common heat sink (similar to the switch). This way we can handle large currents without heavy wires. Of cause such a heat sink must not be directly connected to parts you can touch!!

I routed a combi pcb for the power modules of the dimmer and switch packs with 4 channels and an on-board zero crossing detection. Dimmer and switch modules use different parts.

pcb:


The board is 37mm x 111mm. The picture has a resolution of 300dpi.

placement:

To reduce EMI a toroidal choke (iron powder core) with 1-2mH for each channel is recommended.

temperature monitor

Switching large amounts of current, we have to make sure that the power modules won't get to hot. This is done with an NTC (6k8 @20C) connected to A-In of the transceiver. For calibration move the poti R1 to the right stop and set the NTC to the shut down temperature. Now move R1 back until the red LED goes on and the pack shuts down.

switch mode

To use some channels in switch mode, you have to do the following steps:
1.) shut down the pack
2.) switch on DIP10 and the DIPs of the 'to be switched channels' (1-8).
3.) Start the pack again and wait 2sec.
4.) Choose the start address of the pack (DIP1-9) and switch DIP10 off.

60Hz mode

If your mains are 60Hz (USA), you have to jumper Spare1 to get the correct firing angles.

Debugging

To make the assembling of the dimmer easier, there is a wiring plan of the whole pack.

The change of a relevant channel is indicated by a flashing green LED. The red one repeats error codes until the bug is fixed:

Pattern Error Solution
flashing There is no signal connected to the transceiver. Connect the transceiver to the DMX bus.
double flashing The signal is not accepted as DMX.
The transceiver cannot receive all required channels.
Swap D+ and D- at the DMX connector.
Transmit more channels or choose a lower start address.
fast flashing The ZC-Detection is not connected to the transceiver. Connect the ZCD or repair it.
constant on The power modules are too hot. Wait until the temperature is OK again. Improve the heat sink.
Build the temperature monitor and the A-In of the transceiver.