Upgrading a Power Designs Inc. 6050C with a 4 digit plus mode display

Developing a new 4 dig­it plus mode dis­play for a Power Designs Inc. 6050C pow­er sup­ply capa­ble of dis­play­ing cur­rent at a mil­liamp res­o­lu­tion, and volt­age at a 10 mV res­o­lu­tion.

PD 6050C 4-digit Display old and new boards
PD 6050C 4‑digit Display, old 3‑digit +mode assem­bly, and new meter board

After the suc­cess of upgrad­ing the orig­i­nal dis­play board for the PD 6050C, I still want­ed to look into the fea­si­bil­i­ty of adding an extra dig­it to the dis­play.

Design para­me­ters for the upgrade were:

  • No mod­i­fi­ca­tions to the pow­er sup­ply that could not be reversed by plug­ging the old dis­play board back in, and re-cal­i­brat­ing.
  • Utilize the exist­ing 12 volt AC pow­er to sup­ply the board.
  • Large, deep-red 7 seg­ment LED dis­plays to match the orig­i­nal col­or.
  • Milliamp cur­rent res­o­lu­tion, and 10 mil­li­volt volt­age res­o­lu­tion.

After a bit of search­ing I found a MAX1447 4.5‑digit, sin­gle-chip ADC with LED dri­vers IC by Maxim Integrated. It includes an inter­nal ref­er­ence, and a mul­ti­plexed LED dis­play dri­ver. An inter­nal charge pump gen­er­ates the neg­a­tive sup­ply need­ed to pow­er the inte­grat­ed input buffers for sin­gle-sup­ply oper­a­tion, and the ADC is con­fig­urable for a ±2V input which is the range need­ed for the PD 6050C.

PD 6050C 4-digit display multi-meter and LED driver board
PD 6050C 4‑digit dis­play mul­ti-meter and LED dri­ver board

For the first revi­sion of the board I decid­ed to keep things sim­ple and uti­lized the inter­nal ref­er­ence, instead of the addi­tion­al expense and time of adding an exter­nal high­er accu­ra­cy ref­er­ence. The spec­i­fi­ca­tions looked rea­son­able, and I was very hap­py with the final results.

I uti­lized a near­ly iden­ti­cal rec­ti­fi­ca­tion and fil­ter­ing design from the orig­i­nal board, and then reg­u­lat­ed that down to 5 volts using a high effi­cien­cy Recom buck reg­u­la­tor with some extra fil­ter­ing on its input and out­put.

The MAX1447 requires three sup­ply inputs but can oper­ate from a sin­gle 2.7V to 5.25V pow­er sup­ply with some addi­tion­al fil­ter­ing between pow­er rails. To reduce heat­ing from the LED dri­vers on the MAX1447 IC, I opt­ed for a 3.0 volt MCP1703 lin­ear reg­u­la­tor for the LED sup­ply rail. I also used a 3.3 volt ver­sion of the MCP1703 for the ana­log and dig­i­tal pow­er rails, main­ly because I had a large quan­ti­ty of them in stock.
The ana­log and dig­i­tal rails require less than 1 mA total, so a much small­er reg­u­la­tor can be used.

PD 6050C 4-digit display multi-meter board side view
PD 6050C 4‑digit dis­play mul­ti-meter board side view

Most of the oth­er com­po­nents are bypass capac­i­tors for the MAX1447 and reg­u­la­tors, with the excep­tion of R1, R9, R10, R11, and R12.

R1 is the trim pot for the bright­ness cur­rent con­trol of the 4 mul­ti­plexed LED dig­its, and R11, R12 togeth­er form the sec­ond half of a volt­age divider for scal­ing down the pow­er sup­ply out­put volt­age by a fac­tor of 100.

In a future revi­sion I will replace R12 with a 10K mul­ti-turn trim­pot, and add the oth­er half of the volt­age divider, a 1.62 K resis­tor to the top of the board.

Resistors R9, R10 are pull-down resis­tors for the dec­i­mal point selec­tion inputs of the MAX1447 and are dri­ven by the posi­tion of the front pan­el “Display Function” switch. I also enabled lead­ing zero sup­pres­sion in Voltage mode to blank the most sig­nif­i­cant dig­it for volt­ages less than 9.99 volts.

PD 6050C Multi-meter main circuit board top
PD 6050C Multi-meter main cir­cuit board top

The inter­face con­nec­tor for the PD 6050C uses an uncom­mon 0.15″ spac­ing 13 pin con­nec­tor. I was able to find some 6 pin head­ers with the cor­rect spac­ing and luck­i­ly pin #7 is not used, so I pop­u­lat­ed both sides of pin #7 which worked out well.
I added 5 test points for test­ing the indi­vid­ual volt­age rails, ground and the 5 volt out­put.

PD 6050C Multi-meter 7-segment display assembly
PD 6050C Multi-meter 7‑segment dis­play assem­bly

The 7‑segment LED dis­play is on it’s own sock­et­ed board and includes 5 cur­rent lim­it­ing resis­tors for the seg­ments of the mode dig­it.
I decid­ed to change the orig­i­nal “I” and “E” mode indi­ca­tion to a “c” and “E” dis­play so that the “I” could not be con­fused with a “1”.
On the orig­i­nal board there was addi­tion­al spac­ing between the mode indi­ca­tor and the rest of the dig­its, but with 5 large dis­plays there is no addi­tion­al space to dif­fer­en­ti­ate the mode from a fifth dig­it.

PD 6050C Multimeter display bare board top Rev 1.1 c-E
PD 6050C Multimeter dis­play bare board top Rev 1.1 c‑E

I used mat­ing machine pin head­ers for con­nect­ing the dis­play board to the  main mul­ti-meter board for ease of trou­bleshoot­ing. 12 of the head­er pins are used for the 4 mul­ti­plexed dig­its, and 3 pins for the mode indi­ca­tor includ­ing ground. The three mode “c” seg­ments are always on and con­nect­ed to the 3.0 volt LED pow­er rail thru indi­vid­ual cur­rent lim­it­ing resis­tors. The addi­tion­al two seg­ments that add to the always on “c” to make it an “E” receive pow­er from the front pan­el “Display Function” switch and two cur­rent lim­it­ing resis­tors.

Power Designs Inc. 6050C 4-digit Display assembly installed in power supply
Power Designs Inc. 6050C 4‑digit Display assem­bly installed in pow­er sup­ply

All five 7‑segment LED dis­plays are sock­et­ed using Mill-Max 14 pin DIP sock­ets (110–41-314–41-001000) and the total height of the dis­play from the board is just under 0.5″. I used a pair of hex #6–32 thread­ed stand­offs  0.5″ high to replace the thread­ed clips used to mount the red acrylic lens on the front pan­el. The dis­play board was then attached to the stand­offs using a pair of #6–32 nuts and nylon wash­ers.

PD 6050C 4-digit front panel E 12.02 volts
PD 6050C 4‑digit front pan­el E 12.02 volts
PD 6050C 4-digit front panel c 0.214 amps
PD 6050C 4‑digit front pan­el c 0.214 amps

I still con­sid­er this 4‑digit dis­play mod­i­fi­ca­tion a pro­to­type. It works well, but needs an addi­tion­al trim pot for adjust­ing the volt­age divider to cal­i­brate the volt­age mode. It also needs bet­ter phys­i­cal mount­ing of the mul­ti-meter board to the dis­play board. It should work great if sit­ting on a work bench, but would not be secure for rough ship­ping con­di­tions.

Cost of com­po­nents with­out ship­ping or tax­es was around $57.00 USD

Eagle CAD V9.X ZIP files for both boards

Power Designs 6050C Revision 1.2 dis­play update

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