# USB charger circuit

Started by October 1, 2013
```I am working on a simple USB charging circuit.  Normally, the 3.6V NiMH bat=
tery is connected to the device via the NC contacts of a relay.  When conne=
cted, a 5V relay switch the battery to charge via a diode. So, a fully char=
ged battery is around 4.2V.  As someone suggested, i will put a parallel R =
& C to reduce the hold current of the relay.=20

Questions:

1. What is the minimum safe holding voltage/current of a 5V DC relay?  The =
coil has around 130 ohms, or around 38mA activation current.  What should b=
e the values for R & C?

2. Fully charged battery is around 4.2V (for a short time anyway), but the =
```
```On Tue, 1 Oct 2013 10:56:42 -0700 (PDT), edward.ming.lee@gmail.com
wrote:

>I am working on a simple USB charging circuit.  Normally, the 3.6V NiMH battery is connected to the device via the NC contacts of a relay.  When connected, a 5V relay switch the battery to charge via a diode. So, a fully charged battery is around 4.2V.  As someone suggested, i will put a parallel R & C to reduce the hold current of the relay.

---
Why would you want to do that?
---

>Questions:
>
>1. What is the minimum safe holding voltage/current of a 5V DC relay?

---
Barring the MUST OPERATE voltage, there's really no telling.

For example, looking  at the data sheet for OMRON's G5NB relay at:

http://www.components.omron.com/components/web/PDFLIB.nsf/0/2B5B6EDC22E38AB885257201007DD57B/\$file/G5NB_E_0911.pdf
we find a relay with a coil very similar to yours with a MUST OPERATE
voltage which is 75% of rated voltage (max.), and a MUST RELEASE
voltage which is 10% of rated voltage (min.)

That means that while the relay is _guaranteed_ to make at 3.75 volts,
there's nothing saying it can't make at a lower voltage.

Conversely, while the relay is guaranteed to break at 0.5 volts,
there's nothing saying it can't break at a higher voltage, which just
might be the low voltage you want to use to keep the relay made.

Consequently, my suggestion would be to use 5V on the coil to make the
contacts, and 0V to break them.
---

>The coil has around 130 ohms, or around 38mA activation current.  What should be the >values for R & C?

---
See above.
---

>2. Fully charged battery is around 4.2V (for a short time anyway), but the device is 3.6V max.  How much do i need to worry about this over-voltage?

---
Since the manufacturer probably expects the device to be operated with
fully charged cells once in a while, you probably don't have much to
worry about, but the prudent thing to do would be to ask the
manufacturer and find out from the horse's mouth.

--
JF
```
```On Tuesday, October 1, 2013 12:32:48 PM UTC-7, John Fields wrote:
> On Tue, 1 Oct 2013 10:56:42 -0700 (PDT), edward.ming.lee@gmail.com
>=20
> wrote:
>=20
>=20
>=20
> >I am working on a simple USB charging circuit.  Normally, the 3.6V NiMH =
battery is connected to the device via the NC contacts of a relay.  When co=
nnected, a 5V relay switch the battery to charge via a diode. So, a fully c=
harged battery is around 4.2V.  As someone suggested, i will put a parallel=
R & C to reduce the hold current of the relay.
>=20
> ---
> Why would you want to do that?
> ---

We want to fully charge the battery at 4.2V, without connecting it to the 3=
.6V chips.  It would likely be connected overnight, so the R & C would redu=
ce the relay power consumptions.

>=20
> >1. What is the minimum safe holding voltage/current of a 5V DC relay? =
=20
>=20
=20
> we find a relay with a coil very similar to yours with a MUST OPERATE
> voltage which is 75% of rated voltage (max.), and a MUST RELEASE
> voltage which is 10% of rated voltage (min.)

Yes, but we need a MUST HOLD voltage, perhaps 50%?

> >2. Fully charged battery is around 4.2V (for a short time anyway), but t=
?

> Since the manufacturer probably expects the device to be operated with
> fully charged cells once in a while, you probably don't have much to
> worry about, but the prudent thing to do would be to ask the
> manufacturer and find out from the horse's mouth.
>=20

Let says for example, a PIC32MX575F512 rated at nominal 3.6V and maximum of=
4.0V.  It seems to work fine at 4.2V.  But how much should we worry about =
this extra 0.2V?

Alternative, we can drop the 5V with double diodes, but much shorter batter=
y operating range.

```
```On Wed, 02 Oct 2013 06:28:22 +1000, <edward.ming.lee@gmail.com> wrote:

> On Tuesday, October 1, 2013 12:32:48 PM UTC-7, John Fields wrote:
>> On Tue, 1 Oct 2013 10:56:42 -0700 (PDT), edward.ming.lee@gmail.com
>>
>> wrote:
>>
>>
>>
>> >I am working on a simple USB charging circuit.  Normally, the 3.6V
>> NiMH battery is connected to the device via the NC contacts of a
>> relay.  When connected, a 5V relay switch the battery to charge via a
>> diode. So, a fully charged battery is around 4.2V.  As someone
>> suggested, i will put a parallel R & C to reduce the hold current of
>> the relay.
>>
>> ---
>> Why would you want to do that?
>> ---
>
> We want to fully charge the battery at 4.2V, without connecting it to
> the 3.6V chips.  It would likely be connected overnight, so the R & C
> would reduce the relay power consumptions.
>
>>
>> >1. What is the minimum safe holding voltage/current of a 5V DC relay?
>>
>
>> we find a relay with a coil very similar to yours with a MUST OPERATE
>> voltage which is 75% of rated voltage (max.), and a MUST RELEASE
>> voltage which is 10% of rated voltage (min.)
>
> Yes, but we need a MUST HOLD voltage, perhaps 50%?
>
>> >2. Fully charged battery is around 4.2V (for a short time anyway), but
>> over-voltage?
>
>> Since the manufacturer probably expects the device to be operated with
>> fully charged cells once in a while, you probably don't have much to
>> worry about, but the prudent thing to do would be to ask the
>> manufacturer and find out from the horse's mouth.
>>
>
> Let says for example, a PIC32MX575F512 rated at nominal 3.6V and maximum
> of 4.0V.  It seems to work fine at 4.2V.  But how much should we worry

Um, no. Rated 3.0 to 3.6 with 3.3v being nominal. 4.0v is the absolute
maximum where the chip will not be destroyed but it is not guaranteed to
work properly. 4.2v is way out.
```
```> > Let says for example, a PIC32MX575F512 rated at nominal 3.6V and maximum
> > of 4.0V.  It seems to work fine at 4.2V.  But how much should we worry
>

> Um, no. Rated 3.0 to 3.6 with 3.3v being nominal. 4.0v is the absolute
> maximum where the chip will not be destroyed but it is not guaranteed to
> work properly. 4.2v is way out.

So, we really need a 2 and 1/2 cells battery (3 cells 4.2V/3.6V is too high and 2 cells 2.8V/2.4V is too low).

With 3 cells, we can only use half the capacities.

1. Diode before battery: 1/2 charge and full discharge.

2. Diode after battery: full charge and 1/2 discharge.
```
```On Wed, 02 Oct 2013 07:36:40 +1000, <edward.ming.lee@gmail.com> wrote:

>> > Let says for example, a PIC32MX575F512 rated at nominal 3.6V and
>> maximum
>> > of 4.0V.  It seems to work fine at 4.2V.  But how much should we worry
>>
>
>> Um, no. Rated 3.0 to 3.6 with 3.3v being nominal. 4.0v is the absolute
>> maximum where the chip will not be destroyed but it is not guaranteed to
>> work properly. 4.2v is way out.
>
> So, we really need a 2 and 1/2 cells battery (3 cells 4.2V/3.6V is too
> high and 2 cells 2.8V/2.4V is too low).
>
> With 3 cells, we can only use half the capacities.
>
> 1. Diode before battery: 1/2 charge and full discharge.
>
> 2. Diode after battery: full charge and 1/2 discharge.

A lithium cell and a 3v regulator will do or 2 x 1.5v or 2 x 1.2v and  one
of this range:

There are heaps of options. But you should dump the relay for something
else that more reliable, quieter, faster and generally less problematic.
```
```On Tue, 1 Oct 2013 13:28:22 -0700 (PDT), edward.ming.lee@gmail.com
wrote:

>On Tuesday, October 1, 2013 12:32:48 PM UTC-7, John Fields wrote:
>> On Tue, 1 Oct 2013 10:56:42 -0700 (PDT), edward.ming.lee@gmail.com
>>
>> wrote:
>>
>>
>>
>> >I am working on a simple USB charging circuit.  Normally, the 3.6V NiMH battery is connected to the device via the NC contacts of a relay.  When connected, a 5V relay switch the battery to charge via a diode. So, a fully charged battery is around 4.2V.  As someone suggested, i will put a parallel R & C to reduce the hold current of the relay.
>>
>> ---
>> Why would you want to do that?
>> ---
>
>We want to fully charge the battery at 4.2V, without connecting it to the 3.6V chips.  It would likely be connected overnight, so the R & C would reduce the relay power consumptions.

---
So what?

The relay's only dissipating a couple of hundred milliwatts, and it's
designed to do that forever, so why is that a problem?

Also, if you're _designing_ a charger, why don't you just design it to
maintain the battery at 3.6V and knock off all the bullshit?
---

>> >1. What is the minimum safe holding voltage/current of a 5V DC relay?
>>
>
>> we find a relay with a coil very similar to yours with a MUST OPERATE
>> voltage which is 75% of rated voltage (max.), and a MUST RELEASE
>> voltage which is 10% of rated voltage (min.)
>
>Yes, but we need a MUST HOLD voltage, perhaps 50%?

---
Idiot, there is NO SUCH THING as a MUST HOLD VOLTAGE which is lower
than the MUST OPERATE voltage on any data sheet I've ever seen.

--
JF
```
```> Also, if you're _designing_ a charger, why don't you just design it to
> maintain the battery at 3.6V and knock off all the bullshit?

3.6V is only 1/2 charged.  Fully charged battery is 4.2V.
```
```On Tuesday, October 1, 2013 4:42:04 PM UTC-7, edward....@gmail.com wrote:
> > Also, if you're _designing_ a charger, why don't you just design it to
>
> > maintain the battery at 3.6V and knock off all the bullshit?
>
>
>
> 3.6V is only 1/2 charged.  Fully charged battery is 4.2V.

This is not quite right.  The terminal voltage does NOT reliably indicate
the charge.  A fast-charge might put 4.2V onto the terminals, but
You must remove the charger and deliver current to a load when
doing any useful testing, because that's the way batteries are used.
```
``` > > Also, if you're _designing_ a charger, why don't you just design it to
> > > maintain the battery at 3.6V and knock off all the bullshit?
>
> > 3.6V is only 1/2 charged.  Fully charged battery is 4.2V.
>
> This is not quite right.  The terminal voltage does NOT reliably indicate
> the charge.  A fast-charge might put 4.2V onto the terminals, but