SLA7078 Step Motor Driver

A step motor driver using the SLA7078MPR has several advantages over one using the SLA7062M: one, a newer design, two,seems to be more available with a price of $12ea or $8 per ten, three, SLA7078 series has built in sense resistors and,four, the PR version has built in protection circuits.

The design shown below has two version: with and without opto isolation. An idle delay feature has been incorporated which places the motor in half power mode when idle for 10 seconds. This driver does not set motor current by a selection of sense resistors as with the SLA7062 driver. With that driver the reference voltage was fixed, such that a .5 ohm resistor gave 1 amp motor current, so three .5 ohm resistors in parallel gave 3 amps. In the SLA7078 the sense resistors are located in the chip and current settings are adjusted using the trimmer pot. To set the current, a multimeter is attached to the reference signal, JC1 in schematic above, and the trimmer adjusted to a voltage given in the SLA7078 datasheet.

The delay feature uses a 74HCT4538 retriggerable oneshot. A circuit using a LMC555 was investigated which would probably work, but the board space required is about the same as with the 4538 even though only half the 4538 package is used. The circuit works as follows: as long as step signals are arriving, the oneshot stays triggered and the motor stays at full power, but when the step pulses stop, 10 seconds later the motor switches to half power. The first step after a delay should immediately switch the motor to full power. Whether this switching is fast enough will be determined during testing of the prototype driver.

Stepit 4A: Driver using SLA7078MPR with delay

Schematic:

BOM:

Bill of Material for C:\PCB PIC\SLAgm_04C.PCB
On 4/28/11 at 12:18:18 AM
Count	Label-Value	Pattern	Designation(s)	Mouser
1	330uf 50V	C+400D200	C1	647-UHE1H331MPD
1	47uf	C+200D80H31	C2	647-UHE1E470MDD
4	.1uf	cap100	C3 C4 C5 C7	80-C320C104K1R
1	47uf 50V	C+200D100H31	C6	647-UPW1E470MDD
1	15uf tant	cap100	C8	581-TAP156K016SCS
1		DIP6	DS1	774-2063ST
1		XIDC10	J1	517-D2510-6002-AR
2		SIP2	JC1 JP1	517-647-01-02
1		SIP3	JP2	517-647-01-03
4	10K	AXIAL0.25	R1 R5 R6 R8	299-10K-RC
1	4k7	AXIAL0.25	R2	299-4.7k-RC
1	1M	AXIAL0.25	R3	299-1M-RC
3	1K	AXIAL0.25	R4 R7 R9	299-1K-RC
1	10K	SIP8	RN1	265-10K-RC
2	BC327	TO-92A	T1 T2	512-BC32740BU
1	BC547	TO-92A	T3	512-BC547CBU
1		TB508A_2	TB1	538-39880-0402
2		TB508-3	TB2 TB3	538-39880-0403
1	1K	TRIM2	TR1	858-67WR1KLF
1	SLA7078MPR	SLA7078	U1
1	74HCT4538	DIP16	U2	595-CD74HCT4538E
1		DIP16 Socket		517-4816-3004-CP

Next will be the schematic and BOM for the opto-isolated version, Stepit 4

Stepit 4: Driver using SLA7078MPR with opto-isolation, regulator and delay

Schematic:

BOM:

Bill of Material for C:\PCB PIC\SLAgm_05A.PCB
On 4/30/11 at 2:23:09 PM
Count	Label-Value	Pattern	Designation(s)	Mouser
1	330uf 50V	C+400D200	C1	647-UHE1H331MPD
6	.1uf	cap100	C10 C4 C5 C6 C8 C9	80-C320C104K1R
1	47uf	C+200D80H31	C2	647-UHE1E470MDD
1	47uf 50V	C+200D100H31	C3	647-UPW1E470MDD
1	15uf	cap100	C7	581-TAP156K016SCS
1	FR202	DIODE0.35	D1	583-FR202-B
1		DIP6	DS1	774-2063ST
2	Fuse Holder	Fuse3517	F1	534-3517
1	Socket IDC 10p	XIDC10	J1	517-D2510-6002-AR
2		SIP2	JC1 JP1	517-647-01-02
1		SIP3	JP2	517-647-01-03
1	HLMP1790	LED	LD1	638-HLMP1790
1	240R 1%	AXIAL0.25	R1	270-240-RC
1	1M	AXIAL0.25	R10	299-1M-RC
5	1K	AXIAL0.25	R11 R12 R6 R7 R9	299-1K-RC
4	10K	AXIAL0.25	R13 R14 R15 R16	299-10K-RC
1	720R 1%	AXIAL0.25	R2	270-720-RC
3	330R	AXIAL0.25	R3 R4 R5	299-330-RC
1	4K7	AXIAL0.25	R8	299-4.7K-RC
1	10K	SIP8	RN1	265-10K-RC
2	BC327	TO-92A	T1 T2	512-BC32740BU
1	BC547	TO-92A	T3	512-BC547CBU
1		TB508A_2	TB1	538-39880-0402
2		TB508-3	TB2 TB3	538-39880-0403
1	1K	TRIM2	TR1	858-67WR1KLF
1	SLA7078MPR	SLA7078	U1
1	74HCT4538	DIP16	U2	595-CD74HCT4538E
1	6N137	DIP8	U3	512-6N137
1	HCPL2631	DIP8	U4	512-HCPL-2630/31
1	LM317HV	TO-220V	U5	LM317HVT Digikey
2	IC Socket 8p		U3S U4S	517-4808-3004-CP
1	IC Socket 16p		U2S	517-4816-3004-CP