L298 with discrete parts - Page 1
Forum section: electronics / projects

author: shebu18
author_rank: Frequent Contributor
timestamp: February 26, 2013, 10:40:34 am
content: Hello, i am trying to design a discrete 2A (6V-15V) motor driver using the L298 internal schematic as guide line.This started because the price of the L298P(smd) went up and it is no longer a simple solution for hobby. I thought to use a inverting gate, 2 AND gates and 2 NPN for half of the bridge, this is all 4 times present on the board.What parts would you recommend for the build? I went with smd at first but it is kind of hard to route it on a 2,5cmx5cm board. Now i want to try and use some THD parts, transistors in special.Is there a alternative to L298? same output but with a smaler price?

author: slashguitar
author_rank: Contributor
timestamp: February 26, 2013, 02:30:45 pm
content: Hi,Take a look at the schematic of Digilent Pmod HB5http://digilentinc.com/Data/Products/PMOD-HB5/PmodHB5_D_sch.pdfIt is designed for 2A 12 V. Just replace the mosfets with beefier ones to get the Volt/Amp rating you need.Also take a look at the bridge schematic from Cerebot MC7 (pg 6)http://www.digilentinc.com/Data/Products/CEREBOT-MC7/Cerebot%20MC7_sch.pdfThis one has over current protection and other bells and whistles.P.S. According to the internal schematic of the L298 the high side transistors are driven directly from a logic gate. Isn't it necessary to drive them with a charge pump(to have the base voltage greater than the collector)

author: oPossum
author_rank: Super Contributor
timestamp: February 26, 2013, 07:46:38 pm
content: This is a simple bipolar H bridge that uses only 2 output pins to do off, brake, forward and reverse. Limited to about 250 mA with ordinary NPN/PNP (TIP41/42 or similar) or maybe up to 5A with darlington (TIP142/147 or similar).

author: nctnico
author_rank: Super Contributor
timestamp: February 26, 2013, 09:51:55 pm
content: Note that many power transistors (and especially darlingtons) already have internal diodes so D1 thru D4 can be left out.

author: senso
author_rank: Frequent Contributor
timestamp: February 26, 2013, 10:21:00 pm
content: See this small list:http://www.ti.com/apps/docs/viewdevices.tsp?blockDiagramId=6145&blockId=7722&designOptionId=18996&appId=409They are h bridges, that are identical to the L298, but they are mosfet based, so they dissipate less energy than the old L298.

author: shebu18
author_rank: Frequent Contributor
timestamp: February 27, 2013, 07:42:57 pm
content: Thank you all for your replays.I made a schematic and a PCB that is 25x42mm with 8xMJE802(NPN Darlington), 8x74LVC1G08GW(AND Gate), 4x74AHC1GU04(Inverter).Licgt yellow(cream) is bottom and yellow is Top layer.The board is thought to be in a dip format. The cost of such a board with all the components is arround, 4-5$.

author: fcb
author_rank: Super Contributor
timestamp: February 27, 2013, 07:59:56 pm
content: Quote from: shebu18 on February 27, 2013, 07:42:57 pmSo a bit more than the price of an L298P then?If you were going to bother having vertical THT parts - why not use the cheaper (and smaller...) L298N or HN???

author: shebu18
author_rank: Frequent Contributor
timestamp: February 27, 2013, 08:17:52 pm
content: My starting idea was to use SMD parts, instead of the L298p, smd version. But now i need to redesign the bridge.

author: shebu18
author_rank: Frequent Contributor
timestamp: February 28, 2013, 06:50:48 am
content: I wanted to find a cheap solution for the L298 smd. I now have seen on farnell some sot223 1A 2W NPN darlington transistors that i could use. To output 2A i would put 2 in paralel. I need also to redisign the logic components, i want to use a bit bigger ones.

author: poorchava
author_rank: Super Contributor
timestamp: February 28, 2013, 08:46:30 am
content: You could modify the circuit to use MOSFETS. Nowadays you can get ones with really low Rds in small packages. You can get both N and P types capable of over 4A in SC-70 and SOT23 packages.

author: Kremmen
author_rank: Super Contributor
timestamp: February 28, 2013, 09:12:25 pm
content: Quote from: shebu18 on February 27, 2013, 07:42:57 pmHave you simulated or prototyped that your circuit actually works? Because i see several issues that make me think it won't.

author: fcb
author_rank: Super Contributor
timestamp: February 28, 2013, 10:03:53 pm
content: Quote from: shebu18 on February 28, 2013, 06:50:48 amYour design at present won't work. Not even slightly. I hesitate to explain the main blunders as I think you might not have enough grasp of what's going on in a transistor (and i'm not trying to be rude) and spending your time looking at component specs.oPossum provided a very reasonable H-bridge schematic that you should work through with a transistor & logic theory and perhaps a calculator.

author: shebu18
author_rank: Frequent Contributor
timestamp: March 01, 2013, 06:50:39 am
content: I did not simulate, neither did i prototype the schematic. I also changed the components now and i will simulate it and come back with the results.

author: mrflibble
author_rank: Super Contributor
timestamp: March 02, 2013, 10:03:10 am
content: Quote from: shebu18 on February 27, 2013, 08:17:52 pmAnd forget about the original motivation I presume?Because what I read here is: "The L298 is too expensive! Therefore I shall make a new solution that in total cost will be more expensive in low quantities!". So by now you have decided that thru hole is acceptable again for whatever reason? Well, then can you use the thru hole L298, which is < 3 EUR in single quantities.Anyways, you could check out the DRV88xx range from TI and see if something fits the bill. The DRV8833 and DRV8836 for example fit the lower end of your voltage range and is fairly cheap. Might be enough for some of the smaller motors you want to drive.

author: shebu18
author_rank: Frequent Contributor
timestamp: March 08, 2013, 04:35:36 pm
content: I found the chips on aliexpress with half the price of here. So no need for a discrete design.

author: mrflibble
author_rank: Super Contributor
timestamp: March 08, 2013, 06:06:55 pm
content: L298's, or creative chinese versions?

author: shebu18
author_rank: Frequent Contributor
timestamp: March 08, 2013, 08:09:24 pm
content: Quote from: mrflibble on March 08, 2013, 06:06:55 pmi hope they are l298'sSent from my ONDA MID using Tapatalk HD