H-Bridge - diodes (or rather the lack of)

[Dixo]

G'day all!

I've got an H-bridge here from an old motorised toy.. as I want to use it for something else and the RF receiver is no use to me for this project i figured i' strip the H and stripboard it.

I noticed though that there are no diodes to protect the four transistors (yes they are transistors - looked up the part numbers).

Now I thought this wasn't supposed to work.. but this toy has survived years.

Any ideas?

Thanks,

Dixo

 

[davenn]

I noticed though that there are no diodes to protect the four transistors (yes they are transistors - looked up the part numbers).

what is the type # of the transistors ?

I have seen the occasional bipolar transistor H-bridges without diodes ... it is surprising that they don't fail

if they are mosfets, then they have built in diodes by default and don't require external diodes eg ...

I should qualify that by saying that not all MOSFETS have built in diode, many do

 

[Dixo]

what is the type # of the transistors ?

Hi dave

They are H772 and H882 - bog standard npn and pnp basically.

I have seen the occasional bipolar transistor H-bridges without diodes ... it is surprising that they don't fail

Yes I was quite surprised too because i understood that without diodes they were guaranteed to fail.

 

[davenn]

They are H772 and H882 - bog standard npn and pnp basically.

never heard of them ... can't be very standard

do you have a link to a datasheet please ?

 

[Dixo]

Hi Dave,

Yes, for the H772 its here:
http://www.alldatasheet.com/datasheet-pdf/pdf/334919/HUASHAN/H772.html

For H882 this one:
http://www.alldatasheet.com/datasheet-pdf/pdf/334922/HUASHAN/H882.html

Careful to click the link where it says this:
Part No. H882
Download H882 Click to view

All the rest are spam / adverts so only click that one as it'll send you the pdf file.

Thanks again,

Dixo

 

[davenn]

Hi Dave,

Yes, for the H772 its here:
http://www.alldatasheet.com/datasheet-pdf/pdf/334919/HUASHAN/H772.html

For H882 this one:
http://www.alldatasheet.com/datasheet-pdf/pdf/334922/HUASHAN/H882.html

cheers

definitely ones I have never heard of before
appear to be a Chinese semi manufacturer

will be interested in seeing how well they last for you

you could put diodes across the transistors if you wanted to ... it would give some peace of mind

they used 1N4148 ... I would prefer to see 1N4001 series diodes in there as they are much more robust and commonly used for that and similar purposes
1N4001 - 4007Dave

 

[jim hardy]

I have seen the occasional bipolar transistor H-bridges without diodes ... it is surprising that they don't fail

I think it's a case of getting away with sloppy design.
Remember that a bipolar transistor when polarity is reversed will still act like a transistor, just with poor gain.