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Forum Index : Microcontroller and PC projects : H-bridge in lieu of SSR?

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Tinine
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Joined: 30/03/2016
Location: United Kingdom
Posts: 726
Posted: 11:56am 22 Oct 2021
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Here I go with the high-side switching again  

A couple of years ago I used the LMD18200T instead of an SSR to drive the solenoid of a hydraulic valve. I wanted to try PWM-ing the coil to see if I could achieve proportional flow control and this actually worked (I have a video somewhere).
I left this h-bridge in situ and it works like a champ.

Now I am looking at the DRV8871 and here's what I see:

SSR: Relatively pricey, typically too slow for PWM, requires external overload protection.

DRV8871: Low cost modules from Aliexpress, can handle 100KHz+ PWM, selectable current-limit, over temperature protection.

Can also be driven with a static signal (doesn't need to have PWM)

Just tie one of the inputs low and make it uni-directional


drv8871.pdf
Edited 2021-10-22 21:57 by Tinine
 
Mixtel90

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Joined: 05/10/2019
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Posts: 1196
Posted: 12:26pm 22 Oct 2021
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Nice...
Use active low on In1 and In2 and you have two high level switches. :)
In1 low, Out2 goes high
In2 low, Out1 goes high
Both inputs high, both outputs low.
-- Mick

Zilog Inside! nascom.info for Nascom & Gemini
 
Tinine
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Posted: 12:57pm 22 Oct 2021
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  Mixtel90 said  Nice...
Use active low on In1 and In2 and you have two high level switches. :)
In1 low, Out2 goes high
In2 low, Out1 goes high
Both inputs high, both outputs low.


Yeah, I got thinking about this....external ground, don't return through the FETs  
 
Mixtel90

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Posted: 06:15pm 22 Oct 2021
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Of course, if you try using active low inputs and they are both low the outputs both go tristate - which may not be what you want. Likewise if you use positive logic inpts and both go high then both outputs go low. Hmmm... not really a 2-channel device unless they are alternative inputs. lol
-- Mick

Zilog Inside! nascom.info for Nascom & Gemini
 
Tinine
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Posted: 08:07pm 22 Oct 2021
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  Mixtel90 said  Of course, if you try using active low inputs and they are both low the outputs both go tristate - which may not be what you want. Likewise if you use positive logic inpts and both go high then both outputs go low. Hmmm... not really a 2-channel device unless they are alternative inputs. lol


In actual fact, it couldn't be better because the solenoid valves are double-ended; Solenoid A and Solenoid B. The hydraulic cylinder extends and retracts  
 
Mixtel90

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Posted: 08:46pm 22 Oct 2021
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Sounds good - you even get current limiting thrown in for free. :)
-- Mick

Zilog Inside! nascom.info for Nascom & Gemini
 
Tinine
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Posted: 08:30am 23 Oct 2021
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Well, the really exciting bit (for me) is that; I can give a 20 year-old machine a whole new level of functionality that has tangible benefits.

Being able to control the speed of hydraulic actuators from software, eliminates the manual adjusting of flow-control valves which I have always regarded as ridiculous on a big-ticket CNC machine. Not only does this save precious setup time (short part-runs and many changeovers are more common, today) but also setup-scrap (stainless steel).

I just wouldn't be able to do this with a Siemens system. Well, I could but I would have to replace the valves @ GBP2,500 a piece plus some ridiculously expensive analogue Siemens modules.

With a Mite+Propeller, child's play  
 
Volhout
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Posted: 05:53pm 23 Oct 2021
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Hi Tinine,

The DRV8871 is a nice device, but it is designed to be used as a half bridge.
This means always one of the output stages pulls low. And it is in that particular branch that current limit is sensed (and controlled).

If you use both outputs independently, and drive them high (load to ground) there is no current limit (no protection).

The current limit is a PWM. Be aware if you use it, and additionally want to use your own PWM to control the power to the outputs, your own PWM frequency should either be higher, -or- much lower than the current protection PWM. The chips current limit will depend highly on reactance of the load (could be different per load). So maybe you should just use the DRV8871 as 2 power FET's.

Finally SSR's typically have opto isolation build in. The DRV8871 has not, so you have to apply optocouplers (digital ones like the TLP2745/TLP2345 from Toshiba) that have low input current (2mA is enough) high speed, and can be powered from 5V....30V on the detector side.

When I look at it objectively, when you have to use optocouplers in you application, you better stay with simple MOSFET's. -or- use the DRV8871 as a full bridge. Then you get all the goodies.

The circuit for using simple mosfet's is proposed earlier by me in a thread where someone insisted to use N channel MOSFETS to drive LED's.  
The digital optocouplers proposed above drive PWM isolated in my applications (Arduino PWM running at 31kHz) directly driving the gate of the MOSFET.
Edited 2021-10-24 03:57 by Volhout
If nothing goes right ... turn left
 
Mixtel90

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Posted: 06:28pm 23 Oct 2021
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I'm afraid 'twas I who mentioned the current limit. I only checked the data sheet after and realized that it isn't in circuit for high-side drivers.
-- Mick

Zilog Inside! nascom.info for Nascom & Gemini
 
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