Question about an H Bridge

A

Ad8chd

New Member
Hello and thank you in advance to anybody who helps me out. This is my first post hopefully it’s in the right place. I’m trying to build a little miniature winch mostly for fun just to see if I can, also if it works it would be cool to have something to drag logs up the hill with. I’m using a 24 V motor that was repurposed out of a hoverround mobility scooter. I’m using an on off on switch with a 12 V Li Ion battery pack to trigger 4 automotive relays (2 at each position) that will send 24 V from 2 deep cycle batteries to the motor. I’m under the assumption that I will have to deal with some back EMF from the motor when I change directions, I’m also under the assumption that that can be dealt with with flyback diodes. When I go to Google, I can find schematics that are what I’m doing, but there’s flyback diodes on all the coils of the relays but nothing around the motor. If it was only going One Direction, I know it would go across the motor terminals, but with a bi-directional setup I’m pretty sure that that cannot be a thing. I’m a doofus, and I already had one little fire and blew up a big diode, I’m gonna blame that on ChatGPT. Also, I’ll blame it on me, OK it’s my fault. I did it, if anybody could shine a little bit of light on this for me, I would greatly appreciate it. Thank you again.
 
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Just diodes on the relays should be fine, and if you're just using mechanical switches?, no diodes at all would probably be fine as well - the diodes are there to protect the driver transistors, which can be destroyed by the voltage spike.
 
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If you want to suppress any back-emf from the motors (which may not be needed for the relay contacts but couldn't hurt) requires four diodes, two on each motor lead.
One diode goes from the motor lead to the V+ (cathode to V+) and one from the motor lead to common (anode to common)
 
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Nigel Goodwin said:
Just diodes on the relays should be fine, and if you're just using mechanical switches?, no diodes at all would probably be fine as well - the diodes are there to protect the driver transistors, which can be destroyed by the voltage spike.
Click to expand...
That’s awesome, thank you. I almost used mosfets but I found the relays and figured it would be easier and a bit more robust.
 
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Thanks for the response, I probably should have mentioned in the post (because I already got tripped up on this) that these are automotive relays so they don’t have a common pin. I’m gonna give it a shot like Mr Goodwin suggested and just go Wild West footloose and diode free… I’ll keep you guys posted on my bumblings, and try to get a video if there’s any more fireworks lol
 
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If you use
By "common", Crutschow meant the negative of the supply.
 
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Diver300 said:
If you use

By "common", Crutschow meant the negative of the supply.
Click to expand...
Oh, duh… lol it might be hard to tell from my calm, confident, demeanor, But as far as the electronics goes, I am not what one would call an expert… I’ve been trying to learn some stuff because it’s interesting to me, but it’s almost like they’re trying to keep this stuff a secret, all this damn math and vocabulary words. Who would’ve thought electrical engineering would be so complicated?
 
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There are automotive relays that have a normally open connection and a normally closed one. The terminals are often marked 30, 87 and 87A. If you used those, you only need two relays. Wire +ve to 87 on both relays, and -ve to 87A on both relays. Connect the motor between the terminal 30 on one relay and the terminal 30 on the other relay.
 
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It's the same for all disciplines. People who are familiar with the language don't always realise how obscure it can be to others.

In particular, in electronics, circuit diagrams are all-important but many people who don't use them don't even understand how much information can be in them, or how much can be understood from a brief glance at one.
 
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Mine are unmarked and normally open, I figured that out all by myself with my multimeter and a battery Also (please excuse what I’m sure is about to be an incredibly dumb question) what is ve? The only context that I know of where that was used is when I was messing around with some bjt amplifiers that I’m still scratching my head on…
 
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I do that all the time whenever I try to teach someone to play guitar, doing something the majority of one’s life has a way of normalizing the extraordinary. I’m still working on schematics, I can usually read and build from one pretty well, drawing them out is a whole other monster… simulating is still kind of rough too
 
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Hi A,
With H bridges:
Check N and P type mosfets, this will explain common and negative.

Beware of switching from forward to reverse at the same time, else more magic smoke, and fires.

With your set-up I think you will need some gearing on the motor.
C.
 
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Ad8chd said:
Also (please excuse what I’m sure is about to be an incredibly dumb question) what is ve?
Click to expand...
It's not a dumb question. It's come from me using and abbreviation without thinking whether it would be understood.

It's part of an abbreviation for positive or negative.

positive
+ve
+

negative
-ve
-

They are shorter than the full "positive" or "negative" but less likely to get lost in the noise or misunderstood than + or - which can be seen as "and" or a hyphen (or even as concatenate if Javascript is involved)
 
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You are absolutely correct about gearing the motor, I got a chunk of drive chain and a handful of various sprockets to do some trial and error and figure what works best, I might even get froggy enough to do a couple different ratios. I’ve got an on-off-on switch specifically for that little dead space between the “on’s” to try and mitigate accidentally having them both on at the same time. Would you mind expanding on the mosfet thing you were talking about? I actually almost used a couple of those in this because I didn’t have a switch that could take 12 V for the relays.
 
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Ad8chd said:
I didn’t have a switch that could take 12 V for the relays.
Click to expand...
What type of switch can't take 12V?

Below is an example circuit using two SPDT (Single-Pole Double-Throw) relays to control.
The diodes to suppress the voltage transients are optional.
If you have only SPST relays, then you will need two on each side to make the connections.
 
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