Voltage regulator over 1A

P

pjrebordao

Senior Member
I'm using a 28X1 to drive a stepper. However, the stepper draws quite a bit of current. I'm supplying it with a 7805 regulator, but I find that despite a good heatsink, after a while the regulator goes into termal shutdown, so it is probably drawing more than 1A.

Can I wire a couple of 7805 in parallel (I to I, G to G, O to O) ? or should I go to bigger capacity one ?

Thanks
 
D

Dippy

Moderator
Step #1.
Has this stepper got a Data Sheet which indcates current? (Or is it from Ebay?)
Have you measured the current under worst-case load?
Give us a clue and note there are variants on that old dinosaur.

What sort of power supply are you using? Voltage, current capacity.. etc?
If you are dropping a long way or the current is big then consider a switcher as they can be a lot more efficient.

(I say "can" as that is in the hands of the constructer to a great degree unless you buy a pre-built device... but can't be guranteed if it is some cheap anonymous chod from Ebay though ;) )
 
P

pjrebordao

Senior Member
Stepper is an Astrosyn MY5602 and according to the spec, it draws 0.74A / phase. However they don't specify voltage used !
I measured each phase at about 550mA when driven @ 5V.
I'm using a 7805 from ST.

Both phases are fed from the 7805 but I thought I could get away with that. But since the stepper operates for long periods, the 7805 eventually gets too hot. I use it to drop from 24V (I need 24V for another stepper).
What other regulator could I use for a bit more current (say 1.5A) ?
 
B

BeanieBots

Moderator
pjrebordao said:
Can I wire a couple of 7805 in parallel (I to I, G to G, O to O) ? or should I go to bigger capacity one ?
Click to expand...
Simple answer, NO.
The output voltage of a 7805 varies from device to device. The highest one will supply ALL the current. If you add a small resistor in series with the output of EACH regulator then possibly, but not good practice.

There is another range 78S05. There are two types of that range just to confuse things even more. One is a linear version similar to the 7805 but with 2A capability. The other (the original) is a switcher. ("S" for switcher).
I forget the current spec but it was more than 2A and a LOT more efficient so a lot less heat. VERY expensive though.

Answer Dippy's questions and we can advise better.
 
A

Andrew Cowan

Senior Member
Say it draws 1A.

24V minus 5V = 19V

This means you are dissipating 19W. Are you sure your heatsink can handle this? If it can, and you are really drawing more than 1A, consider the 78xxCV series of regulators - 2A max.

I have paralleled 7805s together sucessfully, but lots of other people say don't do it, as they could stop working. I haven't had this problem.

A
 
S

slurp

Senior Member
I'd recommend using the regulated supply for your PICAXE and the Logic circuits but supply unregulated voltage to your motor.

All the driver chips I've looked at allow you to do this. You might note the data sheet having pins labelled
  • Vs - Supply voltage and Vss - Logic Supply Voltage
  • VM - Motor Drive Power Supply and VDD - Logic Supply

Alternatively, you might find items that are just labelled VDD - IC power supply where you only need the un-regulated supply and the logic supply is handled internally.

In all cases, check the data sheet and look for something that can manage the un-regulated voltage.

Best regards,
Colin
 
W

westaust55

Moderator
5V regulators > 1 Amp

Another form of the linear 7805 is the 78H05 which is rated to 5 Amps and in a TO3 case. But will still need good heat sinking.

While noting that you have measured the stepper motor current, from past looking at stepper motor specs, the inductance is very small relative to the resistance so if you measure the winding resistance then
Amps = Volt / Resistance
 
B

BCJKiwi

Senior Member
Just trying to bring the different comments together here;

1. With a 24V supply, the 7805 is 'burning off' 19V x A in heat to give you the 5V you need.
2. at 10.5ohms per coil, current is 5/10.5 = 0.476A per coil.
3. so even if both coils on at once, it's still within spec IF the HEAT can be dissipated!
4. Changing to a higher capacity regulator or to a transistor will still require dissipating the heat.
5. would suggest either a higher capacity regulator - gives you more 'free board' (1 part) or use the regulator to manage a transistor (2 parts).
6. Whatever you do you need a GOOD HEAT SINK.
Fairchild's LM7805 1A regulator data sheet indicates PEAK load is around 1.6A at a 19V differential between in and out. Ratings at a 25deg C JUNCTION temperature.

As a simple trial, mount the reg on a heat sink and blow some air over it and see if it holds up!
 
Last edited:
M

mickm2au

Member
Another trick I've used in a unit we produced which had 24 v for the power circuitry and 5 v for the rest was to have a 7815 in series with the 7805. Gives a minimum parts count, shares the dissipation and as the 78 series are only a few cents usually works out cheaper.
 
D

Dippy

Moderator
Well, you've had some good advice Paulo.
Any further thoughts on how you are going to progress this.
Personally, I'd use Switchers. Much greater power efficiency but care concerning noise will be required - and this is where total novices can go wrong and end up thinking Switchers are awful.

Remember, we could go on listing regulators ad-nauseum.
The number of suggestions wil be proportional to the anorak size.

I would suggest that you get on to BIG suppliers sites eg. Farnell, RS Components, Digikey and look through their listings.
There are hundreds.
You don't have to BUY from them, merely get some ideas, device numbers .... AND Data Sheets too ;)
 
M

moxhamj

New Member
I'll second Dippy's comment. After spending years playing with bog standard 7805s and big heatsinks, I've just discovered the incredible simplicity of Simple Switchers http://www.national.com/mpf/LM/LM2575.html

1 Amp. $1.55 from Futurlec. Ok, you need that diode which will cost 10c. And you need an inductor that has a fairly low DC resistance. I picked up some for $2 each but they are not as easy to find as other components. Ideally under 1 ohm. Consider, 1 ohm, 1 Amp, =1V drop. And W=IV= 1W heat dissipation in the inductor. But small ones well under 1cc are way under 1 ohm resistance. Here is the (ahem) cool bit though. The chip runs cold. Consider - you want 1A and you are dropping from 24V. So you are using 24W of energy in driving that stepper. And 19W of that is going into the 7805 - no wonder it is getting warm. (ok, yours is a bit less but I bet it is still more than the heatsink can disssipate) But with a switcher running at 80-90% efficiency, you are only drawing 250mA from the 24V supply. Work out the cost of a heatsink vs the cost of the simple switcher components, and the switcher comes out cheaper I believe.
 
Last edited:
M

MartinM57

Moderator
I'm with the above two as well - I've had lots of success with LM2575 - 16 multiplexed seven segment displays and a few other things on a 5v supply from 12-14v in, and it doesn't even seem to get above ambient. It's just bolted to the PCB which acts as a reasonable heatsink as most of either side is either a +5v or 0v powerplane, so there's lots of copper about...
 
Last edited:
P

pjrebordao

Senior Member
Lots of advice ! As usual, the feedback here is superb.
I'm still going through the options, but the switcher really seems the most appropriate and elegant.
However, it requires a few more parts and I was looking for a drop-in replacement, have to think about it...
Cost is no big issue. This is a personal project and it will be a single unit.
 
L

LizzieB

Senior Member
Here's a whole other idea. I assume that the common of the coils goes to +ve and the picaxe outputs drive transistors that switch the other ends of the coils to ground? I wouldn't mess with a regulator, I would drive the motor from the +24v and convert the driver transistor config (suitably heatsinked) to limit the coil current to the max (0.74A was it?). Then you'll be dissipating the power in multiple devices, and you'll get the advantage of hitting the coil inductance with a higher voltage which will allow you to drive it faster.
 
B

BeanieBots

Moderator
The 78SR05 is a switching drop in replacement for the 7805.
It's MUCH thicker but only in the one direction. The T0-220 form, pinout and form are identical. Just make sure you double check it's the switcher version. Like I said earlier, it's MUCH more expensive than the linear 2A version but IS a direct drop-in version.

The last one I bought was from RS about 5 years ago. It cost £17 then.
 
J

jglenn

Senior Member
An easy way to use switchers is the demo boards they make, here is one that puts out 5V at 3A, 10-35V in. Scroll down and see the pcb layout and schem. High freq circuits are hard to get to work on a breadboard. You need the optimal layout and ground planes. Just the positioning of the parts can make or break it. Buy one of these $10 from Digikey, and you know it will work.

http://focus.ti.com/lit/ug/slvu157/slvu157.pdf

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=296-19640-ND

The TPS5430DDA chip itself is $4, but comes on the fully build demo board.

http://focus.ti.com/lit/ds/symlink/tps5431.pdf

Up to 95% efficiency. Note the L1, C filter used. Further filtering can be done with a common mode choke, to really squash the ripple. But you can't quite reach the pure DC of a good old battery, for sensitive circuits.
 
J

jglenn

Senior Member
Not bad, but runs at 50KHz. My later chip runs at 500KHz. This allows smaller inductors. But your chip could more easily be breadboarded. Make sure the inductor has low R. And look for high ripple rated electrolytic caps, way cheaper than tantalums.
 
G

geoff07

Senior Member
Just a thought, but if you have to lose 19v into a reg why not put a resistor in the input line? You could lose say 15v without affecting the regulation. 15 ohms would lose 15v at 1 amp. Would need to be a chunky resistor though, but it would shift some of the heat from the reg.
 
P

pjrebordao

Senior Member
Just went into www.national.com looking for switcher specs.
They have a very nifty site (WeBench) that allows you to generate circuits using their chips, but also testing them electrically and thermally, tweaking them and finally ordering them built or as a kit if you feel so.
 
G

geoff07

Senior Member
Ooops sorry I missed that one when I checked. Still better than frying a reg tho.

My own attempt at stepper motor control was for an ultrasonic radar experiment for a robot subsystem using a floppy disc motor driven by an 18X and the FETs on a CH035 project board. But that motor only uses 200mA. The mechanics work fine. One day I'll programme the echo analysis.
 
P

pjrebordao

Senior Member
jglenn said:
An easy way to use switchers is the demo boards they make, here is one that puts out 5V at 3A, 10-35V in. Scroll down and see the pcb layout and schem. High freq circuits are hard to get to work on a breadboard. You need the optimal layout and ground planes. Just the positioning of the parts can make or break it. Buy one of these $10 from Digikey, and you know it will work.

http://focus.ti.com/lit/ug/slvu157/slvu157.pdf

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=296-19640-ND

The TPS5430DDA chip itself is $4, but comes on the fully build demo board.

http://focus.ti.com/lit/ds/symlink/tps5431.pdf

Up to 95% efficiency. Note the L1, C filter used. Further filtering can be done with a common mode choke, to really squash the ripple. But you can't quite reach the pure DC of a good old battery, for sensitive circuits.
Click to expand...
I decided to take your "advice" and look for a ready made eval board.
I'm in Europe so Digikey isn't the best option. However, Farnell sells that TI board for 11Eur ! I'm ordering it now.
 
B

BeanieBots

Moderator
I have mentioned several times to check & double check that it is a switching type because not all (even with same part number) are. Also, they ARE very expensive (the biggest clue as to which type it is). £9 is a VERY good price for a 7805 switching drop-in replacement.
Compare the cost of that to the cost of a board re-design and it's actually a very cheap solution for small quantities.
 
J

jglenn

Senior Member
Paulo:

Let us know how that TPS5430 board works out. Being high freq, it should be small and light. Get a ripple measurement on the output voltage if possible, into a decent load. I predict about 50mV peak to peak, or more. Need a scope.
 
P

pjrebordao

Senior Member
Actually, I chenged tack and went instead for the switching regulator from Dimension Engineering.
It's reasonably cheap for what it does and shipping is just 1.5USD anywhere.
It arrived yesterday, so in a few days I will let you know how it goes.
 
lbenson

lbenson

Senior Member
5V 3-pin switching regulator, 7805 drop-in (nearly)

Another option for 1A 5V 3-pin switching regulator is here: http://store.gravitech.us/

It's $8.99US with US shipping @ $2.99 + $1 for each additional.

Its efficiency is not given, but from the pictures it's based on the LM2575-5, so would max out somewhere around 80% if other components and layout suit.
 
Last edited:
P

pjrebordao

Senior Member
The one I bought from Dimension Engineering is more expensive, but it is bulkier. Looks like the condensers are built-in.
 
P

pjrebordao

Senior Member
For everyone's information, I've already installed the drop-in replacement from Dimension Engineering and it does what they say in the "tin".
A quick recap: I use it to drop 24V to 5V and drive a 28X1, some switches and leds, a stepper and a servo. Total continuous current is about 0.5A. There was no need for any capacitors (before and after the regulator), and heat buildup is quite moderate, no need for a heat sink.
I'm happy !
 
You must log in or register to reply here.
Top