ULN2803A PCB Design Guide: Footprint, Pinout, and Alternatives
Eight Darlington sink drivers with built-in flyback diodes for relays, solenoids, and LED strings.
The ULN2803A is eight Darlington sink drivers in one DIP-18: open-collector outputs that withstand 50 V in the OFF state and sink 500 mA continuous (600 mA peak) each, with integral suppression diodes brought out to a common free-wheeling pin for inductive loads. Each input has a 2.7 kOhm series resistor sized for 5 V TTL and CMOS, so a bare logic pin switches a relay coil, solenoid, or LED string with no external base resistor. ST's datasheet DocID1536 Rev 4 covers it alongside the ULN2801A, ULN2802A, and ULN2804A, which differ only in the input network.
It earns its place on cost and ruggedness, not performance. The Darlington output saturates around a volt, not millivolts: 0.9 V typ at 100 mA, up to 1.6 V max at 350 mA. The load loses that voltage, the package dissipates it, and the whole chip is capped at 2.25 W. For relay coils running well above the drop, none of that matters; for low-voltage loads, high sustained currents, or a surface-mount board (ST builds this part only in DIP-18), a DMOS array like the TBD62083A or discrete logic-level MOSFETs is the better answer.
Most ULN2803A field failures are wiring and budget mistakes, not bad silicon: a COM pin left floating so the flyback diodes never clamp, all eight channels loaded as if the per-channel rating were a package rating, saturation drop missing from the load budget, inputs driven from rails the 2.7 kOhm resistor was never sized for, and SMD parts sourced under the same MPN from a different manufacturer's lineage. Each is covered below.
What breaks boards
The outputs are Darlingtons: budget up to 1.6 V of saturation drop
A Darlington pair cannot saturate like a single transistor or a MOSFET. VCE(sat) is 0.9 V typ / 1.1 V max at IC = 100 mA and climbs to 1.3 V typ / 1.6 V max at 350 mA. The load sees its supply minus that drop, and the package, not the load, dissipates drop times current, which is what makes the thermal ceiling bite. On 5 V loads the lost volt is real margin. If the drop disqualifies the part, a DMOS array such as the TBD62083A switches the same loads with far less on-voltage.
The per-channel ratings are not a package rating: 2.25 W total
Each Darlington is rated 500 mA continuous, but total package dissipation is capped at 2.25 W, any single pair at 1 W, with an RthJA of 55 C/W and an ambient rating that stops at 85 C. Sum VCE(sat) times IC over every simultaneously-on channel and the budget disappears fast; eight loaded channels at even moderate currents blow past it. Derate for real ambient, stagger duty cycles, or parallel outputs: the datasheet allows paralleling for higher current, and lower per-channel current also means lower VCE(sat). The heat still lands in the same package either way.
Floating pin 10 makes the built-in flyback diodes ornamental
The suppression diodes run from each output to a common free-wheeling pin, pin 10. They clamp inductive kickback only if pin 10 is tied to the supply feeding the coils. The schematic symbol makes it easy to leave unconnected, and then every relay turn-off flies the output past the 50 V absolute maximum until channels start dying. Tie COM to the load supply, not the logic rail, when the two differ, and budget the clamp: the diode drops 1.7 V typ / 2 V max at 350 mA while returning the kickback to the coil supply.
The 2.7 kOhm input is sized for 5 V logic; check VI(ON) below that
Inputs are current-driven through a 2.7 kOhm series resistor chosen for 5 V TTL and CMOS, drawing 0.93 mA typ / 1.35 mA max at VI = 3.85 V. The input voltage guaranteed to switch a channel rises with load current: 2.4 V max at IC = 200 mA, 2.7 V at 250 mA, 3 V at 300 mA (VCE = 2 V). Lower-voltage GPIO works at modest collector currents, but an input that only half-turns-on a Darlington parks the output in linear dissipation instead of saturation. For 6-15 V logic, use the 10.5 kOhm ULN2804A instead.
ST builds this part only in DIP-18; a SOIC on your BOM is another lineage
Every ST order code for the ULN2803A ships in the 18-lead DIP: Table 1 lists one package and the mechanical data covers nothing else. Wide-SOIC 'ULN2803A' parts come from other manufacturers (TI's delisted ULN2803ADW lineage, onsemi) with their own datasheets; TI no longer catalogs the part at all. If your footprint is SMD, re-verify every number on this page against the manufacturer you actually buy from. Also note ST specifies the electrical table at TA = 25 C only, and OFF-state leakage grows with temperature: ICEX reaches 50 uA max at VCE = 50 V, TA = 70 C.
Key specifications
| Parameter | Value | Source |
|---|---|---|
| Output voltage (absolute max) | 50 V (VO); device 'can withstand at least 50 V in the OFF state' | DocID1536 Rev 4, Table 2 Absolute maximum ratings + Description (page 1) |
| Continuous collector current (absolute max) | 500 mA (IC, per Darlington); description states 'a peak load current rating of 600 mA (500 mA continuous)' per Darlington; outputs may be paralleled for higher current | DocID1536 Rev 4, Table 2 Absolute maximum ratings + Description (page 1) |
| Saturation voltage VCE(SAT) | 0.9 V typ / 1.1 V max at IC = 100 mA, IB = 250 uA; 1.1 V typ / 1.3 V max at IC = 200 mA, IB = 350 uA; 1.3 V typ / 1.6 V max at IC = 350 mA, IB = 500 uA | DocID1536 Rev 4, Table 4 Electrical characteristics (TA = 25 C unless otherwise specified) |
| Input drive (5 V logic) | ULN2803A has a 2.7 kOhm series input resistor for 5 V TTL and CMOS; input current II(ON) 0.93 mA typ / 1.35 mA max at VI = 3.85 V; input voltage VI(ON) max 2.4 V at IC = 200 mA, 2.7 V at IC = 250 mA, 3 V at IC = 300 mA (VCE = 2 V) | DocID1536 Rev 4, Description (page 1) + Table 4 Electrical characteristics |
| Clamp diode forward voltage | 1.7 V typ / 2 V max at IF = 350 mA (integral suppression diodes, common free-wheeling pin 10) | DocID1536 Rev 4, Table 4 Electrical characteristics + Figure 2 pin connections |
| Power dissipation / thermal | PTOT 1 W (one Darlington pair), 2.25 W (total package); RthJA 55 C/W | DocID1536 Rev 4, Table 2 Absolute maximum ratings + Table 3 Thermal data |
| Operating ambient temperature range | -20 to 85 C (TA) | DocID1536 Rev 4, Table 2 Absolute maximum ratings |
Verified against the manufacturer datasheet on 2026-07-10. Confirm the current revision before production use.
Alternatives
- ULN2003A: seven channels of the same Darlington sink-driver family in 16-pin packages; far more second sources and package options than the 18-pin ULN2803A. Pick it when you can spare a channel.
- TBD62083A: Toshiba's 8-channel sink-type DMOS transistor array positioned as a ULN2803A successor; MOSFET outputs give a much lower output on-voltage than the Darlington's ~1 V VCE(sat).
- TPL7407L: TI's 7-channel low-side NMOS driver marketed as a ULN2003A-family upgrade; lower on-resistance outputs, but seven channels and a different pin count.
Common questions
- Can the ULN2803A source current or drive a load high-side?
- No. All eight channels are open-collector Darlington sink drivers: the load connects between its supply and the output pin, and the ULN2803A pulls the low side toward ground. Each channel sinks 500 mA continuous (600 mA peak), and outputs may be paralleled for more. For sourcing or high-side switching you need a different part entirely.
- Do I need external flyback diodes when driving relays with the ULN2803A?
- No; each output has an integral suppression diode to the common free-wheeling pin (pin 10). But they only work if pin 10 is connected to the relay coil supply — left floating there is no clamp at all. The diode drops 1.7 V typ / 2 V max at 350 mA.
- What is the difference between the ULN2803A and the ULN2804A?
- The input network. The ULN2803A has a 2.7 kOhm series input resistor for 5 V TTL and CMOS; the ULN2804A is the 10.5 kOhm version for 6-15 V CMOS. The eight Darlington outputs are the same, so pick by the logic rail driving the inputs.
- How much total current can a ULN2803A switch at once?
- Each channel is rated 500 mA continuous, but the package limits — 2.25 W total dissipation, 1 W for any single Darlington pair, RthJA 55 C/W — decide how many channels can carry how much simultaneously. Sum VCE(sat) times current over the on channels (up to 1.6 V max at 350 mA each) and stay under the cap after derating for your ambient.